Just another OTL Headphone Amplifier
It started out as a replacement headphone amp (you know, for work...) and got lost along the way. Current version is about the 6th circuit and sounds pretty good - I tried several variations of tubes and parallel feed using cheap output trannys, but couldn't get things to make music. The current version makes music.
If you are interested in building a Headphone amp you really have to go over to the headwize site and check out all of the really groovy designs and discussions on this interesting sub-universe of amplifier design:
OK, building a headphone amp is exactly like building a low-power integrated amplifier, except for the added complications of needing extremely low noise (good headphones will ruthlessly reveal noise, hum, grunge, whatever) and needing to support a wide range of output impedances since headphones vary all over the map. Diving a pair of 300 ohm Sennheiser HD600 'phones is a very different load from a pair of 32 ohm Grados or Sony cans.
Fortunately, I like Sennheisers and have a pair of HD580 cans, so I was able to focus my design for their higher impedance - Note: my amp is not tested with low-impedance 'phones - no warranty expressed or implied, don't blame me...
Earlier ideas (Yup, I did build and test all of these, but sorry, no schematics of these available):
I used the 6N1P and 6H30pi becuase I've A) used them before, B) had a stock on-hand, C) like their sound. The 12B4 was a well-liked tube from online discussions with a low plate impedance and good power range. All of these tubes are very durable and should last a long time in the schematic shown below. I was really impressed with the sound of the 12B4, and plan to use it again if possible - BTW, it's also a very attractive tube when illuminated.
Headphone Amp: C4S loaded 6H30pi voltage amplifier into C4S sunk 12B4 cathode followers
Frame is paduak and birdseye maple with an aluminum top. Tall cans are Target kitchen department covers for output transformers bypassed in current design.
The final hurdle was getting rid of the cap coupling between the 6H30pi voltage amplifier and the 12B4 cathode follower - making this amp into a really big 'ol Foreplay derivative (sorry Doc).
Just to give folks and idea that this is hobby is not all instant gratification, here's a note I submitted to Bottlehead Forum on 5jul02:
OK, so I've got this little headphone amp (you know, for work...) I've been fiddling with for about 6 months.
Doc B. Kindly replied:
What's the Rp of the 6H30pi? Can't you just dispense with the CF altogether and couple the cans right to them? If not, it seems like maybe reworking the PS so you can direct couple the stages might be a neat way to go.
Aha! I repost:
Sometimes the process of posting makes things fall into place in my head (sometimes I just get more confused), but I'm now thinking that at one point before I added the resistor networks to the CF grids I had been running the thing overnight and the leakage had driven the grid up to 117VDC (you have to be quick to even see it as the process of measuring it made the voltage drop immediately). The output voltage was happy at almost 100VDC. Maybe I should just snip out the coupling cap and voltage divider, and let the C4S ride at a higher voltage and be done with it. The 12B4 has plenty of headroom. Simpler is better.
The next day I reply to Voltsecond:
OK, performed more surgery last night - removed the coupling cap and voltage reference dividers and just directly connected the plate of the 6H30pi driver to the grid of the 12B4 and let the C4S ride and do it's current sinking thang.
I'm still interested in trying to build a transformer output headphone amp, but I've got some other projects to finish first, and this is good enough to enjoy at work for a while...
Finally, more links:
http://www.head-fi.org/ Another Discussion site on Headphones
Bottlehead Doc B.'s site and the home of the Bottlehead Forum
VoltSecond This guy is relentless - some Headphone stuff in the Foreplay links
http://www.audioasylum.com/ Another good Discussion site