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Old 1st June 2012, 11:35 PM   #21
gootee is offline gootee  United States
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I know that you said the buzzing goes away when you short the inputs. but what if you put a resistor across the inputs, say 50 to 100 Ohms.

If it buzzes then, then it's probably due to enclosed loop area between the signal and signal ground wires, between the jack and the board.

If it doesn't buzz then, then it's probably not due to that, but could be, if there is some radiating source that you're only near-enough to at certain times.

Is the buzzing at about 60 Hz? Do you have any fluorescent light fixtures, or dimmer switches, or anything else that might radiate?

If it didn't buzz, above, then try one more thing: Plug in the instrument cable (to the preamp) but unplug it from the instrument.

Check for buzzing with:

1. instrument end open
2. instrument end shorted
3. a low-value (50-100 Ohms) resistor across the instrument end.

It is usually a good idea to not use cable adapters. They have the potential to create problems.

Cheers,

Tom
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Old 2nd June 2012, 12:23 AM   #22
JGAN is offline JGAN  United States
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I used a 220-ohm resistor to test:
Shorting the inputs stopped the buzzing, as did shorting the plug :/

The buzzing is 60Hz, but I have tried different outlets and a ground lifter.

Removing the adapter and installing a permanent 1/4" jack didn't help.

Any more ideas? Should I try a ground loop isolator?
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Old 2nd June 2012, 12:52 AM   #23
gootee is offline gootee  United States
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More ideas? Yes. Read my previous post. Twist those wires together yet?

Different outlets? All I saw was a nine-volt battery. Why didn't you post photos of the AC-powered part's internals? (Or label what you posted, if it's already here.) And an actual wiring diagram might be a big help, and not the way the schematic is drawn (how it's supposed to be) but how it was actually wired, with each ground connection and path shown accurately, etc.

Maybe it's something as simple as letting all of the grounds run back to the supply in the same wire, or sharing part of some wire along the way. Then the 60 Hz stuff makes the signal input ground reference bounce at 60 Hz, which then arithmetically sums with the input signal.
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Old 2nd June 2012, 01:05 AM   #24
JGAN is offline JGAN  United States
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What wires are you talking about?

For outlets, I meant the amplifier, not preamp. The preamp is powered by a 9v. I'm working on a PCB layout that I will add very shortly (no pun intended).
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Old 2nd June 2012, 01:12 AM   #25
gootee is offline gootee  United States
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Quote:
Originally Posted by JGAN View Post
I used a 220-ohm resistor to test:
Shorting the inputs stopped the buzzing, as did shorting the plug :/

The buzzing is 60Hz, but I have tried different outlets and a ground lifter.

Removing the adapter and installing a permanent 1/4" jack didn't help.

Any more ideas? Should I try a ground loop isolator?
So did you use 220 Ohms or did you short it? There is a 220 Ohm difference between the two, and the results might be different.

I also can't tell if you tried the same tests but at the other end of the input cable, as suggested, or not.

From the looks of it, it's probably got something to do with excessive wire lengths and undressed wiring, or a poorly-shielded or grounded input cable, or a layout error, or a solder joint in need of reworking, or the lack of a grounded metal case.
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Old 2nd June 2012, 01:18 AM   #26
gootee is offline gootee  United States
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Quote:
Originally Posted by JGAN View Post
What wires are you talking about?

For outlets, I meant the amplifier, not preamp. The preamp is powered by a 9v. I'm working on a PCB layout that I will add very shortly (no pun intended).
Oh! When I see "outlets", I think of AC Mains outlets on the wall, i.e. wall plugs. Sorry!

The wires I was referring to are mentioned in my earlier post: The input pair, the DC power/gnd pair, the output pair. Twisting them tightly together prevents them from being ANTENNAS for the 50/60 Hz AC that's all around us in the air.

How is your output cable configured? What does it connect to?
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Old 2nd June 2012, 01:25 AM   #27
JGAN is offline JGAN  United States
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Wait, now I'm confused... the preamp goes to an amp, which is powered through a 120v wall outlet?

Anyways, here are some photos. Sorry about the poor image quality; I'm shooting with my iPad... Go figure.
Attached Images
File Type: jpg IMG_0168.JPG (224.6 KB, 29 views)
File Type: jpg IMG_0163.JPG (108.2 KB, 29 views)
File Type: jpg IMG_0167.JPG (231.8 KB, 29 views)
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Old 2nd June 2012, 01:50 AM   #28
JGAN is offline JGAN  United States
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Quote:
Originally Posted by gootee View Post
So did you use 220 Ohms or did you short it? There is a 220 Ohm difference between the two, and the results might be different.

I also can't tell if you tried the same tests but at the other end of the input cable, as suggested, or not.

From the looks of it, it's probably got something to do with excessive wire lengths and undressed wiring, or a poorly-shielded or grounded input cable, or a layout error, or a solder joint in need of reworking, or the lack of a grounded metal case.
I don't have any resistor smaller than 220-ohms at the moment, but I did test both ends of the cable and the buzzing always stopped.

The shielding on the input is braided wire. It doesn't buzz when I plug in directly to my amp. The one thing you suggested that caught my eye was the "grounded metal case." How exactly does this work?

I will try twisting the wires

Last edited by JGAN; 2nd June 2012 at 01:58 AM.
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Old 2nd June 2012, 02:17 AM   #29
gootee is offline gootee  United States
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Quote:
Originally Posted by JGAN View Post
I don't have any resistor smaller than 220-ohms at the moment, but I did test both ends of the cable and the buzzing always stopped.

The shielding on the input is braided wire. It doesn't buzz when I plug in directly to my amp. The one thing you suggested that caught my eye was the "grounded metal case." How exactly does this work?

I will try twisting the wires
Pay special attention to twisting the two input wires together, all the way from the jack to the resistor.

Also look at how your Nine-volt connections are layed out. The + and - should stay as close to each other as possible, everywhere.

But none of that might matter nearly as much if you put the whole thing inside of a sealed metal box, which would provide a lot of shielding against the 60 Hz that's propagating in the air. BUT, you would probably need to make sure that the input and output jacks were the type that are insulated from the case.
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Old 2nd June 2012, 02:53 AM   #30
JGAN is offline JGAN  United States
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Twisting the wires didn't help. I also tried touching a wire to the ground terminal of the battery and a bare piece of metal, but it didn't seem to help.

Is it possibly not the preamp? Because it doesn't have this problem with certain amplifiers/speakers.
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