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VCA810 control voltage leaking onto the output?

Jonathan Arthur
Prodigy 10 points
Other Parts Discussed in Thread: VCA810

I've been prototyping using this VCA for a compressor - for audio purposes. The VCA is controlled by the PWM output of an mcu. The (filtered of course) PWM output sits on -1V until told to boost or cut by the mcu. (The CV(control voltage) can only go to a maximum of 600mVpp - as I don't think I need to use the whole gain range). Now my problem is any change on the CV leaks into the signal path (up to around 200mV or so of it). The control voltage is relatively slow 50Hz-100Hz max but this is difficult to filter out - as my input is roughly 80Hz-5kHz!! I've taken care in isolating the audio signal path from the CV - this is done on a pcb. The CV is smooth. I'm really not sure how to fix it.  

Here's a schematic of the audio signal path. Note I also tried adding in another high pass filter before the pot, there's also another one on the output which is just cut off the schematic .

Here's the control voltage schematic which converts +5V PWM to 600mVpp centred on -1V. Trim pots are used to adjust this accurately.  - Sorry you'll have to click the image to get this for some reason..

If anyone has any ideas or any comments let me know. I'm really keen to get this working.

  • 0
    Guru 21975 points

    Jonathan,

    We don't ordinarily handle the VCA810 on this forum. It is supported on the high speed amplifiers forum. But I will make a comment before moving this tread to the high speed forum.

    I believe that the bandwidth of your control signal may be too wide. This is one of the very tricky issues with compression. Fundamentally you are distorting the signal when the gain is changing. The trick is to do it slowly enough to make the result acceptable. I believe audio compression would normally limit the bandwidth to somewhere in the 20Hz range. Attack and decay characteristics of the control signal are crucial making the compression not appear too obvious.

    I'll move this to the high speed forum where they may have knowledge on the VCA810 that would be helpful.

    Regards, Bruce.