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CP251 - noise on voltage signal

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CP251 - noise on voltage signal

Postby andybutler » Wed Jan 03, 2018 3:43 pm

Tried sending this to the Moog tech support email.
Acknowledged receipt...waiting..

anyway here's the question

A bit of background before I get to the problem.

I've been using the CP-251 along with other Moogerfoogers for a while now, and I'm familiar with how it works.

While using the CP-251 to control other Moogerfoogers I have no problem (although have not done in depth tests for stray sounds).

As I understand it, the Moogerfooger system is intended to be modular, and as such I would expect the the line between control voltage and audio signal would not be clearly defined.

I've made a number of experiments with the CP-251 in this regard. Using the lag processor on audio works well, a nice overdrive for bass signals.

Now here's the problem.
I now have in my possession a passive ring modulator made from schottky diodes and transformers. It functions exactly as hoped, with good rejection of carrier and unmodulated dry signals.
It's possible, obviously, to use the output from the CP-251 as the carrier, for tremolo type effects.
I'm also hoping to use it as a DC offset on the ring to allow voltage controlled distortion.

What becomes apparent, however, is that the CV output from the CP-251 is contaminated by a high pitched signal, regardless of the power supply used.
( in the kHz range, I can measure precisely if required)

So questions

1) is that a feature of all CP-251?
2) is it caused by a charge pump running at audio frequency?
3) and most important of all, how can I fix it?
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Re: CP251 - noise on voltage signal

Postby umbrella » Mon Jan 08, 2018 3:18 am

I think my unit has not such a problem. Maybe yours is faulty?
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Re: CP251 - noise on voltage signal

Postby andybutler » Fri Jan 12, 2018 3:46 pm

Yeah, would be nice to know.
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Re: CP251 - noise on voltage signal

Postby kxx303 » Sat Jan 13, 2018 6:27 am

My 251 produces some high-pitched noise as well when power is daisy-chained with three or more MF-ers. I think it will remain silent when it will be powered up by an isolated supply. I hope it will. Need to try.
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Re: CP251 - noise on voltage signal

Postby stiiiiiiive » Sat Jan 13, 2018 7:31 am

Ah, mine as well. The MF102 and MF101 are culprit too. MF103 and MF107 not that much.
I ended installing a power strip and the MF's PSUs. Silent is the room now says Yoda.
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Re: CP251 - noise on voltage signal

Postby andybutler » Fri Jan 19, 2018 6:39 am

Do we need to send food parcels to Moog Tech Support?
(I mailed them *before* posting here)
I'm getting the problem with isolated power supply.
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Re: CP251 - noise on voltage signal

Postby andybutler » Sat Jan 20, 2018 5:41 pm

OK, how to really hear this.

Plug expression pedal into attenuator input.
Turn attenuator pot full clockwise.
Plug Attenuator out to audio input. (via capacitor if you're worried about DC on your input)

As you turn down the attenuator level the sound cuts fairly sharply, but never goes fully away.
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Re: CP251 - noise on voltage signal

Postby umbrella » Sat Jan 20, 2018 7:11 pm

andybutler wrote:OK, how to really hear this.

Plug expression pedal into attenuator input.
Turn attenuator pot full clockwise.
Plug Attenuator out to audio input. (via capacitor if you're worried about DC on your input)

As you turn down the attenuator level the sound cuts fairly sharply, but never goes fully away.


I think your unit is faulty*, there is no sound coming from attenuator output connected to expression pedal (whatever the knob position) - it gives DC only.

*or there is some other problem in your setup/connection.
Minitaur / MF Delay / MF Trem / CP-251 / Mac Mini / OS X 10.11.6
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Re: CP251 - noise on voltage signal

Postby andybutler » Fri Jan 26, 2018 5:55 pm

umbrella wrote:I think your unit is faulty*, there is no sound coming from attenuator output connected to expression pedal (whatever the knob position) - it gives DC only.

*or there is some other problem in your setup/connection.


Ok thanks, I guess you actually tried this with your CP251?

To my way of thinking it was always faulty ;-)

Presence of Moog Tech Support *could* be a brilliant addition to this thread.
It's NAMM time I guess.
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Re: CP251 - noise on voltage signal

Postby umbrella » Mon Feb 12, 2018 3:27 pm

Hi again,
I have stumbled upon some old thread about high pitch whining sound coming from CP-251 when used as audio mixer and it made me think about your problem. I guess it's the same issue.

https://forum.moogmusic.com/viewtopic.php?f=1&t=5441
https://forum.moogmusic.com/viewtopic.php?f=3&t=4714

I have checked my unit and in case I amplify empty signal (nothing plugged in as input) all the way up I can hear the whining as well. But there is no way I could hear it with usual level signal coming into input.
Minitaur / MF Delay / MF Trem / CP-251 / Mac Mini / OS X 10.11.6
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Re: CP251 - noise on voltage signal

Postby andybutler » Wed Mar 14, 2018 6:01 am

Thanks for the response.
I took the thing apart the other day and tested with an audio probe.

Indeed there's a chip to generate the negative rail voltage.
It has some 100microfarad capacitors around it, to soak up the audio frequency whine the chip makes.
One of those capacitors is surrounded by a halo of EM whine.
The audio probe picks it up just by being close.

anyway, waiting for replacement caps to arrive, apparently it's a common issue with similar powering arrangements.

I'll report back if fix works.

Later: yes seems better now.


umbrella wrote:Hi again,
I have stumbled upon some old thread about high pitch whining sound coming from CP-251 when used as audio mixer and it made me think about your problem. I guess it's the same issue.

https://forum.moogmusic.com/viewtopic.php?f=1&t=5441
https://forum.moogmusic.com/viewtopic.php?f=3&t=4714

I have checked my unit and in case I amplify empty signal (nothing plugged in as input) all the way up I can hear the whining as well. But there is no way I could hear it with usual level signal coming into input.
Last edited by andybutler on Mon Aug 13, 2018 4:21 pm, edited 1 time in total.
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Re: CP251 - noise on voltage signal

Postby Liquid Guy » Sun Apr 01, 2018 5:46 am

I'm having the same exact issue on mine which I just bought used on Reverb.com.
I'll be following to see if the fix works.
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Re: CP251 - noise on voltage signal

Postby Alien8 » Wed Feb 27, 2019 1:56 pm

I recently stumbled across this issue as well, and found the source of the noise was the MF-101 Low Pass filter ENV out.

During some audio probing of the circuit, I also found a capacitor that had a "Halo" of sound around it. Looking further at the circuit, a charge pump is used to create the negative power, and the cap that has the sound is the charging cap. In this application, the frequency of charging for the 7662 chip used is in the 1-2 kHz range.

I sourced an LT1054 charge pump, which is a direct replacement, except that it's max voltage is 15V not 20V of the 7662. The charging frequency of the LT1054 is much higher, greater than 20k, meaning it is not audible.

Replacing the chip has eliminated the noise entirely, and actually improved the noise floor of the LPF slightly.

I have not tested yet with other gear, or checked to see if the noise can also be sourced from their CV signals, however when I do find another I will be replacing the 7662 with the LT1054 when ever possible.
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