Re: Can this circuit be modded to work with lower power input signal?


"Kasterborus" <kasterborus@xxxxxxxxx> schreef in bericht
news:5b96c98c-dbb4-479e-9821-ef3c2ebe9144@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I was looking at the schematic again - do you think that the pin 6 on
the OI could be wired up to do the job?
It seems to provide an electrical input to trigger the switching .

Maybe breaking the connections on pin 1 , connecting this to pin 6.
Then breaking pin 2 and connecting this to GND.

Obviously at this point I would never want to wire it up to a speaker
again, but maybe the half rectified line level signal would be enough.

I'm great with theory, just no so good with number crunching.

That opto-coupler is required to insulate the speaker output from the lamps
and their power supply. I'd leave it that way. Make an error and you blow
your amplifier.

At the same time that opto coupler causes your problem. It's input LED uses
to function at 0.8-1.2V and some mA of current. At normal speaker output
levels the LED will hard produce any light. That's why the output of that
optocoupler is ampified so hard.

Increasing the volume makes sense but you've already found out that you have
a lot of sound at about 2Vpp already. Besides, you introduce some distorsion
as the diodes (the LED and the 1N4148) are a highly non-linear load. That's
one of the reasons that pretty high series resistor is added. As long as the
impedance of the speaker is far below the impedance of the LED-resistor
combination you'll hear not too much of it. You nevertheless will need some
series resistor as you need to limit the current through that LED.

One thing you can try is getting another opto-coupler. Maybe you're lucky to
find one that works better in your circuit. But you can be sure. After all,
the LED is supposed to work way below its normal specs.

Another, better, way is using an input amplifier. See below.

-----+----+--12V
| |
| V Opto
| - LED
| |
| |
.-. .-.
100k| | | |4k7
| | | |
+ '-' '-'
LS #| | |
----#|---+ | |
#| | + | |
10uF .-. ___ |# | |/
| |<--|___|---|#--+--|
| | 1k |# | |>
'-' 10uF | |
| 10k | |
| | |
| | |
| .-. .-.
| | | | |
| 10k| | | |470
| '-' '-'
| | |
-------+----------------+----+-
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Aan de ene kant is het een klassieke schakeling. Aan de andere kant, ik heb
hem zo'n beetje uit mijn blote hoofd opgekrabbeld. Deze schakeling wordt
geacht een keer of tien te versterken. Belangrijker is dat er ongeveer 1mA
ruststroom door de LED gaat. Wat weer tot gevolg kan hebben dat de
versterker achter de optocoupler zwaar overstuurd wordt. Je kunt eventueel
experimenteren door de 100k weerstand groter of kleiner te nemen.

(Uiteraard haal je de 12V voeding uit de versterker en niet uit die van de
lampen.)

Succes

petrus bitbyter





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