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In Reply to: Head Phone amp for K240 posted by Soundripples on May 5, 2007 at 20:40:09:
If the attached link is your headphones, a 55 ohm impedance won't be incredibly easy to drive. I just recently designed, and will build within the week, a headphone amplifier that has an output impedance of around 38 ohms. With a coupling capacitor of 82 ohms 450V, the amplifier's -3dB point is 21Hz. 100uF nets you 17.2Hz, 180uF 9.5Hz, and so on.If you have space, try using a 100uF motor run capacitor of at least 220VAC - danger that they will blow is much reduced, as they're self-healing, and the ESR is a lot lower as well. No real need for a high-quality film capacitor as a bypass, either. If you do want one, follow the 1% rule - 100uF bypassed by 1uF bypassed by .01uF, as far as you want.
Here's the schematic. If you can design your own PSU, go ahead, but if not, I can probably knock one together for you. Personally, I'm using two 6A 6.3V filament transformers back to back, feeding a voltage doubler, a 10H 65mA choke, a 470-ohm resistor, and then B+, then a 4.7K resistor for B++. Add capacitance to those filters as you want.
Follow Ups:
Thanks for the help. I did not expect a custom design.This maybe past what I would feel I could do. It would be an adventure!
Well if your headphones are really 600 ohms, you can use a cap of 13uF or greater for a -3dB point of 19.2Hz or lower. I recommend some nice 15uF motor run caps! They'll sound AMAZING. That is, if you still feel up to construction. As soon as the 157J choke for my PSU comes, I'll be building. I'll tell you how it sounds on some 63 ohm Sony MDR-V6's.
Søren, You must like this tube. You have it in both positions. Why did you choose it to do double duty in this circuit?
because I have a few of them, and they're very cheap.. needless to say the gainstage could easily be a 12BH7, and the CF could use one with a little re-working. If you want more gain from the headphone amp, just sub in a 6922 or something for the gainstage.
NT
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