Discussion in 'Electronic Design' started by [email protected], Apr 13, 2006.

1. Guest

Hello all,
searched and found some great single output amplifiers. Which brings me
to a question. If I build one of these, times four, and connect each
amplifier to the same input, will it work properly?

schematic
http://www.minidisc.org/schem.gif

another hp amp

What I mean is if say I have all of the pots at half value, and then
bring three (6 in stereo) of them down to zero, will it make the output
of the remaining channel go through the roof? I know I should be more
up on my basic electronics (sad because I'm an electronics mechanic,
but thats the keyword, not a technician) Thanks a bunch!
Jester

2. Walter HarleyGuest

No. They'll all be independent of each other.

The load that this circuit presents to the source is 100k. That's very
high. Four of them in parallel, 25k, still quite high. And the position of
the pot does not change the load significantly; so the position of the pot
will not change the output voltage from the source, which means that the
other amps will not notice a difference.

3. BanGuest

Jester, I didn't look at your schematics, but if they are IC based there
shouldn't be any problems.
Normal headphones have an impedance between mostly 32 and 600 Ohms(those
legacy HiFi ones) and require maybe maximal 100mW for a decent loudness.
Some DJ headphones can go up to 400mW, but small earphones are blown with
that level. When we calculate the required voltage Vrms= sqrt(P*R) = sqrt
(32*0.1) =1.8Vrms for those newer ones and 7.5Vrms for the legacy ones. The
DJ type would need 3.6Vrms.
Now we can use a resistor of 120ohms with at least 1W rating in series with
each output to drop the unrequired voltage and connect to a 10Vrms output
amplifier(25W into 4ohms). Any old stereo amp will do.
To adjust the level individually, we put a 1W-250 ohms linear
potentiometer(best is wirewound or Cermet) in front of each headphone. Maybe
a little box for each person with 2 mono pots (dual ones are hard to find,
but try PartsExpress), so you can adjust the balance too. You can wire all
boxes in parallel and connect as many as you need. They do not influence
each other. To avoid damage to the amp in case someone shorts the incoming
wires, put two 10R/10W resistors directly on the red speaker outputs and
your outgoing wires. You only need one black gnd wire, since these are in
parallel.

see diagram with fixed font.

AMP speaker
red ___ terminals
L o-|___|--o----o----. mini jack .---|
10R/10W | L +red | .------| |
| .-. ___ | .---| L |
| 250R| |<--|___|----o----' | '---|
blk | | | 100R .--o----. |
o---------)+ '-' | o----)--+
gnd || -blk | | |gnd | |
|+---o----+----------)--+ | | .---|
red ___ || gnd | | '--)---| |
R o-|___|--))+--o----. | | '---| R |
10R/10W |||R +red | | | '---|
||| .-. ___ | |
||| 250R| |<--|___|-' |
||| | | 100R |
||| '-' |
||| | |
||| '-------------'
|||
|||
|||
||| next box
|||
||'--------o R
|'---------o gnd
'----------o L
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

4. Guest

Thank-you everyone!
Jester