LM386 overdrive: easy, cheap, and fast to make. This is known and popular audio amplifier circuit with a fairly simple but good sounding overdrive guitar effect. Neither the operation of the complex.
BS170 mosfet guitar signal to the already worsened. the R8 potentiometer regulates the amount of signal to the 386. If you have more than that the chip can handle, then the signal will be distorted. That is, the “overdrive“. So it is quite dry and rough sound. And of course it’s good for us. Listen to:
LM386 Overdrive mods
The 2N7000 BS170 instead of perfect. A similar effect can be achieved with transistor preamplifier. You may want to try a different mosfet or 2N2222 transistors as popular, BC109, BC547, etc. The effect sound will be slightly different. Important elements of the D1 and D2 ESD protection diodes, these are not to be missed.
The R11, R12, R13, C8 and R13 potentiometer tone of the section. It is quite a strong tonal control, than to hear the video. Here the value of R13 and C8 parts you may want to change so little different tones. But who does not like it feel free to skip or replace.
How to make?
Video tutorial, part One, preamplifier:
Part second, the LM386 amplifier stage:
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I built this to the schematic, and it was horrible sounding, very broken and poppy/crackly. I added a 2.2uf cap between pin 6 and 4 on the LM386 and it sounds much betyter, but still sounds more like a very nasty fuzz than an overdrive, nothing like the demo video. I’m not sure what else I could be doing wrong here.
Hi coffeeman! The BS170 pinout correct?
Thank you, it is, I have verified both using a transistor tester and the datasheet.
Dear coffeeman.
Soldering problem?
Or try in stages.
Possible problem in the tone circuit: replace resistor R11 with a capacitor of about 10nf.
I also tried listening to the stages, the output from my BS170 does not sound great like yours but it’s not terrible either. I wasn’t able to test it out of circuit so the lm386 loading it down in parallel with the amp may have contributed to the poor sound quality.
Driving the lm386 directly with the guitar and bypassing tone altogether sounds more like a fuzz than an overdrive, and is still a bit staticy (when I play hard it makes a big crashing noise).
The tone circuit is definitely making it sound worse, but I’m not sure if that is a case of garbage in -> garbage out. For now I have the tone bypassed, but of course i have to be careful with the volume control because the tone circuit sucks a great deal of voltage out.
Is that you? 🙂
https://www.youtube.com/@betterl8thannvr/videos
Yes, it is me 😀
Oh that’s great 😀
These videos help a lot, thanks for it, so you can already hear the problem. It’s like you don’t get enough voltage, very interesting.
But we are already working on the problem. 🙂
It should be plenty of voltage, I believe in all of the demo videos I am connected to my pedal power supply, not battery, but it’s the same either way. Connected to a power supply that tells me output current it’s less than 10ma.
I put it all back together today with a new BS170 and replaced R11 with a 10NF cap. Output is now MUCH higher (the tone section was reducing voltage by perhaps 80% before), and the sound is much better, as long as I only play the bottom 4 strings. It still breaks and pops a lot on the low notes.
Baco to voltage, on the scope, injecting a single frequency shows no difference in outputs in the frequencies we care about, everything is clean now, and we can even see some soft clipping at the right settings. Playing the guitar through it into the scope however I notice that the output voltage shifts negative when playing either full chords or low strings – e.g. my peak high drops from (at current volume) 150mv to 50, and my low peak drops from -150mv to -250, but Vpp stays the same. I’m not sure how that would manifest in the output audio though, or what would cause that. I also see this shift on the bias line, e.g. it’s sitting pretty right around 2v, but the harder I play or lower I play the biased audio shifts lower.
Dear coffeeman
This thorough test and description is great!
Back to basics. Try:
1: R6 removed and pin 2 connected directly to negative.
2: Disconnect pin 1 and pin 8
3: Remove C1
4: Remove R16
Reference: as stated on page 10 of the LM386 datasheet (https://www.ti.com/lit/ds/symlink/lm386.pdf)
🙂
1. No Change
2. Won’t this set the gain back to 20, whereas I think connecting them together sets it to 200? I have them bypassed with a 10uf cap per the datasheet currently. I took that out but it sounded terrible, no treble response, and very crackly when it started to break up.
3. Woah, major change. Now the signal is skewed but at very low drive levels I can get no clipping. What’s odd is that this also changed how the signal looked on the BS170, it is no longer so clipped but is very skewed. I just can’t imagine how that impacted the BS170 so much??
4. I actually tried without R16 on accident originally 🙂 That;s just making a high pass filter with a very low frequency right? The cap to block DC is probably sufficient. Removing it seems to make no difference.
After all of that (except I added back my 10uF cap between 1 and 8) it sounds okay, if I twist the knobs just right I can almost get it to sound like an overdrive, but it goes into fuzz territory very quickly.
What should I see on the gate and drain of the BS170? The weird skew and asymmetrical voltages bring me back to your original though about voltages. What should I be seeing on the gate and drain? I believe I have 2v and 4v. New video on my channel with scope traces and audio.
Dear coffeeman
Thanks for the video and the scope tutorial, it helps 🙂
1: not even minimal?
2: yes, 1 and 8 combined 200x amplification
3: this is interesting, in theory it shouldn’t affect the sound.
In fact, it only filters the supply voltage (explained here: https://redeyeprep.wordpress.com/2014/08/22/adventures-with-the-lm386-part-ii/)
4: yes, but it doesn’t really matter much
Oh yeah, it’s pretty fuzzy, but it sounds pretty good 😀
The voltages seem correct.
Tip 1: D1 & D2 are not necessary, they are only safety elements.
Tip 2: 2N7000 is also suitable instead of BS170.
The entire circuit will be reassembled for testing, so you will have accurate reference values. 🙂
No impact at all from removing R2 as far as I can tell. It doesnt make sense to me that removing the capacitor from pin 7 would have changed it that much either. I am not sure what was going on there.
It does sound like a fuss now, and I think the asymmetric clipping is characteristic of a fuzz, and is the main difference between fuzz and od clipping. I actually ran it side by side with my Big Muff and with the right settings I can get them to sound almost identical, so it’s not a bad fuzz. Feeding it into the big muff it can sound even weirder! I hooked it up to the electric piano last night, and feeding that through both pedals I got some really interesting video game sounding notes, with a bit of a flanger woosh at the end.
I had considered pulling the diodes, but I’ll wait to see what your voltages look like. I don’t have any 2n7000 on hand unfortunately.
We are a bit busy at the moment, but we will get back to you soon. 🙂