Guitar piezo pickup reverse engineering

[Vytautas]

Hi, guys,
I tried to reverse engineer a piezo pickup preamplifier for guitars. It is a MiSi type pickup which uses 1.5F supercaps instead of batteries as the power supply. The caps are charged for one minute and then the caps supply voltage for about eight hours of play. I assembled one unit and we installed it into a guitar, and the middle tones sound sort noisy, as if creating additional overtones to the main sound. Guitar itself is at no fault as it has no problems with the original from MiSi. For the piezo pickup which is installed under the guitar saddle, we used a cheap Chinese copy piezo pickup. Could this be the culprit? Or, could it be that the ceramic SMD caps on the board have some piezo effect and infuse noise because of the unavoidable preamp board vibration inside guitar? Or, would anyone spot some glaring mistake in my schematic? It is quite simple, and I just cannot see any problems in it.

[tpowell1830]

It may be possible that your caps are causing distortion, a simple test would be to move the electronics outside the box.

Hope this helps...

[NiHaoMike]

The negative supply connection to the opamps looks very strange. Perhaps the designer got confused and it's actually supposed to connect to ground?

[Buriedcode]

I assembled one unit and we installed it into a guitar, and the middle tones sound sort noisy, as if creating additional overtones to the main sound.

When you say noisy, do you mean noise when you aren't playing? = added noise, or clipping/buzzing when playing, that may or may not get worse the harder/louder you play?

Under saddle pickups always have a brittle type of sound that some love, some hate.  Also, how is it fitted to the inside of the guitar? As the mems mic is on a PCB, could the board be rattling around?

[SiliconWizard]

I assembled one unit and we installed it into a guitar, and the middle tones sound sort noisy, as if creating additional overtones to the main sound.

When you say noisy, do you mean noise when you aren't playing? = added noise, or clipping/buzzing when playing, that may or may not get worse the harder/louder you play?

Under saddle pickups always have a brittle type of sound that some love, some hate.  Also, how is it fitted to the inside of the guitar? As the mems mic is on a PCB, could the board be rattling around?

Yeah. The OP is assuming it's a problem with the preamp, but it could just be a problem due to the pickup itself and/or its installation.

[jonpaul]

Just test the cheap Chinese pickup on another peramp or use an instrument amplifier.



I doubt if the caps have any effect.

Jon

[Vytautas]

Thanks everyone for your kind input. We guess that the type of piezo we used is not the right one. We will try to order a different one and see how it performs.

[Renate]

Under saddle pickups always have a brittle type of sound that some love, some hate.

Seriously? Somebody loves them?

Use an oscilloscope.
Softly pluck (as with your finger) the middle of the string to keep the harmonics at a minimum and see if you have clipping or whatever on the scope.
Piezo can be a single strip or segmented per string.
You might have to shim something to get equal volume from each string.
I had to use a piece of cellophane at one string position of my guitar.

[Vytautas]

Hi, guys,
I made some tests with my reverse engineered MiSi preamp using a signal generator. The preamp is not at fault at all. I include a screenshot of my scope in its "spectrum analyzer" mode (actually, it is fft with a spectrum analyzer interface). It shows the first harmonic that comes out from the preamp to be around 40.8dB down. This is too weak to be heard. Moreover, its frequency is an exact double of the main signal! This means, even if our ear were able to hear it, it would be in complete unison with the main tone! But what we hear coming from the guitar is a far cry from unison. And the installation method we used is not at fault also, as the original MiSi unit, installed exactly the same way creates no noises. So, the only culprit remains - chinese piezo. We ordered another type, hope to get something better.