Blue Hawaii Hybrid Electrostatic Amplifier for Stax Omega II Headphones.

by Kevin Gilmore
(Project Editor: Chris Young)

The Blue Hawaii amp is my latest design in my search for the perfect amp to pair with my Stax Omega II headphones. The genesis for this hybrid electrostatic headphone amplifier occurred when I was in Hawaii on vacation, at a fancy hotel on Maui. Sitting at the bar on the beach, drinking “Blue Hawaiis,” I drew the schematic for the amp on a placemat. The design is my conception of the mysterious and rare Stax T2 amp, which I have never been able to find at anything resembling a rational price.

I searched out any information I could find on the T2 in an attempt to create my own version. I was able to determine that it used EL34s as output tubes in a grounded grid configuration, which is the lowest distortion tube output circuit known. It also used 6DJ8s as input tubes with some solid state in the second and third stages. My design uses the first and second stages from my solid state electrostatic amplifier coupled with a third FET stage and then the final grounded grid stage.

My design ended up with a fairly large amplifier pulling significant amounts of power which results in a very smooth and extended frequency range from DC to over 200khz (-3dB at 400khz). Of all my electrostatic amps, this one has the largest output voltage swing. This is not an amplifier for the timid, nor is it a good idea to build this as your first project, though some, however, have actually done so.

The Circuit

Figure 1
(Click here to see single-image schematic of amplifier.)

Figure 1 is the amplifier schematic. The entire amplifier has a differential topology from input to output to get a balanced input and for lower noise, less ground loop problems. The first stage is a differential amplifier with feedback directly from the output stage. It works equally well with both balanced and unbalanced audio input sources. The step attenuators from Goldpoint make good volume controls for this stage. The JFET device (Q1) is a dual JFET all on one wafer. It is known for extremely low noise and excellent matching, and is used in a number of expensive designs, such as the Nelson Pass amplifiers. Q17 is a current source that sinks 3mA.

Because the amp is totally DC coupled from input to output, drift in the input stage is a bad idea. Since the first two stages run in current mode, the JFET input is more linear than a pair of bipolar transistors. Dual transistors all on one wafer suitable for audio use are hard to find these days. The FETs steer current away from the current sources Q2 and Q3. Together Q2 and Q3 each supply 2mA or a total of 4mA. The Q17 current source takes away 3mA leaving 0.5mA in each of Q4 and Q5, but some of the sink current is coming from the output feedback, so each FET is actually using somewhere between 0.5mA and 1mA.

The approximate voltage gain of this stage is 5; this stage really runs in current mode. The unit was designed to work equally well in both balanced and unbalanced mode. For single-ended signals, ground either the + or – input and apply signal to the other. The much higher impedance of the JFET works better when one side is grounded for unbalanced inputs.

The second stage starts with a constant current source (Q2 and Q3). The current source feeds a common base amplifier (Q4 and Q5). The common base amplifier feeds a modified Vbe multiplier. I believe a famous designer is now calling this circuit a current tunnel. It’s the most linear way of translating the voltage down to the bottom rail. The voltage gain of this section is about 4. The basic idea of the first two stages is to supply the third stage with a very fast low impedance drive signal that is referenced to the bottom rail.

The currents flowing into the common base amplifier (Q4 and Q5) are the difference between what Q2 and Q3 are supplying and what the FET is taking away. The rest of the current goes down the tunnel to the vbe multipliers (Q6 and Q7) that convert the current back to voltage. The current sources in the second stage supply 2 mA each. With no signal, the FETs take 1 mA, leaving 1 mA going through the common base amplifier into the bottom transistors, which are wired as Vbe multipliers (like a zener diode in series with a resistor, except a lot less noisy). This generates the 13 volts (referenced to – rail) necessary to properly bias the third stage.

The third stage is another differential amplifier (Q13 and Q14) being driven via another constant current source (Q10 and Q16). The voltage gain is about 200. Q11 is the power supply for this stage and makes a 100 volt power supply with -400V as the reference. The power supply voltage for this stage is kept down to 100 volts to reduce the Miller effect and keep the frequency response up. The higher output impedance of this stage is lowered by the use of 2SJ79 transistors, which are used as zero voltage gain emitter followers. The use of FETs in this stage coupled with the current source further reduces the distortion and provides for a solid low impedance drive signal for the output stage.

The 4th and final stage is a tube in grounded grid configuration (V1/Q8 and V2/Q15), similar to the common base amplifier in the 3rd section of my solid-state current-domain electrostatic amp. Q9 and Q12 are high compliance current sources and supply 25mA of bias current. Think of them as linear pull-up resistors for current (in fact, one builder has replaced the current sources with large resistors). The use of a current source here instead of load resistors acts to further linearize the output stage and reduce output distortion. V1 and V2 are the equivalent of common base amplifiers and do the entire rail-to-rail output voltage swings.

With feedback, the overall voltage gain of the amp is exactly 1000. The frequency response is kept high due to the low impedance cathode drive. The EL34s are biased at 10 watts and have an 800V voltage swing (by comparison, the output tubes of my original DC-coupled electrostatic amp are biased at 2 watts with a 600V swing), resulting in a frequency response well in excess of 100kHz into a 150pF load. (+0/-0.1dB).

Figure 2

A regulated power supply design is shown above. The ±15V supply is made with the standard 7815/7915 regulators. The high voltage supply is a pair of 400 VDC supplies, glued together at the output (P-channel MOSFETs are a lot more money than the equivalent N-channel MOSFET). In each section, beginning with a 460V raw supply, a PNP transistor (2SA1968) is used as a current source to feed the 400V zener reference. Then a N-channel FET is used as a high impedance, input voltage follower and outputs 400VDC. By the way, the same exact supply with a 350V zener reference string instead and a slightly smaller transformer (without filament windings) is what I use now for the solid state current domain headphone amp.

The bias supply is a voltage doubler with an adjustable reference. It has a range of about 350VDC to 650VDC. For low bias Stax headphones, put a 10M resistor to ground at the end of the 4.7M. to make the output voltage .66 times the voltage before the 4.7M, which puts it in the range for low bias.

Construction

(Click here to download pc board patterns in pdf format. 1)
(Click here to download pc board patterns in pdf format. 2)
(Click here to download pc board patterns in pdf format. 3)

Caution: This project involves working with high voltages, so be extremely careful! Keep one hand behind your back at all times. 800VDC across both arms might possibly stop your heart.

This amp was assembled on three printed circuit boards (two for each channel of the amp and one for the power supply) and housed in separate enclosures. A complete set of pc board patterns (pdf format) can be found above. They could be sent to just about any circuit board manufacturer to have boards made. The top of the board is almost all groundplane. All the parts, including the tubes, are mounted on these boards – the tubes are installed in pc-mounted ceramic tube sockets from Parts Express. The tubes must be exposed through the chassis. They dissipate 20W each (actually 10 to 12 watts of plate dissipation plus another 6.3V * 1.6A = 10 Watts of filament power).

It was so much easier to do a pc board for this amp, but if I were to make a prototype, I would again use the 0.1mm perf board; the layout look much like the circuit board. (Note: For a layout in a single chassis, see the interior view of Headamp.com’s Blue Hawaii amp below.) 99% of the wiring would be on the bottom, and it would be, therefore, rather flat. Mounting the tubes would be trouble though, and would cause mechanical problems. The tubes are fairly heavy and get stinking hot. Each chassis measures 12″ x 10″ x 3.5″. (Note: Headamp.com is selling the Blue Hawaii design in a single chassis measuring about 16.5″ W x 16.0″ D x 3.5″ H and may sell the Blue Hawaii pc boards. Please contact Headamp for more information.)

I have Mullard EL34 tubes, but keep them put away due to what I could sell them for if I wanted. I actually used the National Union tubes from Richardson Electronics which cost $11.50 US each.

All of the parts except the 2SA1968 have lots and lots of sources such as Digi-Key, MCM Electronics and Mouser Electornics. Only B&D; Enterprises has the 2SA1968 in the United States. In Japan and Canada, they can be ordered from Sanyo direct – the minimum order is 100 at a time, but then they are $1.25 each or so.

Q9 and Q12 are each made of six 2SA1968 transistors in parallel with one 2SA1968 as the driver. Matching the transistors is not required – unless one of the 2SA1968s is way off compared to the rest in which case it might get way too hot.

All resistors are 0.25W except where labeled. It is important to have all the pnp current source transistors correctly mounted to a large heatsink with silicon impregnated washers. If any one pnp transistor gets too hot it can short out the whole current source.

Standard tab heat sinks will do for the 2SK216 and 2SJ79 transistors, but the 2SA1968 and 2SC3675 transistors must be mounted a big heatsink (one for each channel), capable of dissipating 20 Watts of heat. I obviously fabricated them, but otherwise they can be obtained from Conrad Heatsinks cut to length. The IRFBC30 MOSFETs in the power supply must be heatsinked too: Mouser part number 532-529902b25.

The Stax SRC-5 headphone jack came from AudioCubes.com. Since the price has gone up to $19 each (I paid $10), it may be more cost effective to use the Allied jacks (see the current domain amp project article). Allied has a $25 minimum order, the cost of three pieces. Then they must be filed down on a lathe. Actually I am buying the male connectors from Allied, because no one else sells them. The male connectors are much easier to convert to standard Stax plugs. The power supply-to-amplifier connectors are the Amphenol military 12-pin connectors. The 4 connectors (two male and two female) were $130.

The custom Victoria Magnetics power transformer has these specs: 2 x 330VAC/150mA, 36VCT/100mA, 2 x 6.3V/5A (filament supplies). Everything with Victoria Magnetics is custom. I paid $110 for the transformer with shipping. They know about the Blue Hawaii design and will supply the correct transformer on request. For safety, I recommend a 2A/110Vac fuse located in the input line to the power supply.

Setup and Results

Test voltages (with the amp at idle) are shown in red on the schematic and are with respect to ground. To set up the amp, adjust the two pots in each channel of the amp. P1 controls the differential output voltage. Put a voltmeter between the 2 stators of one channel of the headphone and set this pot for zero. P2 controls the voltage with respect to ground. Put a voltmeter between any stator and ground and set for zero. Then repeat both adjustments a few times. The plates of both tubes should measure 0 volts with respect to ground when the pots correctly adjusted. Once the pots are adjusted, that’s it – there’s no change from headphone to headphone.

Setting the bias voltage depends on the headphones. For Stax headphones that can accept a high bias voltage, adjust the pot for 560V. I do not think that the Omega II headphones can be damaged by this amp unless the bias is set way too high. If the bias is set right, the outputs are close to 0V at idle, and all the LEDs are lit, then the amp pretty much has to be working correctly. Now if one or more of the outputs is stuck at +400V or -400V, then something is seriously wrong and needs to be fixed. An oscilloscope really helps.

Adjust the pot to 580 volts for Sennheiser HE-90 and HE-70 headphones or leave it at 560V. For Koss headphones, adjust the bias for 600V. To use the Sennheiser HE60 headphones with this amp, I made the adapter shown above. Those are RS-232 female connector pins that fit the HE60 pins perfectly. By hand I cut a circuit board with lands exactly 3.5mm apart put the pins on the HE60 connector, lay them down on the circuit board and solder. Then attach wires and a standard stax plug.

The amp can output 1500 V p-p measured stator to stator. At 800V_p-p, THD is less than 0.004% from 20Hz to 20kHz. The actual frequency response is 0 to 100kHz (-3dB at 150kHz) into an Omega II load. Compared to the sound of my previous tube amplifier, the bass is no longer tubby; it’s very sharp and tight. The high end is no longer rolled off, so female voices sound much more real. If the bias supply is reduced to 280V, the amplifier will drive all electrostatic headphones. I tried it last night on a pair of SRX’s. I never ever heard them sound so good.

Previously with a standard dummy head, I measured the SPL in Omega 2 headphones driven by this amplifier. With a drive signal of 800V_p-p per side, the resulting spl is 106dB. THAT’S LOUD! The amp can put out 1500 volts peak-to-peak, and thats louder! I just ordered a pair of Stax SR-001 MkIIs, which can reach up to 120dB. My ears distort before the amplifer/headphones do. It is quite loud at clipping, but the clipping is a hard clip with no oscillation or ringing. To use the amplifier with electret headphones, delete the bias voltage. And probably keep the output swing under 200V. Electrets phones when driven with this amplifier can probably get very very loud.

Several of my previous electrostatic designs are available in the Headwize Projects section. Comparing the Blue Hawaii to my all solid-state current domain amplifier, they really are more the same than they are different, but in general, the differences are the differences between tubes and solid state, such as a much smoother high frequency response, which in the case of the Blue Hawaii goes well beyond 500kHz. Additionally, the four times power consumption of the Blue Hawaii means a much stiffer and tighter bass response. Even though both are flat to zero and test similar, the BH bass is much more apparent and tighter.

c. 2004 Kevin Gilmore.

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A Simple Tube/Opamp Hybrid Amplifier.

by Alex Cavalli, Mark Lovell and Bill Pasculle

Introduction

After seeing many of the excellent and eye-catching tube-solid state amplifiers of others, we’d like to present a slightly different topology of hybrid amplifier design, using the same two basic components of a tube and an opamp. This amp is simple and has less than $50.00USD worth of parts and no lethally high voltages. It makes an ideal tube headphone amp for those who like the sound of tubes but worry about the safety of high voltage equipment.

Bill was looking for an amp that sounded good and also was a good companion for his Rio Carbon portable player. He also suspected that others might like something similar as well. With high bit-rate MP3 or WMA files, the Carbon can produce excellent sound, but as with many other portable players, benefits by the addition of a decent headphone amp. He also wanted to use the amp in his cubicle at work – requiring good sound without taking up too much space.

The design of the amp started when Bill suggested that he wanted to build one of the YAHA (Yet Another Hybrid Amplifier) amps , but using a different design. Alex, being mostly interested in tube amps, was not initially interested but as Bill and Mark began to suggest requirements for the amp and some options for the input tube, it was obvious that there was room to improve the typical hybrid topology. After more thought, Mark provided a list of some requirements for the design that he felt were necessary to meet in order to improve on the hybrid topology.

Alex went to work taking the design discussions and turning them into a draft design. Our first name for the amp (as a joke) among the team was Stoopid Opamp Headphone Amp (SOHA). The name, as so often happens with skunk-works project names, eventually stuck and finally we are just calling this amplifier the Stoopid or the SOHA. Another name for the amp, with the same acronym, might be the Simple Opamp Hybrid Amplifier.

The original prototypes assured us that it is possible to make a surprisingly good performing amp utilizing a tube at relatively low voltage and while still keeping the build cheap, easy, and reasonably electric shock-free. After altering some of the power supply and circuit values and testing the prototype, we ended up with something that was stable and fun to listen to for extended periods. It applies some compression but that’s part of its charm, and it would be a rather sterile sounding amp without the 12AU7/ECC82 altering the sound the way it does with its 40V plate voltage.

Amplifier Circuit

Most of the hybrid amps that have appeared in HeadWize threads and elsewhere (such as the Millet hybrid) have used the same B+ for both the tube and the opamp. Some of these amps are designed to be portable enough to run from a battery, but most are really constrained to a low voltage DC supply of some kind plugged into the line and so are not truly portable.

In addition, it is generally true that tubes that are not designed for low voltage use will not perform well at 12-24V (which is why the Millet uses special low voltage tubes), so we decided to try to provide the tube with higher B+ to get better performance, while still keeping the voltages fairly low. This meant that the amp could be small and portable although requiring AC power. Like the other hybrid amps, the SOHA is designed to give the sound of tubes while avoiding the high voltage risk that some builders don’t like. Still, providing a higher B+ permits us to get excellent sound from a more commonly available tube like the 12AU7/ECC82, which is in good supply from NOS and current production sources and which gives a wide variety of choices for tube rolling. Having this wide selection also makes it easier for the amp to be constructed in any part of the world.

The way in which most other hybrids use a common B+ for both tube and opamp has two detrimental effects on a hybrid amp:

1. It puts the opamp in a single ended configuration where it needs an output cap to block half the B+
2. It forces the B+ on the tube to be low so as not to exceed the maximum opamp rail voltages.

The first design decision was to decouple the B+ for the tube and opamp. Doing this makes it possible to use a standard bipolar supply for the opamp, eliminating the large output cap (as in the Chu Moy’s pocket amp for example) and requiring only a small coupling cap between stages (see power supply discussion below).

The tube is loaded with a constant current source (CCS) for two reasons:

1. The resulting non-linear performance associated with low voltage operation is partially offset by the high dynamic impedance of the CCS
2. A CCS has a much better PSRR than a simple resistor making it possible to have more ripple in the B+ and, therefore, simplifying the B+ PS.

The original amp is designed to run with FET input opamps. See note 1 (the BJT opamp section) for a version using BJT input opamps.

After extensive prototyping by Mark and Bill, and after posting the first design to the HeadWize forums and receiving feedback from several builders, we modified the original design. The most notable change was to the heater circuit. Originally the heater voltage was supplied directly from the AC secondary with voltage dropping resistors. This approach was implemented initially to maintain simplicity. However, because there is so much variation in transformer regulation, line voltage, and heater characteristics, the simple resistors were replaced with a regulator circuit. A side-benefit is the elimination of power wasted as heat since the dropping resistors reached 105°C – 115°C under normal operating conditions.

The Amplifier Circuit

The basic amplifier circuit uses a very small number of components, as shown:

R1	100k Log Pot	C1	1000u 10V Electrolytic
R2	300R 1/8W	C2	100n 100V
R3	560R 1/8W	D1, D2	1N4148 or similar
R4	2k Trimpot	U1	OPA2134 or similar
R5	1M 1/8W	V1	12AU7/ECC82 or equivalent
R6	150R 1/8W

Figure 1 – Basic SOHA Amplifier

The topology of the amp is a simple grounded cathode gain stage coupled through a capacitor to an opamp wired in unity gain mode. The standard SOHA uses LND150 depletion mode MOSFETs for the CCS for reasons discussed below. The amplifier circuit is, thus, very simple. Trim pots are provided as part of the cathode bias resistors to adjust for variations in tubes to set the plate voltages to ~40V. Each CCS is set to regulate at approximately 1mA.

An advantage of this design over many of the other hybrid designs is that there is no large coupling electrolytic at the output. The required inter-stage coupling capacitor is small making it possible to use good quality film/audio capacitors here at nominal additional cost.

With a 12AU7/ECC82 the input stage has a gain of about 12. This is sufficient for almost any source driving almost any headphones which is why the opamp is simply operating as a unity gain current buffer. However, with such high gain it is possible to exceed the input voltage tolerances for the opamp with just 1Vp at the input. The data sheet for the OPA2134 (and many similar opamps) indicates that the maximum input voltage is (V-) – 0.7V to (V+) + 0.7V. This means that the input swing must not exceed the supply voltage by more than one diode drop. The diodes ensure that this does not happen.

If the diodes conduct, the excess current passes into or out of the bipolar power supply. What happens thereafter depends on the ability of the tube to source/sink current and the ability of the PS to sink/source it. In this case, the tube will source/sink in the range of hundreds of micro amps which will find their way to the output caps of the bipolar supply which are in turn supplying current to the opamp V+ and V-. The output of the regulators will fluctuate some, but at this point the amp would not be operating properly anyway.

The standard CCS for the basic SOHA uses a single LND150 MOSFET in this configuration:

R7

1k 1/8W

R8

360R 1/8W

Figure 2 – Standard LND150 MOSFET CCS

For a discussion of why this was selected as the standard CCS, see note 2 (the CCS comparison section) at the end of the article. One limitation on the standard SOHA CCS is the uneven availability of the LND150 MOSFETs globally. To ensure that this amp can be built almost anywhere, we have created two variations that use other devices for the CCSs. The first uses J113 JFETs and the second uses 1N5297 current regulator (CR) diodes. Here are the schematics for both variations:

R8

1k5 1/8W

Figure 3 – Alternate CCSs using JFETs and CRDs

Care should be exercised when building the SOHA with the J113 JFETs. Their maximum Vdss is 35V. Under normal operating conditions they will see only about 15-20V, but if the plate voltage on the tube is too low it is possible to exceed this maximum and destroy them. To protect the JFETS, the minimum plate voltage should never be set below 20V (see below the warning about adjusting the trimpots). The 1N5297 CRD has a 100V maximum and should withstand all of the normal voltages in this amp. The J505, noted in parenthesis, will also work but has only a 50V maximum. This makes the J505 a little more robust in this circuit than the J113, but less desirable than the 1N5297.

Power Supply Circuit

The key to this amp is the power supply. Initially the amp used an easy-to-acquire 30VCT/200mA transformer. As noted above, during the development and testing process, including builds by several HeadWizers, we changed the heater supply from AC to regulated DC. To accommodate the 150mA DC drawn by the heater it is necessary to increase the current spec on the secondary to 400mA. This will also give some headroom for the amp itself. Eventually, we chose the Amveco TE70053 toroid to replace the original split bobbin transformer. Another benefit to using the toroid is less EM radiation in the box and, since the SOHA also designed to be small, this reduces or eliminates problems with PS buzz. Other transformer possibilities are in the Power Supply section below. You can use a higher current rating transformer without difficulty, but if you increase the voltage be careful about not exceeding the maximum input voltage for the regulators. The bipolar opamp supply is a conventional regulated supply using 78L12/79L12 inexpensive regulators. They have a maximum input voltage of 40V.

The trick to the power supply is the use of a 1.5x full-wave voltage multiplier to generate the B+ for the tube. To make the voltage multiplier, the entire secondary of the transformer is rectified through a pair of coupling capacitors and bootstrapped on top of the V+ of the bipolar supply. With a typical transformer with 25% regulation and with no load on the B+ for the tube, this generates over 80V (this is marginally dangerous and will give you a pretty good sting so be careful).

The power supply, including the heater circuit is shown below:

R9	2k2 1/8W	BR1, BR2	100V 1A Bridge Rectifier
R10	1k3 1/8W	VR1	12V Fixed Regulator 78L12
R11	11k 1/8W	VR2	-12V Fixed Regulator 79L12
C3-C6	100u 100V	VR3	Adj. Negative Regulator LM337
C7, C8, C11	47u 16V	D3, D4	1N4002
C9, C10, C12	470u 35V	T1	30VCT 400mA Transformer

Figure 4 – the SOHA Power Supply

When the B+ is loaded with the tube, with each triode drawing ~1mA, the voltage is pulled down to between +55-65V. This means that there is plenty of headroom in the B+ to run the tube at+40V while still leaving space for driving 7-10V into the opamp. And this seems to give very good performance. As noted above, using a CCS for the plate load relieves ripple requirements on the B+ so a much simplified and less expensive filter section becomes possible. For the components as drawn the B+ ripple is about 1mV. The capacitor values are kept low and, hence, the capacitors are small and inexpensive. Again, for CCS PSRR comparisons see below.

The heater supply uses a full-wave rectifier into a negative 12.6V regulated supply. Pay careful attention to the orientation of the rectifying diodes. The heater supply is attached to the negative half of the bipolar supply. This was done because the heater current will pull down the input to the filter section of whichever half of the bipolar supply to which it is attached. Since we are using the positive supply to bootstrap the B+ for the tube we don’t want the heater supply to pull this voltage down. Therefore, it is derived from the negative supply because if the negative input to filter drops by a volt or two the regulator will not be affected. Pay careful attention to the orientation of the rectifier diodes and capacitors in the heater circuit since it is a negative supply. A power indicator LED can be attached to the heater supply taking care to note the polarity. The negative regulator should be heatsunk to dissipate about 2W.

Construction

The full schematic for both channels and the PS is shown below with the complete parts list less some miscellaneous components such as enclosure, power switch, etc.

Click here to see full-size schematic.

R1	100k Log Pot	C3-C6	100u 100V
R2, R12	300R 1/8W	C7, C8, C11	47u 16V
R3, R13	560R 1/8W	C9, C10, C12	470u 35V
R4, R14	2k Trimpot	D1, D2, D5, D6	1N4148 or similar
R5, R15	1M 1/8W	D3, D4	1N4002
R6, R16	150R 1/8W	U1, U2	OPA2134 or similar, dual or single
R9	2k2 1/8W	V1	12AU7/ECC82 or equivalent
R10	1k3 1/8W	BR1, BR2	100V 1A Bridge Rectifier
R11	11k 1/8W	VR1	12V Fixed Regulator 78L12
R7, R17	1k 1/8W	VR2	-12V Fixed Regulator 79L12
R8, R18	360R 1/8W	VR3	Adj. Negative Regulator LM337
C1, C13	1000u 10V	T1	30VCT 400mA Transformer
C2, C14	100n 100V

Figure 5 – Full SOHA amplifier, both channels and PS

For the J113 version eliminate R7, R17 and change R8, R18 to 1k5 1/8W. For the 1N5297 version simply replace the entire CCS with the single diode.

The amp has been built several ways by different HeadWizers [click here to see forum member Neurotica’s (Jim Eshleman) SOHA build narrative]. Mark and Bill initially built the prototypes using point to point wiring on perfboards and several others did so as well. Bill eventually also made a homemade PCB while Alex designed a PCB using the commercial package ExpressPCB (see below). Most builds to date have been like the prototypes with the PSU and amplifier circuits on the same board. Pictured below is a pictorial drawing showing how the SOHA can be wired point-to-point on a 4 x 6-inch perfboard.

Figure 6a – Bill’s SOHA constructed by point to point wiring on a perf board
Click here to see full-size layout.

Part of our purpose with the design and component specs is to keep everything as small and cheap as possible. The parts list shows the parts from the usual American suppliers. Mark was able to source similar parts from Farnell and RS in the UK.

Part #	Mouser Catalog Number	Description	Qty.	Price	Total
R9	270-2.2K-RC	Xicon 2.2k 1/8W	10	0.11	1.10
R3, R13	270-560	Xicon 560R 1/8W	10	0.11	1.10
R11	270-11K	Xicon 11k 1/8W	10	0.11	1.10
R10	270-1.3K-RC	Xicon 1.3k 1/8W	10	0.11	1.10
R5, R15	270-1.0M-RC (regular CCS)	Xicon 1.0Meg 1/8W	10	0.11	1.10
	270-100K-RC (mu follower)	Xicon 100k 1/8W	10	0.11	1.10
R2, R12	270-300	Xicon 300R/1/8W	10	0.11	1.10
R6, R16	270-150-RC	Xicon 150R 1/8W	10	0.11	1.10
R4, R14	652-3306K-1-202	Bourns 6mm 2K pot	2	0.56	1.12
C7, C8, C11	140-HTRL16V47	Xixcon 47uF/16V	3	0.07	0.21
C3, C4, C5, C6	140-HTRL100V100	Xicon 100uF/100V	4	0.42	1.68
C9, C10, C12	140-HTRL35V470	Xicon 470uF/35V	3	0.3	0.90
C1, C13	140-HTRL16V1000-TB	Xicon 1000uF/16V	2	0.25	0.50
C2, C14	1429-1104	Xicon 0.1uF (100nF)	2	0.44	0.88
	CCS Options
	512-J113	J113 JFET	4	0.24	0.96
R8	270-1.5K	Xicon 1.5k 1/8W	10	0.11	1.10
		OR
	689-LND150N3-G	LND150 MOSFET	2	0.55	1.10
R8, R18	270-360	Xixon 360R 1/8W	10	0.11	1.10
R7, R17	270-1K-RC	Xixon 1k 1/8W	10	0.11	1.10
		OR
	610-1N5297	1N5297 CC Diode	2	4.29	8.58
BR1, BR2	821-DB102G	1A 100V Bridge	2	0.33	0.66
VR1	512-LM78L12ACZ	LM78L12 Regulator	1	0.27	0.27
VR2	512-MC79L12ACP	LM79L12 Regulator	1	0.34	0.34
VR3	512-LM337T	LM337 Regulator	1	0.5	0.50
	567-273-AB	Wakefield Heatsink	1	0.38	0.38
Power LED	351-3310	Xicon Blue Led 3mm	1	1.5	1.50
	271-560-RC	560R 1/4W LED Resistor	10	0.09	0.90
D1, D2, D5, D6	78-1N4148	1N4148 Diodes	10	0.03	0.30
D3, D4	512-1N4002	1N4002 Diodes	5	0.10	0.50
R1	313-1240-100K	Taiwan Alpha 12mm pot, 100k	1	2.84	2.84
	575-393308	IC Socket	1	0.36	0.36
J3	161-3502	3.5mm Headphone Jack	1	0.92	0.92
J1	161-1052	RCA Jack Black	1	0.82	0.82
J2	161-1053	RCA Jack Red	1	0.82	0.82
				Total	38.04
		OR
	DigiKey
U1, U2	OPA2134PA-ND	OPA2134PA	1	2.63	2.63
T1	TE70053-ND	Amveco 30V CT 500mA	1	12	16.22
				Total	18.87
	Optional Sources
	Newark
U1, U2	75C4624	OPA2134-PA	1	2.37	2.37
	18C6948	J113 JFET	2	0.2	0.40
VR1	34C1091	LM78L12ACZ Positive regulator	1	0.28	0.28
VR3	34C1076	LM337T Regulator	1	0.67	0.67
R4, R14	46F1092	Bourns 6mm 2K pot	2	0.27	0.54
Antique Electronic Supply
V1	T-12AU7-JJ	JJ 12AU7 Tube	1	8.95	8.95
V1	P-ST9-511	Tube Socket	1	1.95	1.95
Miscellaneous
	Knob
	Power Switch
	Wire
	Fuse holder and Fuse (0.25A)

The Amveco toroidal transformer (30VCT/500mA) is available from Digikey. Remember with 15-0-15 VAC (nominal) secondaries and the poor regulation of these inexpensive transformers, you will see over 21V with no load at the inputs to the bipolar power supplies and ~85V with no load for the B+. Because of the poor regulation make sure to use capacitors with voltage ratings that meet these off-load conditions. Note that the PS parts table shows 100V capacitors for the B+ section. If you use a higher voltage transformer make sure that the input to the regulators does not exceed their maximums (typically about 37V).

Some other possible split bobbin transformers are: Dagnall D3019 (0-240 pri), D3023 (0-115,0-115 pri). Both are 12VA. Other possible toroids are: MULTICOMP MCTA015/15 (0-115,0-115 pri), MULTICOMP MCFE015/15 (0-230V pri), or MULTICOMP MDCG015/15 (0-230V pri).

This design is optimized for 12AU7/ECC82 and its exact equivalents (5963, 6189, and 6680) rather than a close equivalent (or other types of tubes such as 6922).

All three flavors of CCS provide a degree of PSRR and some immunity from power fluctuations. They differ in availability worldwide and in maximum voltage ratings. The best overall CCS uses the LND150 MOSFET which is not available everywhere. The J113 FET is widely available but its maximum DC voltage is only 35 volts. Normally the FET wouldn’t see more than 15-20V unless the plate voltage gets too low. The 1N5297 CRD has a maximum voltage of 100 VDC but is not as widely available and is also expensive. Nevertheless, working amps have been built using all three types of CCS. Trim pots located at the cathodes are used to adjust the plate voltage. In order to prevent burning out the CCS FETs the cathode trim pots should always be turned to their highest resistance when swapping in a new tube.

OPA2134 and its relatives are fairly common opamps for audio. This was a good place to start. The authors would like to know how other FET input opamps perform and welcome feedback from builders. The OPA551, for example, is a FET input opamp that drops right into the Stoopid. However, it only comes in single packages so you will have to account for this with the build.

FET input opamps are preferred because there is a risk that BJT input opamps may tend to excessively load the tube and defeat the effect of the CCS. To use BJT input opamps see the section below for modifications to do this. The authors welcome feedback on the performance of the SOHA with BJT input opamps.

A BUF634 could easily be put into the unity gain feedback loop of the opamp to give super high output. One change that might be necessary if really trying to pull 200mA is to increase the size of the input capacitors in the bipolar PS to more like 2200uF. Even larger values may be required to get full bass.

Mark added 150 Ohm resistors (R6) at the outputs as this enables the amp to drive low and high impedance headphones without experiencing a large change in volume. These can be left out of the circuit at the builder’s discretion, however, their use is recommended. Likewise the pairs of 1N4148 diodes connected to the non-inverting inputs of the opamps are optional, but serve to protect the opamp inputs from overload and their use is recommended.

Here are a few details to pay attention to during construction and double check before applying power to your SOHA:

• Wiring the Triad transformer is not intuitive; the pins are not numbered consecutively. Study the datasheet carefully.
• The 78L12 and 79L12 do not share the same pinout.
• The capacitors in the heater supply (as well as those in the negative half of the bipolar supply) have their positive leads grounded.
• Use of a star-ground is highly recommended.
• Use of shielded cable from the input jacks to the pot, from the pot to the tube grids, and from the opamp to the output jack is highly recommended. Attaching the safety ground to the star ground is optional. Most builds have worked fine with the star ground floating but an occasional unit has been quieter with the safety ground connected to the star ground.
• Grounding the pot body is usually required to eliminate static/hum.

Wire dress is important in this amp to avoid hum. Keep all signal wires away from the transformer; keep the filament wires as far away from the audio circuit as possible.

PC Boards

We’ve created Express PCB boards for the SOHA. These are related to the full schematics with part numbers shown.

For the J113 version eliminate R7, R17 and change R8, R18 to 1k5 1/8W. For the 1N5297 version simply replace the entire CCS with the single diode.

ExpressPCB and PDF files for both the amp and power supply are included below. The tube socket on the amp board is in the center of the board. Note that the tube socket mounts on the foil side of the board. With this configuration you can easily mark a hole in the center of the standoffs and punch it out to pass the tube through so that the tube can stick up through the chassis while the components are sticking downward.

The copper layer in these PDF and ExpressPCB files can be used for home etched boards.

SOHA Amplifier Board (PDF)
SOHA Power Supply Board (PDF)
SOHA Amp and PS boards (ExpressPCB)

Techniques for making home PCBs were suggested by HeadWizer Bill Blair. Here are some links that Bill used to make his own SOHA boards using the single layer PDFs:

EasyPCB Fabrication
HomeBrew Printed Circuit Boards

The boards can be jumpered to use all three versions of the CCSs and to operate as standard plate drive or as source follower drive. This is the stuffing guide for these possible configurations.

Click here to see full-size stuffing guide.

Figure 6b – Bill’s stuffing guide for the SOHA amplifier PCB

Setup

Wire everything up but don’t put the tube/opamp in yet. Measure the voltages at the B+, V+, V-, and heater. They should be >80V, +12V, -12V, and -12.6V respectively. If they are not then there is a problem that must be fixed before inserting either the tube or the opamp.

If voltages are good and nothing has fried, power down and then insert the tube and opamp. Before powering up again, dial your trim pots so that they are in the maximum resistance position. This will put the maximum bias on the tube. Measure the voltage at the plates (pins 1 & 6) and adjust the associated trim pot until the voltage comes down to 40V for each plate. After these adjustments, measure the B+ again. It should be between 55-65V. Occasionally you may find a NOS tube does not work well in this circuit. You may need to replace the tube to get good results. If so, the tube is probably outside of its published operating characteristics. Each triode of 12AU7/ECC82 draws only 1mA from the B+ supply and at these low currents there can be a wide variation in operating characteristics, particularly among tubes that may be marginally within spec.

Results

OK, how does it sound? Well, in short, stoopidly good. When first powered up, the prototype plate voltage was only 17V and the amp sounded decidedly solid state. Very “steely” and just tonally “off”. As the plate voltage was raised the sound became more lush and tube-like. At 40V the amp began to perform extremely well. The SOHA easily rivals the Cavalli-Jones/Morgan Jones which costs over three times more to build! It’s got decent amounts of bass, classic sweet tube midrange and plenty of top end extension. Also the soundstage is extremely wide and respectably deep. This thing is just plain stoopid fun to listen to!

The amp drives headphones of any impedance between 16 Ohms and 300 Ohms without problems, which covers most that are currently available.

The compression applied by running the 12AU7 with 40V at the plate allows an unexpectedly refined sound with no sharp edges, yet without being mellow. It will reproduce transients when required and has a respectable dynamic range. The overall result is something than can be listened to for extended periods with no “listening fatigue” and providing a pleasingly wide and reasonably deep sound stage.

As is the case with tube amps, a warm up time is required and in this respect the authors agree 20 minutes is required for it to sound its absolute best, but of course it’s up and running after 30 seconds.

Mark has compared this amp to three other headphone amplifier designs available at HeadWize having built them: namely the CJ, the CL MkII, and the BCJ MkI, (the BCJ MkII was unavailable). All of the alternative designs used for comparison tests are more expensive to build, all require potentially lethal voltages, and all are optimized to ensure the tubes are working at their optimum.

Clearly, the SOHA would be the worst of the bunch? Not so. It proved itself to equal the CJ and gets closer than expected to the CL MkII. That’s pretty impressive stuff for a tube amp deliberately designed to be cheap and not use lethal voltages.

Tube rolling in this amp is also a lot of fun. Mark and Bill, who listen mostly through Sennheiser HD-600’s, found that grey-plate 5963’s from GE and RCA and Brimar sounded better than other tubes. Some other Headwizers with low-Z cans seemed to prefer black-plate versions of these tubes. Among the new production tubes, the Electro-Harmonix 12AU7 seemed to approach (but not exceed) the performance of the NOS tubes while the JJ 12AU7 was a somewhat distant second. Differences between tubes seemed to be in the clarity of the top end and the amount and quality of the bass.

Here are some photos of Bill’s SOHA in its final home.

Figure 7 – Bill’s SOHA Top Side and Figure 34 – Bill’s SOHA The Guts

Note 1: BJT-Input Opamps

As noted above the SOHA was designed to use FET input opamps. However, to permit opamp rolling, we’ve created two minor variations that permit the use of BJT input opamps.

Bipolar-input opamps like the TSH22IN, NE5532, or NE5534 can substitute for the 2134. But bipolar opamps will have lower input impedance than the FET input opamps. This will increase the loading on the tube and increase the distortion.

One way around the increased loading is to configure the CCS as an active load source follower. This variation requires only one change in wiring at the CCS and will only work for the LND150 and the J113 versions. An active load source follower is a variant of a well-known tube topology where the CCS that is acting as a plate load is also utilized as the output device in a follower configuration. With this topology the output impedance of the gain stage drops considerably and its ability to supply current increases in proportion. With both FET topologies we can wire the FETs as source followers to make a hybrid follower configuration for the first stage.

If you’re using the LND150 CCS you can convert the CCS into a source follower by simply changing the point where the coupling capacitor is connected. The FET then becomes a source follower with low output impedance and the ability to drive higher currents into the load.

Figure 8 – Changing the LND150 CCS for BJT-input Opamps

If you’re using the J113 CCS you can covert it to a source follower using the same technique:

Figure 9 – Changing the J113 CCS for BJT-input Opamps

Although the 1N5297 CRD is actually a JFET wired as a CCS we cannot access the source of the device so the CRD cannot be used when driving BJT opamps.

For example, a full amplifier schematic for the standard LND150 CCS with BJT opamp is:

Figure 10 – Driving BJT input opamps

If your amplifier exhibits high DC offset with BJT opamps, you can decrease the value of R5 from 1M to 100k or even 50k without overloading the gain stage. Note that decreasing the value of R5 while leaving C2 at 100nF also reduces the low frequency response of the amplifier. To correct for this, increase the value of C2 so that the product of R5 x C2 is the same as 1M x 100nF. For example, if you decrease R5 to 100k, then to maintain the same low frequency response, increase C2 to 1uF.

For BJT input opamps, the full schematic is this:

Click here to see full-size schematic.

R1	100k Log Pot	C3-C6	100u 100V
R2, R12	300R 1/8W	C7, C8, C11	47u 16V
R3, R13	560R 1/8W	C9, C10, C12	470u 35V
R4, R14	2k Trimpot	D1, D2, D5, D6	1N4148 or similar
R5, R15	100k 1/8W	D3, D4	1N4002
R6, R16	150R 1/8W	U1, U2	BJT opamp, dual or single
R9	2k2 1/8W	V1	12AU7/ECC82 or equivalent
R10	1k3 1/8W	BR1, BR2	100V 1A Bridge Rectifier
R11	11k 1/8W	VR1	12V Fixed Regulator 78L12
R7, R17	1k 1/8W	VR2	-12V Fixed Regulator 79L12
R8, R18	360R 1/8W	VR3	Adj. Negative Regulator LM337
C1, C13	1000u 10V	T1	30VCT 400mA Transformer
C2, C14	1u 100V

Figure 11 – Full SOHA with BJT opamp, both channels and PS

Note the part changes shown in red. These are the only changes necessary to use BJT opamps in the SOHA. The layout of amp does not change.

Note 2: CCS Comparisons

The choices for standard CCS and acceptable variations are derived from three criteria:

1. maximum breakdown voltage of the CCS
2. current regulating ability
3. PSRR

These comparisons were done using PSpice simulations. These simulations are not likely to give absolute accuracy, but they are good at providing a relative comparison among the various CCSs.

Simulations were done for the following CCS types:

• Single LND150
• Single J113
• Single PN2907A
• Single 1N5297
• Single PN2907A with CRD bias string
• Cascoded J113
• Cascoded PN2907A
• Cascoded PN2907A with CRD bias string

This table shows the current variation, PSRR, and breakdown voltage (BV) for these various configurations:

	Current Variation	Ripple at Plates		BV
	300mVp 1kHz Input	1mVp 120Hz Ripple from PS
	Delta I (uV)	Delta V (mV)	PSRR (db)
Topology
Cascoded JFETs (J113)	0.01	0.002	-54	35
Single MOSFET (LND150)	0.9	0.0025	-52	500
Cascoded BJTs with CRD	0.26	0.0095	-40	60
Single BJT w/ CRD	1.2	0.0135	-37	60
CRD (1N5297)	6.6	0.017	-35	100
Single JFET (J113)	13.3	0.034	-29	35
Cascoded BJTS (PN2907A)	0.34	0.058	-25	60
Single BJT (PN2907A)	1.2	0.06	-24	60

The cascoded JFETs have the best current regulation, followed by the MOSFET. The cascoded BJTs with CRD and without CRD have the next best current regulation. This might make these the next best choices. But, we must look at the PSRR and BV tables too.

The cascoded JFETs also have the best PSRR but they have a low BV. The LND150 has nearly the same PSRR (indistinguishable as a simulation result) but a very high BV. The LND150’s current variation comes in fourth behind the cascoded BJTs. However, the PSRR for the cascoded BJTs is 12db and 27db less than the LND150. Furthermore the BV for the BJTs is on the margin of where the voltages in the amp may be, and the BJT CCSs require many more parts than the either the JFETs or the MOSFET.

Taking all of these results together, the LND150 rises to the top for the standard SOHA because of its good current regulation, excellent PSRR, very high BV, and low parts count. The cascoded JFETs come in second because of their excellent regulation, PSRR, and low parts count. The CRD comes in third because of its good regulation, high BV and extreme simplicity (only one part).

Appendix: Simulating the Amplifier in OrCAD PSpice

Alex Cavalli has provided the project files for simulating this amplifier using OrCAD Lite circuit simulation software. The simulations will run in OrCAD Lite 9.1 or 9.2 only (later versions of OrCAD Lite and OrCAD Demo are more restrictive and will not run the simulations). The installation files for OrCAD Lite 9.1 or 9.2 can be downloaded from various educational sites on the internet. Search for them using the keywords OrCAD or Pspice and 9.1 or 9.2. OrCAD 9.1 is the smaller download (27MB). If you have trouble finding these files, email a HeadWize administrator for help.

There are 4 programs in OrCAD Lite suite: Capture, Capture CIS, PSpice and Layout. The minimum installation to run the amplifier simulations is Capture (the schematic drawing program) and PSpice (the circuit simulation program).

Download Simulation Files for SOHA Headphone Amplifier

After downloading cavalli2_soha_sim.zip, create a project directory and unzip the contents of the cavalli2_soha_sim.zip archive into that directory. Move the .lib and .olb files into the \OrcadLite\Capture\Library\PSpice directory. These are the component libraries containing the SPICE models for the vacuum tubes, MOSFETs and opamps used in the SOHA. (Note: heater connections are not required for any of the triode models.) In OrCAD’s Capture program, open the stoopid.opj project file.

The two basic types of simulation included are frequency response (AC sweep) and time domain. The time domain analysis shows the shape of the output waveform and can be used to determine the amplifier’s harmonic distortion. They both run from the same schematic, but the input sources are different. For the frequency response simulation, the audio input is a VAC (AC voltage source). The time domain simulation requires a VSIN (sine wave generator) input. Before running a simulation, make sure that the correct AC source is connected to the amp’s input on the schematic.

The following instructions for using the simulation files are not a complete tutorial for OrCAD. The OrCAD HELP files and online manuals include tutorials for those who want to learn more about OrCAD.

Frequency Response (AC Sweep) Analysis

1. Run OrCAD Capture and open the project file stoopid.opj, if not already open.
2. In the Project Manager window, expand the “PSPICE Resources|Simulation Profiles” folder. Right click on “Schematic1-ac” and select “Make Active.”
3. In the Project Manager window, expand the “Design Resources|.\cavalli.dsn|SCHEMATIC1” folder and double click on “PAGE1”.
4. On the schematic, make sure that the input of the amp is connected to the V4 AC voltage source. If it is connected to V3, drag the connection to V4.
5. To add the triode library to the Capture: click the Place Part toolbar button (). The Place Part dialog appears. Click the Add Library button. Navigate to the triode.olb file and click Open. Make sure that the analog.olb and source.olb libraries are also listed in the dialog. Click the Cancel button to close the Place Part dialog.
6. From the menu, select PSpice|Edit Simulation Profile. The Simulation Settings dialog appears. The settings should be as follows:
   • Analysis Type: AC Sweep/Noise
   AC Sweep Type: Logarithmic (Decade), Start Freq = 10, End Freq = 300K, Points/Decade = 100
7. To add the triode library to PSpice: Click the “Libraries” tab. Click the Browse button and navigate to the the triode.lib file. Click the Add To Design button. If the nom.lib file is not already listed in the dialog list, add it now. Then close the Simulation Settings dialog.
8. To display the input and output frequency responses on a single graph, voltage probes must be placed on the input and output points of the schematic. Click the Voltage/Level Marker () on the toolbar and place a marker at grid of U6. Place another marker above R9 at the amp’s output.
9. To run the frequency response simulation, click the Run PSpice button on the toolbar (). When the simulation finishes, the PSpice graphing window appears. The input and output curves should be in different colors with a key at the bottom of the graph.
10. The PSpice simulation has computed the bias voltages and currents in the circuit. To see the bias voltages displayed on the schematic, press the Enable Bias Voltage Display toolbar button (). To see the bias currents displayed on the schematic, press the Enable Bias Current Display toolbar button ().

Time Domain (Transient) Analysis

1. On the Capture schematic, make sure that the input of the amp is connected to the V4 sinewave source (VAMPL=0.4, Freq. = 1K, VOFF = 0). If it is connected to V3, drag the connection to V4.
2. In the Project Manager window, expand the “PSPICE Resources|Simulation Profiles” folder. Right click on “Schematic1-signal” and select “Make Active”
3. From the menu, select PSpice|Edit Simulation Profile. The Simulation Settings dialog appears. The settings should be as follows:
   • Analysis Type: Time Domain(Transient)
   Transient Options: Run to time = 80ms, Start saving data after = 40ms, Max. step size = 0.001ms
4. To display the input and output waveforms on a single graph, voltage probes must be placed on the input and output points of the schematic. Click the Voltage/Level Marker () on the toolbar and place a marker at grid of U6. Place another marker above R9 at the amp’s output.
5. To run the time domain simulation, click the Run PSpice button on the toolbar (). When the simulation finishes, the PSpice graphing window appears. The input and output curves should be in different colors with a key at the bottom of the graph.
6. To determine the harmonic distortion at 1KHz (the sine wave frequency), harmonics in the output waveform must be separated out through a Fourier Transform. In the PSpice window, press the FFT toolbar button (). The PSpice graph changes to show the harmonics for the input and output waveforms. The input and output curves should be in different colors with a key at the bottom of the graph.
7. The fundamental frequency at 1KHz will have the largest spike. The other harmonics are too small to be seen at the default magnification. In the PSpice window, press the Zoom Area toolbar button () and drag a small rectangle in the lower left corner of the FFT graph. The graph now displays a magnified view of the selected area. Continue zooming in until the harmonic spikes at 2KHz, 3KHz, etc. are visible.
8. Harmonic spikes should exist for the output waveform only. The input is an ideal sine wave generator and has no distortion. To calculate total harmonic distortion, add up the spike values (voltages) at frequencies above 1KHz and divide by the voltage at 1KHz (the fundamental).

Note: simulations only approximate the performance of a circuit. The actual performance may vary considerably from the simulation as determined by a number of factors, including the accuracy of the component models, and layout and construction techniques.

c. 2006 Alex Cavalli, Mark Lovell and Bill Pasculle (remove _nospam_).