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VC-LFO as audio VCO
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author VC-LFO as audio VCO
Synthbuilder
Today I spent some time looking into the possibility of using the VC-LFO PCB as a dedicated 1U wide VCO module. As you may know the standard VC-LFO circuit does perform across the whole audio band but 1V/octave tracking goes flat above 1.2kHz or so. This is, of course, not a problem since the unit is designed to be a low frequency oscillator.

However, the VC-LFO does sound rather nice and it has a distinctive sync sound that is useful. With that in mind I tried to speed up the circuit to allow it to work with good tracking across the usual range of the keyboard. I have had some success but it's not as good as I had hoped.

By changing the main timing cap to 220pF one can increase the range of musical operation to just under 2kHz. Tracking is excellent (+/-1cent) below 1kHz and then progressively worsens above this becoming -7 cents flat at around 2kHz. For most modern musical uses this is actually quite acceptable - although nowhere near as good as my normal VCO design and not really good enough for me to create a separate product from it..

The main cause for this flattening of the tuning curve, I think, is the delay(s) in the comparator of the VCO core. These could almost certainly be compensated for but that goes beyond a simple component change for this PCB layout.

Those that are interested can have a go with the following changes to the standard VC-LFO parts:

C12 220pF (or 470pF) - this speeds up the VCO core.

R9 to 390K - this reduces the range of the coarse pot.
R10 to 10M - this reduces the range of the fine pot.

Tony
Godric
Tony demonstrated the difference in synch sound between the normal VCO and the VC-LFO on my Jubilee system at Synth DIY this year.

I can vouch for the difference in sound. The VC-LFO has a synch-sound with a real edge to it.

even though the tracking above 1K might not be perfect if you are using it as a slave osc for synch effects, that's not such an issue (might even be one of the reasons for the edge, as the synched oscillator is typically octaves above the master).

I had one of those "oooh I didn't know it could do that" moments.

Godric
SepticUnderground
FYI if my memory serves me well, I did track the VC-LFOs more over the 2 oct range, into 4 orso i think via Silent Way calibration and MOTU 24IO.

They do sound very raw and cool indeed.
Synthbuilder
SepticUnderground wrote:
FYI if my memory serves me well, I did track the VC-LFOs more over the 2 oct range, into 4 orso i think via Silent Way calibration and MOTU 24IO.


You should get a lot more than two octaves since the limitation is the highest frequency rather than the accuracy and the range of the exponential convertors. If you start your note range at 55Hz (a nice low A), you can easily get 110, 220, 440 and 880. Even on the old issue 1 VC-LFO.

The issue 2 VC-LFO built as stock can go higher still. Whilst the mod above will push that even higher - close to the complete range of a 5 octave keyboard.

I would have thought that any software corrected system should be able to push it to 10kHz. The VC-LFO does not drift as such - the 'problem' is that the expo curve goes flat. But it is repeatable and could be corrected I would have thought.

But it is quite usable as a audio VCO if you don't push that hard. And for as use as a sync slave (and who would not love to be a slave to the sink?) it's high frequency tracking abilities are not so important.

Tony
Neutron7
How low can it go when this modification has been done? is it still a usefull LFO?

Or if a switch was used between the 2 capacitors (with the range pots left alone) would the long wire to the switch from the timing capacitor cause problems?
Synthbuilder
Neutron7 wrote:
How low can it go when this modification has been done? is it still a usefull LFO?


The operation frequency is now 10 times higher so with the timing cap at 470pF you'll still be able to go as low as 0.02Hz - maybe even lower with external CV inputs.

However, if you have changed the sensitivity of the tuning pots by altering those resistors your control range will be much decreased. The problem is that the perfect speed controls on an LFO give you too much range on a musical VCO.

Neutron7 wrote:
Or if a switch was used between the 2 capacitors (with the range pots left alone) would the long wire to the switch from the timing capacitor cause problems?


I don't really know. It's bound to have some negative effect. But whether it would be noticeable I couldn't say. I would recommend using a twisted pair or even screening the cable to reduce as much pick up as possible.

Tony
analog604
I know this thread is old but it seemed like the proper place for the VC-LFO as an LFO.

I've got an VC-LFO Issue 3 PCB and am considering adding the C12 caps to a front panel switch.
I'll tie in the Issue 3 C12 component value 4n7 as well as the extended range 1nF as suggested in Tony's build guide.
I'll keep the wires as short as possible and use shielded mini-coax.

My goal isn't to have the mod'd unit track perfectly above the 2k, but simply to add the range option.

Guys, has anyone found this mod useful or thoughts?

Will it be worth the effort? The front panel is not mounted and I'll only need to make one drill hole into it. My front panel design is complete and ready to be adhered.
Also thoughts on possible most useful values to choose for C12 (other than the 4n7 and 1nF)?

thanks!
Jay
Synthbuilder
You can certainly try it.

However, thanks to the HFT trimmer and slightly different circuit the issue 3 boards do have a greater range and accuracy than the issue 1 and 2 VC-LFO modules. Therefore, the need for a switchable timing capacitor is probably not needed. Also issue 3 has the coarse and fine tuning pots (issue 1 had only tune pot and a range switch) so should already allow for a wide range of frequencies to be controlled.

If you are going to do it then make C12 a 1nF cap and then use your switch to connect a 3n9, 3n6 or 3n3 across it to slow it down. This way the circuit always has the 1nF cap in place when the switch is being moved. If you simply have a switch going between 1nF and 4n7 they'll be a point in the switch's motion that will have no capacitor attached to the circuit and the circuit will momentarily squeal. Some switches do have a make before break action which would stop this intermediate behaviour but most don't.

Tony
analog604
Thanks very much Tony!
terjewinther
Just to add a little:
I have an older VC-LFO (I can´t remember if it is iss.1 or 2) that I uses as VC-LFO, but also as a standard "bass" VCO now and then. I have even added (on veroboard) the PWM circuit from the standard VCO at the time, so (if I remember correctly) I take the square out of the VC-LFO and into the PWM circuit, and have added external CV control of that extra part.
I have no problem using the CV-LFO as a standard VCO. I haven´t pushed it all that way high up, but for standard sequencer work in Berlin school style covering 2-3 octaves, there is no problem at all. It is way more stable than my old moog modular VCOs any day :-)
And yes: I do agree that there is a slightly different sound to the VC-LFO compared to the standard VCO. Subtle, but still there.
The thing I really like about the VC-LFO though, is that I can make these really long LFO curves that are smooth (they follow the curve they are supposed to follow), and not stop. That is very nice.
analog604
hi terjewinther,

interesting! smile

Did you manage to fit that on a 1U x 5U tall panel?


terjewinther wrote:
Just to add a little:
I have an older VC-LFO (I can´t remember if it is iss.1 or 2) that I uses as VC-LFO, but also as a standard "bass" VCO now and then. I have even added (on veroboard) the PWM circuit from the standard VCO at the time, so (if I remember correctly) I take the square out of the VC-LFO and into the PWM circuit, and have added external CV control of that extra part.
I have no problem using the CV-LFO as a standard VCO. I haven´t pushed it all that way high up, but for standard sequencer work in Berlin school style covering 2-3 octaves, there is no problem at all. It is way more stable than my old moog modular VCOs any day :-)
And yes: I do agree that there is a slightly different sound to the VC-LFO compared to the standard VCO. Subtle, but still there.
The thing I really like about the VC-LFO though, is that I can make these really long LFO curves that are smooth (they follow the curve they are supposed to follow), and not stop. That is very nice.
terjewinther
analog604 wrote:
Did you manage to fit that on a 1U x 5U tall panel?


No: in that particular modular I am using Formant sized panels, but cut down to 5U hight (Formant i 6U high). So they are slightly broader, and also I was using small jacks (3,5mm) so there was room for extra jacks.
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