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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Vocoder issues...
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LektroiD



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PostPosted: Fri Jul 17, 2015 4:54 pm    Post subject: Vocoder issues... Reply with quote  Mark this post and the followings unread

I emailed a similar message, but thought I'd repeat the post here, as it may be useful to others, if this gets resolved.

I only hear white noise coming from the output. With no external cables attached, I connect my scope to the inputs. Attached pictures show what I see with the scope at the inputs and outputs.


DS1Z_QuickPrint1.png
 Description:
Measured at the input
 Filesize:  58.98 KB
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DS1Z_QuickPrint1.png



DS1Z_QuickPrint2.png
 Description:
Measured at the output with 50% gain. Frequency increases with gain.
 Filesize:  55.86 KB
 Viewed:  13 Time(s)
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DS1Z_QuickPrint2.png



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LektroiD



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PostPosted: Fri Jul 17, 2015 6:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Now I feel rather silly...

I forgot to ground the PCB to the panel components. Embarassed

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LektroiD



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PostPosted: Fri Jul 17, 2015 7:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

I would like to say that there is one small issue... the lowpass band bleeds into the output. I built the individual band level controls, and the only way to get around it is to shut that band off completely. But it would be nice to have it working properly. Any ideas how to prevent this bleed through?
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Shock_Hazard



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PostPosted: Sat Jul 18, 2015 10:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

signal bleedthrough can be caused by capacitive coupling of cables, make sure you have some power supply filter caps aswell, you could even calculate them to cancel out the kind of frequencies you are working with.
Shielding is the usual solution for blocking out outside/unwanted signals.
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LektroiD



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PostPosted: Sat Jul 18, 2015 11:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

I placed the specified value caps as per the BOM, and used shielded cables on every connection, unless otherwise stated in the build docs…

It's just the lopass which has the bleed. All other bands work as expected.

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Shock_Hazard



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PostPosted: Sun Jul 19, 2015 12:27 am    Post subject: Reply with quote  Mark this post and the followings unread

hnmmm weird... I'd have to get a look inside to really tell you more, you seem to have done everything well.
By capacitive coupling I didn't mean anything to do with the capacitors- it's really that unintentional capacitors are created by close together cables, allowing the passage of AC signals between cables, but if shielded, as you have done, that problem should be prevented.
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elmegil



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PostPosted: Sun Jul 19, 2015 11:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Does the lowpass section have any traces that have long(ish) parallel runs next to the output traces? That's the most common way for coupling to happen. The rev 1 TTSH's had that problem with VCOs 2 & 3, had to cut a trace and run the signal a different way with a shielded cable.
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LektroiD



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PostPosted: Sun Aug 09, 2015 6:08 am    Post subject: Reply with quote  Mark this post and the followings unread

What's interesting is the problem is only evident when the modulation source (mic input) is set to 'mic', when switched to 'line', it works as intended (no bleed).
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elmegil



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PostPosted: Sun Aug 09, 2015 7:17 am    Post subject: Reply with quote  Mark this post and the followings unread

LektroiD wrote:
What's interesting is the problem is only evident when the modulation source (mic input) is set to 'mic', when switched to 'line', it works as intended (no bleed).


That's not surprising: a mic level signal needs more amplification, which means it's more likely to pick up low-level bleed.
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LektroiD



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PostPosted: Sun Aug 09, 2015 8:05 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
LektroiD wrote:
What's interesting is the problem is only evident when the modulation source (mic input) is set to 'mic', when switched to 'line', it works as intended (no bleed).


That's not surprising: a mic level signal needs more amplification, which means it's more likely to pick up low-level bleed.


I'd understand that, but the bleed comes from the carrier (synth). It is still present when the modulator is turned fully anticlockwise, so the signal is ground zero. When I switch the modulator to line and turn the dial to maximum, no bleed.

The bleed only comes from the hipass and lopass bands (I did the individual band mod, so it was easy to determine).

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