LM386 Preamp sounds horrible, do i have the wrong capacitors?

Started by valvefreak, April 22, 2015, 11:42:31 AM

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I've designed and breadborded a simple 12AX7 Overdrive pedal with an lm386 as a Preamp to kick the signal into the tube.
here's a scematic of the preamp section:

C1 = 10µF, 25V, Electrolytic
C2 = 10nF Ceramic
C3 = 10nF Ceramic
R1 = 100K Pot
:icon_exclaim: Pin 2 is not grounded, because its internally grounded through a 50K resistor.
:icon_exclaim: The output goes into a 500K pot which biases the tube and brings the signal in.

The problem is, that the lower frequencies are very distorted, so the lower notes sound horrible when they're palm muted.
I have tried to change the capacitors and got a better sound when i changed C3 from 10nF to 100pF (Ceramic), i could only hear the highs. that was better, but it was not as powerful and it had no bass at all. Have i configured the LM386 wrong? do i need Film caps? ???
The Valve works and sounds perfect, i bypassed the lm386 and i got a wonderful sound which of course had nearly no distortion/overdrive.

So here's the big question: How do i get the LM386 to sound good?
Don't let the magic smoke come out!


Well, i solved the problem... :icon_confused: the guitar has died somehow, thats why it sounded so bassy :D
excuse me for wasting your time  :icon_biggrin:
it sounds great with active pickups! Rock n Roll! :icon_twisted:
Don't let the magic smoke come out!


Well, keep in mind that the 386 has a 50 k input impedance. Why did you decide to use this particular chip and not an op amp, FET or BJT gain stage? It doesn't sound like you're driving a heavy load or are interested in its brand of distortion.
Quote from: R.G. on July 31, 2018, 10:34:30 PMDoes the circuit sound better when oriented to magnetic north under a pyramid?


To be honest, i dont know much about impedance and stuff like that  :icon_confused:
It was just the chip i knew, i got one and tried it out... it seems to work fine. especially the gain control is a great feature, but i don't know if other ICs have one too. (are there any?)
what IC would you recommend if i were to build a distortion pedal (for example)?
Don't let the magic smoke come out!

KR Audio

The gain control on the LM386 is very convenient yes. I enjoy the sound of that IC as well. As FiveseveN remarked the input impedance of this ic is lower than many others that are used for the same application. Though in this case you could build a simple high impedance gain stage with a JFET transistor or even a JFET Source Follower put in front of the existing circuitry and see if there's a significant increase in sound quality and output level. Impedance is the sum of resistance, capacitance, and inductance located at any given junction in a circuit. It's important to keep in mind because it will dictate how much energy is able to flow from one entity -like your guitar- to another - like the input of the pedal. What you have going should do you well, though experiment with a BJT or JFET in front of it and see if you get any better results.


The biggest problem I had with the LM386 for guitar is no pinch harmonics no matter how I modded the designs. I was fortunately able to learn just enough at that point 20 years ago to abandon it, but it did turn out that it's spectacular for a microphone preamp.


Quote from: Jmariner on December 02, 2023, 10:03:19 AMit's spectacular for a microphone preamp.
:o But it has so much noise!

Here are some pinch harmonics through a '386 amp I made a while ago (@ 4 min if the timestamp doesn't work):

Quote from: R.G. on July 31, 2018, 10:34:30 PMDoes the circuit sound better when oriented to magnetic north under a pyramid?