So I now have my triple super simple oscillators working, but they have issues. I think I know what I need to do, but just need some clarification on a couple of things. My understanding of the SSO is that the 100K resistor, 47nF capacitor and 100K tone pot are a simple first order low pass filter. If I have three SSO’s going into a second order low pass filter, do I still need the 100K resistors…? The circuit currently looks like:
Converting the second order low pass filter from passive to active will require the addition of op amps after each stage. Do I also need to buffer the output of each SSO…? Or, should I mix all three SSO’s using the double inverting op amp stage bit from the simple mixer…?
Finally, as alluded to in the post on the build progress thread, when I plug this into a powered speaker, the pots controlling the oscillation only work for about 20 - 25% of their throw, before stopping. Will moving to an active filter, or mixing the outputs of the SSO’s together stop this from happening…?
You might want to look into using a summing amplifier to mix your three oscillators:
Usually people go for the inverting version - for audio signals it shouldn’t matter if it’s inverted or not, and it’s simpler to implement.
You might think about putting a pot between the output of each oscillator and the summing resistor if you want to independently control the amount of each oscillator in the final mix. If you’re just going for a drone kind of sound and you want all three oscillators contributing equally then you won’t need pots, just use the same summing resistor for each one.
Thanks @jaradical I knew I needed some op amps, just wasn’t sure where. Which reminds me that I totally forgot the most important part of my initial question.
So the whole idea was to build a MFOS WSG and then augment that with some banks of SSO’s droning away. As the WSG is setup to run off a 9v battery, I got a 9v wall wart and have been using that so far. So I only have +Ve and GND, no -Ve rail.
All the schematics and videos online, show circuits that have +Ve, GND and -Ve rails, can you power an op amp off just +Ve and GND…? Or do I know have to go away and build a proper +/- 9v PSU…?
Edit: I should’ve said, that I have plenty of TL074 quad op amp’s to hand.
You can power an op amp off +V and GND, but the TL07x would have issues. That one wants input voltages to be several volts above the negative rail, so you’d have to bias your inputs to work around maybe 5 V or so, and then you have only a couple volts swing available.
In fact, in the WSG (schematic here), U2 is an LM741 op amp powered with ground and +9V. That op amp also needs inputs staying away from the rails by a couple volts. However, it’s in an inverting configuration, which means both inputs stay at 4.5 V as supplied by the R16/R17 voltage divider, so that’s okay. But in your schematic the buffers and active filter are non inverting and the voltage swing would be an issue.
A rail to rail op amp like an MCP6002, on the other hand, can take inputs all the way (give or take a little bit) from one rail to the other, so you could have inputs biased around 4.5 V and have a full 9 Vpp swing.
Oh, right, the reason I was looking at the WSG schematic was to see if it obviously could not work with a +12 V supply. I suspect it could, or at least would not require more than minor changes. That would give you better options for the SSO. Then again, you could use a +12 V or even +15 V supply for the SSO and add a 9 V regulator to power the WSG.
Thanks both! I thought I may have to stick a mixer of some sort in before the filter, I wasn’t 100% sure if I also needed a voltage follower on each SSO output as well. @analogoutput I think running a 12v PSU into the whole thing, with a 9v branch for the WSG is probably the best idea. Having said that, David Haillant has done a 12v conversion, so it should be possible…
