Piezo pickup hum problem

[hash]

Hello, I've been reading on how to convert an acoustic guitar to an electro-acoustic for quite a while and decided to try it. I've built the mint-box buffer and connected it to a 1'' piezo transducer. The problem arrived when I tried to amplify the output of the buffer. It works, but in order to hear something I have to crank up the preamp stage on my amp - an old tape recorder - which induces a very loud hum in the speakers. From what I've read it appears to be an earth loop... but the amplifier doesn't have a ground connection (only 2 pins on the mains connector) and I don't know how to connect the wires from the buffer in order to get rid of that hum. The buffer is powered (for now) from a wall-wart ar 9V and when I touch the neutral wire on the buffer the hum stops (probably I'm becoming the ground).
A friend of mine also has this problem with his electric guitar although he has a proper earthed guitar amp (I suspect that the strings aren't grounded because the guitar stops its hum also when he places his hands on the strings or any metal part of the guitar).

Hope you can help me
Cheers!

[black_labb]

your synopsis seems right, you should ground it.

[hash]

Ok, but how? The amplifier has a 2-prong power cord (no ground). The input is a 5-pin circular DIN jack and I don't know where the ground might be connected on that. It now has 3 wires on pin 1 (left), 3 (common neutral) and 5 (right). I'm connecting the left and right together on the positive side of the buffer and the common neutral on the negative side. Where should the ground go besides those... I don't know

[black_labb]

Originally Posted by hash

Ok, but how? The amplifier has a 2-prong power cord (no ground). The input is a 5-pin circular DIN jack and I don't know where the ground might be connected on that. It now has 3 wires on pin 1 (left), 3 (common neutral) and 5 (right). I'm connecting the left and right together on the positive side of the buffer and the common neutral on the negative side. Where should the ground go besides those... I don't know

im not really sure, i havent done anything for amps, try the amp section of the forums. i do want to one day, but not yet.

[madialex]

I would think you could just change power cords. Buy a new cord with 3 wires that has the ground prong on it, rewire the 2 wires where the old ones are and run the green to the chassis I guess as ground? I', no amp guy by far and know even less about electronics and stuff that can shock the living hell out of you so yeah the amp section may be your best bet. David S is pretty smart on this stuff, maybe he will chime in.

[Rob Mercure]

Hash,

Unfortunately I can't easily follow your description of your "buffer" - if you've got a schematic to provide this would help. But there are a few things that I can pick up: 1) I agree that your amp(s) should be grounded ("earthed") as a general safety precaution having been shocked on stage too many times; 2) Your "buffer" either has too low a signal output or your amp lacks sensitivity - I'd vote for the former. Since the "buffer" most certainly uses some active components you might simply have too much NFB in the circuit or be operating the devices at a funky characteristic point - at worst you will need to add another amplifying stage. 3) You don't have a "ground/earth loop" but instead the opposite - you aren't adequately grounded at all! In addition to this what you're probably hearing is the "hum floor" of your amp when you have to crank it up all the way.

I usually hang around the amps section of this forum but decided to check out this section before starting work. If you've got a schemo for your buffer please provide it as well as specifics for the amplifier you've got. While you may not have a schemo for it is it valve/tube? And what is the tube compliment? And, if tube, have you ever had the filters/"smoothing capacitors" changed?

Lastly, why not power your "buffer" from a 9 volt battery? This would make for a smaller/lighter power supply and eliminate any source of possible ground loop from that source.

Rob

[Dave Kerr]

I'm guessing that he used the schematic provided here:http://www.scotthelmke.com/Mint-box-buffer.html

[hash]

Yep, that's right, I've built the mint-box buffer but changed the FET to BF245C. The connections are shown in the attached file (sorry, mspaint novice ). The tape recorder is an old TESLA CM-130, I think it's solid state. I've used it only for testing purposes, I don't have a guitar amplifier and it's the only thing I could think of with a preamp. I'm somewhat reluctant to change the power cord... In any case, I'll have access to a friend's guitar amp in a couple of weeks, but my concern was that there's something wrong with the wiring of the piezo-buffer-input jack.

[Rob Mercure]

Hash,

If there's a problem with the input jack your illustration doesn't help as you show the piezo element directly wired to the buffer. I noticed that the buffer site has a high and low gain option - are you using the high gain one? Anyhoo, any tape recorder amplifier should have sufficient gain for your purposes unless you are using something like an "auxillary" input which would probably be a line level input. If you're using a "mic." input or the tape head input you should have gain to "throw away" - so having to turn the amp up seems indicitive of a problem with the buffer. Again, why not power it with a battery just to make sure that the problem isn't in your wall wart?

If cobbled up the buffer then you can solder. So make yourself a RCA phono plug to 1/4 adapter of some sort - probably the easiest would be to cut a patch cord (called "interconnects" these days by folks with more money than brains) and solder the "other" type jack on. Then you can use this with your home stereo, etc., to test your buffer.

Rob

[hash]

Originally Posted by Rob Mercure

Hash,

If there's a problem with the input jack your illustration doesn't help as you show the piezo element directly wired to the buffer.

The input jack is shown on the right, the 5-pin din thing... I was referring to not knowing if how the shield (gray) is connected to the neutral wire (blue) is alright. I'm using the MIC input on the tape recorder and the hum appears when the preamp is set to about 4/10 volume, at about 6-7/10 the guitar is loud enough to make the windows rattle

I've checked and double checked the connections on the buffer and everything seems to be fine. I've tried the buffer before soldering it to the pcb on a breadboard and it did the same thing.

Um... this IS my home stereo system so I've got nowhere else to try it. Maybe I'll try to make a buffer/preamp stage, as seen on this link: http://guitar-repairs.co.uk/jfet_guitar_preamp.htm
although it has a not-so-high input impedance (1Mohm).

[Rob Mercure]

I believe we've got a terminology misunderstanding. As I view it the "input" is where the piezo signal enters the buffer, the output where it goes to the amplifier and the power supply - which may be what you're calling "input" is another animal. But looking over the posts I've come to suspect that the mic input you're using is low impedance - low enough to "pull down" the buffer circuit which seems designed to feed something around 50-100K at the minimum. Quite adequate for a guitar amp or PA but far too low for a 600 ohm, or lower, microphone input. And I wonder if your tape recorder is "expecting" a balanced input? If so you may have to ground one side of the output.

I dunno, I still don't have sufficient info on you setup to provide a better opinion but the buffer should be sufficient to feed a hi-Z signal level input.

Rob

[hash]

I don't know the input impedance of my amplifier, but I've tried the buffer again with a battery and the noise has somewhat diminished. Anyway, I'll try it in a few days on a proper guitar amplifier, I'll then come back with the results

Thanks for your help!

[David Schwab]

Originally Posted by hash

Maybe I'll try to make a buffer/preamp stage, as seen on this link: http://guitar-repairs.co.uk/jfet_guitar_preamp.htm
although it has a not-so-high input impedance (1Mohm).

The two 2M resistors at the front end of the circuit set the input impedance (in parallel you get 1M). You can replace the two 2M resistors with one larger one... try 10M. Also remove the input cap that was between the two 2M resistors. You don't need that.

There's a lot of piezo buffer schematics on the 'net.