12AU7/12BH7 Headphone Amp

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Postby sorenj07 » Fri Jun 15, 2007 1:24 pm

I get heavy distortion at mid-to-high volumes on low-impedance headphones, but the line output works pretty well. There's still a bit of hum but it seems to be at acceptable levels. I had reconnected feedback when fixing the line output problem. Is there something else I could try to reduce this horrible distortion?

changing the headphone out feedback pots from 25K to 1K just seems to decrease volume - no effect on distortion is detectable.

of course, when I pick up my oscope probes, I can have a better look.. 20Hz, 2KHz, 20KHz sinewave input should help demonstrate clipping, right?
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Postby sorenj07 » Fri Jun 15, 2007 1:45 pm

Just for kicks I dropped a 6N1P in the place of the first 7AU7 gainstage. Sound became a bit less distorted. Hmm.

In addition, I found that increasing NFB actually INCREASES clipping somehow. I'll disconnect it.
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Postby sorenj07 » Fri Jun 15, 2007 3:30 pm

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..

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Postby EWBrown » Tue Jun 19, 2007 7:15 am

THe "Aikido" mono 9 pin preamp design has a viable headphone amp variant, check it out here:

http://www.tubecad.com/Aikido_9-pin_Mono_Rev-A_PCB.pdf

Page 7 has the headphone amplifier diagram and parts values.

There might be some useful information here.

The last two pages (9 and 10) have a "spread sheet" comparing the various dual triodes' operating characteristics. That alone is very useful Yellow_Light_Colorz_PDT_11

/ed B in NH
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Postby sorenj07 » Sat Jun 23, 2007 9:46 pm

yeah the 6N1P looks very tempting. I might well try that but at the same time, a mu follower with a pentode in the top part looks interesting too. Who knows. Unfortunately, I'm blowing 1/2A fast-blow fuses every few times i switch the thing on. I ditched the 350-ohm "idle" resistor because I'd read bad things about leaving tubes at reduced heater voltage, but ever since I removed it, it seems the shock of turning the thing on just pops it. Should I use slow-blow or possibly stick in a current limiter? It's when I turn on the heaters, not the B+, that this happens.
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Postby sorenj07 » Mon Dec 10, 2007 5:07 am

Just wondering. Does anyone know of any smallish noval pentodes with very high transconductance? I'm thinking of trying to rewire this thing as a mu-follower using just one tube, either a 7AU7 or 6N1P, with the other two noval sockets being used for an active current source for each 1/2. My only concern is heater voltage, as my entire PSU is based off of two 6.3V 6A transformers back to back and into a doubler, and any excess filament current steals current away from the B+...

My current setup uses 3 7AU7's, which take .9A total, which leaves at least 125mA or so by my calculations.
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Postby EWBrown » Mon Dec 10, 2007 6:57 am

12GN7 or 12GN7A comes to mind. I've read of these being used in single-tube "spud" amps, with decent results. These have an RP of 50K ohms and a gm of 36 mA/V (mu of 1800).

http://www.mif.pg.gda.pl/homepages/fran ... 12GN7A.pdf

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Postby sorenj07 » Mon Dec 10, 2007 7:21 am

ok, looks pretty cool! Two of these guy plus a 5687 makes 2.2A of heater current, leaving 3.8A 6.3V for the B+. The nice thing about mu-followers is that they have great PSRR, right?

I ran PSUDII with 120V 200mA power transformer, which is the equivalent of the backwards 6.3V transformer minus the heater current. Doubler is two 100uF caps, with regular 1N4007's. One LC filter tacked on, 10H 205 ohms (65mA max), into 650uF bypassed with film caps.

Current draw : B+ Voltage
20mA: 333V
30mA: 325V
40mA: 317V
50mA: 310V
60mA: 304V

It's not great regulation, I know, but since the mu-follower basically draws constant current, it should be OK, right? Now, the only thing is to figure out what values I could try... Does the above seem feasible, by the way?
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Postby EWBrown » Mon Dec 10, 2007 8:18 am

There is another 12GN7A equivalent, it's just a page pr two away from this one in the RCA tube manual. Both have identical or near-identical operating characteristics and relatively high plate dissipation (around 11 Watts). These should be cheap, as they aren't (yet) popular audio tubes...

The other one is 12HG7 sometimes listed as 12HG7 / 12HG7A / 12GN7A, same pinout, and nearly identical characteristics (slightly higher mu) and with the T-9 fat bottle instead of slim T6.5 bottle, 9 pin miniature.


/ed B in NH
Last edited by EWBrown on Tue Dec 11, 2007 8:34 am, edited 1 time in total.
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Postby Geek » Mon Dec 10, 2007 2:43 pm

EWBrown wrote:These should be cheap, as they aren't (yet) popular audio tubes...


*Geek grabs box before audiophool magazine gurus read this thread Yellow_Light_Colorz_PDT_16
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Postby EWBrown » Tue Dec 11, 2007 10:30 am

Here is the canonical "pentode" list from Pete Millet:

http://www.pmillett.com/pentodes.htm

PDF file: http://www.pmillett.com/images/pentodes.PDF

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