A Low-Voltage Class-A Tube Headphone Amplifier.

by Helmut Ahammer

Warning: Like other projects using tubes for amplification, the circuits described in this article contain high voltages, and, therefore, the risk of a lethal accident is evident. Neither the author nor HeadWize is responsible for any damage or harm resulting from the construction of this project. DIYers should be familiar with and follow high-voltage safety precautions when building this amplifier.

This amplifier (which I call the “VR2”) is the follow-up of my previous HeadWize project (Tube Headphone Amplifier/Preamp with Relay-Based Switching) and has been designed with following conditions having in mind:

1) Pure Class-A triode OTL design and only one tube for amplification.
2) The plate voltages should be low.
3) The output impedance should be as low as possible and the maximum output current should be as high as possible.

This amplifier is a stand alone Headphone amplifier without the property of input selection as mentioned in my previous project but could be expanded if desired.

ad 1) Preferring the Class-A OTL design I decided again to implement a long tailed pair for the input section and a parallel connected cathode follower for the output section.
ad 2) The voltages should be as low as possible. This condition was taken into account mainly because the risk of any high voltages at the output of the amplifier, if any damage occurs, should be kept as low as possible. This could be the case if the output capacitor shortens and then the full cathode voltage would be connected direct to the output, damaging the headphone or there would be the risk of touching lethal voltages! In my opinion it’s better that this voltage is 40V and not 150V or even higher.
ad 3) Low output impedance and a relatively high current for the output sections are needed to drive headphones and in connection with the condition 1) and 2) only a few tube types are suitable.

These conditions could be fulfilled very good with the 6DJ8, ECC88 tube type family. An operating point with a plate voltage of only 80V is well between the limits and the gain µ of 33 is well enough. For the parallel connected cathode follower the E288CC is well suited, delivering higher currents and an output impedance of 25 Ohm. The voltage at the output coupling capacitor could be with this tube about 40V.

Circuit Description

Figure 1

Figure 1 shows the whole circuit of the Headphone amplifier for one channel. Even the high voltage decoupling resistors (1k5 and 330 Ohm) and capacitors (100µF/400V and 220µF/400V) are implemented for each channel separately.

The audio input is connected to a volume potentiometer (47K log, ALPS, Noble or Panasonic) and then coupled with a 220nF MKP capacitor to a long tailed pair with the E88CC triodes . The long tailed pair delivers a signal which is not phase inverted and therefore the whole amplifier is not phase inverted. Pin 7 is connected to ground, because the amplifier is designed for asymmetrical input signals. If an upgrade to symmetrical signals is desired pin 7 could be connected to the second input signal. The 1M resistor connects the grid of the first triode to ground and the 100 Ohm resistor blocks RF oscillation of the circuit. The common cathode resistor with 180 Ohm sets the operating current of 5mA for each triode and the plate resistors 7k5 set the plate voltages of about 80V. The anode of the second triode is connected to the 220nF coupling capacitor. The output section is a parallel connected E288CC cathode follower. The 470K, 33 Ohm and 1K/3W resistors set the operating point of the tubes (about 76V plate voltage and 21mA plate current). The cathodes are connected to the output coupling capacitors 470µF/400V and 1µF/400V. The output resistor 4k7/1W pulls the output to ground if no load is connected.

High Voltage DC Supply

Figure 2

The supply was build up for each channel separately, only the power transformer is used for both channels (Figure 2). To save money I decided to serial connect the secondaries of two easy gettable toroidal transformers. The primaries are connected in parallel. Two smaller transformers instead of one big transformer have the additional advantage that they can be placed very space saving into the chassis. The serial connection of 2x18V and 2x55V transformers gave under load an AC voltage of about 167V.

I used a tube rectifier for this project, because the rectifier tube implements a slow turn-on characteristic for the amplifier tubes and eliminates turn-on cracks. In my first project, I used a lot of electronics to do this job. When using tube rectification (EZ80) with only one supply voltage it is necessary to implement full wave rectification with two additional diodes (1N4007). The cathode of the rectifier tube is connected to the capacitor 47µF /450V and a 20Hy choke with at least 50mA current specification. The 47µF capacitor should be of high quality and rated at least with 400V because the periodic current loads are the highest for this capacitor. The output capacitors are a combination of electrolytic (1000µF/400V) and foil (1,5µF and 100nF MKP) types. The 220K/2W resistor should be soldered near and directly to the big electrolytic capacitor to discharge the capacitor when there is no load.

The supply has a slow turn on characteristic of about a half a minute because the rectifier tube gets conducting slowly accordingly to the warm up of the heater filament. The output voltage is about 135V under the specific load of 52mA. Especially the voltage drop of the choke and the tube rectifier is dependent on the load current. If the choke with an actual resistance of 750 Ohm is replaced by a choke with an other resistance the secondary of the power transformers has eventually to be changed accordingly. Using relatively cheap toroidal transformers, it will be not very cost intensive. Independent of the secondary voltage, the minimum VA rating should be chosen so, that there is a maximal current rating of 1A.

Heater AC Supply

Figure 3

The heater supply is simply a 6.3V 40VA transformer with two 100 Ohm/2W resistors connected to ground for hum reduction (Figure 3). Additionally for saving money, I bought a cheap 12.6V 60VA transformer for halogen lamps. Then I rewinded from the secondary so much turns that there were the desired 6.3V under load. Doing so, one have to keep in mind that the VA rating of the transformer is halved if conservatively calculated. The rewinded transformer has at least the same current rating of 60[VA]/12.6[V] = 4.76A and therefore a VA rating of at least 6.3[V]x4.76[A] = 30VA. The actual current draw of all tubes is 2.75A. Therefore there is some margin for the case of using other tubes with higher heater current demands.

Construction

Tubes:

The tube for the input section is the double triode ECC88 or 6DJ8. Better versions are the E88CC, CCa or 6922 or even with less noise the E188CC or 7308. There are many brands NOS and from current productions available. I have good experience with Philips ECC88 NOS and JAN-Philips 6922. This type of tube has µ = 33, S=12.5mA/V and Ri=2k6. Nearly equivalent is the Russian 6N1P which has slightly differing specifications.

The tube for the output section is the double triode E288CC or 8223. This tube is sometimes falsely referred as a replacement for the E88CC type of tubes. Despite that this tube has only µ = 25 the operating current must be much higher. Furthermore the inner resistance Ri = 1k25 is less. The plate voltage is about the same as for the E88CC type. For the headphone amplifier output section this tube is very well suited. Relatively low plate voltage, a current of 20mA for each triode is far below the maximum power limit and with S = 20mA/V the output impedance of the parallel connected cathode follower is 1/(2 S) = 25 Ohm. This tube is a special quality double triode with a tested life time of 10 000 hours, gold pins and with a noval socket. I used Siemens NOS and the operating point matched instantly the data sheet very closely. I can really recommend this tube.

Figure 4

If there is no possibility to get this tube, the Russian 6H30 could be used with slightly changing the circuit. Comparing the data sheets it is possible to implement this tube by changing the 33 Ohm resistor. As this triode needs about -3.4V instead of -1.4V grid voltage for an operating point of 80V and 20mA per triode the resistor should be changed to about 80 Ohms for a common cathode current of 42mA. The exact value depends on the actual tube and has to be examined by a view trials. As S=18mA/V for this tube, the output impedance calculates to 1/(2S) = 27.8 Ohm. The higher heater current demand of this tube should be mentioned too. An other possibility is the use of two E88CC tubes and therefore four triodes connected parallel together. The same current of about 40mA could be achieved and with four triodes the output impedance is calculated to 1/(4 S) = 20 Ohm. I tried it and replaced the two E288CC triodes with four E88CC triodes yielding quite the same operating point. But on the long run I think it is better to split the cathode resistors that there is again one resistor for two triodes to ensure a better equivalent current distribution through the tubes. This alternative output section is shown in Figure 4.

There are many types of the ECC88/E88CC type tubes available with probably different electrical characteristics. Therefore, if the current at the 1K /3W resistor is not about 40mA it is possible to set the operating point by changing both 68 Ohm resistors simultaneously. Increased resistor values will cause a decreased current and decreased resistor values will cause an increased current. Paralleling tubes causes an increase of the input capacitance and therefore paralleling is not very often recommended. Nevertheless I had no bad experience by paralleling tubes, there is no audible high frequency roll of or so. The MOSFET has a higher input capacitance too and therefore there are problems in high speed switching applications, but there are a lot of excellent audio circuits around.

The rectifier tube EZ80 could be replaced with the 6V4 or any other tube with the same or even higher current range. The EZ80 is rated for a maximum current of 90mA. Therefore there is a margin when using two tubes, one for each channel respectively (the current demand for one channel is 52mA) . With less lifetime there could be used one tube for both channels which I would not recommend. Using one tube for both channels the better solution would be to use the EZ81 or 6CA4 with a maximum current of 150mA. Using other tubes than the EZ80 would lead probably to an other voltage drop at the rectifier diodes and therefore the plate voltage would change or this has to be taken into account and the secondary voltage of the transformer would have to be adapted correspondingly.

Transformer:

Any serial combination of transformers could be used and as mentioned above the current rating for both transformers should be at least 1A. As transformers have lower voltages with the load connected, the right combination should be worked out by experimentation.

Capacitors:

All capacitors with values up to 1.5µF should be MKP foil types. For this types I recommend the Epcos MKP capacitors. The electrolytic high capacitance capacitors should be industrial grade types. I can recommend Aerovox BHC electrolytic capacitors. They have larger dimensions as compared to standard brand types but have less resistance and a very long life time. The high voltage ratings for the capacitors are needed because the high voltage power supply is not regulated and with less load current the voltage raises. For instance, if one output tube is disconnected the voltage increases to over 200V!

Resistors:

All resistors should be of metal film type with a power handling capacity of about 0.5W or higher which is then specified in the circuits.

Choke:

At least 20Hy are recommended and the current rating should be at least or in the best case 50mA. If a choke is used with an actual current which is less than the nominal current the inductivity is decreased. The resistance of the choke determines the voltage drop when used with a specific current. If the choke has an other resistance as about 750Ohm, the high DC voltage would be changed or to circumvent this, the secondary voltage of the power transformers should be changed accordingly. I bought the chokes at the German transformer and tube gear seller Welter Electronic located in Ulmen/Eifel (Type: Dr.7 20Hy, 50mA, 720 Ohm). Sowter (www.sowter.co.uk) sells the type CB25 with 20Hy, 50mA and 451Ohm. Hammond (www.hammondmfg.com) sells the type 193C with 20Hy, 100mA and 181 Ohm.

Realisation:

The pictures shows the final realisation with a similar design as my previous amplifier. As I stated in my previous article I like to see the tubes but don’t like to see the transformers and capacitors. Therefore all the transformers and big capacitors are packed into the back part of the chassis. The chassis measures 435mm x 319mm x 122mm and is made from 1.5mm aluminium plates with enlarged hardness. The single aluminium plates are mounted together with L-shaped aluminium profiles and screws. The top of the actual version of the amplifier is built from three pieces of birch plywood which where glued together and sprayed with the same colour as the front panel.

One picture shows the same amplifier with an alternative top made of transparent lacquered beech plywood. The front panel is again made from white clay and has a thickness of 15mm. I drilled the holes after the drying period of about 6 weeks and fired it at about 1000°C in a kiln. After lacquering I engraved the letters with a high rotation mini drilling machine. Because of this ceramic front panel the amplifier is really heavy.

Results

Using good components this headphone amplifier is a high grade amplifier incorporating a really Class-A operation with all it’s sonic benefits. The use of only one amplifying device strengthen this approach. The sonic quality of this amplifiers lies in the excellent reproduction of the music especially with moderate volume levels. All the details are there without being overwhelmed by any special sonic property. Therefore this amplifier is very neutral but without any insistent behaviour. Long term listening with the Sennheiser HD580 or the Beyerdynamic DT770 Pro (250-Ohm) headphones is really a pleasure.

The overall gain = 12dB and with modern CD-players, which have outputs of 1V up to 2V the gain is far enough to get really loud volume levels. With my Philips CD-Player I turn most of the time the volume at the position 1/4 from maximum and the position 1/2 is really loud. I can’t hear with the maximum setting because this is far too loud and ear splitting for me. If really more gain is necessary there is the possibility to connect a capacitor (10µF 200V) to the 120V connection and the pin 1 of the E88CC triode. This capacitor shortens the 7k5 resistor for AC voltages and increases the gain of the first triode circuit which is principally a cathode follower. This capacitor should lead to a gain of 18dB. More gain is even achievable if the E88CC is not wired as a long tailed pair. Instead of the long tailed pair it is possible to build up two common anode amplifiers using separate cathode and anode resistors and an additional coupling capacitor. In this case the phase isn’t inverted too.

Compared to my first project I can say, that the bass control is increased (mainly because of the lower output impedance of 25 Ohm). Especially the bass with the DT770pro is more defined and accurate, but this behaviour is less pronounced with the HD580. Overall the tonal behaviour is very neutral and balanced with a slight bit of warmth and it is not the type of amplifier which is clinically detailed. Compared to the headphone output of the Philips CD-player, the tonal presentation is full of life with a very clear representation.

Measurements:

Finally here are some values from measurements I have done so far:
Frequency response: 10Hz-100kHz
Overall Gain: 4
Output Resistance: 25 Ohm (24.8 Ohm and 26.7 Ohm for both channels respectively)
Maximum Output Current: 300 Ohm or 30 Ohm load: 23.3mA rms, 33mA peak
Maximum Output Power:
300 Ohm load: 163mW rms, 327mW peak
30 Ohm load: 16.3mW rms, 32.7mW peak

c. 2003 Helmut Ahammer.

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