What Causes Squealing?

[thor653]

Recently I built a 5th gear Overdrive and everything is working properly except it's squealing.
I've had this happen on a couple other builds too.
Just wondering if anyone else has had this same problem and what they did to fix it?
Thanks for the help.

[Stephen]

Too much gain

Sometimes a small like a.002 uf cap after the output will help after the cap....It works for me

But than noy always you need to sheild and watch your wires.

Fets in tandum have a common problem or quads I hear because of the oscillation of the input to.the output is the same ....an odd number will work better I THINK...I heard of this and am just passing it along!!!

[brett]

Hi
Squealing is almost always an oscillation.
Oscillation is usually caused by feedbak.
Feedback is caused by high gain and poor layout.

What is poor layout?
Input and output wires in parallel
Input and output close
Input and output in phase (2, 4 or 6 inverting amplifiers in series)
Inputs and outputs not shielded.
Thin earth wires.
Lack of power supply AC de-coupling near op-amps.  ie that 0.1uF or similar cap between +9V and Gnd that all good designers use

Actually, everything that RG says at geofex.com (why didn't I say that first?)

If you want to find out which particular wire is causing a problem, try "chopsticking".  Use a wooden skewer or thin chopstick, and touch or move each wire in turn.

cheers

[thor653]

Thanks for the info.

[jonathan perez]

pulling their tails too hard. or just chasing them...

(pigs)

[col]

I have had some success with squealing by;

1. Shortening all wires in the build by as much as possible
2. Putting caps across the base and collector of transistors in the build. Start with 60pF and work up in values until the squealing disappears. Use the lowest possible value that works ot it will affect tone. try each transistor individually.
3. Ground the board to the case by soldering an earth wire to the back of one of the pots.
4. Using slightly lower hfe transistors from the same batch.

Sometimes one will work, sometimes a combination and sometimes none work perfectly.

Col