LM3886T Mute Circuit

[Bob W.]

The GMX 212 I have has the outputs muted. Can anyone tell me if .6 volts on the output of one of the LM3886T's is enuf to operate the mute circuit? The other IC output is at 0v. I'm not familiar enuf with those IC's to know if that's the problem or not. Thanks, Bob

[Jazz P Bass]

I have attached the power amp schematic.
It shows pin 8 (mute pin) of both ic's tied together.
Then both go to the mute circuit.
At zero volts or open circuit, the mute is 'On'.
You have to pull current through the mute pin to turn it off.

[Bob W.]

I have attached the power amp schematic.
It shows pin 8 (mute pin) of both ic's tied together.
Then both go to the mute circuit.
At zero volts or open circuit, the mute is 'On'.
You have to pull current through the mute pin to turn it off.

Ok, When there is 0 volts on pin 8 the amps are muted. When the outputs have a dc voltage on them I understand the amp goes into mute protection. My question was is .6 volts on the output of one of the amps enuf to drive the amp into mute mode? It's really hard to get in and measure the pins without shorting something out. Bob

[J M Fahey]

Can anyone tell me if .6 volts on the output of one of the LM3886T's is enuf to operate the mute circuit? The other IC output is at 0v.

When the outputs have a dc voltage on them I understand the amp goes into mute protection.

Sorry but no.
Mute is not a protection but a slow turn-on/off to avoid thumps (and allow remote digital controlled muting in TV, Home Theater, etc.).
It does not care about DC voltage at the output.

Mute works by starving current fed to the LM3886 first stage, so to unmute you must turn this stage on.

Since a grounded emitter PNP transistor controls this, you must feed 0.5 mA into its base, coming from the negative rail.
That base has 2 added diodes in series, so you need "3 diodes" drop there plus 0.5V .

Datasheet states that when unmuted you will have around -2.8V on pin 8 ; what do you actually measure there? .

[Enzo]

Are you sure the outputs are muted? Or does the amp just make no sound and you are inferring a mute?

[Bob W.]

Are you sure the outputs are muted? Or does the amp just make no sound and you are inferring a mute?

Enzo, That was a good question, I had to ask myself that after I read yours. When I checked it earlier today it had the -2.5 v to operate. I checked the outputs again and they were working. When I had checked it originally there was sound at the headphones but none thru the amps. I applied an input to the slave ins and got sound thru the HP's on 1 contact but nothing on the second cont, or the amps out. Now I get sound thru everything, have no idea why? I do know that one of the output IC's is bad, the right channel. The longer it's on the more distorted it gets, other channel clear. When I spray the IC with cold spray the volume picks up and totally clears up until it warms up again. I wonder if it has something to do with the the outputs not working, maybe dragging down the mute line and shutting both off. I can't get it to stop now so I can measure the line. That's electronics I guess. I have to order the IC. Bob

[Bob W.]

Sorry but no.
Mute is not a protection but a slow turn-on/off to avoid thumps (and allow remote digital controlled muting in TV, Home Theater, etc.).
It does not care about DC voltage at the output.

Mute works by starving current fed to the LM3886 first stage, so to unmute you must turn this stage on.

Since a grounded emitter PNP transistor controls this, you must feed 0.5 mA into its base, coming from the negative rail.
That base has 2 added diodes in series, so you need "3 diodes" drop there plus 0.5V .

Datasheet states that when unmuted you will have around -2.8V on pin 8 ; what do you actually measure there? .

Juan, Thank you for your response I checked schematic and saw that the outputs are not connected to the mute circuit, other amps I have worked on have been, I just assumed, I know, I know... The -2.8v you mentioned came in handy when checking so your info helped a great deal, read my following post to Enzo. Bob

[J M Fahey]

I do know that one of the output IC's is bad, the right channel. The longer it's on the more distorted it gets, other channel clear. When I spray the IC with cold spray the volume picks up and totally clears up until it warms up again.

I suspect poor cooling on the failing chipamp.
Separate it from the heatsink, clean both metal surfaces, check for burrs/irregularities/some foreign object (as small as a grain of sand or wood or whatever), apply new grease and mica and tighten the nut.
Maybe now it will work properly.

or plan B: maybe that side lost its Zobel network and is oscillating or at least unstable.

In a nutshell: having one overheat and the other normal makes me try to find a reason .... beyond "bad chip".

[Bob W.]

Damn, you were right...... after cleaning it up I have been running it all afternoon, works just fine. Now I have to wonder what the original problem was? I don't think it was connectors cuz I had them apart and back together several times and it still acted up. Oh well, just have to keep an eye on it. Thanks, Bob