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  • Author Author: kk99
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Tube headphone amplifier

kk99

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I would like to present a simple headphone amplifier based on PCC88 tubes. This construction was made a around a year ago. This amplifier gives a very fun from listening of music.

Main construction based on the White's cathode follower. Here is schematic of this part:
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Here you could find more information about White's follower with equations for e.g. gain: http://www.tubecad.com/october99/page4.html

As power supply I have used a boost converter which gives around 180V with 40 mA of current. Here is schematic of boost converter:

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Values of elements:

R1 = 0.47 Ω

R3 = 10 kΩ

R4 = 681 kΩ

L = 1mH

T1 = IRF840

D1 = BYV29-500

C5 = 470uF + 100nF

Case was made from old box for transformer which was polished.


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Here is short video presentation:

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  • Interesting circuit and a nice project built around it.

     

    I can sort of understand the operation, indirectly, by substituting FETs for the tubes. The top tube is a

    follower and the bottom one an active pull-down, driven by feedback from the anode of the top tube via C4 on your schematic (C3 on mine) leading to a low output impedance (for a valve

    circuit without a matching transformer).

     

    Couldn't resist trying it in the simulator. Here's a FET version that I've slapped together. (Important: this isn't a

    properly designed circuit, so don't build it and expect it to be any good as a headphone amplifier. I might even be operating outside the spec for

    the FETs. And, the equation for transconductance of a tube is more complicated than a FET, so the FET isn't quite an analogue of a tube and this

    won't behave in quite the same way as the valve version.)

     

     

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    It does actually work (kind of works), though I can't persuade it to follow properly (so the pull-down might be a bit skewy) - I get a gain of about 0.8. It's

    horribly inefficient - I've got a current of 30mA through the FETs and about 5mA peak into the load - but probably not as

    inefficient as the tube version which I assume has a secondary use as a small room heater. Looks like there's a moderate

    amount of distortion on the output too. All of that could probably be improved with proper design, though it's never going to

    be that efficient because of the standing current through the FETs.

     

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    Frequency response is quite reasonable. Flat to 10MHz. Wonder how that compares to the valve version? Presumably the tubes do better because of lower parasitic capacitance.

     

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  • in reply to jc2048

    I suppose that sometimes characteristics does not show how given amplifier sound. I think that tubes have more the even harmonics which cause different sound reception. I also like germanium transistors which were used on old radios. These radios sound really awesome e.g. ITT Schaub-Lorenz Touring International103.

    Here is nice presentation with description of sound and distortion:
    http://www.pmillett.com/file_downloads/ThesoundofDistortion.pdf

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  • in reply to kk99

    Thanks for the link - it was an interesting read. I like his distortion experiment board - that's cool.

     

    This is what the simulator gives for an FFT with a 1kHz sine wave into the FET circuit but I don't know what signal level it uses for that. [It wrongly labels the amplitude axis - it should be dBs.] It might not be large enough to exercise the more non-linear part of the transfer function,so may not be representative of what you'd actually hear. The harmonics are quite well down compared to the fundamental, so the thd figure wouldn't be as bad as I supposed. The 2nd harmonic dominates, and there's a fair bit of 4th, so you'd probably like the sound if you like tubes.

     

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