midiDAC with MFOS ADSR

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tpoolan
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midiDAC with MFOS ADSR

Post by tpoolan » Fri Oct 16, 2015 4:32 pm

Just built the PSU (which is a work of art) and midiDAC and added them to my very experimental modular (i.e. built on a slab of plywood).

I appear to be having this exact problem:

http://electro-music.com/forum/topic-39 ... highlight=

Anyone else come across this?

Getting about 5V gate from the midiDAC when it's not connected to the ADSR but this more than halves when I connect it up...

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Synthbuilder
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Post by Synthbuilder » Mon Oct 19, 2015 5:13 am

The gate output of the midiDAC has an output resistance of 11K when in the high state and approximately 1K when in the low state. Clearly what is happening is that the MFOS ADSR is loading the gate output too heavily. The input resistance of the ADSR is therefore too low for the midiDAC.

Two things you could do...

You increase the input resistance of the ADSR. There's a 10K on the gate input of the MFOS ADSR circuit - it's R11. You could change this to 100K.

Secondly, decrease the value of R34 on the midiDAC to 4K7.

Tony

tpoolan
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Post by tpoolan » Mon Oct 19, 2015 3:56 pm

Hi Tony,

Many thanks for your response - I'll certainly try what you suggest.

Makes absolute sense when I look at the schematics - which is encouraging for a noob!

Cheers,

Tim

tpoolan
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Post by tpoolan » Wed Oct 21, 2015 5:26 pm

Thanks Tony, the 100K in the ADSR fixed it.

The midiDAC is working perfectly.

Cheers,

Tim

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Sciamano
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Post by Sciamano » Tue Jul 04, 2017 7:16 am

I'm going to build one of these, and luckly I found this thread before starting.
I've understood that the MFOS ADSR has some problems with gate/trigger signals coming from standard eurorack gear.
I'm actually building a MFOS modular system, but I'd like to play it even with my Doepfer DarkLink and, in a future, connect it to other external eurorack modules.
It seems that, even if Ray wrote that it's fine to trigger the ADSR with gate signal greater than 2V due to the input voltage comparator, some of you reported issues using the module with 5V gates...
This seems to be related to the very low imput impedence of the module.

How can I calculate this input impedance ? Is it only dependant to R5-R4 and R10-R11 ?

And if I replace R11 and R5 with a 100K resistor, do I loose compatibility with other MFOS modules ? Has this modification some impact to the connectivity with other MFOS modules ?

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Synthbuilder
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Post by Synthbuilder » Wed Jul 05, 2017 3:08 am

Sciamano wrote:How can I calculate this input impedance ?
It's the standard parallel resistors equation. For example, the gate input is

1/Rin = 1/R10 + 1/R11

Rin = 6K7

Which is way too low. So either remove R5 and R11 completely or replace them with something high like 100K.
And if I replace R11 and R5 with a 100K resistor, do I loose compatibility with other MFOS modules ? Has this modification some impact to the connectivity with other MFOS modules ?
I would not have thought so. Increasing the input resistance (up to a point) can normally help with interfacing.

Tony

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Sciamano
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Post by Sciamano » Wed Jul 05, 2017 3:45 am

Synthbuilder
Thank you!
Very accurate response! I'll go for the 100K substitution

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