[sdiy] simple headphone amp??

[epeasant at telusplanet.net]

> 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??  

I have not seen the circuits in question but would assume 
this to be the case, particularly if they are single 
supply. This is common practice with small single supply IC-
based power amplifiers. Since the AC musical signal has to 
go positive and negative from zero, the zero signal level 
at the output must be half way between VCC and ground. But 
the other side of the speaker connects directly to ground, 
causing a DC offset of half the power supply to be applied 
across the speaker. The capacitor blocks this offset. You 
can either bypass the electrolytic with a higher quality 
smaller value capacitor, or modify the circuit to work on a 
dual supply so no output = zero volts.


> 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 two 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??

Definitely put the volume control on the input, where you 
can use a lower wattage, higher value dual ganged pot. 10K -
 100K is usually OK for the pot resistance. Using a single 
gang pot is more complex and requires extra added 
circuitry, such as VCAs, but can be done.

> 
> Sorry, I haven't had any time to think about tube
> filters.
 
Tsk, tsk. ;-)

Take care,
Doug
______________________
The Electronic Peasant

www.electronicpeasant.com