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mogliaa 20th May 2012 10:47 AM

6e5p WCF headphone amp with no cap!
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I've been thinking about building a headphone amp with the following constraining designs:
1) OTL
2) Capable of driving my GRADO's impedance (32 ohms)

So given the above, looked at a simple white cathode stage using the lovely 6e5p in triode mode and using SY's great servo idea used in his Heretical preamp (The Heretical Preamp, p5) to remove the output cap.

To do so, we need a symmetrical HT supply. I'm sure this can be improved further, but a quick simulation shows that driving the valves at 45mA (a bit too hot perhaps) can deliver a low output impedance to around 16-20 ohms and distortion is an acceptable 0.25% at 90mVrms. I remember measuring my headphones and finding that 100mVrms was loud enough... (if I'm not mistaken).

Yes, 6e5p are running very hot and a lot of power is dissipated in their anodes, but is worth trying this in a breadboard to listen to?

6e5p is a great sounding valve and with no output cap, this seems to be a promising solution.

How could I improve this circuit?


Bigun 20th May 2012 01:18 PM

Looks interesting to me !

I know you like this tube, but perhaps the 6BX7 gives more headroom for anode dissipation.

Dimitris AR 20th May 2012 11:34 PM

Reverse the polarity of HT2 in the scheme .

mogliaa 21st May 2012 06:43 AM


Originally Posted by Dimitris AR (
Reverse the polarity of HT2 in the scheme .

HT supply value is -150, otherwise circuit won't work :-)

Dimitris AR 21st May 2012 09:49 AM

I said there is a mistake in the polarity of HT 2 in the schematic the + pole is connected wrong in the cathode , You don't see it ?

mogliaa 21st May 2012 09:51 AM


Originally Posted by Dimitris AR (
I said there is a mistake in the polarity of HT 2 in the schematic the + pole is connected wrong in the cathode , You don't see it ?

Yes I do see it. But having HT2 with a value of -150 is the same as inverting HT2 (i.e. negative side to cathode) with a value of +150....


DF96 21st May 2012 10:17 AM


Originally Posted by mogliaa
How could I improve this circuit?

Add an output cap? For reasons of safety for you and your headphones. A cap ought to be more trustworthy than a servo, if only because it is simpler.

If you don't like the 'sound' of caps then you may have to invent a cap-less servo too.

R6 at 10M may be high enough to allow for thermal runaway in the upper valve. What does the datasheet specifiy as maximum grid resistor for this valve?

revintage 21st May 2012 01:34 PM

There are still two caps in the signalpath. What about doing it totally capless by using iron at the output?

R6 can as well be 100k.

Still a nice concept.

mogliaa 21st May 2012 06:13 PM

What about adding protection diodes at the output to protect the headphone from an unlikely failure of the servo circuit? I'd be able to avoid an unwanted electrolytic there.

Circuit still have an input cap and the cap on the servo. The input cap can be a good quality one as its value is 100-220nF. Need to tweak value depending on the servo resistor which as suggested with 100K then 220nF is ok.

Not sure what would be the sound impact of the servo op amp feedback cap?

@Lars: Yes going for an OT version could be probably the best option. Just was toying with the idea of an alternative OTL without output cap.

What if I parallel more valves to increase effective gm and reduce HT voltage? HT supply would be cheaper (and smaller that way).

Otherwise I may go for a simple spud using a good quality OT...any recommendations?


TheGimp 21st May 2012 08:10 PM

What if you combined the diodes with a pair of 62mA pico fuses in series with the supplies. You might need to drop your bias current slightly to be sure you don't blow the fuse during normal operation.

Increase R2 to 35R and R4 to 60R and your bias current drops to 52mA, Zo is around 17.6R, and THD is 0.12% at .109Vrms out. Anode plus screen dissipation drops to 7.8W

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