So I'm in the process of speccing out and ordering parts for a MFOS SoundLab Mk. II - the first synthesizer I've built (though not the first kit I've assembled.) All well and good, mostly pretty straightforward save for hunting down a few parts that're getting harder to find, remembering that bipolar and non-polarized capacitors aren't the same thing, etc.
But there's a few things about the design where I see room for improvement, and I'm getting the urge to tinker here and there. Some of the mods I'm considering are dead simple, and Ray even wrote up a few as suggestions, all those years ago - but there's a couple things that aren't quite so trivial, and as a newbie to low-level electronics I figure I oughtta sanity-check myself before I go needlessly blowing anything up...
(Not that MFOS needs any introduction 'round here, but for reference, the schematics/theory-of-operation are here.)
First off, I'd like to normalize the external-input jack to the VCA output, for the usual overdrive madness. There's already a test-point for this (X85 on the VCA schematic,) but do I need to do anything to isolate it from the master volume pot (R196) to avoid having that attenuate the feedback signal, and/or having the ext.-in pot (R165 on the mixer schematic) affect the main audio output?
(I'm also adding a separate VCA-out banana jack; dead simple, but the same question applies.)
More significantly, I'm adding triangle shapers to the VCOs, using the circuit on the MFOS website and a couple of the "Swiss Op-Amp Knife" boards. That part's simple enough - but if I'm following the schematic correctly, one op-amp out of the the four the TL074 provides is unused. Ray has it "tied up," I assume to reduce any chance of noise pollution, but that leaves me with two spare op-amps that I might as well put to use...
One I figure I'll set up as an inverter; that's well-documented enough (there's an example right in MAKE: Analog Synthesizers,) and the only real question I have with that is to what extent the absolute value of the resistors in the standard inverting-buffer op-amp circuit matters. I know their ratio controls the gain, but if I just want a 1:1 inverter, how much difference does it make whether I use, say, two 10K vs. two 300K resistors? I've looked into this a bit and seen some discussion on impedance/loading vs. thermal noise, but I'm not sure how big of a deal any of that is in this context.
And now the complex part: I'd like to add a secondary VCA, since You Can Never Have Too Many VCAs, and after some digging around I found my way to this circuit, a basic (inverting?) linear VCA using only an op-amp and a couple JFETs. Good enough for my purposes, but I'm trying to wrap my head around how it works and figure out what I'd need to do to adapt this for my project.
So...going off the description and what (I think) I understand, Q1 is used as essentially a voltage-controlled resistor, attenuating Vin (shunting it to ground) based on the value of Vc before it goes to the inverting input. I'm...considerably less clear on things from there. What's the purpose of Q2? It also appears to drain Vin to ground before it goes into the non-inverting input, but its gate is tied to ground - isn't that 0V, a.k.a. "wide open" on a JFET? How is this different from just tying the non-inverting input to ground directly? And if the path through Q2 to ground is already "wide open," what's the point of the "k * R" resistor on a separate path to ground?
Also: I gather from the notes at Vout that k is the gain factor, so if I don't need greater than unity and only care about the VCA as a voltage-controlled attenuator, all the resistors can be the same value, no? Do any considerations about the absolute value carry over from the inverter to this application?
Also also: the description lists the properties to look for in the JFETs, but not the op-amp, which is specified as an LT1006; what differences between that and the TL074 might need to be accounted for? And on the subject of the JFETs, the description suggests that a current limiter (resistor?) might be advisable if a forward gate voltage is possible, which (if I'm understanding things correctly) it most likely is. What sort of value would be necessary for that?
Thanks in advance for any advice/commentary![:)]( "smile")
But there's a few things about the design where I see room for improvement, and I'm getting the urge to tinker here and there. Some of the mods I'm considering are dead simple, and Ray even wrote up a few as suggestions, all those years ago - but there's a couple things that aren't quite so trivial, and as a newbie to low-level electronics I figure I oughtta sanity-check myself before I go needlessly blowing anything up...
(Not that MFOS needs any introduction 'round here, but for reference, the schematics/theory-of-operation are here.)
First off, I'd like to normalize the external-input jack to the VCA output, for the usual overdrive madness. There's already a test-point for this (X85 on the VCA schematic,) but do I need to do anything to isolate it from the master volume pot (R196) to avoid having that attenuate the feedback signal, and/or having the ext.-in pot (R165 on the mixer schematic) affect the main audio output?
(I'm also adding a separate VCA-out banana jack; dead simple, but the same question applies.)
More significantly, I'm adding triangle shapers to the VCOs, using the circuit on the MFOS website and a couple of the "Swiss Op-Amp Knife" boards. That part's simple enough - but if I'm following the schematic correctly, one op-amp out of the the four the TL074 provides is unused. Ray has it "tied up," I assume to reduce any chance of noise pollution, but that leaves me with two spare op-amps that I might as well put to use...
One I figure I'll set up as an inverter; that's well-documented enough (there's an example right in MAKE: Analog Synthesizers,) and the only real question I have with that is to what extent the absolute value of the resistors in the standard inverting-buffer op-amp circuit matters. I know their ratio controls the gain, but if I just want a 1:1 inverter, how much difference does it make whether I use, say, two 10K vs. two 300K resistors? I've looked into this a bit and seen some discussion on impedance/loading vs. thermal noise, but I'm not sure how big of a deal any of that is in this context.
And now the complex part: I'd like to add a secondary VCA, since You Can Never Have Too Many VCAs, and after some digging around I found my way to this circuit, a basic (inverting?) linear VCA using only an op-amp and a couple JFETs. Good enough for my purposes, but I'm trying to wrap my head around how it works and figure out what I'd need to do to adapt this for my project.
So...going off the description and what (I think) I understand, Q1 is used as essentially a voltage-controlled resistor, attenuating Vin (shunting it to ground) based on the value of Vc before it goes to the inverting input. I'm...considerably less clear on things from there. What's the purpose of Q2? It also appears to drain Vin to ground before it goes into the non-inverting input, but its gate is tied to ground - isn't that 0V, a.k.a. "wide open" on a JFET? How is this different from just tying the non-inverting input to ground directly? And if the path through Q2 to ground is already "wide open," what's the point of the "k * R" resistor on a separate path to ground?
Also: I gather from the notes at Vout that k is the gain factor, so if I don't need greater than unity and only care about the VCA as a voltage-controlled attenuator, all the resistors can be the same value, no? Do any considerations about the absolute value carry over from the inverter to this application?
Also also: the description lists the properties to look for in the JFETs, but not the op-amp, which is specified as an LT1006; what differences between that and the TL074 might need to be accounted for? And on the subject of the JFETs, the description suggests that a current limiter (resistor?) might be advisable if a forward gate voltage is possible, which (if I'm understanding things correctly) it most likely is. What sort of value would be necessary for that?
Thanks in advance for any advice/commentary
Statistics: Posted by commodorejohn — Wed Sep 11, 2024 4:18 pm — Replies 2 — Views 67