Crackling noise issue

[slirak]

Hi,

I sent the below to Moog's support a few days ago, but thought it would be a good idea to post it here too. Any ideas would be greatly appreciated!

(I'm sorry if this post appears multiple times, I'm having issues getting posts through to the board!)

/Jonas

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I've bought a used early model Voyager, #178.
It has two issues.

As I understand it from an old Sound On Sound article, one of the issues is known to exist in some early models. That one's a minor issue that doesn't bother me much, but I mention it, if the two are related. It's a quiet but distinct click that occurs when you twist the master volume past c:a 5.

The other issue, which is a big problem to me, is a crackling noise, audible at the line outputs and headphone out alike. I think it's always present when the VCA is open, but it's mostly audible if the sound has few overtones, like a filtered triangle wave, especially with lower notes. It's high enough to discourage the use of such sounds. I guess the crackling gets masked by the overtone content in more overtone rich sounds.

The crackling sounds almost like a vinyl crackle. So it's not evenly distributed over time or over the frequency spectrum, like the kind of background noise you would expect to some extent in any analog synth.

I've tried three other Voyagers and none exhibited the crackling.

One experienced synth tech I spoke too said that it sounded like a defect op-amp. I dunno.

My guess is that the crackling's source is after the filter, probably in the VCA or final output stage, because as far as I can tell, the crackling's not present at the effect loop insert point.

I've had it in for service, but the tech couldn't find any info about the issues in the service manual. He followed a tips that I quote below, but that didn't help. If you could give us any hint, I'd be immensely grateful!

Regards
/Jonas

"the IC designated U49 on the analogue board needs to be replaced. It is an LM837 type IC. Also, resistors R301 and R303 need to be changed from 47 Ohms to 330 Ohms"

[thealien666]

Hello Jonas and welcome to the forum.

Don't worry about your first few posts taking a long time to appear, they have to be approved by the moderator until he is convinced that you are a legitimate human being and not a spamming bot.

After that "probation" period, your messages should appear right away.

It is like that for all new members, just so you know...

[slirak]

Thanks!
Yeah, I realized this after my third attempt, when I finally stopped for a moment to read the text about the post being moderated...
Also, I should've guessed already, since I moderate a mailing list in the exact same way...

Ah, well, it just goes to show how eager I am to hang out with you folks!

[Portamental]

^ Really ? ... Well bang my head and call me Alfred

[Portamental]

Ok.. now I understand why posts that were not there suddenly appear out of the blue, inserted above mine (time stamp). Like just here ^

It happened a couple of times this week.

[Peff]

Interesting, I just installed the 3.5 upgrade on my signature voyager which has a serial number below yours, and I'm experiencing the exact same issue.

At the end of your original post it states that the U49 opamp and R301 and R303 need replacement. Is that the recommendation from tech support? I might need to break out the scope and start checking.

thanks,
Kurt

[slirak]

Interesting, I just installed the 3.5 upgrade on my signature voyager which has a serial number below yours, and I'm experiencing the exact same issue.

Are you saying you did NOT experience this issue with an earlier OS?

At the end of your original post it states that the U49 opamp and R301 and R303 need replacement. Is that the recommendation from tech support? I might need to break out the scope and start checking.

thanks,
Kurt

Nope, this was for a different issue, but both issues seemed to involve the final output stage, so it was worth a try. Didn't change a thing though.

[Ljudmakaren]

I have had the same problem with two Sub37 (no solution to the problem). It appeared briefly in the djungle of problems with an voyager performer i got to replace the sub37. And now I experienced the same thing with my newly acquired (used) Old School.
It appeared for a while but disappeared before I got to record it.

It seems that using a pedal ep-2/3 for slow filter sweeps can trigger this behaviour but it appears even without anything connected.

Im getting quite sick of moogs quality control (or lack thereof).


Other than this the old school is the most fantastic synth I've played.

[Ljudmakaren]

This being a problem on the low frequency side of the synth is a huge downaside since I'm a bassist..