Quad VCA response - Lin or Log?

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Isaiah
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Quad VCA response - Lin or Log?

Post by Isaiah » Mon Sep 05, 2011 1:06 am

Hi,

I'm thinking about getting a Quad VCA PCB and maybe wiring the outputs so they're normalled to the inputs of a mixer. Seems like a great multiple VCA / VC-Mixer solution for Eurorack.
Is the response of each VCA Linear or Logarithmic?

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Post by mmm » Mon Sep 05, 2011 12:02 pm

A couple of guys on here have done that. Or, something similar where they grab a quad VCA and a quad VC Mixer and put them behind the same panel. Usually normalling each of the VCA channel outputs to the corresponding VC mixer channel inputs.

That way, they have both hard wired together but can be seperate through patching.

Since the VCA channels are (typically but, not necessarily) expecting a RC generated exponential output from an EG, the input response to the VCA CV inputs are linear.
Didn't want to exponentiate too much. :guinness:

Maybe the guys that did this can post their results.

Thanks for asking Isaiah!

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Post by Isaiah » Mon Sep 05, 2011 3:11 pm

Thanks for your reply, George!

Linear response will be perfect, as the planned module would be used for processing CV (Linear), and Audio with Logarithmic/Exponential(?) RC-generated curves like you said.

It'd likely pair it with one of your 4-channel Mixer PCBs, though I would like to have a 5th input (No VCA, hardwired to the Mixer summing node).
Would the PCB accommodate this, or would it end up getting messy?

Also, would it be possible to add an attenuator to each VCA's CV input?


I've seen a few great examples in the MMM DIY thread, thanks! :bananaguitar:

By the way, how much would shipping to the UK be likely to cost?

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Post by mmm » Mon Sep 05, 2011 8:44 pm

Isaiah wrote:Thanks for your reply, George!
You're welcome.
It'd likely pair it with one of your 4-channel Mixer PCBs, though I would like to have a 5th input (No VCA, hardwired to the Mixer summing node).
Would the PCB accommodate this, or would it end up getting messy?
That would get messy. The summing is done post VCA prior to the final buffer and inverter. Also would be hard to tap.
Also, would it be possible to add an attenuator to each VCA's CV input?
You could if you wanted to. But the existing attenuators limit the signal input so, the output is reduced at full CV. It keeps the response smooth for the full sweep while allowing the signal to be processed to only reach specific values.

I like running a 5V signal into one channel and use a pitch bend or foot pedal sending a 0-5V cv to the CV input. I set the 5V signal to only change a VCO pitch by a minor 3rd when the CV is full on and using the VCM attenuator.

The next channel CV is cascaded from the previous so, on the next channel, I patch in a triangle LFO and set it to only do a slight vibrato to the VCO at full cv.

When I push the pitch wheel or foot pedal that I'm sourcing the CV from, The pitch of the VCO bends up to the minor 3rd while the vibrato portion doesn't start to show up until the pitch gets close to the full bend.

Anyway, boring as it may be, that's how I get my jollies... :help:
By the way, how much would shipping to the UK be likely to cost?
That's easy. USPS international small, flat rate is like $15.00 USD

Thanks Isaiah

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Post by mmm » Mon Sep 05, 2011 8:50 pm

Addendum:
That's USPS small flat rate priority..

And, I forgot to mention, The gain for each channel on each board has a trim pot adjustment to allow you to set the overall gain to whatever you want. I set it to 1:1 at full CV but, you can set it to slam the rails or, just be a preamp if you wish...

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Post by pugix » Mon Sep 05, 2011 9:04 pm

Isaiah wrote: It'd likely pair it with one of your 4-channel Mixer PCBs, though I would like to have a 5th input (No VCA, hardwired to the Mixer summing node).
Would the PCB accommodate this, or would it end up getting messy?

Also, would it be possible to add an attenuator to each VCA's CV input?
I can answer the last question. It's not a good idea to add attenuator pots on the CV inputs. That's because the four CV inputs are cascaded to one another through switching jacks with the first having +5V, if you follow the standard jack wiring. With nothing plugged into the CV inputs, they all act as if "on", so you can use the attenuator pots on the inputs and have a 4-input mixer. You could put attenuators on the CV inputs, if you did not connect the switching lugs on the jacks. But then you would not get any signal to pass through the mixer without patching in a CV. Moreover, adding CV attenuators isn't useful, because a VCA is a two quadrant multiplier. The input signal gets multiplied by the CV input. Since each input already has an attenuator pot, you don't need a CV attenuator. George got the design just right. 8_)

George will have to answer the question about adding a fifth pass-through input. Sounds like a good idea, if it isn't hard. It would have to be non-inverting to maintain the same phase as the other inputs.
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Post by pugix » Mon Sep 05, 2011 9:07 pm

Wow. George and I answered at the same time. :lol:
Richard
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Post by mmm » Mon Sep 05, 2011 9:48 pm

pugix wrote:Wow. George and I answered at the same time. :lol:
Nah, I was slightly ahead of you. You must type slower than I do :hide:

And, I'm an old man... :youkids:

G

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Post by Isaiah » Tue Sep 06, 2011 2:12 am

Thanks for the replies, very helpful!

I can see now why adding CV attenuators wouldn't make much sense as the CV inputs are normalled to operate fully open, and then you attenuate the input signal. I thought the manual control would be a variable voltage offset per VCA.

What's the input impedance of each VCA?

Might have to use on of Ken Stone's DC Mixer PCBs. Looks like it has six inputs.

Cheers!

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Post by negativspace » Tue Sep 06, 2011 6:40 am

Isaiah, you're on the right track. I built myself a 12hp Euro module using one of the Quad VCAs and a CGS04 mixer. I used the switching lug on the VCA's output jacks to run the signal into the mixer - so now I have a 4-channel VC-mixer and each channel can be removed from the mix and used as an individual VCA simply by plugging a cable into its output. It's very much a core module in my 18U.

I'd like to un-cascade the 4 channels' CV... I want 4 fully-independent channels. Fortunately this should be easy - next time I unpatch the module I'm making the change. I'll unhook channel 2-4's CV cascade connection and run the +5v from CH1 straight in instead. White wire dance!

Image Image

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Post by Isaiah » Tue Sep 06, 2011 8:04 am

negativspace wrote:Isaiah, you're on the right track. I built myself a 12hp Euro module using one of the Quad VCAs and a CGS04 mixer. I used the switching lug on the VCA's output jacks to run the signal into the mixer - so now I have a 4-channel VC-mixer and each channel can be removed from the mix and used as an individual VCA simply by plugging a cable into its output. It's very much a core module in my 18U.

I'd like to un-cascade the 4 channels' CV... I want 4 fully-independent channels. Fortunately this should be easy - next time I unpatch the module I'm making the change. I'll unhook channel 2-4's CV cascade connection and run the +5v from CH1 straight in instead. White wire dance!

Image Image
Great work, it looks ace! :bananaguitar:
That's exactly what I'm thinking - 4 VCAs with up to 4 channels of VC-Mixing!

How do you have the polarising attenuator wired up?
Are all the VCA outputs summed to the wiper of the pot on the Ken Stone PCB?

I'd likely wire each VCA's CV input independently, so there's no cascading.

Cheers!

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Post by negativspace » Tue Sep 06, 2011 10:00 am

Thanks. :party:

You'll want the +5v normalled to all channels as that's what opens the VCA (via pot setting) when you have nothing plugged in to the CV input.

Each output jack on the QVCA is wired to an input of the CGS04 mixer, meaning you will use 4 of the 8 available inputs on board. The polarizing attenuator is wired up using the OUT and 'A' (I think?) pads on Ken's PCB, one to each end of a pot. The wiper of that pot goes to the MIX jack.

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