> -----Original Message-----
> From: media.nai@... [mailto:media.nai@...]
> Sent: Thursday, July 25, 2002 1:08 PM
> To: motm@yahoogroups.com
> Subject: [motm] simple headphone amp??
>
>
>
> 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??
Yes, those caps are there to keep DC from frying your headphones or anything
else downstream. You cannot simply AC couple the input and expect there to
be no DC on the output. Look at the data sheet functional diagram and you'll
see regardless of what comes in, the output is still at some DC level. This
device can be powered bipolar, and that might allow you to circumvent the DC
blocking caps.
> 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.