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Cable w/resistor to Interface 8v Eurorack to 5v MU?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Cable w/resistor to Interface 8v Eurorack to 5v MU?
renny3c
I've only built guitar pedals, amps
basically just a solder components kind of guy... (noob)

What I need is a way to connect my Eurorack Envelopes /Lfos into
5U / MU (Mos-Lab 8a) without dialing in necessary attenuation using a separate attenuator...

Using an external attenuator, I would need to "dial-in" a max of 5v, which would give the full range sweep

from what i understand all a passive attenuator really is is a sweepable resistor...

My idea is to build a cable (1/4 to 1/8) with a resistor inside the large connector in order to attenuate the full range of (0-8v) into (0-5v)

this way, it would be easy to interface these two modulars

is this at all possible? what resistor value would i need?
thanks, hope not to get completely flamed for my dumb idea!
thermionicjunky
If you can fit in two resistors, you could run the signal through a 120k followed by a 200k to ground (or 60k and 100k, or anything with the same ratio).
renny3c
so basically something like this? (ignore the '.' if not the drawing came out wrong)

Input --> --[res1]---> Output
...|.......................|
...|.....................[res2]
...|........................|
...v........................v
-------------------------->

with:
res1=120k or 60k
res2=200k 100K

the ratio used is of .6
so basically this would give me a max output voltage of 4.8V? (4.8/8=.6)

which is more than close enough, but just to make sure i understand the science of it

(8/5 = .625, .625 would be the perfect theoretical value?
thanks!
thermionicjunky
renny3c wrote:


(8/5 = .625, .625 would be the perfect theoretical value?
thanks!


R2/(R1 + R2) must equal 0.625.
renny3c
cheers, and thanks again!
daverj
The input resistance of whatever module you plug this into must be known and used in the calculation. The input resistance of the module it is going to acts as a resistor in parallel to R2. So R1 and R2 must be calculated with that in mind. Other wise the actual voltage will be smaller than you expect.

Also, I would probably go with lower value resistors so that module input resistance has less effect on the result.

One advantage of a variable attenuator is it is adjustable, so you can turn it up as needed to compensate for input resistance.

For example:

feed 8 volts into a 60K with a 100K to ground, as described above, and you theoretically have 5 volts. But patch that into a module with a 100K input resistance and now you have 3.6 volts. If the module has a 50K input resistance, you get 2.8 volts.

If you go with much lower resistor values it puts more load on the module driving the signal, and a second issue comes into play which I'll get to in a second. Lets say instead of R1 being 60K we make it 10K. To turn 8 volts to 5 volts R2 would then need to be 16.666K. But we need to figure the input resistance into that. So if the most common modules on the 5U have 100K input resistances, then you need to factor that into the 16.666K value.

R2 = 1 / ( (1 / R_ideal) - (1 / R_input) )

So if R2 is 20K and the input module is 100K, those two in parallel equals 16.666K

So R1 = 10K and R2 = 20K going to a module with 100K input will turn 8 volts into 5 volts. If the module it's going to has a 50K input resistance, then the result is 4.7 volts. A lot less difference than the voltages shown earlier with large value resistors.

Now we come to the "second issue" I mentioned above. As we go with lower resistors, any output resistor on the module supplying the voltage comes into play. It's common for Euro modules to have either a 1K, or 300 ohm, or (a few) have no output resistor. If there is an output resistor, it's value needs to be subtracted from R1. So if the module has a 1K output resistor, then R1 really should be 9K instead of 10K. Or R2 needs to be recalculated based on R1 + R_output instead of just R1.

Another consideration is that you can't feed this cable into a passive mult on the 5U system and then patch to several modules. If you do then the input resistance of each module combines to reduce the voltage further.

It may save space putting a couple resistors in a cable, but has the potential to make things a bit unpredictable.
renny3c
Wow, that does complicate things a bit...
especially because I can't find (in/out) specs for these modules...

the actual modules i am trying to interface seamlessly are:
intellijel Dual ADSR using an Intellijel Triatt as an attenuverter into a Mos-Lab 904a Filter

the Triatt states that it's: "active/buffered attenuator/attenuverter/inverter"

does this mean that the output resistance shouldn't be taken into consideration?

My main concern is not to feed voltages that the Mos-Lab Filter might not "like" or that i might damage it in some way

Links to the Modules in Question:
http://www.intellijel.com/eurorack-modules/dual-adsr/
though a
http://www.intellijel.com/eurorack-modules/triatt/
into a
http://www.mos-lab.com/#!904-a/c1zq

cheers and thank you!
J3RK
I'm curious, are you running into any issues just running the higher (8V) signals into the modules expecting 5V? I could see this possibly being an issue with an envelope maybe, but a typical LFO should be bipolar -5 to +5 in either format. (though I'd need to look up the MU specs to be sure)

Purely from an "is it safe to plug one into the other" it would strange to me for any of the MU module to suffer from an 8V signal. (especially for something like a gate or trigger pulse) If you're getting a clipped envelope or something like that, I could see wanting to do this, but in that case, maybe just a simple attenuator module would be useful.

Just a thought anyway.

Edit: Based on the .com electrical specs, I don't see any reason why you'd need to attenuate anything other than maybe an envelope CV, and even then it might work fine. (you'd need to try it to see if the response sounds off or not)
renny3c
I haven't tried...yet
I ordered the Mos-Lab, but won't get it for a couple of months...
this was intended more as a "getting ready for when it arrives"

I know an attenuator is the way to go, i already have some to control the voltage into the 904a Filter (it has no pots to control any of the inputs, just jacks)

but i wanted to have the module have the expected signal and have the knobs work their full range and not "loose" some of the range just getting rid of the extra volts i dont need

my guess is the envelopes would just clip and would have to dial-it-in to get the full range without clipping... but this is exactly what im trying to avoid
daverj
renny3c wrote:
Wow, that does complicate things a bit...
especially because I can't find (in/out) specs for these modules...


If you have an ohm meter you can measure resistance between the signal input and ground in the input jack of the module in question. If it is a DC coupled input (most CV inputs) this should give you the number needed. It doesn't work for AC coupled inputs, but those are mostly audio inputs not CV.

renny3c wrote:
the actual modules i am trying to interface seamlessly are:
intellijel Dual ADSR using an Intellijel Triatt as an attenuverter into a Mos-Lab 904a Filter

the Triatt states that it's: "active/buffered attenuator/attenuverter/inverter"

does this mean that the output resistance shouldn't be taken into consideration?


No. Buffered outputs often have that output resister I mentioned.



renny3c wrote:
My main concern is not to feed voltages that the Mos-Lab Filter might not "like" or that i might damage it in some way


It is very unlikely to do any damage at all. 0-5 and +/-5v inputs on 5U equipment should not be damaged by 0-8v signal.

Your other concern about clipping and having the full range of the pots is valid. You mentioned the Triatt. If the signal is going through that before going to the 5U, then that is an attenuator and allows you to adjust the levels as needed to match what the module it is going to is expecting.
renny3c
i'm really astonished by doing searches that no one has tried to "seamlessly" interface these two very popular modular synth formats

my guess is most people just connect and adjust as needed but don't mind the trouble of doing things 'right' to interface 'properly'

i do have a multimeter which allows me to measure ohms...
i could try that, or forget the "MiracleCable" and just
build an external Passive attenuator box with pots hidden inside that I 'calibrate' to make sure the envelopes i'm using output a max of 5 volts, and then send that to my attenuverter to control the 904a filter...
that way i just patch thru that "magic box"

i would need to check the voltage at the filter, to make sure it is receiving 5v?

Envelope---->MagicBox----->Attenuverter----->Filter
thermionicjunky
Since you're using the attenuverter anyway, cut out the extra attenuation stage. 8v won't hurt anything and you'll always want control of magnitude and sign.
renny3c
that's probably what i'm going to end up doing,
I just wanted to have the full action of the knob and not loose part of the turn of the knob in removing the extra voltage
thermionicjunky
renny3c wrote:
that's probably what i'm going to end up doing,
I just wanted to have the full action of the knob and not loose part of the turn of the knob in removing the extra voltage


If you ever have extra attenuators, you could patch them in series for finer control.
daverj
If you do build a box to put between, don't hide the pots. Just as there's no simple combination of fixed resistors that is perfect for all situations, there is no adjustment of an attenuator that is perfect for all situations. Attenuators are passive, so have the same problems with the levels being different depending on the output resistor on the module going into it, and the input resistor on the module it is going to. So keep them available to adjust in case you want to patch things differently in the future.

Or just go the more obvious route and build a simple DIY attenuator panel in the 5U system and patch through that when you go from the Euro to the 5U. Then you can adjust those as needed to scale incoming signals, as well as use them for patchable attenuation within the 5U.

Plus this is modular. Don't rule out what might happen if you do feed an oversized signal into a CV on the 5U. You might get a result you like. Perfect signals aren't the only ones that sound good.
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