LM386 woes

[Byff]

I'm trying to take my circuitry skills to the next level by combining a number of VCOs, filters, and special-effects generators into a "sound card" whose functions I can individually address digitally. (The intended application is robotics.)

In order to tie together the various elements, I'll need to route the output signals to a mixer and amplifier. My problem is that while I do a good job of wiring up VCOs and filters, I've never made an amplifier based on the LM386 that actually worked correctly.

What are some of the gotchas that might be messing with me? I've put together a number of the Forrests Mims 386 circuits, as well as others I've found online and in magazines. Either I suck at using this chip, or all my 386s are somehow damaged.

There are no special handling requirements for this chip, are there? I know the instructions to various circuits often advise us to "keep capacitor leads short" and "bypass the power connections with capacitors". Being on a budget, I'm loath to trim leads on capacitors unless I have spares on hand. Are long leads really that problematic? Also, when Mims says "bypass the power connections", is he telling me to wire a capacitor in parallel with the power lead, or replace the power connection completely with a capacitor? (That would block the power, since it's DC, right?)

Finally, is it possible that in misinterpreting (or ignoring) these instructions on previous projects, I've damaged the 386 or the 741s I've tried to use as preamps? Are there any nifty troubleshooting tactics I can take to verify that the chips at least are undamaged?

Thanks in advance!

 

[Hero999]

Just build the circuit on the LM386 datasheet.

The LM741 isn't good for audio unless the gain is low and it's really simple siren sounds <8kHz.

 

[MRCecil]

There are a few tips in taming the LM386. One is to ENSURE an independent ground and an independent Vcc trace/line/wire from the supply as short as possible. Second, use a .1uf ceramic bypass cap right at the Vcc input (pin 6?). Third, ensure you are not overdriving the input. It can be configured for 20db or 26db as I recall, and too high an input will start it singing even unloaded. Fourth, ensure you are not trying to drive an impedance lower that 8 ohms. And as Hero has already stated, configure it just as the datasheet recommends/shows.

One last thing, be sure to use a blocking cap on the input (2.2-10uf tant.). Any dc component will be amplified and could send the 386 to the rail.

 

[Byff]

Thanks, both of you. I'll try to apply these as soon as I get home from work.

The use of the 741 as preamp stems from the 2-stage amplifier and 3-input mixer circuits in Mims' Op-Amps experimenter's notebook. I'll start with the datasheet circuit, then see about substituting some other sort of preamp if I need one.

 

[Hero999]

Whilst Forrest Mims does have some interesting ideas but it's important to realise he's not an electronics engineer and there are much better books to learn electronics from. I initially learnt electronics from his books, then went to college and joined forums, like this one and I learned that many of his circuits are either poor or won't work.

Attached is a classic example, when Q1 turns on LED is supposed to light and LED2 is supposed to go off. This won't happen, LED2 will remain lit and LED will light very dimly due to the tiny base current.

Forrest, loves the 741 but you should replace it with a TL071 or TL072 which is a much better audio amplifier.

I just read the Wikipedia article about him and I seriously doubt he was the first one to discover that LEDs can be used to detect light, I discovered that for myself when I was about 12 and I can't believe that the engineers who invented LEDs wouldn't have known they can be also used as detectors.
Forrest Mims - Wikipedia, the free encyclopedia

 

[Byff]

I've got about a dozen of his experimenter's notebooks, plus the RadShack Learning Lab that he evidently had some input into designing. I agree that not all of his experiments work; I guess I've tended to assume that I've been at fault when I couldn't get something working.

(Side note on Mims: dunno about anybody's politics and / or beliefs, and so won't state specifics, but some of what I've read about him gives me pause. Nonetheless, his name is all over the place when digging into the recent history of digital electronics, so he certainly appears to be a luminary of some sort.)

As for getting that full education, I'm on it. I'm in my second year of ELEE at UH (and have been for some time, unfortunately, due to intervening military service). Haven't yet had any actual electronic coursework, but I do have access to some reasonably advanced resources.

Looks like I've gotta hit the parts store on the way home to get some of the right caps for this thing...wish me luck.

 

[Hero999]

I'd recommend The Art of Electronics, 2nd edition.

You can download it from many places but you should buy the book as well.