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Old 9th April 2007, 02:15 AM   #1
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Default 12AU7/12BH7 Headphone Amp

I've been thinking about using triodes with toroidal power transformers. I also have a shipment of a few 7AU7's coming. I'd originally biased for use with 12AU7's in the output stage but figured that a little more power (hopefully 3-ish watts per channel) can't hurt. My headphones are 32 ohms, which is VERY convenient for output impedance switching (4:1 turns ratio between series/parallel). Does this thing seem feasible? I designed an overkill PSU with 4 chokes, two LC's for the B+ and another for the B++, but I'm not even sure that critical inductance will be reached.

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Now the question is, what will 3 watts into 32-ohm headphones sound like
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Old 9th April 2007, 09:18 AM   #2
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The toroidal option sounds very workable. I have a single ended parafeed headphone amp which uses 12V toroidals _ they certainly aren't the limiting factor.


Quote:
Now the question is, what will 3 watts into 32-ohm headphones sound like
Meet you at the hospital when they try to stitch your ear drums back together.

Shoog
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Old 9th April 2007, 12:02 PM   #3
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Hi Soren,

I also would apply maybe 6dB of gNFB. The grid of the lower 12AU7 section gives you an ideal injection point (use 10k resistor from grid to gnd).

Much more important than that, you must give the LM334Z constant current source a negative voltage of at least, say, -12V to breathe, or you certainly will encouter massive distortion due to stalling of the CCS, from which especially the LM334Z recovers very badly (MJ explicitely mentioned that LM334Z behaviour at ETF06 when discussing a quite similar circuit shown below).

Since it will be a headphone amp, you could use DC to power the heaters, which also will conveniently give you the negative voltage for the CCS. I have done so in a similar design, although using a normal PP OPT.

Link to amp schematic.

Link to PSU schematic, showing combined DC-heaters and splitter CCS negative voltage feed.

Regards,

Tom
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Old 9th April 2007, 05:03 PM   #4
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Quote:
Originally posted by Shoog
Meet you at the hospital when they try to stitch your ear drums back together.

Shoog
yeah I'll watch out.

Quote:
Originally posted by Tubes4e4
Hi Soren,

I also would apply maybe 6dB of gNFB. The grid of the lower 12AU7 section gives you an ideal injection point (use 10k resistor from grid to gnd).

Much more important than that, you must give the LM334Z constant current source a negative voltage of at least, say, -12V to breathe

you could use DC to power the heaters

Regards,

Tom
I assume you mean to the bottom grid of the output 7AU7, right?

As for supplying the negative voltage, the only way I can see is to run the ~6.8V filaments through a doubler, which is bulky. I'd rather not use DC power simply because I believe in AC filaments.

My PSU at the moment is two 115:6.3VCT at 6A filament transformers, run back to back with the second "115" (closer to 125) fed into a doubler, a 157M, etc etc. for B+. The 125-ish house AC means that the 6.3V runs closer to 6.8, perfect for the 7.2V of the 7AU7.
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Old 9th April 2007, 05:07 PM   #5
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Quote:
Originally posted by Shoog
The toroidal option sounds very workable. I have a single ended parafeed headphone amp which uses 12V toroidals _ they certainly aren't the limiting factor.

Meet you at the hospital when they try to stitch your ear drums back together.

Shoog
Cool, I have big hopes for them. I only wish they weren't so expensive ($25 each plus shipping). Anyone know a good source of dual-115V primary, dual 6V or 7V secondary, 15-25VA toroids?
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Old 9th April 2007, 10:19 PM   #6
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Quote:
My PSU at the moment is two 115:6.3VCT at 6A filament transformers, run back to back with the second "115" (closer to 125) fed into a doubler, a 157M, etc etc. for B+. The 125-ish house AC means that the 6.3V runs closer to 6.8, perfect for the 7.2V of the 7AU7.
Tubes4e4 Hi Soren,
I to use back to back transformers for my HT supply on my headphone amp. A bit wasteful and a proper HT transformer would be better though. Mine are 18V toroidals, and I use one of the 18V windings for the series connected heater supplies - with a little resistance padding to get to the 12.6V AC.
I agree with you about AC heaters, but a headphone amp might be one of those rare instances where DC is appropriate. My little headphone amp is very compact and unfortunately suffers from a slight bit of hum - which I think is inductively coupled off the power transformers. If I find a nice case I will definately be looking for a complete rebuild at some point.

Shoog
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Old 10th April 2007, 03:50 AM   #7
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Quote:
Originally posted by Shoog

A bit wasteful and a proper HT transformer would be better though.

I agree with you about AC heaters, but a headphone amp might be one of those rare instances where DC is appropriate.

Shoog
Yeah, but I wasn't going to use the transformers elsewhere, and I like the look of the two identical transformers next to each other, symmetry is big for me.

I'll try AC first. I'll be pretty anal about the wiring, and hope for the best.
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Old 10th April 2007, 10:47 PM   #8
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2 problems. I'm guessing that your headphones are Grados given the 32 Ohm impedance. You have way too much gain, the volume pot's going to be stuck at the very bottom of the range unless you listen at some insane volume. Consider changing the tubes and/or redoing the circuit to give an overall voltage gain of around 2-5, that's all you'll ever need. A gain of 2-3 is plenty given the ouputs of most CD players.

Other potential issue is the output transformer. I've had the 15W Amvecos and there was enough capacitance & resistance in the windings to severly roll off the high frequency response. The 50W ones I'm using right now work just fine and have no issues. The 25W ones which you propose to use may have issues, but I don't know for sure.
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Old 11th April 2007, 12:10 AM   #9
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Quote:
Originally posted by aerius
You have way too much gain, the volume pot's going to be stuck at the very bottom of the range unless you listen at some insane volume. Consider changing the tubes and/or redoing the circuit to give an overall voltage gain of around 2-5, that's all you'll ever need. A gain of 2-3 is plenty given the ouputs of most CD players.
My biggest concern. Would changing the LTP stage to a split-load work? Gain of a bit less than one. I don't know what the gain of the output stage is, though. If I look at my load line, is there an easy enough way?


Quote:
Originally posted by aerius
Other potential issue is the output transformer. I've had the 15W Amvecos and there was enough capacitance & resistance in the windings to severly roll off the high frequency response. The 50W ones I'm using right now work just fine and have no issues. The 25W ones which you propose to use may have issues, but I don't know for sure.
Well at 4W of plate dissipation for two 7AU7 triodes, I think 25W is the correct ratio. All else will just be left to change, I guess


I might also just use different tubes. I only have a limited amount of power to work with. Is there a recommended way to use the 7AU7's without OPT's to power 32-ohm headphones? Mine are Sony MDR-V6's btw.
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Old 11th April 2007, 02:54 AM   #10
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Here's another crack. The constant-current White CF, according to TubeCAD, has an output impedance of under 50 ohms, which should be pretty cool for my headphones and most other amplifiers etc. I tagged the grounded-cathode stage at the beginning to get a bit of gain, you never know when you might need it. Plus, using 3 tubes total instead of four gives me more headroom for the B+ with my back-to-back filament trafos

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