OLC English Channel....squealing, oscillating, wrong J201 voltages....help!!

[jamiefbolton]

I finished building the OLC English Channel last night and today I got busy adjusting trim pots and began testing the pedal.  After getting all of the Drains to +4.5 I plugged it up  and got a lot of squealing and noise.

The squealing goes away when I play a chord. Guitar still comes through and sounds decent, but the extra noise and squealing is unbearable.  Something is wrong. I've checked the voltages for the Source and those are way off.  ROG says the Source should be Q1=.174   Q2=.164   Q3=4.79   Q4=1.41   Q5=27mV
Mine are more along the lines of .3 - .5 for the Source.
Any ideas where to start? OLC says to sub MPF102 for J201 if you get squealing, but of course, I don't have any of those. So what do you guys think?


jamie

[TheNixon]

Hi,

First try to localize error. Find out when squeel start and at what gain. If you don't have an oscilloscope to look at signal take a piece of wire and check different points in pcb to find out at which section squeel begin. If there are few parts (ussualy 3-4(5) transistor parts) then check signal at each section. I am not familiar with OLC english channel but I have experienced same situation wih Dr. Boogey. So I findout that everything start to squeel around last transistor and I found out that my output was too close input and while GAIN is greater than 1 this makes positive back loop and this is heard like squeel.

I learned few very important things about noise and squeel reduction:

- make connections as short as possible
- make sure input signal is away from output signal (to prevent loop)  !!! (very common reason for squeel)
- check all grounding and shielding
- if possible for input and output signal use shielded wires (microphone cables) or make twisted pair with GND wire
- if you have potentiometers with metal shield then connect it to GND
- shorten wires
- if possible solder pots directly to PCB (to shorten connections)

I hope this will help you!

[jamiefbolton]

good idea. i'll get the audio probe out soon and report back

thanks

[mrstar]

This a great troubleshooting guide. I have my input and output wires twisted together and it was causing the squealing. I unwrapped them and it went way.

[ricothetroll]

Hi !

I have the same oscillation problem witn my english channel. When you talk about feedback between input and output, do you mean by induction/microphony between them ?

It seems like this one is VERY noise sensible...

Best regards.

Eric

[jamiefbolton]

i still have not done a thing to the pedal. i've been really busy and have since taken a break from the DIY. hopefully i can resume one day soon. first thing is to remake with shielded wires and then shorten and untwist the wires for the pots. i bet that takes care of it....i hope