What's wrong with this?

[SaBer]

Ok, so I built a blender for a piezo-transducer and magnetic pick-ups. I'm using an old computer power-supply, giving me +/- 12V and TL072:s for op-amps.
My problem is, that about the third time I used it, it started to make terrible noise. I couldn't find anything wrong, so I replaced the op-amp which has the piezo-preamp and mixing stage in it. And the hiss was gone. Then I put it into an enclosure, and the same thing again: Terrible hiss. I didn't dare blow a third op-amp, so...

Is there something wrong with this?



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I'm also thinking I don't need the unmarked capacitor, do I? (it's 1uF at the moment)

And still one thing: would some other op-amp handle the high impedance in the piezo-side better than the TL072? It's a bit noisy with the TL072. The  original design, which I found on the net had some other op-amp, but I can't find it anymore.

P.S. Thanks to RG and whoever designed the piezo-pre-amp, for their designs!

[Johan]

what you could try is to put two back-to-back 3volt zenerdiods between the +and the - pins on the input. that way they will never be more than 6 volts apart ( check out R.G's "when good chips go bad" )
eventhough piezzo's are very high impedance and loads down easily, when they are not loaded down they can actually give suprisingly high peak voltages, that might be what wreks your IC's

the 3,3M resistors on the piezo input will likly generate theire own noise, just from being so big. also they are too big to help with the biasing...have you tried to connect the piezo directly to the input. it might help keeping the noise down, but then you will need somekind of protection like the one described above....

good luck

Johan

[SaBer]

the 3,3M resistors on the piezo input will likly generate theire own noise, just from being so big. also they are too big to help with the biasing...have you tried to connect the piezo directly to the input. it might help keeping the noise down, but then you will need somekind of protection like the one described above....

good luck

Johan

Thanks! I'll try that. I don't know much about piezos, so I just built it like it was in the original design, but I guess that's worth giving a try.

[Boofhead]

The computer power supply could be going bananas - many don't like running into very low current loads.  Try running off batteries - even if it's just to prove the point that the noise goes away with batteries with a suspect opamp.

[SaBer]

The computer power supply could be going bananas - many don't like running into very low current loads.  Try running off batteries - even if it's just to prove the point that the noise goes away with batteries with a suspect opamp.

I've been using it for other projects also, and it works fine. It's pretty old, only 73 Watts. Found it used for 3 euros  

[smoguzbenjamin]

Its 73 watts and you are pulling maybe 10mA from it That might be a problem, mate! Try a battery just in case.

[R.G.]

A couple of the earlier comments are dead on. I would use some protective diodes on the inputs. Also, the power supply likely needs some loading. Switching power supplies get horsey with almost no load.

I believe you have the input labels reversed. Magnetic inputs usually need amplification, piezos don't. I didn't post the piezo preamp, but what you need is just low gain but very high impedance.

I would change the 3.3M series resistor on the gain input to 10K.

would some other op-amp handle the high impedance in the piezo-side better than the TL072? It's a bit noisy with the TL072.

The TL0x2 series is pretty good for run-of-the-mill low noise. There are quieter chips, but its unusual to need them. The TL0x2 series is JFET input, so the input impedance is quite high enough. I think the hiss is actually coming from that series 3.3M resistor.

I don't need the unmarked capacitor, do I?

You don't need any of the capacitors except one you don't have there yet if you are running from +/- 12V. The caps are only to block DC. I might use the input and output caps just for belt-and-suspenders, but with bipolar power, you can bias opamps at 0Vdc and leave them there. The cap you're missing is from the 10K on the inverting input of the piezo side to ground to avoid amplifying DC.

[SaBer]

Thanks alot everyone!

As I mentioned before, the power supply is NOT the problem. I was just a while ago running a signal balancer and booster with only one TL072, and it works just fine!

The piezo is also DIY, with not a very high output, nor a very good low end. I just use it to add some edge to my fretless bass with flatwouds. It's kind of a project bass...

So this is what I'll do:
-Add the protecting diodes
-Change the series resistor on the piezo side to 10k
-Add the capacitor in series with the 10k resistor to ground
-Hope it will work better!

[SaBer]

I did all the mods and now it works great!
I also cut a bit of highs from the piezo side. I had to add some components on the bottom of the board, because it was so full on the top. But, I did get it done!

[Nasse]

:shock: You mentioned diy piezo attachment

Is it o.k. to ask how you did it and what you used??? :mrgreen:

[SaBer]

:shock: You mentioned diy piezo attachment

Is it o.k. to ask how you did it and what you used??? :mrgreen:

Yes, it is probably not the best possible way to do it, but anyway:

1. You take a piezo element out of a piezo siren, or something like that.
2. Detach your bridge.
3. Make a little hole, that's just the same size as the piezo element in the middle of the area where the bridge was.
4. Try to get the leads to fit through the same hole, as the bridge ground lead. If they dont fit, make it bigger.
5. Take a little strip of wood (or something else), that's about as thick as a couple of sheets of paper and put it a bit off centered (towards the neck) under the piezo element, so that it rocks a a little. You could split a match or something. The element should rock towards the neck.
6. Re-attatch the bridge, so that the front (neck-side) is a teeny bit loose, just enough to get a piece of paper to fit in between
7. Try to get it to work well. Adjustments, adjustments...

I hope you understood  :?

I did this to a de-fretted Ibanez gsr200, which costs less than 300$. It is my project bass. I don't recommend doing this to an expensive guitar, because you might not like what you get.  It may affect your sustain or tone or... But since those werent so great in a de-fretted gsr200 in the first place, I gave it a try. Works well for me!

I'll record some samples, if I ever get to it.

[Nasse]

Thanx for the info. I have toyed with similar piezo elements, and think you can even buy those cheaply in some places. Just thought I should sometimes give a new try with these on my old small steel string which I changed nylon strings guitar, so I could plug it in to some effects and reverb. It is not any Stradivarius and I´m never gonna hurt her... So many projects, so lazy me...

But different piezo systems exists even as commercial ready made retrofit kits too, so it is useful info for someone, and nice to know for even more