Miasma Function Generator Modifications

These notes document the modifications I made to my build of the Sin Phi Miasma Function Generator. The panel and boards were kindly given to me by a friend and apart from the changes noted here the module was built according to the documentation on the Sin Phi site.

4QX output modification

I decided to go for the AD633 4 quadrant multiplier build version and although it worked I noticed that the output was inverted and clipped slightly as the gain seemed to be to much on the op-amp following the AD633. This would mean envelopes would range from 0 volts down to about -10 volts. As this wasn’t much use to me I decided to modify the circuit so the output would be a positive signal.

Here we have the original inverting circuit and the modified non inverting circuit with the gain adjusted so it no longer clips.
My modification in all its messy glory!

Firstly, ignore the link wire to pin 12 of U8 – I removed a pad here while removing the chip I’d trashed while experimenting and had to make a little fix!

It will be easier to implement this mod before fitting U8. Pin 10 needs to be lifted as its normally connected to 0v. This is easier to do by straightening the leg before fitting the chip. Alternatively, the traces below the chip could be cut so the pad is no longer connected to 0v – but do check this with a multi-meter before fitting the chip.

  1. R67 need to be replaced. It’s originally 220k and needs to be swapped for a 24k.
  2. R60 isn’t fitted buts its pads are used for the mod. Solder a link from the R60 pad connected to pin 5 of U9 to pin 10 of U8.
  3. Solder a through-hole 62k resistor between the other R60 pad and the end of R41 which is connected to 0v.
Same modification fro a slightly different angle!

Cycle Input modification

I’m not sure how the Cycle input was intended to work but it did nothing unless a trigger was applied to the trigger input at the same time as applying a gate to the Cycle input.

This modification changes things so that applying a gate to the cycle input also triggers the circuit to start it cycling.

The modification is very simple and involves fitting 2x 1N4148 diodes between the cycle inputs and the trigger switch legs for that channel. Only thing to watch out for is the switch legs being on the opposite side from the cycle inputs. Wire is soldered to one of the diode legs so the mod will reach!

You may also notice the wire link in the bottom left corner. This is a modification highlighted in the MuffWiggler build thread as the 0v pin of one of the jacks isn’t connected to 0v.

Trig Input modification

So after a lot of use I discovered that this module wouldn’t trigger properly with all trigger signals. I tend to use pretty short trigger signals in general so thought I’d take a look at the trigger inputs to see if I could get the module to work reliably with all my other modules.

Miasma Trig Input
Schematic for the original Miasma Trig Input

To be honest it didn’t take a lot of work! I simply went back to the CGS/Serge drawings for the CGS75 Voltage controlled slope and tried the input from that with a few tweaks to the values.

Modified Trig Input
Modified Trig Input based on the CGS74 Voltage Controlled Slope

To make the modifications I simply put 10nF capacitors where D2 & D3 would have gone and SMD 1N4148 diodes where C1 & C14 would have gone. I then used through-hole 100k resistors between the input connector of the Trig Input jacks and 0v.

Now everything works as I feel it should – and as this is DIY and it’s just for me that means I must be right!!