LM386 Getting Super Hot

[turdadactyl]

Sometimes the simplest things cause the most trouble. I'm using a very basic LM386 setup (identical to the front end of the Escobedo PWM) and for some reason it's getting super hot. Anybody see an obvious screw up here?

Thanks

[duck_arse]

it looks to me very much as tho that white wire connects pin 5 (output) hard to ground via the black jumper to the bus-strip. there is not enough information in the image to say otherwise.

[turdadactyl]

The output is connected to ground, but also connected to the BS170 at Vin in this sample and hold circuit block.




So here's a thought - if my sample frequency is too high, maybe the switch at Vs is essentially always open and making the connection from pin 5 to ground essentially hard wired?

[duck_arse]

you need to put your load between the output and ground. on the circuit shown, the amp is driving 10k, until something else is connected to. the amp doesn't want to be driving ground. (I just spotted that was a 741 circuit, not a 386.)

also, pin 5 will sit at 1/2 supply, so it should probably always be AC coupled. what voltages do you measure around the IC?

[turdadactyl]

Good news and bad news.  I moved the LM386 drive circuit block to after the S&H block and, as @duck_arse suggested, no more overheating.  That said, I now have:

Input buffer |->  Sample & Hold -> Drive
LFO----------|

...But no audio output.  Full schematic and voltages coming in a minute.

[turdadactyl]

[turdadactyl]

U1 (TL082)
1 = 4.42V
2 = 4.42V
3 = 4.27V
4 = 15mV
8 = 8.76V

U2 (555 timer)
1 = 0
2 = 50 mV
3 = 58 mV
4 = 9.44V
5 = NC
6 = 60 mV
7 = NC
8 = 9.3V

Q1 (BS170)
D = 10 mV
G = 13 mV
S = .49V

U3 (LM386)
1 = 3.52V
2 = -44mV
3 = 5.7V
4 = -33mV
5 = 8.27V
6 = 8.86V
7 = NC
8 = 3.45V

U4 (TL071 in Sample and Hold - Mislabeled in schematic)
1 = 10mV
2 = 5.75V
3 = .58V
4 = 0V
5 = 5.6mV
6 = 5.70V
7 = 9.14V
8 = NC

[turdadactyl]

One more bit of info. The LFO is working.

[PRR]

[duck_arse]

do like ^ he says.

why the lm386 at all? is it driving a speaker or something?

[turdadactyl]

I only used the 386 because I wanted to try this sample and hold idea with distortion and the 386 was the drive section I thought of with the fewest parts (essentially 1).

I very quickly made Paul's suggested changes this morning with no luck, but I had to head out to work, so not much time to triple check everything.  Back to it this evening.

Paul, just so I'm clear, are you suggesting I eliminate the connection to ground at the end of the drive section altogether, or just the resistor?  Same question about the connection between pins 1 and 5 in the S&H section.

[turdadactyl]

So, I'm about ready to quit on this idea...at least in this incarnation.  I tried all of Paul's suggestions.  I messed with the resistor values in the sample and hold section a bit, bringing them down to 6.2K and mixing and matching that with both the 10K and with a ground connection from pin 6.  Nada.  I really just threw these pieces together in a bit of Frankenstein brainstorming.  Not so sure it's going to do anything.

[duck_arse]

the oscilloscope!

- does the input buffer pass signal?
- does the BS170 sample, and hold? what is your gate drive voltage, the screenschott don't mention?
- does your smaple and holt oppy (less the offset trimpot and the output resistor to ground) pass signal?
- does your lm386, wired as per Paul's, pass ignal, provide gain, sound like a drive?

break all the sections apart and test them as sub-sections, then string the working parts together again. (I'm not confident about the mosfet doing what you think in this circuit, but I'll be happilly prooven otherwise with results.)

[turdadactyl]

That's what I've been doing.  The 386 drive section works perfectly on its own.  The LFO (as you saw in the scope pics) works as well.  The input buffer passes signal - no significant gain, which I believe is to be expected.  So it seems to me the sample and hold section is the issue.  Unfortunately, I keep giving it a little time here and there at night.  Hate it when life interrupts building toys!

I'll have to get new measurements next time I'm back in the basement.

[turdadactyl]

- does the BS170 sample, and hold? what is your gate drive voltage, the screenschott don't mention?

Ya know...this seems like the obvious answer now that I look at it.  The voltage at pins 2, 3, and 6 of the 555 are all just a few mV.  No way that's gonna get it done!