Piezo Control voltage source

[(*steve*)]

Sorry, I read those as a Schottky diode symbol. Your original is fine if you use normal diodes (Schottky are better)

Audioguru's method will also work, but a pair of back to back zeners will do the job without needing the additional diodes.

 

[Audioguru]

The datasheet clearly says the max allowed input voltage is +/- 15V or the supply voltages.
What value is your input resistor to ground?
Please post your schematic.

 

[Audioguru]

I forgot. You probably do not need RS because the current from a piezo transducer is very small. The input of the opamp must be biased with a resistor to ground if you have +/- supplies. This resistor is the load for the piezo and it determines the highest voltage produced when the piezo is hit hard.

 

[darren adcock]

It's a whole bunch of different cv ins on the synth modules. It might be that I just need to standardise them or have a switchable option. Here's the chematic from MFOS VCA.

 

[Audioguru]

Audioguru's method will also work, but a pair of back to back zeners will do the job without needing the additional diodes.

Won't back-to-back zener diodes act like ordinary diodes and limit the input voltage swing to only +/- 0.65V?

 

[Audioguru]

The first opamp in the VCA is a mixer with an input impedance fairly low for a piezo transducer of 100k ohms and no gain. The output of this opamp is 1/50th the input level. It feeds either a pair of Mickey Mouse log transistors (affected by temperature) or a linear opamp with a voltage gain of 50.

 

[darren adcock]

The first opamp in the VCA is a mixer with an input impedance fairly low for a piezo transducer of 100k ohms and no gain. The output of this opamp is 1/50th the input level. It feeds either a pair of Mickey Mouse log transistors (affected by temperature) or a linear opamp with a voltage gain of 50.

So i folllow the voltage path see what active components it hits alo the way and make sure it stays within their tolerence lvls? that would be both transistors u1c/d lm13700n pins 1,5 and 7.

My signal is coming in at potentially 11 times gain, then added to u1d's x50 gain seems rather too much,

 

[Audioguru]

Your signal is attenuated to 1/50th by the first opamp in the VCA. If you switch it to Linear then U1D amplifies it back up 50 times.

 

[(*steve*)]

Won't back-to-back zener diodes act like ordinary diodes and limit the input voltage swing to only +/- 0.65V?

To zener in series, either anode to anode or cathode to cathode.

 

[Audioguru]

This is the corrected input protection circuit:

 

[darren adcock]

Your signal is attenuated to 1/50th by the first opamp in the VCA. If you switch it to Linear then U1D amplifies it back up 50 times.

Thanks for explaining that, I wasn't sure what "1/50th" meant but understand now.

Can I put D1 and D2 on the output of the signal conditioner instead? rather than having to go into the pcb's

 

[Audioguru]

Can I put D1 and D2 on the output of the signal conditioner instead? rather than having to go into the pcb's

The input of the signal conditioner opamp must have D1 and D2 to prevent a high voltage from the piezo from zapping it.

 

[darren adcock]

Thanks AG.