A simple speaker protection circuit

[AndrewT]

Hi,
do you have any comparators?
I think they would work here instead of an opamp.

The whole detection thing could be done with a pair of comparators set up as a window comparator. Just one dual 8pin chip and a switching transistor pair.

Worth breadboarding to see the difference (if any) from your proposal.

[mhennessy]

Originally posted by AndrewT
Hi,
do you have any comparators?
I think they would work here instead of an opamp.

The whole detection thing could be done with a pair of comparators set up as a window comparator. Just one dual 8pin chip and a switching transistor pair.

Worth breadboarding to see the difference (if any) from your proposal.

Interesting thought - in this context comparators are the same as op-amps, only their outputs are open-collector (not all, I know). So connect the outputs together for a wired-OR connection, use this to discharge the power-on delay capacitor...

The only "But" I can think of is that a single transistor following on from this delay-cap will get us back to an earlier problem, whereby if the transistor turns on slower than the relay power-saving capacitor discharges, the latter might not perform its function properly. But I've never played with this particular variation myself, so maybe I'm worrying about nothing...

Of course, you could add another dual comparator to do the power-on delay separately, and use the spare section as an over-temperature detector

Russ, if you put the two resistors around the op-amp, you probably won't need an extra zener. Glad you're enjoy this

Mark

[mhennessy]

Originally posted by mhennessy
The only "But" I can think of

Just thought of another one - you have to create a -ve supply for the -ve threshold of the window comparator, not to mention the supply for the chip itself.

Not a big deal, but an extra diode/capacitor/dropper resistor...

There's also a chance that the max supply voltage the IC's run from could become an issue, requiring supply regulation. That's why I generally prefer transistors in these sorts of roles - no such limits if you choose the right device, and no cost-hikes either (unlike high-voltage op-amps!)

Mark

[Nordic]

I don't think the speed thing is such a big issue...
For instance looking at the mauro penasa amp, he has a check along the line of +xVDC for y seconds and another set for the negative rail.

I mean even low frequency sounds like a 10hZ tone could also be called DC, so haveing too fast a response is not nessecarily good is it?

[Russ White]

Originally posted by Nordic
I don't think the speed thing is such a big issue...

so haveing too fast a response is not nessecarily good is it?

Nordic,

As you know i know that circuit (MyRef) inside and out. This is actually significantly different in multiple ways.

The place we want things to happen very fast is when the relay switches when DC "is" detected. That is why the schmidt trigger is there as well as the R and C around the relay coil. The faster the relay witches to GND the faster the speaker is relieved from the DC.

The speed at which the DC is detected is determined by adjusting the RC filter. That would be adjusted to be reasonable for the application with some general starting value which should work for most amps. The triggering of the detector will be determined by the voltage and frequency of the signal coming from the amp's output.


Cheers!
Russ

[Russ White]

I made a simple PCB (will post later)

Here is the circuit I settled on.

Since the opamp will not swing all the way to GND I added the diodes and resistor at the output.

I test the circuit with 35V on input and it takes about 5ms for it to kick off the relay. Better than I hoped.

Cheers!
Russ

[AndrewT]

Hi,
now test it with AC signals.
try full voltage 20Hz and progressively lower the frequency until the filter lets through enough signal to trip the relay.

If satisfactory, then try to see how little DC is needed to trip the relay.

My targets would be
1. pass 10Hz full voltage and stay untripped.
2. detect 1.5Vdc and trip.

[Nordic]

Much appreciated Russ...

[Russ White]

Originally posted by AndrewT
Hi,
now test it with AC signals.
try full voltage 20Hz and progressively lower the frequency until the filter lets through enough signal to trip the relay.

If satisfactory, then try to see how little DC is needed to trip the relay.

My targets would be
1. pass 10Hz full voltage and stay untripped.
2. detect 1.5Vdc and trip.

Hi Andrew.

Seems to easily pass those tests.

Been running only with 35V rails though, will try higher rails later. I can actually get it below 3hz without issue at these rails.

1.5Vdc does indeed trip it, after a half second or so. (simple AA battery enough to test this).

The nice thing is that the Schmitt Trigger with positive feedback does its job nicely keeping the relay from chattering in the gray areas.

Cheers!
Russ

[Russ White]

Here is the supply circuit I am thinking of using.

Easily tuned for 110 or 220V mains.

No dependency on amp transformer at all.

The circuit will be adjust to provide a constant load even when the speaker relay is off.

Cheers!
Russ