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MOSFET for audio power amp output control?

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eTech

Well-Known Member
Need suggestion for circuit to control output of audio power amp.

Requirements:
1. Circuit is placed between output of power amp and speaker.
2. Circuit should be able to "disconnect" amp from speaker (similar to a relay contact, but solid state).
3. Circuit should be switched on or off by 5v control voltage via CMOS or microcontroller.
4. 5v control voltage must be isolated from audio signal
5. Would like ability to optionally control circuit so that audio signal is applied gradually to speaker
to prevent damage.
6. Amp max rated output power will vary (say...between 200W and 800W RMS).
7. And, of course, must not introduce any noticable degredation/distortion of the original audio signal.

thanks

eT
 
Last edited:

ChrisP58

Well-Known Member
Most Helpful Member
Need suggestion for circuit to control output of audio power amp.

Requirements:
1. Circuit is placed between output of power amp and speaker.
2. Circuit should be able to "disconnect" amp from speaker (similar to a relay contact, but solid state).
3. Circuit should be switched on or off by 5v control voltage via CMOS or microcontroller.
4. 5v control voltage must be isolated from audio signal
5. Would like ability to optionally control circuit so that audio signal is applied gradually to speaker
to prevent damage.
6. Amp max rated output power will vary (say...between 200W and 800W RMS).
7. And, of course, must not introduce any noticable degredation/distortion of the original audio signal.

thanks

eT

I would recommend a mosfet based solid state relay. Do not use the triac/scr ones, as they will cause big distortion in the audio, and won't even conduct the lower voltage signals.

To get a mosfet based one, search for DC contact type.
 

crutschow

Well-Known Member
Most Helpful Member
I second ChrisP58's suggestion.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I don;t even think I would go the MOSSFET route. I built a system about 30 years ago that I ended up really happy with.

It does not do DC protect, per see, BUT it will do it's thing even if the output transistors are switched, with NO harm except a single resistor. It could be better, but at the time mico's were not even available.

I had 40,000 uF of capacitance total on 50 V rails and I watched the same amp sizzle when I had a CVT 18 A powersupply connected and a 100 uF cap shorted and popped a fuse. I said, I have to do something.

So, I did.

#1. Because of a design issue with my circuit, I had to put ZNR's across each caps.
#2. The end things are a relay for the speakers and a VTL5C1 optocoupler (it has leads) that's used in series with the audio path.
#3. The OPTO removes the signal quickly
#4. The Optos were arranged with Zeniers such that they turn-on at about 2/3 of Vcc. An AND gate was made with the OPTO's in series.
#5. There is a "selected" flame proof resistor in series with the AC line.
#6. So, with the Audio off, the caps charge through that small resistor. Once they reach 2/3 VCC, the resistor is shorted out.
#7. A BETTER design would be to have another way to cut AC power after a "timeout". So if it never reaches 2/3 Vcc in a specif amount of the the "protect" should be active. This would prevent loosing the resistor.
#8. Now when the voltage is > 2/3 of Vcc, then the OPTO acts as a voltage divider to the input. 200 ohms which, I believe, is the On resistance, doesn't seem to affect anything.
#9. I used a current regulator for the OPTO.
#10. So, on turn on, a Cap charges and a quad OPAMP (LM324?) amplifies the voltage across the cap and is applied to the OPTO.
#11. So the Audio ramps up logarithmically in about 10 seconds. It catches the tail end of when the pre-amp mute circuit enables.

So, the AMP protection relies on a power supply rail fuse blowing. There are 4 of them. Once one of these blows, the amp will shut down very quickly. Another part of the protection is an AGX fuse as the speaker fuse.

I had planned to add clipping indicator, but never did. I did at one time design a clip circuit for a scientific instrument, that glowed red or green depending on the polarity of the clip and it also extended the blip for a second or so. I was thinking of a circuit based on a Norton OP Amp.

I also wanted to add a thermal on the heat sink, but the AMP runs way too cool.

A computer readable schematic, I don;t have. I may have my notes. I did make a PC board. The system was powered by a small control transformer and a 12 or 15V regulator.

SO, I would have liked a clip indicator, overtemp indicator and a protect regulator.

Internally there is a LED in series with the opto and a LED for each of the power supplies, so it's easy to troubleshoot. I have an extension cable to tke the amp modules out of the case. I would have also added a test point across the resistor. Troubleshhoting with a Variac means you have to short out the resistor. There was an alternate input available if the AMP is removed that bypassed the audio path of the OPTO.

I'm really happy with it, even after 35 years.
#9.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I would say simply use a normal mechanical relay - as every commercial amplifier does.

Anything else is just a poor imitation,and more expensive.

There's no need for it to vary the power, that seems a really silly idea.
 
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