Squealing ruby

[Locrian99]

Hello,

Was hoping I might be able to pick some brains for answer to the cause of squealing with the ruby.   If I have the volume knob turned up at all past the first little bit I get an absolutely awful squeal from the speaker.   I used my audio probe and the signal traces out fine until lug 2 of the volume pot at which point the squeal begins.  If I turn the volume knob down to the point I don't have the squeal I can hear guitar through my speaker although it sounds somewhat distorted.  If I have the volume knob turned so there is no squealing and audio probe it I get a fairly clean signal all the way to the speaker terminal.   This was not really a planned project so I got the transistor and ic at my local shop.   For the mpf102 I am using an HEP802 which I was told and haven't been able to back up is a nte312 equivalent which I can see is an equivalent for the MPF102.   In digging around I found this thread https://www.diystompboxes.com/smfforum/index.php?topic=97634.0 where he was having a similar problem (messing with the gain pot can cause squealing on mine when I'm audio probing it).   Also I am using a lm386-n1 also what I got from local place last one they had from Tandy corp lol.  Both pots are as listed in schematic 10kb for volume 1kb for gain.   I tried a 1k for volume due to reading online that using a 25k for volume could lead to squealing no change.  Also I did my own breadboard layout, could get it to work at all using beavis audio layout, noticed some errors in that layout vs the schematic.  Output cap size, volume 2 pot going to pin 1 of ic.   Gain pot two should be going to ic pin 1 with just 3 going to 8.   Here are my voltages:

D- 9.27
S- 1.88
G- 0

1- volume knob maxed (full squeal) 1.13 squeal stop 1.27, all the way down 1.31
2- 0
3- 0
4- 0
5-1.32
6-4.69
7-9.27
4.47


And some pictures:

[antonis]

I should start from Q1 Source voltage by making R2 4k7 (or better 5k6)..
(not relevant to squeal at a first glance but let's try to fix LM386 input signal amplitude and go on..)

[Locrian99]

Alright we now have
9.27
1.95
0

[antonis]

Not good..

We aim to over 3V..

[Locrian99]

Hmm.  Maybe I should put this on hold until I do another parts order and can get the j113

[Vivek]

Would connecting R1 to some kind of Vref help ?

[Vivek]

Try these anti squeal tips

Put a cap in parallel to R1

Put a 100uF and 0.1uF across your power rails

Reduce R1 to 1M

Reduce length of all cables / use shielded cables

Make sure input is far from output

[Locrian99]

Thank you.   I'll try reducing the resistor on r1 I already have the 100uf cap running from + to ground not in schem.   With no connection to ground I'm only getting 2.32 there.

[Vivek]

Isn't pin 6 of LM386 going to +9, hence you should measure +9 at that point ?

[Locrian99]

Oops 5=8, 6=7 etc

Tried a 2nd one of those transistors.  I bought 2.  It dropped to 1.27...

[PRR]

Looks to me like your volume control wires run right over the '386 OUTPUT, then back to the '386 input.

Just like wells and outhouses, layout matters.

[Vivek]

Re FET voltage, please try connecting one end of R1 to a voltage divider with cap (same configuration like a vref for Opamp)

[Locrian99]

Looks to me like your volume control wires run right over the '386 OUTPUT, then back to the '386 input.

Just like wells and outhouses, layout matters.

That's actually the gain control.  But you are correct and the input wire is joining the party.   I can shuffle that around to see if there's any improvement in the stuck pig effect I'm getting as well. 

[Locrian99]

Re FET voltage, please try connecting one end of R1 to a voltage divider with cap (same configuration like a vref for Opamp)

I'll have to look that up.  Still very green, but I'll check it out tomorrow.   Just trying to make a little amp for my bench so I'm not lugging my other one back and forth. 

[Clint Eastwood]

Place the 100uf power supply cap as close to pin 6 as possible.

[Locrian99]

Re FET voltage, please try connecting one end of R1 to a voltage divider with cap (same configuration like a vref for Opamp)

I'm trying to wrap my head around exactly what you are suggesting here.   Are you suggesting attaching r1 to something like this.

And then adding a capacitor to that in series prior to connecting r1 to it?   And using r1 to bias to the desired place of at least 3v?   Am I aiming for 3v or is there a window I am shooting for here.   I'm assuming the purpose of the resistor to ground (r1's current function is to pull excess voltage from the source pin).  So r1 would no longer be going to ground but the + voltage source from the vref?   Which I can set to be whatever I want by changing the resistor values with that ohms law calculator?  Pretty much a total beginner here I'm sure this is pretty basic stuff. 

Or was it more something like this idea. 
https://www.electronics-tutorials.ws/capacitor/capacitive-voltage-divider.html

[Locrian99]

Nevermind you mean the power cap between the positive and ground  I think. 

[Rob Strand]

Nevermind you mean the power cap between the positive and ground  I think.

Paul Marossy's LM386 battle,
https://www.diystompboxes.com/smfforum/index.php?topic=120343.0

[Locrian99]

Thank you I'm going to try moving that cap on the power rails right next to where pin 6 comes off.   Funny though he mentioned the new chip behaving differently than an Old.   Here's the package my chip came in.   Nothing new about it. 




I should have a few minutes here in a bit to dink around with it. 

[Locrian99]

The squealing was fixed by changing layouts.   I just went back to the spread out way on beavis audio and fixed the errors in it rather than doing a whole new way.   No more squeal.   Low volume and distortion though.   I'll play with the voltage divider in a while I got it to raise the voltage but was getting no sound.  Does the source pin have to have some ground connection?