Project Help-8 Channel Piezo Signal Booster/Mixer

[FredF]

Hi Everyone,
I would like to construct some type of signal booster/mixer for my electronic drum set. My idea is to be able too adjust the output signal from each drum and cymbal pad individually, before the module. The goal being, the ability to adjust the volume of all the pads "on the fly", rather than having to stop and adjust the internal parameters. Since the pads create the signal via piezoelectric disks, what would be suggested method to achieve this. I have tried regular audio taper pots, from 10K up to 500K, both before and after the amplifier, but was unable to adjust the volume. This is my first time working with piezos, so I'm really lost at this point. Any help would be greatly appreciated. Thank you.
FredF

 

[Audioguru]

There are many audio mixer circuits in Google with a single opamp that allow each input to have its own volume control.
Please post the schematic of your mixer circuit that did not work.

 

[FredF]

Hello Audioguru,
Thanks for the quick response. Since I "cheated" a little bit, and used pre-assembled amplifier boards, the schematic that I am including is really more of a diagram. I also included pics of the board and slide pot for reference. Thanks in advance for any help.
Fred

 

[Audioguru]

The schematic and parts layout drawing of your Chinese amplifier do not match.
Does it have and where are Ri, Rf, Cin, and a 100k resistor and switch to SD pin 1?
It probably will not work if the Mode pin 3 is not high or low, if pin 1 is not grounded and if Rf is missing.
I also downloaded its datasheet and introduction but they are written in Chinese.

I do not know which pin is what on your volume control.

You show an amplifier with only one input. Where did you connect the other inputs? Where is the mixer circuit?

 

[FredF]

Sorry, I did not convey some details that you would need to diagnose my problem. First, I downloaded the photo of the amplifier from the seller's site. I had never seen the schematic till then. I assumed it was just a sample circuit of how it could be used. The markings on the board seemed pretty straightforward(5V+- IN+- OUT+-),so I just went with that. Second, the diagram is for just 1 individual channel. As you see in the new pics, it will be repeated seven more times. Drum pads plug in to input on mixer where they are amplified and the volume adjusted (hopefully).Then the amplified and adjusted signal are sent to the drum module via the mixer outputs. Having thought about this since I first wrote, could the problem be my slide pot values? Too low or too high?

 

[Audioguru]

The very cheap no-name-brand amplifier is sold on ebay and at Bangood. They have no audio spec's for it like its input impedance and its voltage gain.

1) The 101 resistor is connected to the input pin 4 of the IC but 101 is only 100 ohms and it must be at least 100 times higher at 10k ohms.
2) Pin 1 and pin 3 on the IC might or might not be connected correctly.
3) It looks like you have the (-) outputs of all the amplifiers shorted together and shorted to the inputs grounds which is completely wrong. Look at the amplifier schematic that shows two amplifiers, an amplifier for each wire on a speaker. Why are you using speaker power amplifiers instead of opamps?

 

[FredF]

Like I said, I sort of "cheated" by using the pre-assembled amp boards. I am only looking to boost the volume of a selected channel enough to make it stand out above the others. The amplifier in the drum module takes care of the listening volume. I figured this would be the quickest, and yes, the cheapest route to achieve my goal.
In your opinion, can I make this work using the components that I already have, or should I go back to square one, going the opamp route. If so, which would you recommend? I don't mind putting my ignorance on display in the quest for knowledge.

 

[Audioguru]

The cheap power amplifiers you have are completely wrong to be used as the preamps you need for the piezo transducers.
The volume controls have unknown pin functions.
What does the drum module do? Does it make a "snare" sound when the "snare" input has a signal? For the different functions?

 

[FredF]

Yes,that's correct. Piezoelectric sensors produce an analog output voltage signal proportional to the pressure applied to the piezoelectric sensor.This triggers a sampled sound of the drum or cymbal that the output is plugged into on the module. Tap it with force,more volume,gently,less.They are also commonly used to amplify acoustic guitars, trumpets,saxophones,etc.