LM13700 needs -Vsupply to be quiet?

[gabbagabi]

hi guys,

after burning some 13700 back in the days i wanted to manage to use them now.

so i breadboarded a simple vca with a 10k resistor on the gain input, a voltage divider 1:200 on the signal input and a inverting opamp-buffer on the output.

As i switched it on ive got a heavily overdriven signal but the cv was at 0V?

After some testing ive applied a negative cv and the signal got quiter, as the cv was -15V it was quiet.

Is that normal behavior, or what iam doing wrong here?
My simulator multisim is more or less confirming that behavior.

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[JovianPyx]

If I understand the question, it would seem to be about the Iabc pin.

Iabc current is referenced to -V not ground.

So to completely quiet a VCA made with LM13700, the Iabc pin needs to be at -V.

Also, the Iabc pin can withstand no more than 2 mA of current. Any more current will blow the amplifier (without any magic smoke fun either). So always make sure there is an appropriate resistor or other current limiting device in series with Iabc

[gabbagabi]

thanks for readindg my mind!
of course the question was about Iabc.
Good to know that neither iam stupid nor the chips

[JovianPyx]

I remember having the same question years ago.

The bit about Iabc being referenced to -V was the crux.

[gabbagabi]

ive searched the datasheet up and down, but ....

[JovianPyx]

Yeah, I've looked too. At that time, another DIYer on SDIY told me about it. He was an active EE and would have been able to look at the chip internal schematic and see that.

[Grumble]

it is not that hard to see, and it is in the datasheet as well...

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[JovianPyx]

I have a National datasheet that doesn't have the text you show, but it does have the schematic and other clues.

I do now see how it works, but when I looked at it (25+ years ago) I didn't see it, though my mentor did. There is also a block diagram that shows an Iabc current sink dumping into -V.

[ixtern]

It is in LM13700 datasheet - VCA and control voltage needed for it:

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