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Old 11th April 2007, 08:23 AM   #11
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Looks like a clever design. Can't really comment on technical issues. The only slight concern I have with direct feed from a tube headphone amp is the remote possibility that your headphones could go live if your output cap fails !! Unfortunately caps are more likely to fail in a dangerous way than transformers.

Have you been over to the Headwize website, they have some very good worked up tube headphone amps.

Shoog
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Old 12th April 2007, 01:40 AM   #12
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Quote:
Originally posted by Shoog
Looks like a clever design. Can't really comment on technical issues. The only slight concern I have with direct feed from a tube headphone amp is the remote possibility that your headphones could go live if your output cap fails !! Unfortunately caps are more likely to fail in a dangerous way than transformers.

Have you been over to the Headwize website, they have some very good worked up tube headphone amps.

Shoog
Thanks! Is there some kind of fuse or circuit breaker that I can put to prevent a spike in current over 10mA or something? I'd like to use a really really reliable film output capacitor, I'm thinking a Solen 630V as they're reasonably priced, are available in big uF-age, and I haven't heard any disaster stories about them...

I have checked out Headwize and this is kind of inspired by the ideas I saw there ( http://www.headwize.com/projects/sho...train1_prj.htm ) but I wanted a much lower output impedance, <50 instead of around 500.. Plus I have boatloads of 7AU7's so I wanted to use those. I'll build it pronto and report on how it sounds! I'll try AC wiring but I'll leave room for some hefty 2200-ish uF 16V caps in case hum is intolerable.

Until next time, I'm enjoying my newly modded PSX. You'll see a bit more about that in an upcoming thread.
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Old 12th April 2007, 05:52 AM   #13
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I agree with the comments on gain,and eardrum-damage.
Seriously,be careful!

I once made a 1W/chan headphone amp with some 60FX5 tubes,and it was wwaayy overkill. 3W is insane!
Go easy on the volume control!

Another small piece of advice- NEVER 'debug' a circuit using headphones! If it were to break into oscillation,the volume control was accidentally set at MAX,or some other "unseen issue" -you could be deaf in an instant!
Use a small speaker on the workbench instead.
Not to mention you could damage the headphones with that much power.
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Old 12th April 2007, 06:06 AM   #14
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Quote:
Originally posted by DigitalJunkie
Use a small speaker on the workbench instead.
Not to mention you could damage the headphones with that much power.
Yeah, that's the plan. I don't know what impedance the iPod headphones are but I'll start with those. Leave them on the bench, I should still hear something coming from them, can put my head a bit closer for clarity. I have a scope coming in the mail though (Tek 2213) so a nice 50-ohm load resistor or something for the later, much-less-than-1-watt design should be OK to work with.
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Old 14th April 2007, 05:16 AM   #15
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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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Old 26th April 2007, 04:49 AM   #16
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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 ) 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 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.

Click the image to open in full size.
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Old 26th April 2007, 06:12 AM   #17
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have you built this circuit yet?
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Old 26th April 2007, 12:47 PM   #18
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the parts are on their way.
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Old 26th April 2007, 08:40 PM   #19
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have you considered the 12BZ7?
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Old 26th April 2007, 11:38 PM   #20
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I didn't even know that that tube existed. Looking at a datasheet, it seems a little mismatched for my purposes, though. Amplification factor of 100 vs 20 for the 12AU7. 1.5W plate dissipation vs 2.75 for the 12AU7. Its place resistance is around 32K as opposed to 7.7K. Finally, the plate curves looks INCREDIBLY non-linear, even compared to the 12AU7

Do you mean 12BH7? I might try them later but their higher heater current directly sucks power from my B+ (I'm using two filament transformers back to back)
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