12AU7/12BH7 Headphone Amp

[Brian Beck]

Way too early to flail about with other tubes. The next step is to get those DC operating points right. I doubt they are. See my post above (we might have cross-posted).

[EC8010]

Yes, Brian is right. I meant to say, "Replace the AC coupling to the White cathode follower with DC coupling." This morning was a bit panicked.

[sorenj07]

Originally posted by Brian Beck
OK, first of all leave the feedback disconnected until we can get this playing, and even after that probably. I hope you left the 1M grid "leak" resistor on the lower tube of the WCF. I think that EC8010 might have misspoken when he suggested removing the 1M grid leak along with the 0.47 uF cap. I think he meant for you to remove the 100K grid leak on the upper WCF tube along with the 0.47uF cap. You did then connect the upper tube's grid to the first stage's plate, right? Now, after making sure all this correct, can you post a new schematic with revised values and with measured DC voltages at the plates and cathodes, as well as measured values of B+1 and B+2? I don't think we're ready for music or even square waves just yet.

Thanks for all the help! We did indeed cross-post.

I actually DID disconnect the 1M grid leak, and left the .47uF/100K RC. However I just clipped the 1M at one point and bent the lead away so I can use a spot of solder to reconnect. I'll DC couple the gainstage to the top triode in the WCF. My MM actually gives me the low-battery sign now and then, but it seems to be working other than that.

[Brian Beck]

Originally posted by EC8010
Yes, Brian is right. I meant to say, "Replace the AC coupling to the White cathode follower with DC coupling." This morning was a bit panicked.

Killing mice with a Dyson will do that to a man.

[sorenj07]

OK. A new signal schematic, and much better news. In the 32-ohm iPod headphones in my workshop, I couldn't hear any distortion even at very high volumes. That doesn't mean there isn't any, of course, but it's not ridiculously bad. What's weird is that B+ has fallen almost 50V below projected..

[EC8010]

Not weird at all. You are now drawing a sensible current in your output stage. You are now in a position to try beefier output valves (provided you bias them correctly), and re-instate the global feedback.

[sorenj07]

Unfortunately, these beefier output valves (I have a quad of 6CG7's in my EL34 Williamson, for example) will probably draw more than .3A of heater current. As the PSU schematic I posted earlier shows, it's a contest between heaters and B+ headroom. I'll pop those 6CG7's in but hopefully B+ won't sag too much..

In order to provide some extra voltage swing to feed some NFB, might I try dropping a 6DJ8/6N23P into the first stage? I'd do a quick bias recalculation, but the extra gain might help, right?

also, what value resistor might I try for NFB? I took out the pots already because I might want less NFB than 25K would afford..

[EC8010]

Have a look at the headwize site - there's extensive discussion and development of this circuit over there.

[Brian Beck]

Good, you've now got sane DC operating points. If and when you try global NFB, please do something to block the DC from the output. I couldn't readily find the referred-to circuit on Headwize, but they must show some means of providing DC-blocked feedback.

[EC8010]

Originally posted by Brian Beck
I couldn't readily find the referred-to circuit on Headwize

It's a generic circuit - I doubt if the exact circuit and values is there.

As for DC on the output, I'd expect headphones to be particularly sensitive to it and produce unnecessary distortion as a result. Zero that DC offset!