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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Ultimate VCO Instabilities (Solved)
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Gaetano_AU



Joined: Oct 08, 2011
Posts: 52
Location: Australia

PostPosted: Wed Jul 25, 2012 8:40 pm    Post subject: Ultimate VCO Instabilities (Solved) Reply with quote  Mark this post and the followings unread

I meant to post this ages ago but forgot. There was a weird drift problem with the Ultimate VCO's, which I noticed. The VCO's seem to get affected by random noise, which I suspected was (obviously) the noise generator, but it was not. The problem is that the 3 VCO Sync inputs need to be grounded when not in use, which they are not by default in my unit as I use banana sockets; otherwise they pick up stray voltages. I tried various simple solutions, but they screwed with the pitch and sync etc. You can just use switched jacks or a switch to ground or a triple banana lead with a ground on one end and plug them all into ground when not in use, problem solved.

To check if you have the same problem just touch wires from the grounded front plate or 6.5mm sockets to all of the Sync inputs to see if they are more stable. I would be interested if anyone else has had this problem.

Last edited by Gaetano_AU on Sat Mar 01, 2014 8:15 am; edited 1 time in total
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iopop3



Joined: May 28, 2010
Posts: 95
Location: Malmö, Sweden

PostPosted: Thu Jul 26, 2012 11:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

The sync input is already pulled down via the 100k resistor going to ground (R27 on VCO-1), so in theory it should not be any problem.

This is a longshot, but maybe the pull down resistor needs to be closer to the bananajack?

Im in the process of fixing a new and dedicated PSU for my SL, will make some tests when that is sorted.
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Gaetano_AU



Joined: Oct 08, 2011
Posts: 52
Location: Australia

PostPosted: Fri Jul 27, 2012 12:08 am    Post subject: Reply with quote  Mark this post and the followings unread

I thought that and tried it already, as I have used pull down resistors for other things with no problem, I even tried different values. I also tried shielded cable on these sockets, but there were still stray voltages. Grounding them when not in use seems to be the only solid solution. You can see the drift on an oscilloscope, but ground them when not in use and they are very stable, actually surprisingly so. This drift is not a criticism of these MFOS units however, I would recommend them to anyone. I am thinking of designing bigger front plates for mine anyway, and if I do I'll just put a solution on there like three switches (I have a few spare). Still, if you do not have the same problem I would be interested to know why, even the AC centre tapped plug-pack I use for the power supply, is external to minimise any stray hum and noise.
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Gaetano_AU



Joined: Oct 08, 2011
Posts: 52
Location: Australia

PostPosted: Sat Mar 01, 2014 8:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Problem seems to have gone away (it wasn't that bad anyway), I looked and I think there was some sort of interaction between the noise source and the ground. This was of course my error, from a wire leaking and not a design flaw of the Ultimate (or Expander). These are in my opinion the best value analogue synth kits I have seen anywhere, nowhere can you make a synth this flexible and useful for this price. One of the reasons I stopped designing my own system was Ray had already invented the wheel, with these units.
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