THAT 2162 VCA Control Circuitry

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dfiction
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THAT 2162 VCA Control Circuitry

Post by dfiction » Wed Aug 15, 2018 6:51 pm

Hey everyone!

I'm new to this forum and I have enjoyed reading everyone's wisdom here. So many cool circuits. I wanted to share a problem I ran into with a VCA i'm working on–There are so many wise mentors here, and I'm sure many of you have licked this problem before.

I'm doing some work with the THAT2162, and in my experiments and trials I've managed to get a very good sounding, quiet VCA design that I'm quite happy with. I've designed a circuit that controls the gain sent to 6 power amps with a single potentiometer, by using 6 different VCAs, implementing the THAT2162. I'm using a single 12v supply, and the whole vca section of the circuit is running in reference to vcc/2. I opted to use differential drive because I wanted to maintain a wide dynamic range in my circuit. This might have been overkill, but I ended up with a clean circuit, using some rather expensive semiconductors.

While I have a great sounding circuit with wide dynamic range and low noise, I am having a lot of trouble controlling the contour of the volume pot, which should not scale linearly, but rather with an approximately log curve, so that I can use the single potentiometer to control some "long fadeouts" with the volume knob. I did measure the gain at the Ec- port My circuit tracks linearly, scaling between -120 -> +6 dB at the full extremes of the volume pot (the extremes are configurable by two resistors in the control circuitry).

But the volume pot doesn't track correctly, the way a master fader VCA should, and I don't know how to modify my circuit accordingly. At first I kept increasing the resistor R5, which lowers the threshold of the "off" position of the volume knob P2, and I thought that the right approach would be to raise the minimum threshold to around around -70 db or so (R5 to 100k). But finally I have learned that I want a log response of the pot, which would enable me to keep my dynamic range, and give the circuit a "good response" when the user adjusts the volume control.

So, here's my circuit. I have already read the THATCORP design note 116, which details the diode breakpoint application, but I'm not sure how I should adapt that principle to my current circuit, for P2 is controlling a very small span of approx 300 mV. Where to interrupt, or stick a diode?

I don't really need a perfectly logarithmic response in this circuit. I just need the volume knob to be "useful" in the lower positions—right now the linear response I'm getting means that most of the action is in the top rotation of the pot, not very good for fades.

Any wisdom you might have would be much appreciated.

best,
Daniel
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VCA_ControlVoltageGen2.png

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JR.
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Re: control circuitry for THAT 2162

Post by JR. » Wed Aug 15, 2018 7:02 pm

it's after beer o'clock here in MS, so I will give you a better answer tomorrow, but VCAs generally provide a log gain law from a linear control Vc, so should not require diode break tricks for a decent audio fade.

JR

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mediatechnology
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Re: control circuitry for THAT 2162

Post by mediatechnology » Wed Aug 15, 2018 8:51 pm

Thanks for joining us!

Have a look here and see if this is useful "Resistors Provide Nonlinear Pot Tapers," Mark Rumreich EDN November 13 1986. : viewtopic.php?f=12&t=866#p10260

Though the VCA control taper is linear dB I can see how some range expansion would help the overall "feel."
You might be able to get there using a bowing resistor to slug the pot.

I may have a look at the SSL master fader as well and see if they did anything there you might use.

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JR.
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Re: control circuitry for THAT 2162

Post by JR. » Thu Aug 16, 2018 8:01 am

dfiction wrote:
Wed Aug 15, 2018 6:51 pm
Hey everyone!

I'm new to this forum and I have enjoyed reading everyone's wisdom here. So many cool circuits. I wanted to share a problem I ran into with a VCA i'm working on–There are so many wise mentors here, and I'm sure many of you have licked this problem before.

I'm doing some work with the THAT2162, and in my experiments and trials I've managed to get a very good sounding, quiet VCA design that I'm quite happy with. I've designed a circuit that controls the gain sent to 6 power amps with a single potentiometer, by using 6 different VCAs, implementing the THAT2162. I'm using a single 12v supply, and the whole vca section of the circuit is running in reference to vcc/2. I opted to use differential drive because I wanted to maintain a wide dynamic range in my circuit. This might have been overkill, but I ended up with a clean circuit, using some rather expensive semiconductors.

While I have a great sounding circuit with wide dynamic range and low noise, I am having a lot of trouble controlling the contour of the volume pot, which should not scale linearly, but rather with an approximately log curve, so that I can use the single potentiometer to control some "long fadeouts" with the volume knob. I did measure the gain at the Ec- port My circuit tracks linearly, scaling between -120 -> +6 dB at the full extremes of the volume pot (the extremes are configurable by two resistors in the control circuitry).
linearly? Applying a linear control voltage to VCA generate log gain changes.
But the volume pot doesn't track correctly, the way a master fader VCA should, and I don't know how to modify my circuit accordingly. At first I kept increasing the resistor R5, which lowers the threshold of the "off" position of the volume knob P2, and I thought that the right approach would be to raise the minimum threshold to around around -70 db or so (R5 to 100k). But finally I have learned that I want a log response of the pot, which would enable me to keep my dynamic range, and give the circuit a "good response" when the user adjusts the volume control.
linear control V generates log gain changes
So, here's my circuit. I have already read the THATCORP design note 116, which details the diode breakpoint application, but I'm not sure how I should adapt that principle to my current circuit, for P2 is controlling a very small span of approx 300 mV. Where to interrupt, or stick a diode?
diode break amplifiers involve scaling up the voltage swing to several volts so steering diodes can turn on and off, then afterwards scaling it back down to mV range for Vc use. Not sure how that would help.
I don't really need a perfectly logarithmic response in this circuit. I just need the volume knob to be "useful" in the lower positions—right now the linear response I'm getting means that most of the action is in the top rotation of the pot, not very good for fades.

Any wisdom you might have would be much appreciated.

best,
Daniel
Control V generally ranges from 0V (unity gain) to xV which can be scaled for whatever max attenuation you desire.

If you want it faster/slower at unity gain, or max attenuation end you can slug the pot to pull the taper one way or the other. I have messed around with unconventional laws to perform pan circuits, but simple volume control seems fine with standard log response law.

JR

dfiction
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Joined: Wed Aug 15, 2018 6:19 pm

Re: control circuitry for THAT 2162

Post by dfiction » Thu Aug 16, 2018 1:14 pm

mediatechnology wrote:
Wed Aug 15, 2018 8:51 pm
Thanks for joining us!

Have a look here and see if this is useful "Resistors Provide Nonlinear Pot Tapers," Mark Rumreich EDN November 13 1986. : viewtopic.php?f=12&t=866#p10260

Though the VCA control taper is linear dB I can see how some range expansion would help the overall "feel."
You might be able to get there using a bowing resistor to slug the pot.

I may have a look at the SSL master fader as well and see if they did anything there you might use.
Thanks so much for responding so promptly! I'll start with law steering right now—reading this lovely article from 1986...

Well, the volume control in my circuit is P2, which is connected to the output of an op amp stage, as well as R5, the "minimum attention" setting resistor. P2 does not connect to ground. That kind of defies the circuit in the article you attached.

But trying it, I haven't has as much luck. P2, which is the volume control, doesn't connect to ground, but rather R5. The link I posted shows the voltage divider version connecting to ground. When I connect a resistor between the wiper and the R5 junction, it makes no observable change. Tried 1k resistor, tried 2k, 4.7k, and 330 ohms. No difference. Well, connecting between the wiper lug and the other pin of the potentiometer does make a change—but it reduces the swing of P2 to be basically 0v and -6 db—and then the volume never shuts off.

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mediatechnology
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Re: control circuitry for THAT 2162

Post by mediatechnology » Thu Aug 16, 2018 1:47 pm

Make the lower end of P2 fed from a low-impedance voltage source. (Op amp output.)
(The upper end of P2 should continue to be a voltage source as well.)

dfiction
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Re: control circuitry for THAT 2162

Post by dfiction » Thu Aug 16, 2018 2:29 pm

mediatechnology wrote:
Thu Aug 16, 2018 1:47 pm
Make the lower end of P2 fed from a low-impedance voltage source. (Op amp output.)
(The upper end of P2 should continue to be a voltage source as well.)
Do you mean, stick another buffer in there? ie, the bottom end of P2 would go into an op amp buffer, and then the output of that would drive R5 into ground? While the middle of the pot would continue to drive R14?

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mediatechnology
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Re: control circuitry for THAT 2162

Post by mediatechnology » Thu Aug 16, 2018 5:29 pm

Yes, use a second op amp to drive the lower end of the pot. You don't need R5 - a voltage divider at the input of the lower op amp will set the potential of the end stop. You can then bow it by resistors from one end stop to the wiper.

Also C4 can be connected to ground rather than Vcc/2.
Functionally they're the same but ground is a better AC ground than Vcc/2.

dfiction
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Re: control circuitry for THAT 2162

Post by dfiction » Thu Aug 16, 2018 5:38 pm

Okay, I'm starting to get a feeling for what you're saying.

so this new op amp would be a unity gain buffer like the other three—but how do i figure out the values of this new voltage divider you're suggesting I add? simply calculate the max attenuation I want and calculate it Vin as Vcc? And, should the bottom of the divider reference ground or vcc/2?

best,
daniel

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mediatechnology
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Re: control circuitry for THAT 2162

Post by mediatechnology » Thu Aug 16, 2018 5:45 pm

Yes, just use Vcc as Vin to the divider and set the divider output to whatever maximum attenuation you want.
Then buffer that divider with another op amp.
I would use Vcc/2 as the reference for the lower end of the divider.

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