12AU7/12BH7 Headphone Amp

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12AU7/12BH7 Headphone Amp

Postby sorenj07 » Sun Apr 08, 2007 8:18 pm

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 :D
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Postby TomMcNally » Sun Apr 08, 2007 9:55 pm

Someone asked me if the 3/4 of a watt Darling would work well with heaphones, so I put together a little adapter to take dual banana plugs to a 1/4" stereo headphone jack, via the traditional 100 ohm series resistors. With my 600 ohm AKG's it has plenty of volume, and no hum (with just one lil choke) so yours should be good.
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Postby sorenj07 » Sun Apr 08, 2007 10:26 pm

I've been thinking that there's no real point to using the 12BH7. Primarily, I'm looking to put together a small tube amp that I can take to college or something. ~3W is probably too soft to power the kind of small speakers I'd be taking with me. The ~2W using 7AU7's instead of the 12BH7 don't make much difference.

With a simulated B+ of around 290V, I can run each 7AU7 triode output at around 10Ia, 200Va, -6V bias, 2.2W Pa.

The problem for the driver is, do I need to swing +-6V? I was planning to use a split-load as I only have 3 7AU7's total, but maybe I can just bite the bullet and get another couple before the Soren effect happens :D
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Postby sorenj07 » Sun Apr 08, 2007 10:33 pm

if I wanted to be funny, I could call this the Lucky Seven amplifier. Use two back-to-back 7V toroids for the 7AU7 filament and B+, then use two more for the OPT's. Make the B+ like 277V and 267V or something. Bias at -7V, 7mA. Maybe I should be quiet now.
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Postby sorenj07 » Sun Apr 08, 2007 11:11 pm

TomMcNally wrote:I put together a little adapter to take dual banana plugs to a 1/4" stereo headphone jack, via the traditional 100 ohm series resistors. With my 600 ohm AKG's it has plenty of volume, and no hum (with just one lil choke) so yours should be good.


My headphones are 32 ohms, though, and will be directly coupled to the transformer, no resistors or anything. 2W into 32 ohms would probably do interesting things with my ears if volume was all the way up. Is there any safe way to deal with this? Could I add some resistance to the input to protect my ears in case the volume is all the way up?
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Postby EWBrown » Mon Apr 09, 2007 5:58 am

I know that 2WPC using SET 6DN7s, into a pair of Sennheiser HD600s sounds amazingly, even astonishingly, good! These have 120 ohm resistors in series between the 8 ohm output and the headphone jack, which is considered an "industry standard". Direct driven (without the resistor) and with a 32 ohm output, a half-watt should be sufficient to cause near ear-damaging SPLs. So, be careful...

If you look over on the Gary Kaufman tube ppage (under "Links", he has a 5WPC PPP 12BH7A / 12AU7 amp, based on a design by Nobu Shishido, who was a genius of the small tube amp design world. I have built up a pair of the boards, but, this is just another of many projects in the "pipeline", for now...

Details here: http://www.the-planet.org/Shishido.html

The OPT can be anything from 6.6K to 10K P-P, B+ is about 300VDC.
FWIW, I'm sure that a 7AU7 will run just fine with 6.3VAC on the filament, and the 6GU7 or 8GU7 is functionally equivalent to 12BH7A, just with filament connected to pins 4 and 5 only.

If you want a little extra "oopmh", substitute the 12BH7As with 6MD8s, and lower the cathode resistors from 500 ohms to 330 ohms, 3 to 5 W. The 6MD8 is a nice desirable "plinker" in a 9 pin Novar jug, it's equivalent to three 12BH7 or 6GU7 triode sections, with a shared common cathode. Reduce the OPT primary Z to 5K to 6.6K range. For a little fine "tweeking" of the phase splitter, replace the 18K resistor with a 25K pot, and the balance can be dialed into perfection... :o

6MD8: http://www.mif.pg.gda.pl/homepages/fran ... 6/6MD8.pdf

There is also a 12.6V filament version of the 6MD8, the 12MD8.

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/ed B in NH
Last edited by EWBrown on Sat Jul 05, 2014 8:17 am, edited 5 times in total.
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Postby sorenj07 » Mon Apr 09, 2007 8:59 am

EWBrown wrote:Direct driven (without the resistor) and with a 32 ohm output, a half-watt should be sufficient to cause near ear-damaging SPLs. So, be careful...
Yeah, this is my biggest concern. I'm probably going to have a big, clearly-labelled switch for headphone use. I like the idea of so many watts of headroom though, it should help the frequency response.

EWBrown wrote:The OPT can be anything from 6.6K to 10K P-P, B+ is about 300VDC.
I'd rather not bother paralleling tubes, it means more plate current, and a bigger PSU, and possible instability with unmatched tubes. I don't need the power. The toroid at 17.3K a-a should be fine.

EWBrown wrote:I'm sure that a 7AU7 will run just fine with 6.3VAC on the filament, and the 6GU7 or 8GU7 is functionally equivalent to 12BH7, just with filament connected to pins 4 and 5 only.
/ed B in NH
Yeah, I figured that most US AC runs the 6.3 at closer to 6.6-6.8V, and that's well within 10% of 7.2.

Last night, I realized that I can use a couple back-to-back 6A 6.3V filament transformers I have lying around, with a voltage doubler feeding an LCRC filtered B+ at ~293V. They're nice enclosed models, the bells of which which I will probably strip of varnish and paint a fiery red-orange. I'll probably post a schematic later today.

Two questions: What current set resistor would I use to a 7AU7 in LTP, 6mA between the two? B+ is 280V dropped to ~100V with two 62K resistors, if that helps, to bias at -4V quiescent. The swing remains pretty linear (I know 12AU7's don't have a great reputation for this) as long as you stay under -2V or so, which should be fine for digital sources (-3V to -5V swing). |

Thanks!
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Postby EWBrown » Mon Apr 09, 2007 10:32 am

The 12BH7s don't have to be parallel, just use one and go with a couple watts out, and an OPT primary impedance abour 2X higher, so 17.83K a-a should be just fine..

For the 7AU7, If you're using the LM234/334 regulator, try 11 ohms for 6 mA. It normally works out to R (ohms) = 68/mA so 68/6 woud be 11.33 ohms. There is a small temperature-afected "fudge factor", but it is small enough to ignore.

/ed B in NH
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Postby sorenj07 » Mon Apr 09, 2007 11:09 am

Thanks, 11 ohms sounds fine. Should I try to supply some negative voltage to feed the LM334, to prevent clipping or whatever? Jones warns against this in Valve Amplifiers, and someone just reminded me of this at diyAudio... I'm thinking maybe yet another doubler-rectifier off the 6.8V filaments? how well-filtered would such a supply have to be?
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Postby EWBrown » Mon Apr 09, 2007 11:24 am

You'll have enough voltage headroom, that any negative voltage rail for the LM334 won't be necessary. I get away with no neg voltage with the 6EM7s though it is cutting it about as close as I can get away with... The lower-mu triodes like 12AU7, 6CG7, 6SN7, etc, will "float" at a higher cathode value than will high-mu triodes like 12AX7, 12AT7, etc.

You could always generate -6 to -8V from one of the filament windings, but it shouildn't really be necessary.

In addition to 12AU7, 6AU7, 7Au7 there is a 9AU7.

7 or 9 AU7 and 8GU7s, the tubes for this could be dirt cheap!

HTH

/ed B in NH
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Postby sorenj07 » Mon Apr 09, 2007 1:01 pm

Yeah just check the 'Bay. Hopefully they'll last to tonight, when I'll get my supply :) Anyway, glad this amp seems to be working. I re-worked out the bias for a 310VB+, at 520 ohms you get around 11.5mA at ~220Va, I think. 2.4-ish watts per, comfortably under 2.5, and with a tiny bit more power :)
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Postby EWBrown » Mon Apr 09, 2007 1:22 pm

Hi Soren

If you're in the Boston area, check out the MIT hamfest / swapmeet that is held everh third Sunday all spring and summer long. I believe it starts this coming Sunday, and every month through October.

15 April Cambridge MA FLEA at MIT Nick 617 253 3776
Third Sunday April thru October


/ed B in NH
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Postby sorenj07 » Mon Apr 09, 2007 9:59 pm

EWBrown wrote:Hi Soren

If you're in the Boston area, check out the MIT hamfest / swapmeet that is held everh third Sunday all spring and summer long. I believe it starts this coming Sunday, and every month through October.

15 April Cambridge MA FLEA at MIT Nick 617 253 3776
Third Sunday April thru October


/ed B in NH

wow, I'd love to be there! Unfortunately for my selfish self, it's my dad's birthday very soon after that, so it might be mostly window shopping :D Who knows.
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Postby sorenj07 » Tue Apr 10, 2007 8:57 pm

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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Postby WA4SWJ » Thu Apr 12, 2007 5:56 am

Soren,

I was messing around last night with your latest schematic on Multisim doing a SPICE simulation of the circuit. It has an interesting set of characteristics. The simulation began with a very large distortion result - in the 80% range - that eventually settled down to the 2% range plus or minus a little bit after a long period of simulation. Distortion at both ends of the audio spectrum was extremely high - in the 70% range or so with the built in distortion analyzers in the program. There was also a huge DC offset at the output, which is also interesting since the output is capacitively coupled, but it is a rather large coupling cap. I was also not able to simulate a DC operating point successfully nor was I able to do a complete AC analysis of the circuit. Neither would converge properly. A very interesting and different response from what I've seen before. For example, Shannon's eiclone driver as well as the poseidon both show extremely good linearity, frequency response and low distortion. Distortion in those circuits up to the outputs of the phase inverters simulate in the .001% range. The major frequency response limiter in those circuits appears to be the size of the capacitor in the small R-C network to ground after the voltage amp. That chops off the upper end response quite nicely but is in there for stability reasons. Smaller C values extend the frequency response of the circuit at the expense of other types of behavior.

I would be interested to know if you've constructed this circuit and if so, how it sounds. It doesn't simulate well in SPICE - of course that could be pilot error on my end or some less than perfect tube models. So the analysis itself could well be flawed. When I have some more time I might post some of this stuff but, with some more international travel coming up, it won't be for a couple of weeks. It would be interesting to hear how this circuit works out in real life. Simulation is not always the same. Good luck with it.

Regards,
Ed Long
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