EL38 Triode Connected HPA Pentode Drive

[Cressy Snr]

I can rig up my other headphone amp with 6EM7 if you're curious Paul

0.9W is incredibly loud on headphones.

[Paul Barker]

You have two?

[Cressy Snr]

Yep

It’ll take quite a while to mod, but I’ll report back once I’ve got it sorted.

[Cressy Snr]

Back to the EL38 HPA and I've just introduced it to the RTZ DAC I got from Ray last year.
I mean, the sound is absolutely superb! This DAC brings out the excellence of the EL38 like none of my other DACs.
I can't believe how good this headphone system now is. It’s the best head-fi I’ve had to date.

When first I used the RTZ downstairs with my old valve amp and Mission floor standers (before I got the little Denon system) I didn't feel the amp was revealing enough to do the DAC justice. Sounded a bit flat.

This has changed completely with the EL38 HPA. The RTZ sounds stunning, not only giving an analogue-like warmth and subtlety, to the proceedings but also showing the musical details without pulling the performances apart. Everything hangs together, nothing shouts or gets in your face, though the sound remains fast and dynamic. Bass goes very deep. A lot deeper than my MF M1 DAC.


The combination of EL38 headphone amp, Mac Mini and RTZ DAC is synergy personified.

I like.

[Morgan Jones]

So you're counting it as a success, then? Well done!

I've just had a look and find I have a pair of IEC/Mullard EL37 (whatever they are), not EL38.

[Cressy Snr]

Yep, it seems to be a success.

[Paul Barker]

So you're counting it as a success, then? Well done!

I've just had a look and find I have a pair of IEC/Mullard EL37 (whatever they are), not EL38.

Quite similar. Its little sister the el33 partners it well as a pentode driver and itself will push pull drive a pair of output valves, is it’s role in life.

[Cressy Snr]

I now have single-triode, RCA 12B4As running at 11mA as the input stages:

IMG_0632.jpeg (112.75 KiB) Viewed 17574 times

It was still slightly too loud with the ECC82s, but with the 12B4As, the gain is now just about spot-on.

[Cressy Snr]

I've ramped up the HT by fitting a higher voltage, 150VA mains transformer.
So now we have more realistic voltages for the EL38s and the 12B4As.

Power supply and audio circuits below:

Screenshot 2025-03-31 at 19.46.39.png (64.39 KiB) Viewed 17268 times

Screenshot 2025-03-31 at 19.31.39.png (42.94 KiB) Viewed 17268 times

[Cressy Snr]

I've bypassed the cathode resistor on the 12B4A.
There was a bit of residual hum between tracks with the cathode un-bypassed. That sort of noise drives me up the wall after a while with headphone amps. Some people will put up with it but it gets on my bleed'n wick!
A completely silent background when nothing is playing, is my bag and that's what I have now.

New schematic:

12B4A-Bypass_Cap.png (40.42 KiB) Viewed 17128 times