VCA's and PCB's

PCB

The PCB's from Osh Park came yesterday. They look like this:

PCB's from  Osh Park

Nice!

 I haven't started mounting them yet, but I immediately saw that the MIDI contact didn't fit. This wasn't so surprising, I didn't have the part when I made footprint, and the schematic of the part was a bit strange (I'm not that good at mechanical drawings).

VCA

 I need a voltage controlled amplifier for the synthesizer. This is to be used for generating the envelope. I looked at a few ways of doing this, but using the LM13700 seemed the most reasonable. Remember that I burnt an LM13700 not long ago? I realised that I probably burned just one of the channels. So I wired up a VCA circuit i found in the data sheet (page 10, fig 23):

VCA from LM13700 data sheet

The circuit, and it's output, looks like this:

Input to VCA is the sine, output is the clipped sine.

VCA from LM13700

The VCA looks to be working fine, but I might tweak some resistor values after testing it with the Attack and Release envelope generator I intend to build soon.

Broken Power Supply

When I was fiddling with the VCA the power supply to the gain control was overpowered. This surprised me, since it can drive 3A, and I calculated max current to 1.5mA. Seems like there is something wrong with the current measurement in the old power supply. It can, however, give up to 1.2mA, which is just fine right now. So I'll need to fix it, but first I've got some more interesting things to do...