noise of low level sensors

[FreeTheDrums]

Hi Drummers,
a little tip for all users of low level sensors (i.e. finger drums, small piezos,…) have problems with noise.
The problem has two reasons:
The piezos have very high impedance. The result of this is:
- that the mux input has become voltage offsets, especially number X7.
- the input circuit is very sensible for noise (external and internal from board)

This is a very bad combination.
The offset increase the threshold (Sensor sensitivity goes low)
The noise on the offset (now very high) leads to auto midi events.
I found a very simple solution without board modifications.
I needs only a parallel resistor to the piezo.
This has 2 effects:
- The offset voltage goes down
- The impedance of the input circuit goes down.

The value of the resistor must be in a balance with the sensitivity of the sensor.
A good value is 100k
An additional capacity (parallel to resistor and piezo) reduces the RF interference from board.
A good value is 3.9nF.

So, now you have very sensible and SNR robust sensors.

Regards

[ignotus]

Sounds interesting! I have the occasional random one-off triggering of some inputs (quite seldom but still there) and had it down to electrical noise coming from somewhere. I take it that this approach reduces sensitivity to noise while increasing sensitivity to hits?

Would it be possible for you to post a simple diagram showing how this is wired?

Cheers!

[FreeTheDrums]

Very simple: Resistor and Capacity parallel to piezo.

|- Piezo -|
|- 100K -|
|- 3.9nF-|

|- MD -|

regards

[ignotus]

Sorry, but does that mean the resistor and capacitor are wired between the positive and negative leads of the piezo? I already have 10k trimpots wired in to tame the signal, so I suppose wiring in a capacitor won't harm to see if it helps.

[FreeTheDrums]

Yes.