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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
CV & Gate Expander circuit voltage issue
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sixbyseven



Joined: Jan 28, 2016
Posts: 22
Location: hamilton, Ontario

PostPosted: Sat Sep 16, 2017 10:02 pm    Post subject: CV & Gate Expander circuit voltage issue Reply with quote  Mark this post and the followings unread

Hi There,

Has anyone built Ray's MFOS CV and Gate Expander circuit?

I am using an Arturia Keystep. and the voltage is fairly decent up and down 7 octaves, but the circuit seems to have an inconsistent drift

I am getting a voltage drift on the CV voltage outs.

Example (steps in octaves)

Voltage IN _> Voltage out from circuit
0.746 _> 0.746
1.746 _> 1.747
2.745 _> 2.735
3.741 _>3.607
4.740 _> 4.139
5.735_> 4.436


The drift after 5 octaves is almost 1.3 volts!

The circuit is here http://musicfromouterspace.com/analogsynth_new/CVNGTEXPANDER2015/pdf/cvngtexpand2015_schem.pdf

I am thinking it has to be either with the zener diodes (double checked the are both 1N5231B, oriented correctly and the resistors are all 10K 5%). The voltage on all 4 outputs (out 1 thru 4) is exactly the same. They all drift the exact same way. Ray mentions that there could be some offset voltage, but it should always be the same offset as the input voltage changes.

Should I try a different manufacturer for the 1N5231Bs?

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Young enough to enjoy techno, old enough to remember a Z80 and tiny-C.
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sixbyseven



Joined: Jan 28, 2016
Posts: 22
Location: hamilton, Ontario

PostPosted: Sun Sep 17, 2017 5:50 am    Post subject: Solved! Reply with quote  Mark this post and the followings unread

The 1N5231B Zener diodes have a reverse current leakage of 5 micro amps at 2 Volts. Ray mentioned that he did not use 5Volt zeners, but that he used 10 volt ones. The 1N5240B is the 10 volt version and does not starts the reverse leaking until 8 volts. This is why I got an exponential looking shift to the output voltage

I removed Z1 and Z2 from the board and retested and the voltage outputs are perfect, exact copies of the input. Anyones else building this take note:

Choose a Zenier where the reverse current leakage starts above the range you need, so if you want 7 octaves, use the 1N5240B instead of the 1N5231B.

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LukeLabs



Joined: Jan 11, 2020
Posts: 3
Location: Sydney, Australia

PostPosted: Sat Jan 11, 2020 10:51 pm    Post subject:   Reply with quote  Mark this post and the followings unread

I made two of these recently and can confirm replacing the Zeners with higher voltage versions drastically improves the performance.

I'm using 1N5240B's too and now have 10 octaves of usable CV output.
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