Linearize 'log' VCA's response

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JR.
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Re: Linearize 'log' VCA's response

Post by JR. » Sun Jun 18, 2017 8:53 am

But if we drive the VCA control ports differentially does that work out with simple unity gain inverter? 3 + 3= 6 ?

I recall last time I thought about this, issues with wanting low source impedance when driving the VCA (for lowest noise and distortion).

JR

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mediatechnology
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Re: Linearize 'log' VCA's response

Post by mediatechnology » Sun Jun 18, 2017 9:24 am

Yes it would. Forgot we were driving it differentially so 3mV/dB/port works out to be 6 mV/db single-ended.

And you do want low source impedance that's low over frequency.
An op amp output that's "non-inductive."

Igor Dockx
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Re: Linearize 'log' VCA's response

Post by Igor Dockx » Mon Jun 19, 2017 1:33 pm

Hi Wayne and John,

Thanks for your valuable comments. I'll certainly look at John's proposed economic solution; a good opportunity for me to further explore LT Spice.

I simulated the EDN-circuit with inverted reference current; applying +5V DC as reference voltage gives 0dB thru the VCA if the input voltage is +5V DC; lowering Vin linearly attenuates the signal thru the VCA (+2.5V DC = -6dB, +1.25V = -12dB etc...).

The automation system that I'm looking into is designed to work with SSL circuitry; they have chosen to have the VCA fully open (0dB) at 0V DC in, and gradually attenuating the signal as the input voltage raises up to +5V DC (VCA at max attenuation).

-How can the EDN circuit be modified to react to 0V in = 0dB out / +5V in = max attenuation?

-SSL uses the same control line which steers the VCA with 0 <> 5V DC to also read / reproduce the mute status; the ADC / DAC react to anything above +7.5V DC as a mute signal. What would be the least complicated way to have the VCA circuitry ignore any incoming DC voltage above +5V?

Thanks Gentlemen!

I am by no means an electronics designer; at best an analog audio maintenance tech. But full of enthusiasm to learn as I go :-)

Igor

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JR.
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Re: Linearize 'log' VCA's response

Post by JR. » Tue Jun 20, 2017 10:28 am

Igor Dockx wrote:
Mon Jun 19, 2017 1:33 pm
Hi Wayne and John,

Thanks for your valuable comments. I'll certainly look at John's proposed economic solution; a good opportunity for me to further explore LT Spice.

I simulated the EDN-circuit with inverted reference current; applying +5V DC as reference voltage gives 0dB thru the VCA if the input voltage is +5V DC; lowering Vin linearly attenuates the signal thru the VCA (+2.5V DC = -6dB, +1.25V = -12dB etc...).

The automation system that I'm looking into is designed to work with SSL circuitry; they have chosen to have the VCA fully open (0dB) at 0V DC in, and gradually attenuating the signal as the input voltage raises up to +5V DC (VCA at max attenuation).
Most VCAs are 0dB at 0V control voltage.

To increase attenuation from increasing voltage wire the faders backwards... Ground at the top, +5V at full down. Of course and audio taper pot will not result in a useful control law that way.
-How can the EDN circuit be modified to react to 0V in = 0dB out / +5V in = max attenuation?
don't know
-SSL uses the same control line which steers the VCA with 0 <> 5V DC to also read / reproduce the mute status; the ADC / DAC react to anything above +7.5V DC as a mute signal. What would be the least complicated way to have the VCA circuitry ignore any incoming DC voltage above +5V?
You can clamp or detect and clamp for voltage above 5V..
Thanks Gentlemen!

I am by no means an electronics designer; at best an analog audio maintenance tech. But full of enthusiasm to learn as I go :-)

Igor
I am unclear about what you are trying to accomplish... your original post sounded like you wanted to control a VCA from an audio taper pot. I provided a circuit to support that, albeit with +10dB gain at the top of the fader, typical gain structure for consoles. You can make the top 0dB by scaling the current in offset transistor.

What kind of faders are used SSL with that automation? That may be the real difficulty.

JR

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