Moog Transistor Ladder Filter Resonance not Working

smokestacksproductions · Joined: Aug 24, 2012 Posts: 18 Location: USA

Hello,

I could not find an Antilog pot at my supplier so I used a linear pot instead. The resonance pot has no effect at all, the filter will not self-oscillate, etc. - I have tried adjusting the trimmer, but there is no difference. Any tips?

kkissinger · Posted: Tue Dec 18, 2012 9:50 am Post subject:

The use of a linear pot will not prevent the resonance control from having an effect.

Check your solder joints and polarity on C11, and where your pot connects to the pcb. Check your solder joints on T3 and R18. Also, check soldering on R15 and R17.

If you etched your own PCB, make sure that there are no breaks in the traces at or near these components.

Hope this helps -- this filter is really nice and you will enjoy it once it is up and running.

analog_backlash · Posted: Wed Dec 19, 2012 3:32 am Post subject: Pseudo Antilog Pot

Hi,

As Kevin has already said, the pot choice is not going to cause the problems that you have encountered. However, if (sorry, when) you do get it working, you could try the following alteration to give a pseudo-antilog 50K pot. You just need to replace the Emphasis pot with a 100K lin pot and connect a fixed 100K resistor in parallel across it, as shown. I've also shown the graph of resistance vs. rotation (theoretical) showing the shape of the resistance curve.

I only discovered this after looking at a circuit that Uncle Krunkus had posted, so this web site is very educational!

Gary

WRONG! See below

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smokestacksproductions · Joined: Aug 24, 2012 Posts: 18 Location: USA

ok,

I got the filter working properly, it was bad wiring....

anyways, I tried a 100k linear pot with a 100k resistor across the two lugs that are wired to the board, but It seems like there is little effect until the last 1/4 of roatation (kind of like a regular LOG pot)

I'm thinking I might be better off with the 50k linear. Or does anyone else have a source for the 50k antilog pots?

analog_backlash · Posted: Fri Dec 21, 2012 4:07 pm Post subject:

Strange that, I thought that it should work better than the 50K linear. I've seen other descriptions where they wire the 100K fixed resistor across the 2 outermost lugs of the 100K linear pot, which might give a better result (I haven't done the maths yet).

Anyway, Small Bear have 50K reverse audio pots:

http://www.smallbearelec.com/servlet/Detail?no=116

See also these two articles which cover changing the taper of a pot:

http://sound.westhost.com/pots.htm

http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

Gary

ADDITIONAL: Just checked that out on my spreadsheet and you do get a different shaped curve (see attached). It's steeper at first and it flattens out more towards the end. It might be worth trying.

WRONG! See below

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yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

analog_backlash : this is a very common mistake. You obtain a log curve not a rev-log curve as required. One definitely need a reverse log pot C curve, the taping trick won't work !

analog_backlash · Posted: Sat Dec 22, 2012 3:27 pm Post subject:

Thanks for that Yves Embarassed

Just goes to show that you can't believe everything you read on the internet! That would explain the problem. Those antilog pots do seem difficult to obtain now. I noticed that you said you get them from Das Musikding, but I looked there and they don't seem to have any at present. Are there any other sources?

Gary

Cardinen · Joined: Sep 15, 2009 Posts: 39 Location: Italy

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

Cardinen said it Wink

analog_backlash · Posted: Sun Dec 23, 2012 3:00 am Post subject:

Thanks for that Cardinen/Yves. I clicked a box labelled "rev log" on Das Musikding and it replied "Product not found!", that's why I thought that they didn't have any.

Gary

analog_backlash · Posted: Mon Dec 24, 2012 9:19 am Post subject: I WAS WRONG!!!

Hi Yves (et al.).

I was initially a bit baffled by this antilog pot thing (probably because I believed certain posts on the internet which were also wrong), so I thought about it and realised my error! I was not comparing like with like. My plots are basically resistance vs. percentage turn of the pot, whereas I should have been thinking about percentage output vs. percentage turn. When I tried this out (for my first circuit idea), the graph I got looked more like a (pseudo) log plot (see attached - blue = linear pot, red = linear pot with parallel resistor).

I've admitted my error and edited my posts above, so that other people don't read them and fall into the same trap!

Apologies, Gary

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LFLab · Posted: Fri Nov 27, 2015 2:50 am Post subject:

This is an ancient thread, but regarding "log-faking" of potmeters, see RG Keen's "Secret Life of Pots".

You can do a log or reverse log taper on a linear potmeter, and you can choose the curvature as well, based on the ratio of the resistance of the added resistor vs the potmeter.

Secret Life of Pots