I made an outboard preamp and...

KenToby · #1 12-20-2008, 09:39 PM

I made an outboard preamp from a Fender Jazz Deluxe preamp.
It was pretty easy, to make a long story short, I left all pots (except the balance pot) and wires in place and added a mono 1/4 inut jack and hooked it up to the two wires that used to run from the balance pot in to the circut board.
Problem, there is a high frequency buzz when I boost the treble. What ever the problem is, it isnt obvious. All grounds are as they should be etc.

Any ideas??

Thanks,

KT

ROCKWOOD · #2 12-20-2008, 10:37 PM

Try swapping the 2 wires that used to run from the balance pot?! Polarity & grounding
are pretty much all you have to work with. Might seem a waste, but, try a temporary
groundwire between your axe, preamp & amp.

slyjoe · #3 12-20-2008, 10:45 PM

Hard to tell without a schematic. Fender's diagram doesn't show two wires from the balance pot to the circuit board.

KenToby · #4 12-20-2008, 10:57 PM

The wireing diagram shows the ground running along side of the stacked pots to the PC board. It's drawn as a very thin line (all grounds are drawn as thin lines) and therefore it's easy to miss. The other wire (positive which I connected to the "tip" lug of my input mono jack) runs along the right side of the stacked balance pot to the PC board. See detail "A" of the schematic and you'll see that this is simply layed out...

Seems simple enough but the buzz is terrible with the treble boosted.

Thanks,

KT

slyjoe · #5 12-20-2008, 11:09 PM

Can you post it? The wiring diagram I have shows the ground going to the volume lug. Which year of preamp?

What are you connecting to the input jack? Is it buzzing with nothing connected?

I'm not sure how Fender preamps are electrically - there aren't schematics, so I don't know what is on the other side of the balance pot on the preamp board. You may have a problem letting an input float, or not loading it properly.

The way around this is to leave the balance pot as is, and just attach the mono plug to one of the pickup leads as shown in the diagram.

KenToby · #6 12-20-2008, 11:27 PM

keep in mind, the blend pot is removed in this set-up, therefore I'm only interested in the hot and ground wires that lead from the stacked blend pots into the PC board (these are now the "input" wires from my input jack); the cross wiring of the blend pot lugs are a moot point since it has been removed. At the Fender site, I'm seeing where all blend pot configurations have ONLY two wires to the PC board - black for ground and white for hot (sleeve and tip respectively). What's frustrating is how simplistic this is. Yes, it buzzes without a bass being plugged into the input jack.

Thanks,

KT

cdawzrd · #7 12-21-2008, 01:21 AM

Is it in a shielded enclosure?

chicago_mike · #8 12-21-2008, 02:04 AM

Star ground, Shielding and how noisy was it in the fender? If it was noisy in the fender it will be noisy outside the fender, it could be a noisy circuit from the get go.

kipsus · #9 12-21-2008, 02:37 AM

As everyone suggests - ground and shielding problems are #1 and 99% of buzz causes

vin*tone · #10 12-21-2008, 03:21 AM

Are you using a power supply or a battery?
Are your jacks metal- and mounted in a metal case? or are they insulated?

jbrew73 · #11 12-21-2008, 07:51 AM

you may need to put a resistor the size of the balance pot between the hot and ground wires on the input. a schematic would help if you can provide one.

slyjoe · #12 12-21-2008, 08:05 AM

Quote:

Originally Posted by KenToby

keep in mind, the blend pot is removed in this set-up, therefore I'm only interested in the hot and ground wires that lead from the stacked blend pots into the PC board (these are now the "input" wires from my input jack); the cross wiring of the blend pot lugs are a moot point since it has been removed.

I understand, but you have to remember that when the preamp is in the bass, there is a bunch of input loading from a) the pickups and b) the balance pot.

Quote:

At the Fender site, I'm seeing where all blend pot configurations have ONLY two wires to the PC board - black for ground and white for hot (sleeve and tip respectively).

Hmm. I am seeing a wire from the blend pot to the ground lug on the volume pot. There is note that indicates the volume pot is rotated for clarity.

Quote:

What's frustrating is how simplistic this is. Yes, it buzzes without a bass being plugged into the input jack.

Thanks,

KT

I can understand the frustration. It's frustrating to try to figure anything out on the internet

It may be a matter of shielding as others have suggested. I would also try an experiment - is the buzz better, worse or the same when an instrument is connected to the input.