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Piezo Pickups Problem - HUM!

#21
If by "common preamp" you mean the garden variety Bart or such that are destined for magnetic pickups, no. Piezos dwell in the upper reaches of impedance, as in Millions of ohms, while mags are in the low thousands. The impedance mismatch gives you a sort of quacky, brittle response, with thin lows and highs with bad tonal characteristics. Even if it's just a no-feature buffering preamp, it's a must.

If I am not using one of my amps with a 1 or 10 megohm input impedance, I'll use one of my outboard K&K Sound preamps (Preamps Page - [Invalid or Expired Link Removed] ) or one of the tiny onboard GraphTech ones - GraphTech Piezo Systems - [Invalid or Expired Link Removed] for the matching assignments.
 
#22
Have found exactly the same thing with my Carlo Giordano EUB. I'm investigating now and like everyone else, am back to the piezo wires direct to jack (preamp temp removed) and thru a hi Z buffer to my amp. Yes, still hum; still reduces when I touch a ground point like the jack body.
OK well I really think this is all about the ultra Hi Z presented by a piezo pickup. Effectively we are running almost an open circuit ... so it's like plugging in a longish guitar lead to amp, nothing on the other end and cranking on some gain and LF boost! Oh, Humm! I am thinking about this and shall return with results if any, but possibly will try a load resistor at pickup. 5Mohm or so? Also wondered about polarity etc. but not sure. Hum can be so illusive. Wish me luck! :) CL
 
#23
The hum with piezos is due to the amp not being properly grounded. I have seen this a few times. Old 2-prong outlets are replaced with new 3-prong outlets so you are able to plug in newer devices but if a ground is not connected to that 3rd prong then its not doing anything. The fact that this is happening with multiple piezos makes me 100% certain that the outlet is not grounded and thats your problem.
 
#24
hurleypickups said:
The hum with piezos is due to the amp not being properly grounded. I have seen this a few times. Old 2-prong outlets are replaced with new 3-prong outlets so you are able to plug in newer devices but if a ground is not connected to that 3rd prong then its not doing anything. The fact that this is happening with multiple piezos makes me 100% certain that the outlet is not grounded and thats your problem.
Click to expand...

I was crying ******** when I read this. I have a system setup in my basement. I have a sound board with inputs from my computer, synth, guitar pedals etc. I don't hear any perceptible hum with these devices. When i hooked up my ghost pickups I had a annoying 60hz hum. As with the author of this thread, when I touched the bridge etc., it went away. Even though i didn't think hurleypickups was right, I thought I'd give him the benefit of the doubt. I got cheesy little amp (with a sign on the front that said 'Fender Champ' and second had guitar .... but that is a whole other story). When i plugged into the outlet in my basement and plugged in my ghost equiped Godin Z1 I got the HUM. I then took it upstairs and plugged it into an outlet in my living room (which is on a circuit that runs from a separate power distro) there was no HUM.

The difference with the piezos I think is the signal to noise ratios. The signal from a piezo pickup is very weak compared to a magnetic pickup (even passive).
 
#27
I just wanted to toss in my 2cents and confirm the grounded outlet suggestions.

I recently picked up a Washburn AB-20 acoustic bass with fishman piezo and Equis II preamp. My house is an 1870 victorian with no ground on most of the outlets. I was getting a lot of hum from the piezo ABG through the amp and also when connected directly into my DAW - the hum would vanish if I touched the jack (or even a mic connected to same DAW). But I have nylon tapewound and my bridge isn't grounded even if I did have metal strings - so there is no body contact like you get through strings on a solidbody bass. (I recorded a few tracks sticking a ground wire into my sock - haha - got rid of the hum).

Anyway ... I was skeptical of the grounded power outlet suggestions (I don't get hum with anything else - only the piezo ABG - so I was sure it was the pickup/preamp). As a test I listened to the bass through headphones and a battery powered device and no hum at all. So that made a believer out of me. I grounded one outlet and tested through the bass amp - no hum. Now I have about 30 more outlets around the house to ground.
 
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#28
Great reply-
whole neighborhoods can have ungrounded outlets if they're older and have not been upgraded,
so plugging in next door, or down the street doesn't help.
As noted prev'ly, just because the plug or socket has a ground hole or terminal- doesn't mean that it is connected to anything.
 

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