Relay near amp...why not/what to look out for

[petemoore]

  I want to use a relay switch near an LM3886 amp, using it to switch the mute function.
  Why might this be a bad idea..er..what should I be knowing?

[R.G.]

Why would it be a bad idea?

[black mariah]

Why would it be a bad idea?

That's what he wants to know!

pete: AFAIK... it wouldn't be a bad idea at all.

[R.G.]

OK, I read the rest of your LM3886 stuff.

This isn't rocket science. We just have to cobble up some way to have pin 8 go from 0V (that is, at the middle of the power supplies) to having 0.5mA pulled out of it. The amp is muted if pin 8 is open (read that as "no current is coming out of it") and is on if you pull 0.5mA out of it.  What the spec sheet does not say directly, only indirectly, is that you have to pull pin 8 more than 2.6V negative for any current to come out of it.

So let's design circuits.

An NPN sitting at the - rail makes a good pulldown. All we need to do is make sure that it can withstand the full -V power supply between collector and emitter and stay off, and that it can pull 0.5ma when on. The critical factor there is the Vce being more than -V. As long as your power supply is only +/-18.5V, you can use a 3904, which is rated at 40V. If you were using a higher power supply, like +/- 40V, you'd need a higher voltage transistor. The higher voltage MPSA part works too.

So the NPN is tied to -18.5V. According to the datasheet, we need a resistor of Rm = |-Vee|-2.6V/Im = (18.5V-2.6V)/0.5ma =
31.8k. That's a maximum. So let's use 22K. The current pulled is then Im = 0.0007227A, or a bit more than 0.7ma. The transistor's gain will be low down there, but let's assume it has a gain of 25, so the NPN needs a base current of 29uA to fully saturate with a 22K resistor. How do we get 29uA to the NPN base?

Let's try through a resistor from ground. We need (18.5V-0.7V)/29uA = 614k. So any resistor less than 613K will supply enough base current from 0V (ground) to the base of the NPN to turn it on, and turn the amp on.

By using this circuit (NPN +22K and, say, 470K base resistor) we have converted the problem from how to switch pin 8 from 0V to -V through a resistor into a problem of how to either source or not source 30uA from 0V. The nice thing is that if we get it wrong, it blows the NPN, which typically shorts under fault, and the amp will fault to playing, not muting.

So how to provide 30uA? How about - a PNP? We tie a PNP in a strange way. Tie its collector to the 470K base resistor of the lower NPN, and it base to ground through a 33K resistor. The emitter is an input. With the emitter below 0.5V, nothing happens with the PNP, the NPN is off, and the amp is off (that is, no current comes out of pin . When we pull the emitter of the PNP above 0.5V, a current flows through the base-emitter of the PNP, limited by the 33K resistor to 30uA per volt for all volts over 0.5V. We only one of these to make the NPN work, so raising the input above 1.5V turns the amp on, lowering it below 0.5V turns the amp off.

What we have done is convert a switch closure to -V into a positive-on voltage above 0V to turn on the amp. And we've introduces some limiting resistors. What happens if we feed the PNP emitter input +20V? The base emitter goes up to 20V, the current that flows there is 20V/33K = 606uA. Not exactly a killing kind of current. The PNP is saturated HARD. So the collector goes up to +19.5V, maybe. That means that the 470K base resistor into the NPN is letting (19.5 +18V)/470K = 80uA flow into the NPN base. Again, the NPN is not going to die with that happening. It just switches the pin 8 0.5m harder.  So the switching setup is relatively impervious to input faults.

A final fillip is to tie the PNP emitter to +V with a 100K resistor instead of accepting a voltage into a 33k resistor. Now the PNP is on (and NPN is on, and amp is on) UNLESS you pull the PNP emitter to ground with a switch. The switch can be anywhere physically, and it can have a 0.1uF cap hung across it to keep transients from messing with the circuit.

I recommend you do something like the NPN/PNP setup to do your on/off muting. It's cheaper and more reliable than the relay, which is after all a mechanical part.

[QSQCaito]

Hey! Wait a minute.. I'd better go for some coffee before reading this. It's time to learn a bit.. as I try to do every day.. but it seems that this might need special atention, hence the cup of coffee.


Sorry for the OT