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Old 20th Nov 2018, 11:57 am   #1
Martin Bush
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Default 386 amplifier "overdrive"

I posted a while back on 386 amplifiers so if the mods want to put this as part of that thread that's fine.

Bear in mind my level of ability/ knowledge with this one...

I was wondering whether, if I build two 386 amplifiers and take the output of the first and put into the output of the second, I could achieve an overdrive type effect?

I know that it will potentially be very primitive and, to avoid clipping etc, there may be modification to the circuit required, but I thought I would ask if it's a non-starter.

I'm planning to build a very simple and cheap guitar amplifier (cigar box type thing) and thought that it might be fun to try out a distortion effect given the very low cost of the 386 kits.
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Old 20th Nov 2018, 12:24 pm   #2
paulsherwin
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Default Re: 386 amplifier "overdrive"

You will get plenty of clipping distortion, but it won't sound like a conventional valve guitar amp being overdriven. There are lots of designs for LM386 based effects boxes on the net if you google.

As you say, they are dirt cheap (especially if you buy them in quantity from China) so it doesn't matter if you blow a few up while experimenting.
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Old 20th Nov 2018, 2:25 pm   #3
Martin Bush
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Default Re: 386 amplifier "overdrive"

Thanks Paul.

I have been looking at a design for what's claimed to be a very clean design online which I may have a go at and thought there may be other possibilities. I will look up what you suggest as regards distortion.

I seem to recall buying 10 or so ICs.

Cheap and plentiful is the order of the day for me - it's good to be able to experiment at low non-mains voltages!
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Old 20th Nov 2018, 11:42 pm   #4
Craig Sawyers
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Default Re: 386 amplifier "overdrive"

For the ultimate of what is possible with 3886's have a look at this https://www.neurochrome.com/modulus-686/ . 360W into 4 ohms with ppm distortion across the audio band.
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Old 22nd Nov 2018, 2:38 pm   #5
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Default Re: 386 amplifier "overdrive"

Quote:
Originally Posted by Martin Bush View Post
Cheap and plentiful is the order of the day for me - it's good to be able to experiment at low non-mains voltages!
Wow, you have slipped up with this philosophy in this case.

I would be extremely cautious with the LM386. I found this out recently when I built this project that uses the LM386:

http://worldphaco.com/uploads/VOTRAX...TS_HERO_JR.pdf

I had to acquire a genuine vintage LM386.

The originals could tolerate a high capacitive load on their output and were completely stable.

The new generation clones/copies/fakes ( and that is being kind) out of the far east are "hopeless" and unstable.

Occasionally you will find genuine vintage LM386 for sale, but they are getting a lot rarer and sellers know this now, and they are more costly.

The reason this has happened is the popularity of the part, so a plethora of poor quality fakes has flooded the market diluting the real ones near out of existence, as if they were a grain of slightly different colored sand, thrown on a beach.
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Old 22nd Nov 2018, 2:57 pm   #6
Craig Sawyers
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Default Re: 386 amplifier "overdrive"

Just looked up the data for the LM386. Wow - what a difference between an LM386 and an LM3886! A single digit does it ;-)
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Old 23rd Nov 2018, 11:07 am   #7
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Default Re: 386 amplifier "overdrive"

I've NEVER come across "fake" LM386 ICs, and I've been buying them since the late 70s. There are different suffixes for the '386 - N and M - which give differing output powers.

Many "designs" omit the bypass capacitor on pin 7, and forget to decouple the supply close to the power pin 6. It's also necessary to connect a "Zobel" network to the output - pin5. This network should be a 10Ω resistor in series with a 47n capacitor from pin 5 to ground. The grounding of the IC is also important - a low impedance path is essential. The "gain set" pins - 1 and 8 - also need to be dealt with.

There are many "second source" suppliers of these ICs, but they're all good copies of the original. It's such a cheap and simple IC that it's pretty much impossible to get wrong!

If you pay close attention to the datasheet and design accordingly, the results are usually excellent. It's not a particularly low noise or low distortion IC, but it's good for its intended purpose - a cheap and simple Watt battery-powered audio amplifier.

The datasheet is available at http://www.ti.com/lit/ds/symlink/lm386.pdf

If you want to make a simple guitar distortion pedal, the '386 isn't the way to go - it's too hissy.

Get a TL071 or similar low noise op-amp, and build a simple voltage amplifier with it - high impedance input (~500kΩ), low impedance output. Feed the output through a DC blocking capacitor of 10, through a 4k7 series resistor, into a pair of 1N4148 diodes to ground. Put a 10k volume control across the diodes, and take the output from the wiper. It's usually a good idea to make the op-amp gain variable too, by using a logarithmic pot in the negative feedback loop. There are loads of circuits online for this kind of thing.

Last edited by mictester; 23rd Nov 2018 at 11:09 am. Reason: putting in a missing URL
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Old 23rd Nov 2018, 11:31 am   #8
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Default Re: 386 amplifier "overdrive"

We had loads of LM386s available as 'generic op-amp' in the component boxes in labs when I was at university in the early 1990s. Needless to say several of us built guitar practice amplifiers using them.

They can be a little hissy, but if you overdrive them (and experiment with non-linear components in the feedback loop) you can create some great distortion effects. And if your style of guitar is more thrash metal than clean pop, the sound can be perfect. Some of mine sounded amazing when I was playing Metallica or Megadeth rhythm parts.

Do consider a heatsink if really overdriving it though, I've had some pop, and one partially unsoldered itself (but sounded really good up to that point). I did make notes on component values and circuit layouts that worked well, but I suspect after 30 years I've no chance of finding them. And unlike then - LM386s aren't cheap or even free now...
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Old 23rd Nov 2018, 8:36 pm   #9
Martin Bush
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Default Re: 386 amplifier "overdrive"

Thanks to all for the feedback (no pun intended)...

I have a couple of amp kits on the way and, fake or not, it will be fun assembling and messing with them. It really is a bit of fun for me. If I need a 'proper' amp at some point I will probably buy one.
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