Help with EFM 3544 VCO - FM and Sine shaper

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tardishead
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Help with EFM 3544 VCO - FM and Sine shaper

Post by tardishead » Wed May 15, 2019 1:00 am

I built 3 of these 15 years ago. Its a copy of either Moog Prodigy or Taurus can't remember which
The waveforms are great - the sync is great - but the FM is very weak. How can I get more out of the FM?

Image
Last edited by tardishead on Fri Nov 08, 2019 5:16 am, edited 1 time in total.

mtrnn
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Post by mtrnn » Wed May 15, 2019 2:50 am

I'm not 100% sure but the fm input on schematic looks like it is linear fm. What you probably want is exponential fm. That you can get easily either adding one resistor & attenuator alongside the v/Oct input, or just by using external mixer for v/Oct input to mix the modulation and your v/Oct voltages together. The fm input in the schematic is good for vibrato and other subtle modulation.

tardishead
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Post by tardishead » Wed May 15, 2019 9:00 am

gotcha
thanks for the help

tardishead
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Post by tardishead » Fri Nov 08, 2019 5:20 am

Can someone help with the sine shaper
At the moment I am getting a Spikey glitch at the top of the sine
Do the diodes in the sine wave shaper have to be perfectly matched?

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keninverse
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Post by keninverse » Fri Nov 08, 2019 9:44 am

Yes they should be matched but are you sure there's isn't a glitch in the tri?

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Post by jorg » Fri Nov 08, 2019 10:23 am

It looks like the FM input is linear, but not only for subtle or vibrato effects. It should be pretty full-range. Linear FM is the more common kind for creating tonal variations without losing tuning accuracy.

A saw core oscillator like this will always have a glitch in the triangle and sine outputs. You can tweak it a little with some lowpass filtering, but it's inherent in the design.

tardishead
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Post by tardishead » Fri Nov 08, 2019 6:20 pm

jorg wrote:It looks like the FM input is linear, but not only for subtle or vibrato effects. It should be pretty full-range. Linear FM is the more common kind for creating tonal variations without losing tuning accuracy.

A saw core oscillator like this will always have a glitch in the triangle and sine outputs. You can tweak it a little with some lowpass filtering, but it's inherent in the design.
Yes the triangle is flawed also
It sounds pretty good but I wanted to extend it down to low range to do LFO duties as well
With the glitch in the sine and triangle it has limited use as LFO
As regards the FM it is very weak - there must be a problem on the pcb or with the BOM or designators. EFM are notorious for such discrepancies. They’re not the only VCOS I have but I do like the sound.
So on the schematic there’s no reason why the FM shouldn’t work well? If that’s the case it must be a problem in the build docs.

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SphericalSound
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Post by SphericalSound » Fri Nov 08, 2019 6:37 pm

They have told you. That´s linear fm which is naturally subtle. Just use one of the 1v/oct as exponential FM and add a attunuator to dial the amount.

Modulator>attenuator (or attenuverter)>1v/oct.

That´s it

tardishead
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Post by tardishead » Sat Nov 09, 2019 3:16 am

MapacheRaper wrote:They have told you. That´s linear fm which is naturally subtle. Just use one of the 1v/oct as exponential FM and add a attunuator to dial the amount.

Modulator>attenuator (or attenuverter)>1v/oct.

That´s it

I know what Lin and EXP FM is thanks.
What I am asking is very specific - if anybody knowledgable in design can see if this Lin FM input on the schematic looks OK or is flawed. I have plenty other VCOs with Lin FM and this is very weak in comparison to others I know
I am getting 2 types of response
It looks like the FM input is linear, but NOT only for subtle or vibrato effects
and
The fm input in the schematic is good for vibrato and other subtle modulation.
mmmmm

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paperCUT
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Post by paperCUT » Sat Nov 09, 2019 3:46 am

You could always just reduce the resistor on the input. R11 is 470k which is pretty high, could switch it for 47K and get much deeper modulation with no side effects.

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