4p and SVF voicecard zener replacements

I got a few zeners short in my Mouser order, and I sourced some locally.

Their code is different and was wondering if they would be ok to use for the voice cards?

The 3.6 says BZX 55C 3V6

The 4.7 says IN5230B

Your mission: look in the datasheets of both parts (the “official” one and your suggested replacement) and try to find the parameters by which they differ by more than 10%. Post here about your findings and we’ll discuss if this is relevant or not. This is probably not going to be relevant but it’s worth checking :slight_smile:

Ok! Here it goes:

Zener 3.6 volt, BZX 55C 3V6:

It has a lower Zener current of 105mA (vs. 250 mA)
It has a lower Maximum Reverse Leakage Current of 2uA (vs. 5uA)
It has a lower Maximum Zener Impedance of 85 Ohms (vs. 90 Ohms)
It has a higher Voltage Tolerance of 5% (vs. 2%)

As for the 4.7 volt, IN5230B:

Higher Voltage Tolerance 5% (vs. 2%)
Lower Zener Current 200mA (v,. 250mA)
Much lower Maximum Zener Impedance 19 Ohms (vs. 80 Ohms)
Higher Maximum Reverse Leakage Current of 5uA (vs. 3uA)

The 3.6v I could compare at Mouser.
The 4.7 I found a datasheet which I attach, and I compared against Mouser.

First, let’s have a look at the Zener voltages.

The 3.6V Zener is used as a soft-clipper. The Zener voltage will (roughly) define the maximum voltage the self oscillation tone will have. So a difference in Zener voltage will only create a difference in the self-oscillation tone amplitude. Before you panic, keep in mind that all those 1% resistors and 2% or 5% capacitors can conspire to create greater variations than a 2% error in the Zener voltage.

The 4.7V Zener is used as a cheap reference voltage for scaling the filter cutoff. Difference in Zener voltage between voicecards will cause slight cutoff frequency detuning between them. Not a big deal again since other parts are also influencing the frequency (2% or 5% capacitors, 1% resistors).

Zener current doesn’t matter - you will never pass that much current through it (250mA is more than all the voicecards are going to use!).

Leakage current is not really a problem here - it is negligible compared to the current you are going to pass through it (in the range of a few mA. You can check that easily for the 4.7V Zener : it gets 8V, regulates at 4.7V and receives current through a 2.2k resistor - so it gets 1.5mA (okay 10% of it will be routed towards the op-amp, we only care about order of magnitudes here). A few µA leaking here and there won’t be a problem - at this level you might start worrying more about the offsets of the op-amp doing the CV scaling.

Zener impedance has, again little influence here. This number tells you how good the Zener is at “regulating” its output in response to changes in the current flowing through it. The Zener gets its current through a 2.2k resistor, so with an internal impedance of 19 ohm, you’ll get 19/2219 = 1% of the -8V supply voltage ripples “leaking” into the output voltage ; and with an internal impedance of 80 ohm, this will be 3.5%. Before you panic, the -8V rail is already regulated with only a few mV of ripples.

Nothing to worry about here!


Thank you Pichenettes!

I never thought I would get a very practical, very informative quick course in electronics when I bought my first MI synth!

I am amazed and enormously thankful at MI and this forum.