Fall 2008

Manufacturer Article
A Hybrid Tube/MOSFET Headphone Amplifier
Article By Erno Borbely

Difficulty Level

    

  In the 1/98 issue of Glass Audio, I wrote about a hybrid tube/MOSFET line amp, which, because of its musical sound, became a very popular amplifier ("Low-Voltage Tube/MOSFET Line Amp," which also appears on borbelyaudio.com under Spe­cial Articles). DIY amateurs wished to use it in many different applications, such as CD buffer, I/V converter, power amplifier, and headphone amplifier. It worked very well in all line-level ap­plications, but the second stage was not laid out for high-current operation, so driving headphones was not possible. I have therefore redesigned the circuit to allow high-current operation.

The result is the EB-804/421, a single-ended (SE) pure Class-A amplifier, ca­pable of driving headphones between 32 and 600Ω. The amplifiers need ±15 to ±24V regulated supplies at 160/100mA and 6.3V DC at 300mA for the tube heat­er. I recommend feeding the amps from separate supplies. The PCB for one amp is 90 x 80mm.

Circuit Description
The schematic is shown to the right. The topology is the same as the hybrid tube/MOSFET line amp. Q1 is a double triode that operates as a differential amplifier, with approximately 2mA in each of the triodes. A constant-current diode D1, which supplies the source current to the differential amp, includes two J508 or E-202 diodes in parallel. You can also use a single J511, which delivers 4.7mA.

The two anodes, which produce out-of-phase signals, are converted to a single-ended signal using a current mir­ror composed of Q2, D2, and resistors R3/R4. Q3, a P-channel MOSFET in TO-220 package, is used in common-source mode as a Class-A single-ended second stage. I replaced its drain resistor with a second constant-current source, sup­plying the Class-A current of 100 or 160mA.

The constant-current source, which increases the gain and im­proves the linear­ity of the second stage, is made up of Q4, an N-chan­nel MOSFET in TO-220 package, and its associ­ated components. I used the Hitachi 2SJ79 and 2SK216 for Q3 and Q4, respectively. You can also use the Toshiba 2SJ313 and 2SK2013, but note that the pinout is different from the Hitachi (GDS versus GSD).

The amplifier can work with a ±15V to ±24V supply. The maximum dissi­pation allowed for Q3 and Q4 is 2.4W each, so the supply voltage determines the maximum current. At ±24V the cur­rent is 100mA and at ±15V it is 160mA. Resistor R13 sets the current: it is 6R8 for 100mA and 3R9 for 160mA.

You must heatsink Q3 and Q4. I am using the SK76−37.5 with 8K/W ther­mal resistance. The temperature on the heatsinks is about 55° C, so proper ven­tilation is absolutely necessary! The PS/regulator I recommend for the hybrid tube/MOSFET headphone amplifier is the EB-802/243.

The input tube requires a 6.3V/350mA heater supply. Use a well-regulated/low-ripple supply for this (EB-793/204 is recommended). I recommend that you ground the negative side of the heater supply to the PGND on the PCB.

Linearity Notes
The input tube dominates the overall distortion characteristics of the amplifier. Tubes of different manufacturers produce different amounts of distortion. I have tested the ECC86 from Telefunken and Ultron, ECC88 from AEG, E88CC from Tungsram, and 6922/6H23Π, a Russian military tube. All worked fine, but the difference in THD can be 6 to 10dB!

The Russian 6922/6H23Π produced the lowest THD. We are shipping the kits with these tubes. Nevertheless, I recommend that you try different types of tubes and select the one you like best.

Note also that the tube can pick up hum from mains fields. Again, tubes from different manufacturers show different sensitivity to these fields. It would help to use a shielded tube socket; however, it is difficult to find one for PCB mounting.

Finally, it is a good idea to switch on the heater before you apply the ± supply to the amplifier. This has noth­ing to do with cathode stripping, but with the DC operation of the amplifier. As long as the heater is off, the input does not function even if you apply the ± supply. Consequently, the DC feed­back loop is inactive and the output is not sitting at 0V.

Only after the heater is on can the output stabilize to 0V. Alternatively, you can leave the heater on all the time or in a stand-by mode withsay4V, in which case you can apply the full heater voltage and the supply voltage simultaneously.

The feedback resistors R8 and R9 set the closed loop (CL) gain of the amp. Normal gain is 10×, or 20dB. Changing R9 can change this gain. CL output impedance is 15Ω. Equivalent input noise depends on the tube used and is 1.2 to 1.5μV!

The maximum output power into dif­ferent loads depends on the supply volt­age and the available current from Q4. With ±24V and 100mA in the second stage, the amp delivers >100mW into 32Ω and >250mW into 600Ω at 1% THD. With ±15V and 160mA the power into 32Ω increases to 300mW at 1% THD.

The maximum power is limited by the available current at low load imped­ances and by the available voltage swing at high impedances. If your headphones are low impedance, you should operate the amplifier at ±15V with 160mA in the second stage, and if they are high impedance, use a ±24V supply with 100mA. Since high impedance head­phones require less power than the low impedance ones, the ±15V operation will probably give more than enough power for ear-shattering SPL over the whole impedance range.

Headphone Power Requirements
There appears to be much misunderstanding concerning the power required to drive a headphone. This is usually due to the fact that headphones have different impedances, the lowest is around 30Ω and the highest 600Ω. The headphone impedance is no indication of the quality of the headphone, but it has a major influence on the amplifier from which you can drive it.

Headphone sensitivity is specified in sound pressure level (SPL) when you apply 1mW of power to it. Given the impedance of the headphone and the maximum SPL you would like to achieve, you can easily calculate the necessary drive power.

For the sake of illustrating the power requirements, consider a low impedance headphone first, for example, 40Ω. To produce 1mW into 40Ω you need a current of:

I=√ (P/R)=√ (1mW/40Ω) = 5mA

The necessary voltage to produce this current in 40Ω is:

U = I × R = 5mA × 40Ω = 200mV

So far so good. I am sure all headphone amps can deliver this much current at this voltage swing.

Now for the maximum SPL. This particular headphone is speci­fied at 256mW maximum power, which is achieved at a current of I = 80mA and a voltage of U = 3.2V. The SPL difference between 1mW and 256mW power is given by the formula:

SPL diff. = 10 log (P1/P2) = 10 log (256/1)= 24dB

So the maximum SPL with 256mW power will be 100dB + 24dB = 124dB.

You can draw some general conclusions from these results. You can see that you need a relatively moderate voltage swing, but a rather hefty current to produce this SPL in a low impedance head­phone. In fact, some 40Ω headphones need even more power to achieve maximum SPL. One, in particular, is specified at 102dB at 1mW and 440mW for maximum SPL. The 1mW current/volt­age requirements are the same as the previous one, but to achieve 440mW you need:

I=√ (440mW/40) = 104.9mA

U = I × R = 104.9mA × 40 = 4.2V

The SPL difference from 102dB will be:

SPL diff. = 10 log (440/1) = 26.4dB

And the maximum SPL will be: 102dB + 26.4dB = 128.4dB.

Note that you now have almost ½W of power here, with a relative­ly moderate voltage swing, but quite a lot of current! Of course the question is: do you ever need an SPL of 128dB? Many headphones operating at maximum power might cause damage to your hearing!

Now let's look at the other end of the impedance range: 600Ω. A typical example includes sensitivity of 98dB SPL at 1mW input and a maximum power of 80mW.

The current requirement for 1mW is:

I=√ (1mW/600Ω) = 1.29mA

And the necessary voltage is:

U = 1.29mA x 600 = 0.77V

For maximum power you need:

I=√ (80mW/600Ω) = 11.55mA

U = 11.55mA × 600Ω = 6.93V

The maximum power will produce an SPL difference of:

SPL diff. = 10 log (80/1) = 19dB

And the maximum SPL is: 98dB + 19dB = 117dB.

Although the maximum SPL is relatively low for this headphone, the voltage has increased considerably compared with the 40Ω headphone. On the other hand, the current requirement is relatively low. Obviously headphones with impedances between these values fall between these two as far as current and voltage requirements are concerned.

Portable Headphone Amplifiers
Headphone amps, just like speaker amps, are available in many varieties: tube-based, semiconductor-based, and mixtures of both technologies. Most mid-fi CD players, receivers, and amps also offer headphone outputs. And, of course, all portable Walkman-type CD players, cassette players, and radios use headphones.

The most problematic of these is the last group, because they are operating from batteries. Of course, nothing is wrong with batter­ies per se, except for the amount of voltage/current available for the headphone amp.

Consider for a moment the voltage/current requirements for the two types of headphones described previously. The 40Ω unit required 3.2V RMS to generate 124dB SPL. Since we are talking about sine waves here, the 3.2V RMS is equal to 3.2 x 2.82V peak-to-peak, i.e., 9.024V for the amplifier. And this is a theoretical value.

Practical amplifiers that operate with a 9V supply cannot deliver 9V peak-to-peak audio signal, because most audio amps are not ca­pable of working "rail-to-rail," i.e., from zero to 9V. In addition, the 9V battery would need to deliver 80mA for just the audio amp, not 

taking into consideration the rest of the electronics.

And this is not the end of the story. The Walkman-type devices are usually operating with two 1.5V batteries, for a supply of 3V total. Assuming that the audio amp would be able to work "rail-to-rail," the equivalent audio signal would be 3/2.82 = 1.06V RMS, and you could generate a maximum current of:

I = 1.06/40Ω = 26.5mA

This would give a maximum power of:

P = U × I = 1.06V x 26.5mA = 28.1mW

And SPL difference would be:

SPL.diff. = 10 log 28.1 = 14.5dB

And the maximum SPL: 100dB +14.5dB = 114.5dB, which is actu­ally "only" 10dB less than the maximum. However, remember that in most cases the audio signal would be less than the one calculat­ed, or the amp would already be clipping at a lower value. In a 32Ω headphone with 100dB SPL for 1mW input, the maximum power would be 35mW and the maximum SPL would be 115dB!

Real-life ICs, made specifically for low-voltage operation, will usually deliver less than this. Look up the National LM4911, which is a stereo headphone amp -- it delivers 25mW into 32Ω at 1% THD from a 3V battery (12mW from 2.4V). This means that just a bit over 80% of the bat­tery voltage is "converted" into audio! I bet most of the Walkman-type devices don't deliver much more than 10−15mW of "clean" audio!

What would happen if you connected a 600Ω headphone to this amp? The maximum current would be: 1.06/600 = 1.77mA, the max­imum power: 1.06V x 1.77mA = 1.88mW. The SPL difference is:

10 log 1.88 = 2.74dB and the maximum SPL is 98dB + 2.74dB = 100.74dB.

Obviously, 600Ω headphones are less suited for this kind of ap­plication. For amps with low supply voltage, you need to use low-im­pedance headphones, assuming, of course, that the amp can deliver the necessary current.

Headphone amps operating from ±9V batteries fare much better in terms of maximum power. Assuming an 80% ratio between battery voltage and audio signal, such an amp could deliver over 600mW into a 40Ω headphone. Of course, the battery would also need to deliver the necessary current (over 120mA!), and the question is how long it would be able to do that? The same amp would manage only about 40mW into a 600Ω headphone, so even an amp working with ± 9V power supply cannot cover the whole impedance range.

In addition to the problem of available power, most of the low-voltage, battery-operated headphone amps are working with very low bias current to save battery life. This means in most cases Class-B operation. Now it's well known that Class-B is far from ideal in terms of sound quality due to crossover distortion, but there is really not much you can do when the amp must be portable and operate from low-voltage batteries. Still, there are many people listening to portable devices, so it cannot be all that bad!

Assembly
The schematic to the right shows the stuffing guide for the hybrid tube/MOSFET headphone amplifier. Start the assembly by install­ing the solder pins, jumpers, and then all the resistors (including the trimpot P1). If you have selected ±15V operation, then resistor R13 = 3R9 and R10 = 7R5. If the supply voltage is ±24V, then R13 = 6R8 and R10 = 33R.

Next install Q2, Q5 and diodes D1 (A/B). Mount Q3 and Q4 on the heatsinks with insulator and install them on the board. Make sure the MOSfets are properly tightened to the heatsink. Then install the tube socket and all the capacitors, with C4 and C5 being the last ones. Finally, plug the tube into the socket.

Setup Procedure
If possible, test each amplifier sepa­rately before installing it in the chassis. This simplifies measurements, ad­justments, and, if necessary, component changes. If you have access to a scope, connect it to the output of the amp and check whether radio frequency (RF) oscillations are present. If you have a complete audio instrumentation in your workshop, perform the usual gain, frequency response, noise, total harmonic distortion (THD), and intermodulation distortion (IM) measurements.

Connect the +INP and the −INP to SGND. Apply the appropriate supply voltage (±15V or ±24V) and the 6.3V DC heater voltage to the amplifier. Connect a digital voltmeter (DVM) across R13 and check the voltage drop. It should be 0.62−0.65V. This sets the current to approximately 100mA or 160mA in the second stage, depending on the value of R13.

Let the amp run for about 20 minutes before you adjust the offset. Connect the DVM to the output of the amplifier and set the offset voltage to 0V with P1. This completes the DC adjustments.

The EB-804/421 kit is available from directly from Borbely Audio at www.borbelyaudio.com.

Specifications
Type: Hybrid tube/MOSfet headphone amplifier

Pricing:
EB-804/421 SE HYBRID TUBE/MOSFET AMP. FR-4 PCB, DALE res. Dual: $149
EB-804/421 SE HYBRID TUBE/MOSFET AMP. TEFLON PCB, DALE res. Dual: $199
EB-802/243 Power supply/regulator 1x±24V/200mA reg., needs 2x22VAC. One:$123
EB-802/243 Power supply/regulator 1x±24V/200mA reg., needs 2x22VAC. Pair: $199
EB-793/204 Dual Filament regulator Two independent 1A slow turn-on regs Dual $60
VESA 2 260 FISCHER 19” 2U high 260mm deep, silver color, not drilled One:$155

Manufacturer
Borbely Audio
Angerstr. 9, 86836 Obermeitingen
Germany

Voice: +49/8232/903616
Fax: +49/8232/903618
E-mail: borbelyaudio@t-online.de 
Website: www.borbelyaudio.com