dual conf / off-board connections

dual conf / off-board connections

Postby dnode » Sat Nov 26, 2011 5:47 am

Firstly, what a great project this is, great sound, great possibilities! My respect and compliments, even more for the legal conditions and design principles ... May The Source Be With You! ('Us' actually! #8-)

Along the lines of the former thread on modular system integration ... and together with the immediate thought of combining two units: one custom box (or panel) with larger/more precise knobs, normalized out1->in2 for serial processing, separate L/R jacks, (more) space for modifications (bypass, MIDI, extra knob, accessible programming connectors (USB?))

My questions ...
Are there any problems/challenges to be expected when using off-board connections (possibly using shielded cabling for all stuff analog)? ... I AM talking 'as short as possible' cabling, of course.
Are there any problems/challenges to be expected when trying to 'open' the feedback path, so one effect can be used in the other's feedback path?

Also, being a electronic-engineering noob kind of guy, I would be most thankful for an example circuit for handling a modular system's requirements (using audio only!) ... even if it's only for 'padding' the inputs. Or is this provided for in the gain stage, and did I not read correctly?!

kind regards, dnode
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Re: dual conf / off-board connections

Postby guest » Sat Nov 26, 2011 8:38 am

thanks for the enthusiasm
we will be putting up a tutorial for modular usage very soon
hopefully next week if time permits

it works fine to run the jacks and pots off-board
and there are even jumpers on the board to help out with this

the main thing that needs to happen for modular usage
is level shifters for the amplifiers
the input is easy as a few resistors can knock down the signal
but the output needs an opamp to boost it back up to 10v

what are the signal levels on your rig
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Re: dual conf / off-board connections

Postby dnode » Wed Nov 30, 2011 6:55 am

Well, quite 'normal' I think, it could max out to +/- 10V ...

I'm not sure about 'total' integration, if(!) I want to use it in my modular I probably end up using it 'last in chain', so only input padding would be necessary. It's not that I'd not like to see such an example circuit, it would be most appreciated!

Any other comments on my ideas on combining two units?
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Re: dual conf / off-board connections

Postby guest » Wed Nov 30, 2011 3:06 pm

do you want to be able to overdrive the microdec?

the input gain stage is variable from x1 to x50
so you can knock the signal down a fair bit
and then amplify it back up
so a 100k and 2.2k in series should do the trick

im running a bit behind on the documenatation
hopefully this weekend
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Re: dual conf / off-board connections

Postby schmidtc » Fri Dec 02, 2011 2:01 pm

Opening the feedback loop and sticking another Microdec in is a cool idea.

I also envisioned the Microdec as the last element in the audio chain of my MOTM format modular. But I don't have a good pre-amp module, so oddly enough the Microdec often comes first when I'm plugging patch cables.

Separating the L/R jacks has definitely been useful for me. The ring of a TRS jack can be wired through the switched tip of the other channel. Plugging in the other channel breaks the switch so you can get stereo I/O with either 1 balanced cable or with two mono cables for a bit more flexibility in the interface.
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Re: dual conf / off-board connections

Postby dnode » Tue Dec 13, 2011 3:27 am

Thinking about the implementation of the 'feedback loop mod' ... and getting a bit confused with all the GAIN, FEEDBACK, MIX and LEVEL controls. I'm looking at the schematic.

What was the idea again?
Using two units, 'opening' microdec1's feedback loop and inserting microdec2

I think there are two options?!

1) most flexible
Cut trace on CW side of FB on microdec1, connect to normalling jacks (normally reconnecting the made cut), jacks could be called FB1 SEND and FB1 RETURN. Use mod by externally connecting FB1 SEND to IN2 and OUT2 to FB1 RETURN. MIX2 controls 'depth' of mod.

2) more elegant?!
Cut trace on wiper side of FB on microdec1, use switches (what kind?) to connect wiper FB1 to IN2-P$4/5 and OUT2-P$4/5 to 'after the cut on microdec1'. FB1 controls 'depth' of mod.

Am I thinking correctly? Does it matter what option, although meaning of knobs could be different? Or should I use another RETURN connection point?
... and, any ideas on what kind of switches would be needed for option 2)

Some 'modular' gut feeling tells me that option 1) implemented on both units would give me the most flexibility and usability, although I am not sure on where the cut should be made and what the implications are. It would be nice to still be able to use the units serially (and this mod) ... or am I talking nonsense now?!

Enlightenment is much appreciated!

On the input attenuation implementation: use a switch to insert 102k2R before (or after) R3 and R4 would do the trick. Do I understand correctly?
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Re: dual conf / off-board connections

Postby guest » Tue Dec 13, 2011 12:01 pm

looking at the schematics again
i dont think there is a good way
to send your signal directly into the feedback loop
at least not without affecting the sound quality
or doing a fair bit of rework

the issue is that the amplifiers in that section
perfrom the 3-pole anti-aliasing filters
and they are very sensitive to component variations
and sticking another signal in
requires attaching more resistors
which changes the effective input impedance of the circuit
and changes the gain and cutoff frequency

you can still send signals into the microdec
but they will have to go through the normal input jack
and im not sure if there would be much difference
between the two setups


for the input attenuation
you will have to use two resistors
you can cut the traces under j8 and j9
which will open up the input signal pathways
then put a 100k resistor in series with the pathway
and an 18k resistor from the output side of that resistor to ground
this will make a resistor divider at the input


we put together a control input jack for mod1
that ran through the mod1 pot
so you could attenuate the signal
ill put schematics up soon for that
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Re: dual conf / off-board connections

Postby dnode » Wed Dec 14, 2011 2:09 am

OK, that clears up a lot! Thanks ... so, no FB-loop mod for me. I understand the bit about the divider (although I'll have to draw up something to make this switchable), thanks!

I don't understand the last remark on mod1, or are you referring to a possible CV input? ... I'll just wait for the schematic.
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Re: dual conf / off-board connections

Postby guest » Wed Dec 14, 2011 7:00 pm

so i finally got the info up
http://wiki.openmusiclabs.com/wiki/MicrodecModular
let me know if anything is confusing
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Re: dual conf / off-board connections

Postby dnode » Thu Dec 15, 2011 5:42 pm

Looks understandable to me! Thanks!
My system is powered by +/-15V, so for the cv input mod I need to change the upper resistor to 30k? Or, both to 30k?

I'm eager to see the use of an opamp circuit ... (I should probably be looking that up in a book!)
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