12AU7/12BH7 Headphone Amp

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Postby sorenj07 » Fri Apr 13, 2007 9:40 pm

WA4SWJ wrote: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.

This is kind of unfortunate. Do you still recommend that I use this topology? The individual parts in TubeCAD had very little distortion... I'm a bit hesitant to build the circuit, but my one consolation is that if I leave myself enough room, I might be able to build Fred Nachbaur's phono stage with the space given.

WA4SWJ wrote: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,


I haven't built it. I have the filament transformers, 3 7AU7's and and 4 more coming in the mail, and a 10x7" aluminum plate and top cage. I'd love to build something of my own design but if there's another pretty high-current, lowish-gain circuit that can drive low-impedance headphones, I'd love to see it :D
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Postby sorenj07 » Fri Apr 13, 2007 11:20 pm

Oops. I realize that I had TubeCAD simulating paralleled 12AU7 triodes. This would explain the crappy frequency response and stuff when simmed in SPICE. Fixing this problem and recalculating gives the following:

Ra: 12.4K --> 14K 1W
Ra2: 21.5K --> 37.4K 2W
Rk: 143 --> 590, bypass with 31uF 16V nominally, but probably ~470uF 16V
Rk2: 8.87K --> 23.2K

Gain remains at .94. The cathode bypass cap enables a lower output impedance of around 38 ohms, despite only using single triodes.

B+ is still at 300V but total current draw for the CF section is now 10mA. As voltage increases to potentially 350V, cathode resistance increases to bias the tube more negatively to stay at 10mA dissipation.
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Postby WA4SWJ » Sat Apr 14, 2007 5:51 am

Soren,

Don't let me discourage you. As I mentioned, the SPICE analysis could be flawed. Once you have it built I'd like to know how well it works.

Have fun with it!
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Postby sorenj07 » Sat Apr 14, 2007 10:23 pm

Indeed. I'm ordering the bulk of the parts tonight. I'll put together a little pictorial Yellow_Light_Colorz_PDT_06
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Postby sorenj07 » Wed Apr 18, 2007 7:49 pm

here's an updated schematic, how does it look? someone from DiyAudio said that bass would roll off starting at around 700Hz but I find it kind of hard to understand why...

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Postby EWBrown » Thu Apr 19, 2007 6:26 am

First thing, that 10K linear "balance" pot will lower the input resistance to 5K or less... Which may or not be a problem, depends on the intended source.

Presuming that the headphones are the "standard" 32 ohms impedance, this will start rolling off at 700 Hz, or even 1000 Hz.

Xc for 4.7 uF, at 700 Hz, works out to 48.4 ohms impedance.

In round numbers, 700 Hz will be about 9 dB down.

Xc for 4.7 uf at 1000 Hz, works out to 33.9 ohms impedance.

In this case, 1000 Hz will be about 6 dB down.

Used as a line stage, the load impedance will be much higher, so there should not be any significant high frequency roloff.

HTH

/ed B in NH
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Postby sorenj07 » Thu Apr 19, 2007 11:34 am

I think it's time to use those 100uF 330VAC motor run caps I have lying around. Is a 50Hz -6dB point reasonable? Or should I risk ear and limb by using a 470uF electrolytic for a 10Hz point? Is there any way to put a fuse or something to protect from a short?
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Postby sorenj07 » Thu Apr 19, 2007 11:40 am

Could I try using an impedance matching transformer like the ones here?

http://edcorusa.com/products/transforme ... index.html

They aren't meant to be used single-ended, but the low current that headphones/line output requires wouldn't saturate it, right?
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Postby TomMcNally » Thu Apr 19, 2007 1:42 pm

It seems like it would be better to use a pair of the small Edcor single ended transformers at this point. They are like $ 19 each. I have four of them on the way, gonna give them a try and see how they compare to the Hammonds.
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Postby sorenj07 » Thu Apr 19, 2007 2:05 pm

Actually I've been messing around some more in TubeCAD with this schematic except with a 32-ohm load. Everything holds constant, and I get a 10.4Hz -3dB point using a 220uF capacitor on the output. A 470uF capacitor nets me just under 5Hz for this point. Of course I'll use capacitors that meet safety requirements, but space is also a concern. The plate voltage on the tube, TubeCAD doesn't saw which, is 121V. Does this mean that I can use a capacitor with a rating of 200V or something?
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Postby TomMcNally » Fri Apr 20, 2007 12:59 pm

I'm not sure about the voltage rating, it seems like it should be rated for the full B+ ? 470 mfd at that voltage will be big and expensive and hard to find, won't it ?

I just bought Tube CAD and SE CAD, gotta play with them !
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Postby sorenj07 » Fri Apr 20, 2007 2:51 pm

it's not so bad, under $10 for each.
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Postby WA4SWJ » Fri Apr 20, 2007 3:27 pm

Hi Soren,

Unless you're into music with low organ pedal content do you really need frequency response that low? Not much content at those low frequencies in most music.

I hate to say this, but if you need that kind of response down low you might be better off with a solid state DC coupled amp of some kind (OK - kill me now). My Sherwood S-6040CP's might go down that low - those are 125 watt DC coupled MOSFET amps manufactured in the 80's. They're great solid state amps but they are different. Overkill for headphones too.

Also, will your headphones respond at those frequencies? How about the ears? I think you feel that stuff more than hear it. I can sure feel those vibrations from some of the cars that are around on the street. I can also hear the vibration based harmonics from the trunk lids and other metal parts that aren't nailed down! Even the license plates contribute!

Regards,
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Postby sorenj07 » Sat Apr 21, 2007 12:23 am

No, I really don't need that kind of bass response. Isn't it worth it for a few extra bucks to increase the bandwidth of the linestage? The -3dB point would be at just under 5Hz but the 0dB point would be above it, and I'd rather know for sure that the entire audio spectrum is linearly reproduced...

Running the sims again I see that the 100uF motor run caps give a -3dB point at 15.8Hz. It turns out my headphones have an impedance of 63 ohms, not 32. I'll still "design" for 32-ohm headphones though because I might get myself some Grados in the future.
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Postby sorenj07 » Thu Apr 26, 2007 12:04 pm

Final schematic, both channels. I borrow Rod Eliott's balance pot idea from his website ("Better Balance Control " at http://sound.westhost.com/project01.htm ).

After a crash course in RC filters (yes, I didn't actually know how they worked :bigeyes: ) I learned that the nice big 4.7uF Solen lets pretty much no bass through. Replace the bypassed 4.7uF with a 220uF 450V electrolytic with negative connected to output, bypassed by a 2.2uF 630V Solen. It could have been 400V but whatever, it's a 20 cent difference at PCX.

I simulate around 10.4Hz -3dB into a 32 ohm load (Grado's) with TubeCAD. With my 63-ohm Sony MDR-V6's, the -3dB point drops to 7.2Hz. Both figures are acceptable, and as you can imagine, with higher impedance headphones, the frequency response just keeps getting better. There isn't a whole lot of room but maybe for headphones of 300 ohms or better I could switch that 220uF electrolytic out for 22uF for 21.4Hz -3dB. Otherwise, it's a 2.1Hz -3dB point which is all well and good but pretty impractical.

Image

My bid for ear safety in this case is using 450V 105C electrolytic couplers even though B+ is 315V, and the plate voltage is around 120. Better safe than sorry I guess.
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