"ChipDrive Overdrive" LM386 knobless overdrive

[Earthscum]

Just when you thought it had all been covered...

This sounds great on bass. Maybe could benefit from a boost for guitar. I tried taking a pic of the o-scope, but I gave up, lol. Suffice to say, this is a true overdrive, in the sense of part of the original signal gets mixed in with the distorted signal, creating basically trapezoidal waves with domed peaks instead of flat. The corners are still sharp, giving a nice fuzz.



BIG NOTE: No, I didn't mess up the schem. The output really is taken off of pin 1!

You can add a switch to switch in and out the 10uF cap for a "clean" with a bit of distortion on hard picks (or some resistance or gain flavoring), and full gain WITHOUT having to compensate with a volume knob... awesome, huh? 

[DavenPaget]

No longer 1 knob but 0 KNOB FTW !
Nice one , you impressed me with the "Sewer Pipes Ring-Verb" Simple PT2399 One-Knob Effect and now this .

[Earthscum]

Thanks! This one was wierd... looking at the 386 internal schem always made me wonder if I could bypass the output stage and just take sound straight off the gain pins. I was REALLY suprised to find it was a 1:1 signal, and distortion was still about 1:1. It's like a Clean and Drive channel in the least amount of parts, lol.

One thing I forgot to note... if you want to put a tone cut on this, the 10k at the end can easily be swapped out for a pot to make a SWTC, or a 1n parallel to the 100k to ground does a nice job of shaving off the highest highs without muffling the sound.

[DavenPaget]

Thanks! This one was wierd... looking at the 386 internal schem always made me wonder if I could bypass the output stage and just take sound straight off the gain pins. I was REALLY suprised to find it was a 1:1 signal, and distortion was still about 1:1. It's like a Clean and Drive channel in the least amount of parts, lol.

One thing I forgot to note... if you want to put a tone cut on this, the 10k at the end can easily be swapped out for a pot to make a SWTC, or a 1n parallel to the 100k to ground does a nice job of shaving off the highest highs without muffling the sound.

Somebody could as well be brewing a raw preamp with this 

[tubelectron]

Interesting, Earthscum : I will try to find the time to test it on guitar "as is"...

A+!

[PRR]

Is a very elaborate way to get an emitter-follower with 40mV/400mV clipping.

I don't see that the R-C-C network at the input does anything?

[Earthscum]

Thanks, Bruno... looking forward to your opinion!

Paul: yeah, it is, lol.

The RC network on the front is just typical antipop resistor and AC coupling. The 220p to ground parallels the internal 50k to ground, shaving off the highest highs. I was getting some bleedthrough from something, and I decided to drop it on there, and it worked great. Might not be needed once it's in a box. You could certainly do straight guitar into pin 3 and straight out of 1 with the 10uF between 1 and 8.

[Earthscum]

Here's a super simple, stripped down version. Doesn't sound any different. 

[karter2000]

I just breadboarded the Simple ChipDrive as above, and tried it with my bass.  It sounds great IMHO, as a bass overdrive!  I'll have to play around with this one further.

[therecordingart]

Here's a super simple, stripped down version. Doesn't sound any different. 

Does the inverting input float or should I tie it to ground?

[Earthscum]

Glad to hear that, Karter!

TRA: Cool thing about LM386, both inputs are tied to ground through a 50k resistor internally. You just leave it floating. On the first version, I mentioned to Paul that I take advantage of that, the 220pF cap is basically in parallel with the 50k to cut out highs at around 14kHz.

[aron]

Neat! Might have to make this one for the bass player!

[karter2000]

Bit of a noob question, because, I'm a bit of a noob.  Is it possible to control the amount of gain and/or fuzz with this circuit?

[Earthscum]

Bit of a noob question, because, I'm a bit of a noob.  Is it possible to control the amount of gain and/or fuzz with this circuit?

Yep, you can use a pot across pins 1 and 8 instead of a 10u cap just like other 386 based distortions. Also, as always, you can put a volume pot at the end instead of the 100k to ground.

[deadastronaut]

and maybe 2 leds in series from pin 5 to ground for sound to light too....for extra coolness...

[Earthscum]

Just tried it... didn't work. Basically, what happens to the output stage gets fed back, and that's what is coming out of Pin 1, along with the clean signal mixed in.

I tried a 1uF to a pair of 4148's, though... adds a nice fuzzy top to the sound! Anything above 1u made some gnarly feedback, and 100n didn't do much. 220n you can start to hear the crispies coming in.

ETA: Scope pic of the in/out if the simple version

[PRR]

> Does the inverting input float or should I tie it to ground?

Then it is a "floating input" and picks up buzz. If the box and wiring are good, this may not matter. OTOH grounding it is fine, as long as it goes to a clean ground (not the garbage from the latest LED mod). I guess I'm saying: try it! (either way)

[Earthscum]

I always kind of wondered about that, Paul. If you tie the input straight to ground, what does it do to the other input, or anything at all? I notice in the datasheets they have a 50k pot where the wiper will take the input directly to ground. The only thing I can actually see happening when I pull the other input to ground is that the output will float up or down before stabilizing back to it's happy spot. (the real output, not my made-up pseudo output, lol).

[Ronan]

From the August 2000 datasheet:

When using the LM386 with higher gains (bypassing the
1.35k resistor between pins 1 and 8 ) it is necessary to bypass
the unused input, preventing degradation of gain and
possible instabilities. This is done with a 0.1 μF capacitor or
a short to ground depending on the dc source resistance on
the driven input.

Additional external components can be placed in parallel
with the internal feedback resistors to tailor the gain and frequency
response for individual applications. For example,
we can compensate poor speaker bass response by frequency
shaping the feedback path. This is done with a series
RC from pin 1 to 5 (paralleling the internal 15k resistor).

Don't know if this helps, or not. Interesting thread...

[Earthscum]

Thanks... that's what I had done with the 220p, essentially.

And, on that note... I just turned it on to try it out. I put a .1u cap from the inverting input to ground, and it actually created MORE noise. I put the 220p back on the non-inverting input, and the noise is gone. I pull both caps, and it's even quieter. I would say, in this circuit, leave it floating.