[sdiy] simple headphone amp??

[media.nai at rcn.com]

>No, since to work with a single supply you have to bias the output so
>that it sits at half the supply voltage. So you need the cap, unless you go
>for a symmetrical powering scheme. The cap is the price you have to pay for
>the single sided supply.

Thanks, now I understand.  I didn't realize that the quiescent output would
sit at half the supply voltage (actually, in the case of the 2135, its
maximum output is 4V with a supply voltage of 5V, so for maximum dynamic
range it would sit at 2V, not 2.5V).

>>Is there any trick to use a single pot, or do I need to buy a dual pot??
>
>Well there would of course be the option of using one of the ICs designed
>>for this purpose. They're used often in TVs and such. But a dual gang pot
>>is the simpler solution.

It's too bad I can't find conductive plastic or fully sealed dual pots.

>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.

OK :)

>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.

THANX!! :)