Little Gem MKII not working on the BB

[Ben Lyman]

Sounds bad, gated, splatty, LM386's getting painfully hot to the touch. Ive stripped it out, double checked, triple checked, shortened up, neatened up, rebuilt 5 times... same results.
Got 2 LM386N and 2 LM386L from Tayda and they all worked fine for my Smokey with several different speaker combos.

Can one of you please take a close look at this schematic and tell me what could be missing/mis-labeled? Does it look perfect as it is? I think people have made this amp before
Scemo:  http://www.runoffgroove.com/littlegemMK2.png
Thanks!

[boppy100]

Speaker polarity??

[Ben Lyman]

Looking at the schematic linked, I hooked up the LM386 pictured above to the "plus" side of the speaker and the lower LM386 to the "minus" side labeled speaker. tried a 4" 8ohm, 2x 4" 8ohm in series and parallel. Also tried a 4" 30ohm and a 12" 8ohm, then 2x 12" 8ohm in series.

[PRR]

Speaker polarity does not matter.

The schematic is fine.

What did you *really* build (repeatedly)?

Great heat does suggest reversed *power* polarity (how would I know that?).

[Ben Lyman]

Speaker polarity does not matter.

The schematic is fine.

What did you *really* build (repeatedly)?

I don't know Paul, I'm mystified. I moved on to the Ruby, seems the same but simpler, only 1 chip and works fine as I BB it, as does the Smokey. Ruby has a 100nF pin 7 to ground and a 220uF from out pin (5) to speaker+ ...remove it and chip melts down.

Please tell me where the Little Gem MKII differs in that it does not need a cap there or why there is nothing on pin 7
Thanks

[PRR]

> I moved on

Movin too fast for me.

Is this one of those nebulous projects that has no Volts in it? LINK

The cap on pin 7 (bypass!) defines gain and suppresses power supply crap. It really should be used.

Pin 5, Output, sits at DC voltage of half supply. If you run this through 8 Ohms to ground, way too much useless DC current flows. "All" single-rail grounded-load audio amps need an output cap. The bridge-tie (two outputs) variant has both outputs at 4.5V DC, so no DC current flows in speaker or amplifiers. (Why I was askin about Volts.)

[Ben Lyman]

Sorry, "moved on" was poor choice of words. I just mean that I'm trying simpler things for a minute to make sure I understand this chip.

Right now the Ruby is kicking serious butt from my BB into a 12" 8ohm Mesa cab, loud and clean or louder and raunchy.

So I am almost prepared to tackle the LG MKII again. No caps needed on the out pins of both chips, got that.

So... pins 7 and 7... I should have some caps there on each chip going to ground?
Sorry if I misunderstood that part but I will give it a go with caps if that's what you mean.
thanks

[PRR]

First figure out why the dual-amp plan gets HOT.

Voltages are clues.

[Ben Lyman]

ok, will do, thanks
I had to work fast because the chips were heating up, I think it is only number 2 chip that gets hot.
Readings were moving around a lot and I just had to grab a number sometimes and move on.

Lets call the upper pictured IC #1 and the Lower pictured #2

Q1
D 2.0
S -4.0
G -4.5

IC1
1 4.2
2 5.0
3 5.0
4 5.1
5 2.0
6 4.1
7 -3.6
8 -7.0

IC2
1 5.1
2 5.7
3 5.9
4 5.9
5 .06
6 4.5
7 -1.0
8 3.2

[balkanizeyou]

the voltages look way off - are you powering it with a dual power supply? You should be getting supply voltage (9V) at pins 6 of both ICs, pins number 4 should be at ground - in your case pin 4 is at HIGHER potential than pin 6, which is weird to say the least.
Check the power supply connections, also a picture of the build would be a great help

[bluebunny]

+1

Bad (or no) ground connection?  There's nothing much that looks like 0V, like IC1.2, IC1.4, IC2.3, IC3.4.  No obvious power at pin 6 either.  And given that IC1.3 is connected directly to IC2.2, they should have exactly the same voltage.  Plus what Paul said about the output pins.

If this is all on a breadboard, I'd be inclined to take a deep breath and start over.

[deadastronaut]

i was bored...here ya go,

[Ben Lyman]

Thanks guys, ya I know it's all way off, numbers were changing with the temperature, I have taken several breaths and start-overs already. I'm using a 1-spot, it measures 9.4v

I just used these chips to BB a Ruby on the same BB and everything worked at that time.

Thanks DA, I will lay it out one more time exactly as you show.

Thanks

[deadastronaut]

no probs, thats how the schemo says it is..

good luck.

[Ben Lyman]

Same results. Seems like IC2 is always the culprit, I switched speaker wires around and IC1 got hot. I thought maybe my 4ohm cab and/or speaker cable was loading it down or something? I don't know, I tried the shortest speaker cable I have, about 3 feet, and same results.

I thought it might be cool to try a panoramic shot of the BB:

[Kipper4]

How come you dont have jumpers from the top power rails to the bottom ones mate?

[deadastronaut]

+1 and make sure you have continuity across those rails

some breadboards are split in the middle...

[Kipper4]

Not just the jumpers in the middle Rob.
My BBs dont have continuity from the top power rails to the lower (+ and -)

[Ben Lyman]

good eyes, that was a test 
actually, this bb has continuity horizontally across and the reason I didn't put a vertical jumper was because I only used the top for plus and the bottom for minus... but, umm... oops, I just noticed the 100uF needs some attention. I'll fix that but rest assured it will not change anything.
Thanks again guys

Updated pic with 100uF corrected. Same problem

[Kipper4]

I burnt my thumb print off earlier tonight by badly connecting LM78l05 regulators. Dammed those Underside view datasheets and my stupid old eyes.

At least I could commit a burglary without Thumb print evidence now.

Sorry I cant be more help with the debug.
Have you tried it with some different speakers.
Are some of the designs resistor rated higher than our std 1/4" watt?

I hope you get it sorted mate.