[sdiy] diagnosing polysix output noise (fwd from AH)

[Jimmy Moore]

On Sat, Mar 28, 2026 at 3:22 PM Richie Burnett <rburnett at richieburnett.co.uk>
wrote:

> I would have said that those strange squeaking sounds were likely beating
> between the high-frequency BBD clocks in the chorus section, but you said
> it's audible on the audio before if goes into the effects section.  The
> squeaking sound just sounds similar to the swooshing that you can
> sometimes
> hear at a very low-level in the noise-floor of a normal BBD chorus.
>

I think your intuition makes sense. My experimentation has shown the noise
seems to be added post-BBD chip (IC3 specifically).



Definitely sounds like a loose connection somewhere though from all the
> cracking.  Are Polysix synths particularly prone to battery leakage
> damage?
> I have a friend who is also currently grappling with some really quite
> extensive corrosion issues due to a leaked battery in a Polysix!
>

very much so, but lots of people have already weighed in here!

> Out of interest have you looked on the positive and negative supply rails
> in
> that area of the circuit?  You said there were glitches visible on various
> clock signals as well as the audio when you hear the crackles.  Just
> wondered if there is a bad power supply rail connection somewhere.
>

I've spot-checked the voltages at different points and things *seem* ok.
 not sure if anyone has a working specimen to provide spot measurements but
I can report back with some specific voltages if that'd be helpful.



> -Richie,
>
>
>
> -----Original Message-----
> From: Jimmy Moore via Synth-diy
> Sent: Saturday, March 28, 2026 9:00 PM
> To: Adam (synthDIY)
> Cc: synth-diy mailing list
> Subject: Re: [sdiy] diagnosing polysix output noise (fwd from AH)
>
>
> Great catch, I hadn't listened too critically back to back. Here are two
> recordings where you can clearly hear that the noisehas higher frequencies
> when the note is playing
>
> Not playing:
>
> https://drive.google.com/file/d/1eYx_YXt7WBBX25fwzZ2FOYc6I9fTDbf0/view?usp=drivesdk
>
> Droning note:
>
> https://drive.google.com/file/d/1Rp-peon3I2BJ0aEmvOlD7LLR-jI8AMf4/view?usp=drivesdk
>
> In both cases resonance was at zero and cutoff was about 25%
>
> I don't know the signal path or how it corresponds to stages of the
> schematic, maybe someone can help out here. But would this imply that the
> noise source could be between the filter and envelope generator?
>
>
>
>
>
>
>
>
> On Fri, Mar 27, 2026, 3:34 PM Adam (synthDIY) <synthdiy at adambaby.com>
> wrote:
>
>
>
>
>
> On 28 Mar 2026, at 00:50, Jimmy Moore <jamoore84 at gmail.com> wrote:
> Modifying the filter cutoff or resonance does NOT change the noise timbre.
> There may be a slight relationship between changing the resonance and that
> triggering the the noise. But only very slight.
>
>
>
> But you'll agree that in your example recording, the noise has more high
> frequency content when the note is playing, than later when the note dies
> away?
> If not affected by the filter directly, then there may be some
> relationship
> with the note envelope
>
>
>
>
> On Thu, Mar 26, 2026, 11:35 PM Adam (synthDIY) <synthdiy at adambaby.com>
> wrote:
> Interesting noise!
>
> Sounds to me like the noise is being affected by the filter - would you
> agree? If so, it's not coming from the Effect Board BBD
>
> A
>
>
>
>
>
>
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>
>
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