V2164 VCA CV summing / clipping

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ansome
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V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 11:31 am

I am building a VCA using the v2164, everything is working fine but I have a manual gain control +5v (voltage divider with pot) and an external cv control +5v, both on there own give unity gain (10vpp) but when summed it goes above this. I need unity gain to be the maximum gain could I simply use a Zener diode in the feedback path of the summing op-amp to make this CV clip at 5v? have everything breadboarded but no more zen diodes at home and do not have the V2164 in LTspice.

based on this schematic.
Image


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Re: V2164 VCA CV summing / clipping

Post by oldenjon » Mon Jun 29, 2020 11:54 am

Depending on what format you've build this in, you can normal the CV input to 5V so that the potentiometer acts like an offset when nothing is plugged in, and an attenuator for the CV signal when something is. A lot of the CV inputs in Mutable modules are done this way if you want to see an example.
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Re: V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 12:03 pm

Screen Shot 2020-06-29 at 18.12.23.png
so the pot will be an aux control, the cv will control over the amount of signal sent. I would like them both to function at the same time so you can fine-tune the effect. It would be cool to see what you mean, but not sure it will be what I'm looking for.
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Last edited by ansome on Mon Jun 29, 2020 12:14 pm, edited 1 time in total.

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Re: V2164 VCA CV summing / clipping

Post by oldenjon » Mon Jun 29, 2020 12:13 pm

You can also use a standard diode to limit the CV to ~-600mV/+20dB, but you'll still get clipping. If you want to fine tune things, you can have a pot for both offset and attenuation of the CV signal. You'll most likely want to keep the offset at a minimum when using a CV.
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Re: V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 12:26 pm

oldenjon wrote:
Mon Jun 29, 2020 12:13 pm
You can also use a standard diode to limit the CV to ~-600mV/+20dB, but you'll still get clipping. If you want to fine tune things, you can have a pot for both offset and attenuation of the CV signal. You'll most likely want to keep the offset at a minimum when using a CV.
I'm basically just looking limit the cv control to 5v, if someone puts in a 10v CV the gain will be way above unity and will affect how the rest of the module is working.

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Re: V2164 VCA CV summing / clipping

Post by guest » Mon Jun 29, 2020 12:33 pm

you could add an active clamp. its just one opamp and diode (per channel). this would go after the 4.7k in the lower righthand corner of your second schematic. tie the noninverting input to ground, and put a diode in the feedback loop, with the VCA input connected to the inverting input.
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Re: V2164 VCA CV summing / clipping

Post by oldenjon » Mon Jun 29, 2020 12:44 pm

ansome wrote:
Mon Jun 29, 2020 12:26 pm
oldenjon wrote:
Mon Jun 29, 2020 12:13 pm
You can also use a standard diode to limit the CV to ~-600mV/+20dB, but you'll still get clipping. If you want to fine tune things, you can have a pot for both offset and attenuation of the CV signal. You'll most likely want to keep the offset at a minimum when using a CV.
I'm basically just looking limit the cv control to 5v, if someone puts in a 10v CV the gain will be way above unity and will affect how the rest of the module is working.
That's why having an attenuator for the CV signal is useful. Just get in the habit of keeping the controls at a minimum and dialing in the amount of offset and CV signal you need.
Last edited by oldenjon on Mon Jun 29, 2020 2:08 pm, edited 1 time in total.
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Re: V2164 VCA CV summing / clipping

Post by BugBrand » Mon Jun 29, 2020 12:55 pm

Just put a pair of diodes in around U5C - one across the cap (C38) and then one from the opamp output in series before the 4k7 (R7?)

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Re: V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 1:47 pm

BugBrand wrote:
Mon Jun 29, 2020 12:55 pm
Just put a pair of diodes in around U5C - one across the cap (C38) and then one from the opamp output in series before the 4k7 (R7?)
Just put a diode before the 4k7 resistor and works perfectly, just stops at unity gain with no adjustments almost seemed too easy :hail: it's working great without the diode in the opamp feedback path just I just ignore that one?

thanks :hail: :hail: :hail: :hail:

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Re: V2164 VCA CV summing / clipping

Post by oldenjon » Mon Jun 29, 2020 2:13 pm

No you need a diode in parallel with the op-amp as well, anode to - input and cathode to output, before the other diode and 4.7k resistor. You'd get full-wave rectification without it.
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Re: V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 2:26 pm

oldenjon wrote:
Mon Jun 29, 2020 2:13 pm
No you need a diode in parallel with the op-amp as well, anode to - input and cathode to output, before the other diode and 4.7k resistor. You'd get full-wave rectification without it.
shes working great, thanks every one :sb:
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Re: V2164 VCA CV summing / clipping

Post by oldenjon » Mon Jun 29, 2020 2:27 pm

D7 should be pointed the other way, anode to op-amp output, cathode to 4.7k. Google 'precision rectifier'
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Re: V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 2:33 pm

oldenjon wrote:
Mon Jun 29, 2020 2:27 pm
D7 should be pointed the other way, anode to op-amp output, cathode to 4.7k. Google 'precision rectifier'
I got it running on the breadboard and is only working this way round.

Edit- YEP was the other way around. thanks!

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Re: V2164 VCA CV summing / clipping

Post by BugBrand » Mon Jun 29, 2020 3:18 pm

Good.
I was going to say you didn't need the resistor to V- before U3C summing stage -- but then saw you have it marked as 10M which is a good value just to null out any possible small offsets and ensure the VCA fully closes.
I would always say to upgrade and avoid using the voltage rails for references - on the resistor to IC2A and on the Gain pot.
eg. I use LM4040 10V (on for +ve, one for -ve) and then all the summing resistors etc. can simply be 100k which I find nice & easy in approach.

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Re: V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 3:49 pm

BugBrand wrote:
Mon Jun 29, 2020 3:18 pm
Good.
I was going to say you didn't need the resistor to V- before U3C summing stage -- but then saw you have it marked as 10M which is a good value just to null out any possible small offsets and ensure the VCA fully closes.
I would always say to upgrade and avoid using the voltage rails for references - on the resistor to IC2A and on the Gain pot.
eg. I use LM4040 10V (on for +ve, one for -ve) and then all the summing resistors etc. can simply be 100k which I find nice & easy in approach.
Thanks for the advice, I was sure what I was doing wasn't correct.

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Re: V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 4:47 pm

BugBrand wrote:
Mon Jun 29, 2020 3:18 pm
Good.
I was going to say you didn't need the resistor to V- before U3C summing stage -- but then saw you have it marked as 10M which is a good value just to null out any possible small offsets and ensure the VCA fully closes.
I would always say to upgrade and avoid using the voltage rails for references - on the resistor to IC2A and on the Gain pot.
eg. I use LM4040 10V (on for +ve, one for -ve) and then all the summing resistors etc. can simply be 100k which I find nice & easy in approach.
Screen Shot 2020-06-29 at 22.45.59.png
Something like this? I took the resistor value from a mutable schematic as I wasn't sure how to calculate it.
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Re: V2164 VCA CV summing / clipping

Post by oldenjon » Mon Jun 29, 2020 5:07 pm

I think you can get away with just a -5V reference, if not you can use an op-amp to invert the reference voltage for +/-5V. First you need to calculate the total current requirement for each reference voltage, then find the quiescent current requirement for the reference you're using. You can use the equation (Vcc - Vref) / Rs to find a suitable Rs that supplies enough current for the circuit and quiescent current requirement + some headroom. If you use an op-amp to buffer the reference voltage, you can just use a 10k resistor for Rs. You should also have a decoupling capacitor for each reference.

*edit* You'll want to swap the pins for the negative reference. Cathode should be at ground.
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Re: V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 5:46 pm

oldenjon wrote:
Mon Jun 29, 2020 5:07 pm
I think you can get away with just a -5V reference, if not you can use an op-amp to invert the reference voltage for +/-5V. First you need to calculate the total current requirement for each reference voltage, then find the quiescent current requirement for the reference you're using. You can use the equation (Vcc - Vref) / Rs to find a suitable Rs that supplies enough current for the circuit and quiescent current requirement + some headroom. If you use an op-amp to buffer the reference voltage, you can just use a 10k resistor for Rs. You should also have a decoupling capacitor for each reference.

*edit* You'll want to swap the pins for the negative reference. Cathode should be at ground.
Great thanks, I have some spare opamps in this design so it makes sense to use them. ill get some ordered and test em out
Screen Shot 2020-06-29 at 23.50.07.png
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Re: V2164 VCA CV summing / clipping

Post by oldenjon » Mon Jun 29, 2020 5:57 pm

Looks good, you just need to add some resistors for the inverting amplifier
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Re: V2164 VCA CV summing / clipping

Post by ansome » Mon Jun 29, 2020 6:01 pm

oldenjon wrote:
Mon Jun 29, 2020 5:57 pm
Looks good, you just need to add some resistors for the inverting amplifier
ah yeah, you think 2 10k's to give unity gain?

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Re: V2164 VCA CV summing / clipping

Post by oldenjon » Mon Jun 29, 2020 6:08 pm

You have 12-5V /10k = 700uA supplied current. 5V/10k = 500uA drawn by the inverting amplifier. The quiescent current requirement is around 100uA for the LM4040, and that leaves you with about 100uA headroom which is not much. Anything between 15k and 100k would be better. And yes you want a gain of -1.
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