Heaviside hybrid headphone amplifier

[kwantam]

Fellow diyers,

I've just finished a preliminary writeup of my new hybrid amplifier design, which I've dubbed "Heaviside."



Lab results are excellent: 102 dB dynamic range, .026% THD+N. Sounds great into both my RS-2s and my DT-880s. I'm thinking I'll swap in some 5751s for the 12AX7EHs at some point.

If you're interested in reading more, take a look at my preliminary writeup.

Comments are welcome and appreciated.

-kwantam

[Brian Beck]

Hmmm, interesting use of a 12AX7. You’ve used the 12AX7 in transconductance mode (cascode), not its favorite setting for lowest distortion because the transfer function depends entirely on gm rather than on mu. That it works for you, sounding good to your ears and measuring OK, is testament to the gobs of feedback used. A tube more commonly seen in cascode mode is the 6DJ8/ECC88/6922. Biased properly, a 6DJ8 would give about 8 times the transconductance (gain), which may or may not be useful. It might not be any more linear though. It would need a lot more plate current, but you could cut plate voltage down to 100 volts or even less. I’m not sure it would work any better in the end; just musing…

[kwantam]

Very interesting!

Honestly, this is the first time I've designed a linear amplifier with tubes (previously I'd only built RF stages), and my choice of the 12AX7 was somewhat arbitrary---I remembered that some Fender preamp or other that I'd owned at some point used them, did some preliminary poking around, and settled on them because they were cheap, plentiful, and did the job.

I'll give the 6DJ8 a look, and maybe give it a rip in a similar setup.

Thanks for the info!

-kwantam

[zome]

I bet the amp sounds nice..but it's very nice to see someone using LaTeX :-)