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TM3030 - tuning help!
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author TM3030 - tuning help!
bwithwings
Hi list

I'm hoping someone can shed some light on an issue with tuning on a TM-3030.

Basically the whole thing tunes great, except for the very bottom 7 notes which are flat - C0 to F#0. They get progressively more flat the lower you go, as well. It seems like this is an error with one or more of the 0.1% resistors. I can't get my head around how the CV ladder circuit works, though... too many nodes pulling various voltages here and there!!

Can anyone suggest which resistor might affect the very bottom 7 notes? I don't want to go desoldering these 0.1% guys if I don't have to. wink

Thanks!
--
Darren
New Zealand
Synthbuilder
Could be the DAC resistors but...

What are using for U7? Not using a TL072 are you? That would cause exactly the problem you are describing. If so use a AN6562 or LM358.

Tony
bwithwings
Hi Tony

Thanks for your suggestion, I swapped the LT1358 that was in there for a LM358 and now my tuning woes are gone!

Can you please explain what happened? I'm intrigued...

Cheers
Darren
Synthbuilder
bwithwings wrote:
Can you please explain what happened? I'm intrigued...


The DAC's output voltage goes very close to zero volts. The op-amp that buffers this voltage needs to copy this exactly. But most op-amps cannot swing close to their negative supply voltage and thus their output would be higher than it should be. This is what you you were hearing. The DAC was correct, but the op-amp in the slide circuit could not replicate it.

So you need to use ones which can. Both the AN6562 and LM358 allow for the outputs to go very close to their negative supply voltage.
bwithwings
Ah, yep that explains it!!

Thanks for your advice, yet another happy TM-3030 owner =)
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