Extra hiss when using AC adapter with GGG EQ

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Extra hiss when using AC adapter with GGG EQ

Postby Dave Simpson » 13 Jan 2008, 14:32

Greetings everyone! :D

First post here, so be nice!

I just completed a build of the GGG EQ and noticed there's a little more hiss when using an AC adaptert rather than powering the pedal with a battery. Is there some kind of value change in the filtering section I can do to compensate for the extra noise? Any help would be appreciated.

Right now it uses a 10K resistor, 10uF capacitor and 1N4005 diode.
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Postby soulsonic » 13 Jan 2008, 15:17

You could bump the cap size up a little bit, but I can't say that would necessarily help.
Here's what I would do:
-Get 4 little ceramic caps, the yellow bead type work perfect for this. The values can be anywhere from 10nF to 100nF - the 100nF (.1uF) ones are one of the most common used for this.

-Now, on the trace side of the board, carefully attach one leg of each to pin 4 and pin 7 on both of the TL074 opamps. Try to get the cap as physically close to the pin as possible. The result should be the pin 4s and each pin 7s should each have one cap attached to it with one leg hanging free.

-Finally, connect the free legs to the ground trace that goes down the center of the board (the one that connects to pad "C" in the layout). You may want to put some shrink tubing on the leads to keep them from shorting against anything.

Viola! Hopefully, this should take care of any extra hiss. I'm actually surprised it's not already included in the design because it's common practice. Look at the pics of the A/B box in my build blog - the big blue ceramic caps are there for this reason. It removes high-frequency interference and noises right at the opamp pins. It's actually what those little bead caps are best at - I've even seen IC sockets with them pre-installed.

Cheers!
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Postby Dave Simpson » 13 Jan 2008, 15:38

I'll give it a try. Thanks for the tip!

I know what you mean about the design of the PCB. There's also an issue with using this pedal along with other pedals on a single power supply. Luckily there's a pretty simple fix for that problem.
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Postby soulsonic » 13 Jan 2008, 16:03

You could rig it up with a charge pump circuit like a MAX1044 to give it the bipolar supply it wants with a single battery - it would also let you share supplies with other pedals without trouble. It looks like a good circuit, but I think some little function tweaks here and there could really bring it up to another level of quality.
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Postby Dave Simpson » 13 Jan 2008, 17:35

Thanks again! :D

Hopefully in time I'll be able to figure out how to do the voltage pump mod. Lot's of things to absorb around here.

I agree, with a little work it should be a very usable pedal. Sounds good, just needs the little problems worked out before it's prefect.
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Postby bajaman » 13 Jan 2008, 22:54

When I have hiss problems using a dc power pack here is what I do. :wink:
Insert a 100 ohm resistor between the + dc input and the circuit and connect at least a 220uf electrolytic capacitor from the circuit end of the resistor to ground(+ terminal to resistor of course) :wink: Result: no more hiss!!!
cheers
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Postby Dave Simpson » 14 Jan 2008, 18:58

Thanks bajaman! I'll give that a try as well. :D
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