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Old 28th May 2012, 10:33 PM   #11
gootee is offline gootee  United States
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You would need to post a photo of the circuit, for best results.

Is the cable from the guitar a single inner conductor with an outer shield? Test with an ohmmeter between the outer (shield/ground) parts of the plugs of the two ends of that cable. And also test between the inner/center plugs of the two ends. Both readings should be quite low.

Is the input jack on the preamp electrically insulated from the case/chassis? (Probably should be.)

Do you have the ground connection for the input jack connected to the case/chassis, inside the unit? (Probably should not be.)

Did you use shielded twisted pair cable between the input jack connections and the circuit board? If so, you should probably have the shield connected to chassis ground (and not to the jack) from near the input end only, and only the wire pair inside the cable connected to the signal and ground connections of the input jack.

If you used a single-conductor cable with a shield to go from the input jack to the circuit board, then the shield would connect to the jack's ground lug and the wire inside would connect to the jack's signal lug. The jack would probably need to be insulated from the case/chassis, i.e. with no other ground or chassis connection where the input signal and input signal ground reference enter the case.

If you used two plain unshielded wires to go from the input jack to the circuit board, then they should be tightly twisted together (at least six turns per inch), ALL the way from the jack to the PCB connections.

At the circuit board, where the input ground reference from the jack connects (probably at the base of a resistor across the actual input of the active circuit, with the input signal connecting to the other end of the resistor), there should be a connection from there to the star ground point. It is very important that No part of that input reference ground connection's conductor should be shared with any other ground return (except possibly a nearby closely-related small-signal ground, but if you're not sure then run it completely separately), at any point, ALL the way back to the star ground point (which might be a stub of pcb trace just off of the ground trace for the main smoothing caps in the power supply, for example).

Other types of ground returns should also be run separately from each other, all the way back to the star ground point.

Besides the input signal/ground pair, there are other types of conductor pairs that should have the minimum-possible enclosed loop area between them, so that they will not act as receiving or transmitting antennas (see Faraday's Law, if interested). So they should all be tightly twisted together (or shielded twisted pair), or, in the case of PCB traces, kept as close together as possible, at ALL times, e.g. always fully overlapping on opposite sides of the PCB, or if on the same side they should have a minimum-possible gap between them, always. e.g. Never run anything and its corresponding ground or paired conductor apart from each other, even a little.

That includes the AC Mains pair, the rectifier-to-filter caps pair, each DC power/gnd pair (and maybe the +, -, gnd triple, sometimes), the output signal/gnd pair, and any others I might have omitted. Don't leave 5 mm of separation where you could leave only 1 mm.

Also, keep the small-signal stuff, and DC power, well away from anything with larger or more-dynamic currents, such as the AC and rectifier stuff. If two types of pairs MUST approach each other, keep them at a right angle to each other.

Cheers,

Tom

Last edited by gootee; 28th May 2012 at 10:38 PM.
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Old 29th May 2012, 12:05 PM   #12
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Quote:
Originally Posted by JGAN View Post
I'm not exactly sure what that means?
I know it's a tube site, but download the PDF from here, there's some very good tips. They apply to solid state amp design too.
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Old 29th May 2012, 08:08 PM   #13
JGAN is offline JGAN  United States
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Old 29th May 2012, 09:23 PM   #14
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From the looks of it you have the shield (ground) and signal wires reversed on the jacks. I'm just guessing because I haven't seen those exact phone jacks before and I can't tell exactly how the circuit is wired on the PCB. I'm just going by the wire that is closest to where the gate would be if you are using a J201. Check with an ohm meter between the minus side of the battery connector and the shell of a guitar cord plug.

The Drain and Source are interchangable on the J201 and the vast majority of JFETs.
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Old 29th May 2012, 09:37 PM   #15
JGAN is offline JGAN  United States
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That's highly possible, I have never used these jacks before. Check them out here; 1/4" mono. I didn't know what to do with the third tab, so i just soldered it to the same wire as the shield. I can still get signal from it, so I don't think i reversed the shield and tip?
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Old 30th May 2012, 02:00 AM   #16
JGAN is offline JGAN  United States
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Should I just solder a wire between the two ground tabs on the jacks? Or do they have to be grounded to the battery or something?
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Old 30th May 2012, 02:08 AM   #17
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Study the schematic. It shows what needs to be connected to what. The order things are connected isn't to important in this preamp because the gain isn't very high, and the power is low. If you can get things connected correctly, it should work. Those jacks have a third terminal. It's probably a switch contact arranged so it is connected to the tip terminal when there is no plug inserted. If you connect that terminal to ground, it will keep the output silent when you unplug the guitar cord from the input.
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Old 30th May 2012, 08:28 PM   #18
JGAN is offline JGAN  United States
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So it's not the cause of the buzzing?
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Old 1st June 2012, 03:14 AM   #19
JGAN is offline JGAN  United States
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Ok I know for a fact that it's not the jacks. Also, I rebuilt it with following the schematic exactly, but am still getting a buzz/hum. I will post a diagram of my PCB soon.

The buzz/hum only comes from certain amps and cables, it seems. It's not my outlet, as I have tried different outlets. I didn't get a buzz when plugged into my car stereo (don't ask) or when using a Behringer bass amp (don't remember the model though).

Any ideas? Also, if I were to hunt for a ground loop using a multimeter, where would be a good place to start?
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Old 1st June 2012, 09:39 PM   #20
JGAN is offline JGAN  United States
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I just tried a couple things. First off, I cut the ground prong on a spare cable for my amp and tried that, but it didn't help. In fact, it only made it worse.

I tested for continuity across the sleeves (negative) on the jacks and there is continuity... [s]Does this mean I have a ground loop?[/s] Nevermind, I googled and and found out that should be normal.

I also tried some different inputs on the preamp, including a computer and microphone. I realized that the buzzing is extremely minimal with those inputs. My main input is a cello pickup, which is RCA. I convert it with an adapter to 1/4" mono so I can plug it into the preamp. Could this adapter be the problem? Should I try replacing the RCA with a permanent 1/4" jack?

Last edited by JGAN; 1st June 2012 at 09:58 PM.
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