THAT 2162 VCA Control Circuitry

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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 5:54 pm

mediatechnology wrote:
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.
I'll give it a try. However, I'm confused, because R5 is connected to GND, not vcc/2...

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mediatechnology
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Re: control circuitry for THAT 2162

Post by mediatechnology » Thu Aug 16, 2018 6:29 pm

What matters from a control viewpoint (maximum attenuation) is the voltage. From that perspective the divider reference can be either ground or Vcc/2 as long as you calculate the divider with the correct end-point voltage.

If you have other things that reference Vcc/2, like the VCA, op amps etc., then you'd likely want the voltage divider endpoint reference to also be Vcc/2 so that if the Vcc/2 is not the exact voltage, or drifts, e.g. battery discharge, then the voltage divider reference drifts with it.

You might want a divider string that has three or four resistors.
The first divider tap would set maximum gain, the tap second would set maximum attenuation, the third Vcc/2.
The top of the divider references Vcc the bottom ground.
Then have two or three unity gain op amps buffer the divider to provide a low impedance to the pot and Vcc/2.
Then bow the pot to compress or expand the "linear dB" control curve.

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

Re: control circuitry for THAT 2162

Post by dfiction » Thu Aug 16, 2018 8:03 pm

mediatechnology wrote:
Thu Aug 16, 2018 6:29 pm
What matters from a control viewpoint (maximum attenuation) is the voltage. From that perspective the divider reference can be either ground or Vcc/2 as long as you calculate the divider with the correct end-point voltage.

If you have other things that reference Vcc/2, like the VCA, op amps etc., then you'd likely want the voltage divider endpoint reference to also be Vcc/2 so that if the Vcc/2 is not the exact voltage, or drifts, e.g. battery discharge, then the voltage divider reference drifts with it.

You might want a divider string that has three or four resistors.
The first divider tap would set maximum gain, the tap second would set maximum attenuation, the third Vcc/2.
The top of the divider references Vcc the bottom ground.
Then have two or three unity gain op amps buffer the divider to provide a low impedance to the pot and Vcc/2.
Then bow the pot to compress or expand the "linear dB" control curve.
I'll give this a try. actually, my vcc/2 reference for the entire circuit is a little different than the schem—i'm using an LM8272 for vcc/2 because earlier iterations of this circuit oscillated. it was easier to just plop this down, pay the extra buck, and not worry about the capacitative load. (very messy eagle schem attached...mea culpa!)

so i've got an extra gate from this op amp lying around i can use on my board—what do you think about using the LM8272? It's not as low noise as the 2196, so i'm wondering what noise its offset will add to the control circuitry.

df
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