# Gain using the LM13700

**URL:** <https://lookmumnocomputer.discourse.group/t/gain-using-the-lm13700/2929>\
**Category:** DIY STUFF\
**Created:** [February 13, 2021, 10:59am UTC](https://lookmumnocomputer.discourse.group/t/gain-using-the-lm13700/2929 "2021-02-13T10:59:31Z")\
**Posts on this page:** 1\
**Showing post:** 8

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**Author:** ![jaradical](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/jaradical/32/12069_2.png) [@jaradical](https://lookmumnocomputer.discourse.group/u/jaradical)\
**Post date:** [February 13, 2021, 4:02pm UTC](https://lookmumnocomputer.discourse.group/t/gain-using-the-lm13700/2929/8 "2021-02-13T16:02:27Z")

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The thing to note about OTAs is that they output current - not voltage. Resistor RL in the schematic is there to convert the output current to a voltage for driving the darlington pair buffer.

Here’s my take on an lm13700 based VCA that accepts control voltages up to about 10V:

 ![dual_lm13700_vca](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/1/1ace001a2b0a3fbc0c30f1036651ebf6c5c49c68.png)

U1A and U1C do the current to voltage in this design, I don’t use the lm13700 buffers.  
The bias current (I\_ABC) that does the volume control is provided by the op-amps driving the pnp resistors which act as voltage controlled current sources.

Cheers,  
Jared

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