Ring modulator LTSpice simulation

Before breadboarding the circuit again I thought it would be a good idea to simulate it to see what effects to expect when replacing pots and caps.

The circuit as found on the Yusynth pages (http://yusynth.net/Modular/EN/RINGMOD/index.html)

The LM1496-model was found here: 

https://forum.allaboutcircuits.com/threads/mc1496-monolithic-balanced-modulator-spice-model.47212/

With the pots centered, two 10Vpp input waves result in a 8Vpp output.

Signal and carrier

Green is output

With the 220Ohm pot fully turned to one of the sides, we get this:

 

With the 500Ohm pot fully turned to one of the sides, we get this:

It looks like the wave with the longest wavelength has gotten a DC offset

Now, if we replace the 220Ohm pot with a 500 one we get the exact same waveform when centered, byt at one of the sides we get this:

If we reduce the wiper to half way between center and one of the sides we get back to what we saw with 220Ohm:

It is thus likely that the 220Ohm potentiometer may be replaced with a 500Ohm one.

Cap biasing

The 1uF input cap on the left looks like it has one side DC biased between -3V and -4V:

The 1uF input cap on the right looks like it has one side DC biased around +7.5V

As for the 100uF cap, it is biased at around 6.5V. I cannot see any changes to the output whether I use a 200uF, 100uF, 10uF or no cap here, not with a combination of 1MHz and 10kHz, nor with the combination 1kHz and 10Hz. I think I will try using a 100uF ceramic even with the capacitance dropped to much lower by the DC bias issues such caps have.

Now, these are just simulations of course, so things may be different in practice, but it looks promising - I may be able to standardise on 500Ohm pots and use ceramics instead of electrolytics for the other caps. 

UPDATE: I have even used 500Ohm pots on my prototype so I will be fine :-D

EDIT: Input and output DC blocking / offset caps

I simulated the frequency response for the input. A 1uF cap gives a 3dB point at around 1Hz.

 

As the cap is DC biased at 7.5V, its capacitance may in practice be as low as 0.1 to 0.2uF. It would then have a 3dB point around 10Hz.

If we use a 10uF cap instead, this would in the worst case have a capacitance around 1-2uF, matching the specified 1uF. If id is NOT reduced, a 10uF cap should have a 3dB point even lower than 1Hz and we should still be fine - I hope!

Ring modulator tested, seems to be working

I tested my version of the yusynth.net ring modulator today.

I tried connecting VCAs on the inputs but it doesn't seem very necessary, they affect the signal almost exactly like the output VCA does, so a single output vca will do fine.

1uF input caps seem like a good idea, perhaps an output cap too

I used a 500R pot in place of a 200, looks like I got away with it.

Trimming is done as described in the picture (same as on yusynth.net):

Noise and ring modulation boards

Ring modulation board

Three types of noise - white, pink and red

Summer 2018 - DAC Mux, Ring mod

Summer is always kind of productive for me. While off on summer holiday to the western parts of Norway, I get time to do some electronics work. This time I've drawn up and ordered a 16 channel sample and hold circuit (DAC mux) to replace the 32 channel one used in the prototype. This one is easier to place and makes it possible to use only 16 channels for parts of the synth.

I also did a v3 of the linear VCA. This time, as for the first version, the trimmers are off-board, but the components are placed on both sides as in v2. The whole card is sort of a double width DIL thingy with the pins for one VCA on one side and the other on the other side. I imagine it possible to place it in a socket and have other parts of the circuit below it - we'll see.

Finally, I did a ring modulator based on the one from yusynth.net, which in turn is almost entirely a copy of the one in the datasheet. This one uses a surface mount MC1496. The circuit and board are finished (but untested), including logo.

Oh, and I have written a midi/sysex-based bootloader - it's almost finished. I will feel much better if I can remotely update any MPG-200 I sell. The bootloader is very simplistic and does not have any flow control, so sysex uploading must be done through a program made by me. Still better than not being able to update at all.