Archive of the former Yahoo!Groups mailing list: MOTM

It seems like synth-diy is down again, so I'm posting this here.

I need a simple headphone amp that can run from the RCA outs of a VCR.

The SSM2135 is a dual op-amp specialized for audio and single-sided
supplies, and is used to drive the headphone outputs in a number of
commercial products. Since I already have a SSM2135 in a DIP package, I'm
thinking I could put it on a perfboard in a little project box. That
should take me less than a half hour, which is about all the time I have.
According to AD, it is designed to "tolerate poorly regulated" power
supplies, so it should be happy with couple of bypass caps and a 9VDC wall
wart.

Anyway, I have a few questions. All of the examples of 2135-based
headphone amps that I could find, both inverting and non-inverting, show
enormous caps (eg. 100uF-470uF) in series with the output. Is that only to
prevent DC offset from damaging the headphones?? The bigger the cap the
better the bass response, so that explains the huge caps, but electrolytic
caps sound horrible. I would like to keep them out of the signal path.
The 2135 has a typical input bias of 300nA. If I build a non-inverting amp
with an AC coupled input, shouldn't that be sufficient??

Also, what is the best way to add a volume control -- the input?? the
output?? the feedback path?? I'm thinking that a divider at the output
might be best, that way I can fix the input impedances to minimize offset
with fewer resistors. Also, the output impedance of a headphone amp
doesn't matter much. Any thoughts?? Mouser sells a Xicon dual-ganged
24mm 1/4W audio-taper 1K pot with solder lugs for $2.35. That was the
lowest value dual audio pot I could find. I'd rather have conductive
plastic with a lower value and I higher wattage rating, but what can I do??

Is there any trick to use a single pot, or do I need to buy a dual pot??

Sorry, I haven't had any time to think about tube filters.