Class A MOSFET Headphone Driver.

Editor: The circuit described in this article is a MOSFET follower for driving headphones. FET followers can supply high current, but have a voltage gain of slightly less than unity. They are most suitable in applications where the input signal does not require voltage amplification – such as the output of a preamplifier or a portable stereo. If the input signal is too low, you can add voltage gain stage (see Designing an OpAmp Headphone Amplifier).

The class A headphone driver in figure 1 is ultra simple, except it does need a regulated supply and input and output capacitors. Most of the parts are not critical. I used a pot instead of the 100K bias resistors, so I could vary the output side of the MOSFET at 1/2 the supply. This maximizes the ouput potential of this driver. R1 limits the input current. The diodes are reversed-biased protection only. They short out spikes greater than 9 volts and negative 1 volt. In the original I used a 9 volt zener diode instead, but I figured it’s harder to get zeners. The circuit will work without the diodes.

The volume control, Rp, is optional, because the driver is meant to be fed from a source with a volume control. If the volume control is not used, then it would be a good idea to put a discharging resistor in front of the input coupling capacitor C1 to ground – around 100K ohms. Sometimes the preamp is capacitor outputted, but may not have a discharging resistor. So, in the worst case, there would be capacitors back to back, and the voltage bias in between the capacitors would not be necessarily predictable (e.g., at or near zero volts to ground), or could cause some pops when connecting with the power on. A volume control accomplishes the same thing with its resistance to ground.

All capacitors should be the highest quality, especially the output capacitor. The output capacitor is an easier-to-get size capacitor of 470 microFarads, but one could at least double that for better low response. Bypass it with a quality 1 microFarad polypropylene cap for the best sound. I included the discharge resistor R5 to help eliminate pops when inserting the headphone plug. The input resistor would do the same if the input was inserted live. It might also help eliminate turn-on thumps.

The MOSFET (Q1) should be mounted on a heat sink. Almost any heatsink could be used. Q1 dissipates up to 2.5 watts. I think about 2-3 square inches surface area is minimum, and probably should be more. I just mounted them to the chassis with insulators in my model. A LITTLE silicone grease is nice. The MOSFET and R4 will get warm during operation, which is normal. With R4 = 20 ohms, the source resistors and MOSFETS will dissipate up to 7 watts in the stereo pair using a 15 volt supply. R4 could be changed to even lower values – its simply a matter of delivering enough current out of the supply and providing enough heat dissipation.

The IRF513 is an N-channel power MOSFET, rated at V(ds) = 80V, I(d) = 4.9A and r(ds) = 0.74 ohms. I chose a device with a reasonably small resistance. A higher resistance, lower-rated device could also be used. There is a large selection from which to choose. Q1 draws less than 200 ma. at 9 volts. One thing I didn’t try was to use two circuits per side, so one could use direct coupling on the output side by feeding the signal to one side, and using the other side to provide a positive but equal resting voltage creating an effective zero-DC operating point. I wanted something simple though.

A 15V supply could also be used, but then the power rating of R4 should be increased a little. [Editor: Radio Shack sells a pre-assembled 13.8V regulated supply (RS 22-504). If this supply is used, make sure the MOSFET heatsinks can dissipate 5W.] C3 (usually a ceramic type) decouples the power supply from the circuit.

I put the amp in a Radio Shack box with vents (about 5″ x4″ x2″). Since the boxes sold change so often, I don’t know if Radio Shack still carries them. The binding posts on the front were used as auxillary drive amp for experiments. There is a 1/4 stereo phone jack on the front and RCA input jacks hidden on rear. The power supply is built-in.

This headphone driver design is simple, but it sounds pretty good to me. Using a pair of Grado SR60s, I compared the MOSFET amp to the built-in integrated phone amp in my modified Hitachi HCA 8300 preamp. The MOSFET amp was fed by the Hitachi preamp outputs. There was an immediate noticeable difference in sound. The MOSFET had a cleaner overall sound, not as muddy. Connecting the MOSFET amp directly to my Sony CD player, it sounded good and had more than adequate volume.

c. 2002 Greg J. Szekeres.

Addendum

7/16/98: While not absolutely necessary, small gate resistors (Rg) can be added to prevent high frequency problems with the MOSFETs. They should be placed as close to the gate leads as possible.

6/24/99: Updated value of R3 in figure 1 to 220K ohms when using 9VDC power supply. The author adds these comments:

As shown operating with a 9 volt supply, the value of R3 should be changed to 200K ohms, or 220K, a more common value. When using 9 volts with R3 = 100K ohms, the circuit does not get very warm, and this value of R3 should be avoided. If using a 13.8 volt supply, R3 should remain at 100K.

To make sure this device is operating at optimum, regardless of the power supply voltage, the output DC voltage should measure approximately 2.75 volts in front of the output capacitor – not more than 3 volts for a 13.8 volt supply, and closer to 2.5 volts for a 9 volt supply. The value of R3 can be adjusted so the output voltage is correct, a higher value representing a higher output DC voltage set point.

These voltages are the result from a higher than expected gate-source differential. I am measuring approximately 4.5 volts. It’s possible this circuit would benefit from even higher supply voltage and higher bias potential, but power dissipation will start to multiply rapidly, and there is some hazzard of gate blowout if care is not taken.

6/27/99: Power supply range increased to 9 – 15VDC with R3 at 220K ohms. R4 is 20 ohms, 5W. If things seem to get too hot or the supply can’t deliver a good clean 1 amp, use a higher R4 value: 39 ohms or something in-between. The author adds these comments:

The output DC voltage on top of R4 will vary between 2.5 – 6 volts depending on the supply voltages from +9 to +15 volts. For those who have previously built this device to operate at the lower voltages, R3 should be changed from 100K to 220K. In fact, it is recommended that all designs to +15 volts supply also change this resistor value. A noted increase in dissipated heat will result.

7/25/99: Added image of finished project.

9/7/99: Tomohiko Ishigami built this verion of the Szekeres MOSFET headphone amp in a spacious metal enclosure that once housed a computer power supply. It features the acoustic simulator circuit by Jan Meier (see A DIY Headphone Amplifier With Natural Crossfeed). Note the “star ground” wiring (all ground wires soldered at a single point on the chassis) to avoid ground loops.

I feel it is very good idea to use modular approach. I love this approach so much, I rebuild my Class-A MosFet amplifier with the same technique. I biased each MosFet on one small board while I lumped up my large polyethylene caps on another. Some may think this will take up a lot of room, but the result was much simpler and cleaner boards less susceptible to noises caused by, I guess, interferences. This amp ended up being very pleaseant sounding headphone amp. It is milder and warmer sounding which I love.

1/7/00: For DIYers who will be using Jan Meier’s natural crossfeed filter as a front-end to the MOSFET amp, here are some tips from Jan re: selection and placement of the volume control:

It all depends on the specific circuitry. Generally it might be better to place the pot after the filter instead in front of it. The influence of impedance changes might be less pronounced. A 10 kOhm pot will certainly be too small. 50 kOhm will be a kind of minimum I think.

1/21/00: The IRF513 has been discontinued. Greg Szekeres suggests substituting a MOSFET with an input capacitance of less than 200pF, such as the IRF510, IRF610 or IRF710.

2/15/00: Tomohiko Ishigami wrote:

I am sending you the picture of the upgraded Szekeres amplifier. As you can see the size of heatsink is rather too big. I punched up the power supply voltage to 15V using the LDS-X-15 PSU from Lambda Electronics. Also, the same PSU can output 5A. The resistors that control the quiescent current is replaced with gold metal-looking resistors. (deviation 1% and takes 20W) You can also see the large capacitors. All the film caps have been replaced with Audyn caps (1.5uF) and electrolytic caps with ELNA Cerafine caps. The headphone jack has been replaced with gold plated 1/4 inch socket. It was very cheap but very tough.

The very accurate quiescent current controlling resistors will give you very ideal current supply to the Mosfets. You can use voltage regulators here, but I think those are overkill and hey! I am lazy (valid excuse!). The capacitor upgrade in the path of signal always result in improvement. Especially, the electrolytic must be a good quality. I found Black Gate and other super-exotic parts, but ELNA Cerafine was reasonable in cost and quality. Black Gate rivals or is better than ELNA Cerafine but I hear that the chemicals within have very strange characteristics. (Ok, Ok, my pocket did not like Black Gate …)

The decision as to which headphone jack I would use took a while. I obtained a Neutrik socket. But it was not realistic. The Neutrik Socket is too large and requires you to make a major chasis modification. So I went for cheaper gold plated jack. I took a reamer and enlarged the 3.5mm socket hole I made a while ago. Personally, I feel more comfortable with this cheap one. Neutrik socket does not seem much more rugged. And the cheap one can be replace without modification at any time in future.

Right now, it has reference quality. Of course, not as refined as Melos SHA-1, but considering that it is 10th of the price, it is reasonable comparison. Although it is cheap and very simple, it still is a full blown class-A amplifier. I have to admire how it sounds so good.

5/01/00: The author has submitted several updates to his design:

The first update is a change in the MOSFET gate resistor (Rg) value from 100 ohms to 220 ohms. Also, the author now recommends a power supply of at least 15VDC. A 9VDC supply may not give the amplifier enough headroom.

The next update is an optional AC-coupled voltage-gain front-end for the MOSFET amp based on the Pocket Amplifier by Chu Moy. The OPA132 opamp is configured to run from a single supply by biasing the input at 1/2 the power supply voltage:

I got a modified pocket amplifier circuit to drive the MOSFET amplifier with gain. I’m not advocating the use of the combination to replace the original, but for use by those who need the gain. The 4.7K ohm resistor from the opamp output to ground is one way of making sure the opamp output rides in class A. It provides a somewhat constant DC current flow. The 4.7uF capacitor in the feedback loop decouples DC and also causes a bass rolloff – I think at about 10 Hz. The 1.0 uF capacitor stabilizes the DC bias point in the bias circuit. The protection diodes D1 and D2 are not required in this circuit.

The AC-coupled front-end allows for maximum voltage swing because the MOSFET can be powered and biased separately from the opamp (although both are shown in the schematic being powered from the same 15V supply). With the 15V supply as shown, the opamp will be able to swing between +7.5 to -7.5V. However, the MOSFET (which is biased at 2/3Vcc) will limit the output to about +5V/-5V with a 15V power supply. A +5V/-5V maximum output voltage swing is adequate for all but possibly the highest impedance headphones (e.g., greater than 600 ohms). For driving high impedance headphones, the output voltage swing can be increased by increasing the power supply for the MOSFET. The MOSFET bias voltage should be adjusted (via R3) until Vds is 1/2Vcc.

In those cases where a +5V/-5V output voltage swing is sufficient, the author has designed this DC-coupled version of the front-end gain stage. The MOSFET’s bias voltage is provided by biasing the opamp output at 2/3Vcc.

Finally, several DIYers have asked for a DC-coupled version of the MOSFET amplifier itself. It uses a dual ±7.5V (or higher voltage) regulated supply. The MOSFET Vgs is adjusted via a 10-turn potentiometer until the output voltage is 0V. The schematic show the bias pot value to be 100K ohms, but the author suggests that it could be higher: 250K to 500K ohms. Note: the MOSFET should be allowed to “warm up” for a few minutes and the output voltage checked again and readjusted if necessary for 0V.

Note: There have been reports that this DC-coupled circuit is difficult to stabilize due to thermal drift. Benny Jørgensen’s DC servo (below) automatically compensates for DC drift.

7/15/00: Eduard Orvisky built this DC-coupled version of the Szekeres headphone amplifier. He used a ±7VDC regulated power supply, and says that the amp “achieved excellent sound” in combination with his Parasound Class A preamp. More details of this design can be found on his website.

1/26/01: Benny Jørgensen created this version of the MOSFET amp with an opamp servo that keeps the DC output of the MOSFET at 0V. He has not built it, but did test the circuit in a SPICE simulator. He writes:

The DC servo opamp keeps the MOSFET output at 0 VDC (during idle). With the opamp power supply at +18V and -7V and the MOSFET supply at ±7VDC, the servo can compensate for variations in Vgs from around -1 V to 8 V (it will normally operate in the range from 3V to 7V – the absolute maximum). You can a single set of power supply voltages for both the opamp and the MOSFET, but the servo will not have the same range. For example, if the opamp and MOSFET are both powered from ±12V, then the servo will only compensate for Vgs changes up to 5V. If the opamp can’t deliver enough voltage, there will be DC on the output. MOSFET Vgs vary from model to model and I have tried the make a general servo that will work with most N-Type MOSFETs. The opamp’s negative supply should be less than -3V in order to compensate for any negative voltage at the output.

The idle current in the circuit shown is 7V / 20 ohm = 350 mA. To increase the power supply for the MOSFET to ±9V, just make the DC servo run on +18 V and – 9 V. The DC servo needs a supply that is more positive than the MOSFET itself. Using ±9V MOSFET supply and a 22 ohm resistor (for example) result in an idle current of 9 V / 22 ohm = 409.1 mA.

The opamp should be a FET input type like the TL071 or better the OPA132/604. All capacitors should be poly types. C2 forms the integrator together with the R6 = 330 Kohm. C3 is a low pass filter with R5 = 10 Kohm. C4 makes sure that the servo does not disturb the sound at high frequencies.

The SPICE simulations below are for the version of the circuit, where both the opamp and the MOSFET have the same ±12V power supply. The color of the markers corresponds to the measurement points on the schematic. The input is 1VAC. V(M1:s) is output of the MOSFET. V(C2:1) is output voltage of the opamp – the “working zone” of the servo. V(R4:1) is the bias voltage on the MOSFET gate. The impact of the DC servo falls off above 2Hz (see the Frequency Response). The Time Response shows the servo at startup going from -3.5V to 0V in 2.0 secs.

3/20/01: Tomohiko Ishigami replaced the source resistor on his MOSFET amp with a constant current supply (current source) made from a LM317 voltage regulator. Originally, he set the MOSFET idling current at 500mA (R_ref = 2.5 ohms/1W), but lowered it to 250mA (R_ref = 5 ohms/0.5W) because the MOSFET and LM317 were running too hot. He writes:

Constant Current Sources are good and allow active elements like MOSFETs, transistors, or even tubes to operate better. The modified Szekeres has survived 120 hours. It sounds surprisingly good. I recommend 250mA idling current. This is done simply by removing 2 of 4 stacked resistors. I used 10 ohm, 1%, 1/4W metal film resistors, because of the very precise value and temperature-stability. Carbon is not precisely valued and not temperature-stable. Certainly I would not use wirewound; I don’t want extra inductance.

500mA was a little too much, so I brought it down to 250mA, which decreased the heat production to 1/4. The amplifier as a whole is warm but not dangerously hot. LM317s should have heatsinks rated at least 10W. MOSFETs should have heatsinks rated at least 25W. If you are insistent on 500mA, you could expose the heatsinks or you could use a fan inside chassis. The sound quality does not change in going from 500mA to 250mA. My PSU is happier as well.

All other parts values are the same EXCEPT for C2. The value of output capacitor C2 in my amplifier was calculated for the AKG K240M 600-ohm headphones. When you drive K240M with this amplifier, you will need a high output CDP or other audio source (~2vrms), easy for most non-portable CDPs. If you have lower impedance headphones like Gradoes or even the Sennheiser HD580/600, you must recalculate the capacitance with Apheared’s equation:

C2 (uF) = 1,000,000 / [2p (corner frequency) Rheadphones]

The corner frequency (in Hz) is the 3dB low frequency response drop for the amplifier. So with my choice of 600 ohm phones with 10Hz corner frequency, the value of C2 = 26uF (I used 24uF + 1.5uF – all polypropylenes).

C2 is only for the AC-coupled amp. DC-coupled designs can be modified with a constant current source. Please do not forget to use large heatsinks. Using a dual power supply allows us to make MOSFET source voltage zero. If you have built DC-coupled Szkeres properly, you should need NO output capacitors.

Summary of amp upgrades:

1. Noble Pot
2. RCA inputs
3. Teflon internal connections
4. Less wiring
5. Added 1000uF + 47uF + 1uF + 0.47uF resevoir to PSU – smaller than the original 10,000uF (it died, after being fatally wounded with a reamer during case work). Will add new 10,000uF capacitor may be later.
6. No more crossfeed circuit inside amp chassis. I have external one instead.
7. DIY interconnect

First stage of upgrade:

You see smaller output coupling caps and 10000uF reserver bypassed with few lower value caps. The Noble 50K pot is used and connected close to RCA inputs. Note that those metal clad resistors are Non-Inductive DALE NH series resistors. Ninety-five percent of wiring is, of course, teflon insulated cables.

Second stage of upgrade:

In the center, there are two moderately-sized heatsinks attached to T0-220 chips, close to 4 blue metal film resistors. Those are the constant current supplies that set the MOSFET idling currents to the original 500mA. Both the current supplies and the MOSFETs heat up enough to boil water drops.

Third stage of upgrade:

Now I have 24uF Solen output coupling capacitors. Please forgive the messy wiring at the output. I was barely able to fit the caps in the box. Idling current is now limited to 250mA which is enough to make heatsinks hot, but cool enough so the chassis stays lukewarm. This was done by increasing the current controlling resistors (R_ref). The metal clad resistors are removed permanently.

The DIY interconnect is 3 braided 22 gauge cables. This sounds fresher and more dynamic than RCA coaxial cables. Probably due to low capacitance. Teflon insulation and 3-wire braiding makes a subtle but audible difference.

WARNING: All versions of Szekeres amp may burn your skin since MOSFETs run very hot. So be careful when constructing and handling this amplifier.

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