Hissing from 907 Fixed Filter Bank

System 55 • System 35 • Model 15
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Rezin
Posts: 29
Joined: Tue Dec 10, 2013 11:54 pm

Hissing from 907 Fixed Filter Bank

Post by Rezin » Fri Apr 08, 2016 10:24 pm

Hello,

I've got (to my great excitement) all the filter bands working on the 907 fixed filter bank, but have noticed some hissing/static emanating from the module, which I'd like to avoid.

Video here: https://www.youtube.com/watch?v=j2L-iVuQXW4

I know these old filter banks use inductors (which can pick up magnetic hum), but I moved the PSU off the system entirely and the hiss is still there. I'd like to finish restoring this unit as soon as possible to move on to other things.

It's not the same unit (it's the system 55 version), but it's reasonably similar: https://youtu.be/n3K_fZDvINs?t=2m50s

Any thoughts?

Best,

~Rezin
++

[Jesus loves ya.]

Synth photos: https://www.flickr.com/people/136277463@N07/

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dingebre
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Re: Hissing from 907 Fixed Filter Bank

Post by dingebre » Fri Apr 08, 2016 11:16 pm

You won't avoid some hiss. The final gain in the output stage in the 907 and 914 (I designed a clone of the 914 with custom inductors) is huge, in the hundreds. The biggest source of noise is probably thermal noise from the attenuator pots on the inputs. It's a design "feature". (Blasphemy you say!!!!!!! Moog designs are perfect :) ). I also found the power supply needs to be really really far away, feet if possible. The inductors as well as the signal wires going to the summing resistors pick up stray noise and hum very easily. Make sure you don't have any fluorescent light fixtures on the same mains circuit either or close to the system. Is that speaker battery powered or AC? If AC, you might be getting some hum from it's power supply. It's really close. You also don't show any other equipment that is close to the system. What else is within 3 or 4 feet that is on? Those filters are very sensitive I've found.

Just some thoughts. Others will have better ideas I'm sure.

David
David

http://analoguerealities.com
http://analoguerealities.blogspot.com

Rezin
Posts: 29
Joined: Tue Dec 10, 2013 11:54 pm

Re: Hissing from 907 Fixed Filter Bank

Post by Rezin » Sat Apr 09, 2016 9:32 am

Good to know. Yes, I'm finding out that Moog designs are perhaps a bit more idiosyncratic than I had previously considered ;)

The speaker is AC powered- I'll try moving it away and see what happens. PSU is feet away from the unit, and there's really nothing else around from an electronic point of view.

The Moog is in a room with fluorescent lights, though- the power source should be isolated from that, but it would probably be reasonable to assume that there's some kind of interference (probably somewhat similar to what electric guitarists experience).

For the time being, I guess I'll just let it lie. Does anyone know if there are any shielding measures I could take in order to reduce it?

Best,

~Rezin
++

[Jesus loves ya.]

Synth photos: https://www.flickr.com/people/136277463@N07/

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dingebre
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Re: Hissing from 907 Fixed Filter Bank

Post by dingebre » Sun Apr 10, 2016 2:50 am

Rezin wrote:Good to know. Yes, I'm finding out that Moog designs are perhaps a bit more idiosyncratic than I had previously considered ;)

The speaker is AC powered- I'll try moving it away and see what happens. PSU is feet away from the unit, and there's really nothing else around from an electronic point of view.

The Moog is in a room with fluorescent lights, though- the power source should be isolated from that, but it would probably be reasonable to assume that there's some kind of interference (probably somewhat similar to what electric guitarists experience).

For the time being, I guess I'll just let it lie. Does anyone know if there are any shielding measures I could take in order to reduce it?

Best,

~Rezin
Glad to help. I'll bet some of the hiss/hum is from the speaker. I tried all sorts of things with my clone even as far as mu-metal. Hiss can't be shielded. Hum, the best I found was to put the filter as far away from the transformers as possible and keep any other source of stray EM emissions away from it, too. As I recall, Moog installed their filters on the on the other side of the cabinet away from the power supply.

David
David

http://analoguerealities.com
http://analoguerealities.blogspot.com

Stevie Ray
Posts: 254
Joined: Sun Apr 17, 2016 2:25 pm

Re: Hissing from 907 Fixed Filter Bank

Post by Stevie Ray » Wed Apr 20, 2016 8:50 pm

I think you may need to replace the capacitors. At least the electrolytic caps in the input and output stages of the 907. The electrolyte in old caps can break down over time and cause noise. The other caps in the passive circuit too. Replacing them with modern polyester caps would probably improve the situation.

If in doubt, bypass the input and output on the 907 and inject a signal directly into the passive filter array then take the output off before the 0.1uF cap on the output amp and see how that sounds.

My guess is that the noise is mainly caused by bad electrolytic caps the input and output amplifier stages.

https://www.muffwiggler.com/forum/viewt ... 378bfca9ff

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