LM3886 chip - why no work?

[Alex R]

This LM3886 11-pin amp chip is in a Tech21 Power Engine 60. The amp came to me because another tech had fitted the chip and failed to get the amp to make a sound. Given what I write below I think it's the right chip or should work anyway. But I too have so far failed to get an output despite fitting a new chip.

So my question is, why might a LM3886 not amplify despite the following conditions:

Pin 1 +32v
Pin 4 -32v
Pin 7 grounded
Pin 10 healthy input signal
Pin 3 goes to the speaker

- Hmm? Even if the LM3886 was the wrong chip to fit I would have thought it would be ok with the above connections. But Pin 3 flatlines.

Pin 8, the mute pin, has a few volts on it. There is no connection to that pin.

I don't have a schematic, I'm afraid. Any possibilities springing to mind?

[Amp Kat]

There are two versions of that chip. One requires a heat sink insulator and the other doesn't I don't have the data book but there is a suffix you will find on the chip. If it's not exact it won't work. I keep thinking it's 3886f but I may be wrong.

[Alex R]

The one I took out had an uninsulated body, but there was an insulator between it and the heatsink. The one I fitted was fully insulated. Neither worked. The heatsink is grounded. Could it be that the chip needs to be grounded?

[Enzo]

Nah.

But that unterminated pin 8 bothers me, that is the mute after all.

Have we contacted the Tech21 people to see if schematics are a possibility?

Pin 3 goes to speaker, but is there continuity from pin 3 to that speaker? And is the speaker return intact?

[R.G.]

You're on it Enzo.

According to the LM3886 datasheet, you must pull 0.5ma out of the mute pin to ground to get it to not attenuate. Leaving the pin open makes it mute.

See page 8, http://cache.national.com/ds/LM/LM3886.pdf, item 12, value of Rm.

[jrfrond]

You can freely swap the LM3886 with the "-TF" ("Fullpack") version, and omit the silpad or mica, but you still need to use heat sink grease.

Forget about Tech 21 schematics. I'm friends with them and I can't even get them! However, the LM3886 is ubiquitous, used in lots of gear, and should be easy to troubleshoot. An open MUTE pin is trouble right there.

[Enzo]

Oh well, a schematic would have cleared up whether the 3886 was the right chip for the amp, and also what they did to terminate the mute pin.

ALex, look up the data sheet for the 3886, it is informative on what the IC needs to operate.

[Steve Conner]

When you say there's no connection to the mute pin, is that by design? I think that's unlikely, Enzo's previous comment about "Oh jeez look at our goof" springs to mind.

Or did the last LM3886 just take the mute circuit out with it when it blew? I'd be looking for burnt things in the neighbourhood of the mute pin. Or even missing things: the last guy who worked on it might have removed them in an attempt to make the amp unmute.

[Alex R]

Thanks Steve - there's no connection to pin8 by design. But I though that with the input being on pin10 it had to be the 11-pin chip with the mute, as I think the non-mute ones have it on pin7 - but I really don't know much about these chips...

The output pin3 connects to the speaker and the speaker back to ground ok.

The only possibly missing thing I can see is a SMD that's also on the output pin3. Part of a feedback loop I guess. The possibly missing device is two-pole and in parallel with a 47K resistor which is properly connected. I reason that if it is part of a feedback loop it will be a little cap and its absence shouldn't stop the amp working. Anyhow there are several empty SMD positions and as this one is right by the chip the previous tech's iron has probably brushed it making it look like something was there.

[Steve Conner]

Oh well, in that case just rig up a resistor to unmute it like they show in the datasheet, and see if it works without any horrible power-up thuds and bangs. It was probably meant for another chip without a mute.

[R.G.]

Thanks Steve - there's no connection to pin8 by design. But I though that with the input being on pin10 it had to be the 11-pin chip with the mute, as I think the non-mute ones have it on pin7 - but I really don't know much about these chips...

There's a great tool for figuring this stuff out. It's the internet, and you can look up datasheets for anything, any time. For the cost of a few minutes, you can know anything you need to about the chips.

I just stuck "LM3886" into google, and up popped National Semi's page on it, first choice. Likewise for "LM3875", the non-mute version. The pinout differences are on the pages.
See http://www.national.com/mpf/LM/LM3886.html
and http://cache.national.com/ds/LM/LM3886.pdf

Some of the pin-functions line up, some don't. You need to check where the power lines go to the chip pads in question, as the LM3886 and 3875 are close but not the same. The stock LM3886 does not work with pin 8 open. The LM3875 does not work with pin 8 open either - it's Vin- on that one.

Is it possible that the chip is not even supposed to be an LM3886? Maybe it's one of the STK amps or even one of the TDA72xx series.

[jrfrond]

Originally, that chip was spec'ed as an LM3876, and superseded by the 3886, which is just a beefier version, with higher design center ratings, so your IC is correct.

I would suggest you call Lloyd Schwartz at Tech 21. Blame it on me if you have to! Lloyd is a good guy and knows those amps inside-out. He won't send you a schematic, but he will talk you through over the phone.

I will say this about Tech 21 products though: they are pretty damn reliable.

[Alex R]

Oh well, in that case just rig up a resistor to unmute it like they show in the datasheet, and see if it works without any horrible power-up thuds and bangs. It was probably meant for another chip without a mute.

- needs a resistor to allow 0.5mA to pass to earth I note from RG's link (thanks RG!). The chip can't work with a few volts sitting waiting on pin8, I didn't know that, I assumed (yes I know...) that you'd ground it to mute it but it's the other way around. Jrfrond says it's the right chip, so there maybe there's a trace lifted or something, I need to check with a magnifier. Thanks JR - I'll bung a mute resistor on first before bothering Mr Schwartz.

Great, lots to go on, thanks again all, I'll have a bash in the morning.

[Alex R]

- needs a resistor to allow 0.5mA to pass to earth I note from RG's link (thanks RG!). The chip can't work with a few volts sitting waiting on pin8, I didn't know that, I assumed (yes I know...) that you'd ground it to mute it but it's the other way around. Jrfrond says it's the right chip, so there maybe there's a trace lifted or something, I need to check with a magnifier. Thanks JR - I'll bung a mute resistor on first before bothering Mr Schwartz.

Great, lots to go on, thanks again all, I'll have a bash in the morning.

Oops it's not to earth it's to the -ve supply, dur. More assumptions.

Anyhow it worked... a 10K resistor from pin 8 (mute) to pin 4 (V-) turned off the mute, no on/off pops so no extra caps necessary.

Thanks for helping me through the various examples of ignorance and stupidity I have donated to this thread!

[Enzo]

Oh, it is an election year over here, so we are drowning in ignorance and stupidity. We certainly won't be affected by a tiny little amount from the UK