Zero Feedback Headphone Tube Amplifier

Posted on Aug 25, 2012

To get a low output impedance I needed to use quite a high step-down ratio (20:1); after all, the amplifier may be used with headphones of lower impedance that the 300 Ohms of the HD600. The output valve is the 12B4A which has a anode impedance of about 1000 Ohms, so the output impedance of the amplifier is about 2.5 Ohms, which is very low for a valve amplifier. You can use other valves here but watch that the anode impedance is not too high or the output impedance and bass-cut off frequency will both rise. It is usual with power amplifiers to match the impedance of the loudspeaker to the impedance of the anode circuit using the equation turns ratio = square root (impedance ratio). Under these conditions, maximum power transfer takes place from the amplifier to the load. With headphones, the power involved is so small that it is not necessary to operate under such conditions.

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It is possible to make the turns ratio such that the output impedance of the amplifier is much smaller than the load. Power transfer is not optimum, but the output valve is tending to run under constant current conditions which has the advantage of lowering distortion. The output power will depend on the impedance of the headphones; the amp gives about 10 V output before clipping. The gain of the output stage is 0.33, so quite a bit of amplification is needed before it. I am a fan of SRPP because it improves linearity, lowers output impedance and allows for a large voltage swing. The first stage uses a double triode directly coupled to the output valve, giving an overall gain of about 20 dB. The circuit is shown in figure 1. The 12B4A cathode resistor tends to run quite hot so use a 7 W component here. There is no feedback from the output to the input. The gain can be increased slightly by decoupling the cathode of the SRPP input, or alternatively try a ni-cad battery bias. Although I use the 6072A for this stage, suitable alternatives are the 5965 and 12AT7. The 12AT7 is slightly less linear. The 6922 can also be used with a change to the heater wiring but the gain will be lower. The input phono jacks are located round the back, along with the umbilical between power supply unit and the amplifier. The Switchcraft headphone jack is on the front (left) next to the volume control (right). The plan was to have a...

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