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 Forum index » DIY Hardware and Software » YuSynth
modding elka organ with moog ladder filter
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rothers



Joined: Jun 24, 2013
Posts: 6
Location: Bpool

PostPosted: Wed Oct 09, 2013 11:06 am    Post subject: modding elka organ with moog ladder filter Reply with quote  Mark this post and the followings unread

Hi. Ive modded my elka organ which has a ladder filter. Cut off works great with a 10k pot and a 10k trim pot, with both internal sound (brass) and using external input.
But i can't get resonance to work , ive put a 1k trim after the last transistor in the ladder , a 50k log pot and swopped the capacitor round next to the 50k log pot. The resonance self oscillates but theres hardly any control its like its on or off, i can just barely get some squelchy sounds out but it tends to oscillate too easily! Any ideas would be grateful. ive enclosed the circuit diagram with the trim and log pot mods, the dual op amp is an old u739.


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gdavis



Joined: Feb 27, 2013
Posts: 157
Location: San Diego

PostPosted: Wed Oct 09, 2013 1:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

You want a reverse log pot, not regular log.
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rothers



Joined: Jun 24, 2013
Posts: 6
Location: Bpool

PostPosted: Wed Oct 09, 2013 1:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi thanks for reply, I've also tried a 100k linear pot with a 100k resistor across the centre and outer pin , which I thought makes a tapered pot,but has same effect slight adjustment and it starts oscillating.
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gdavis



Joined: Feb 27, 2013
Posts: 157
Location: San Diego

PostPosted: Wed Oct 09, 2013 3:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

Assuming your pot taper isn't the problem, the only other thing I can think of is that your feedback signal coming from the opamp output is too strong and needs to be attenuated. A voltage divider or maybe even a larger trim pot might work. I don't know what the levels are in your circuit to tell you what a good value might be so you'll have to experiment. Do you have any experience with LTSpice? You might want to try messing around with the circuit in simulation to get a better idea of how it behaves.

Can you put up the original schematic without mods?

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analog_backlash



Joined: Sep 04, 2012
Posts: 391
Location: Aldershot, UK
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PostPosted: Wed Oct 09, 2013 3:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi there.

rothers wrote:
Hi thanks for reply, I've also tried a 100k linear pot with a 100k resistor across the centre and outer pin , which I thought makes a tapered pot...

I was told by the man himself (Yves) that this doesn't work:

http://electro-music.com/forum/viewtopic.php?highlight=reverse+log&t=56139

I thought that I understood why, but then I doubted my own explanation Embarassed . Perhaps M Usson could explain why - I could do with a bit of tutoring on that one.

Gary
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gdavis



Joined: Feb 27, 2013
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PostPosted: Wed Oct 09, 2013 4:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

I totally forgot about that discussion, but yes, it's true.

Because the res pot is setup as a variable resistor, not a voltage divider. If you look at the resistance of the pot + parallel resistor then yes, you get something resembling a reverse log curve. Which would be fine if resonance increased with resistance, but it doesn't, it decreases. Maximum feedback (self osc) occurs when the pot resistance is 0. If you plot feedback level vs how much the pot is turned, you end up flipping the curve and getting a log taper.

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rothers



Joined: Jun 24, 2013
Posts: 6
Location: Bpool

PostPosted: Wed Oct 09, 2013 10:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

hi ive enclosed a unmodded version of the circuit. There was a 100k pot between the o/p of the op amp and the 10uF capacitor. I think you could be right that the o/p is too high so i will put 100k resistor back or increase trim pot.

Im struggling to find a 50k reverse log pot in the uk! any sources?


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rothers



Joined: Jun 24, 2013
Posts: 6
Location: Bpool

PostPosted: Wed Oct 09, 2013 11:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

ive ordered one of these off the bay

rev log c pot

http://www.ebay.co.uk/itm/120746208325#ht_613wt_927

if i can get this to work , there is another 2 moog ladder filters to go at!, there are two joined together on the monosynth on the organ , like a high pass and low pass setup.
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rothers



Joined: Jun 24, 2013
Posts: 6
Location: Bpool

PostPosted: Mon Oct 14, 2013 12:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

hi all, i got the rev log pot and resonance works alot better now more controllable, i also put a 100k variable resistor on the op amp output as the feedback was too high.

Im waiting on a 10k trim to replace the 1k trim for the resonance. As the 1k is having little effect and i may be able to remove the 100k from the op-amp o/p.


Is is normal when resonance is turned up the overall loudness drops a little?
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yusynth



Joined: Nov 24, 2005
Posts: 1229
Location: France

PostPosted: Mon Oct 14, 2013 1:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

yes this is the normal behavior of this filter
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rothers



Joined: Jun 24, 2013
Posts: 6
Location: Bpool

PostPosted: Mon Oct 14, 2013 2:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi thanks , I've put a 25k pot on opamp output and 10 k trim so its a bit better balanced now, think I will leave it as is.
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