GGG 5 Band EQ Problem

[ranchak]

I just finished housing my 5 Band EQ that worked fine when it was sprawled over the floor. I used all shielded wiring for the input and output. I used Marshall style jacks and tried to be neat with my wiring. I fired it up and I had a terrible hum! I was using a wall wart so I tried another one... same problem. So I tried a battery and my hum was gone. I thought well this is a good time to finish the power supply that I started months ago. I used the PS layout from GGG, but I am putting a 9v, 12v and 18v in one housing. So I finished the 9v PS, hooked it up and ..... HUM. So I tried the 12v PS and ..... HUM. By now I am getting upset so I remove the voltage regulator and I have 22v. I have a voltage regulator inside the pedal so I hook it up and .... NO HUM! But, when the effect is on I have less volume than when bypassed. I didn't notice this before housing the pedal, but I was using a guitar and amp (I have been using an audio probe since I discovered the hum problem). My signal is clear until I get to the first op amp, from here I will get some hum, reduce volume and also will pick up some AM frequencies. I guess I have a two part question, why does the hum go away with higher input voltage and any ideas why I have decreased output? I will try to get some voltage readings later today. I don't think I have two more op amps or I would try to switch them.

[Dean Hazelwanter]

Hi,

Take a look at the power supply section of the schematic. Note that it is setup for a split power supply (+/- 9v). If your power supply is connected to ground either through V+ or V-, and the signal ground for the EQ is connected to that same ground, you are shorting out part of your power supply voltage to ground.

With the board in the case, check your voltages relative to signal ground.

Let me (us) know how you make out! 

[ranchak]

Hi Mark, you're right I used a common ground. I separated the grounds. I still have a nasty hum and distortion. One place that I might be getting my hum from is I still am using the the ground from the input signal to the bypass switch. I should probably change that and use the ground from the power supply. I'll probably try that tomorrow, I've had about enough of this project for one day. Ron

[Dean Hazelwanter]

Mark?

When you are getting frustrated, it's always a good idea to take a little break.