Square & Random LFO for Small Stone???

[moosapotamus]

Could this LFO work as a replacement for the standard LFO in a Small Stone?
http://www.cgs.synth.net/modules/psycho_lfo.html

If so, any hints about modifying so it will perform well on 9volts?

Alternatively, can anyone point me to a multi-waveform LFO scheme that would work well in the Small Stone?

Thanks!
~ Charlie

[mattv]

Isn't that practically the same thing as R.G.'s Pseudorandom LFO?

[moosapotamus]

Yes, with actual parts values specified, and a few nice add-ons like the speed control on one of the oscillators.

Any idea if it will put out the same voltage range as the Small Stone's stock LFO?

I'm guessing that it might need to be modified in some way. Since it is using a bipolar supply, won't the output swing from -V to +V? And, since the Small Stone uses a single, +9V supply, doesn't the stock LFO only swing from 0V to +V? Or, am I talking sideways? ~%)

Can someone help straighten me out?

Thanks!
~ Charlie

[drew]

The stuff on the Catgirl site is mostly intended for use with modular synths... I actually have a "Psycho LFO" built up and it's good although not really random.

For a real random circuit I'd try the one taken from the Roland SH-09: http://members.tripod.com/urekarm/synth/sh09_shn.pdf

While I haven't tried it, I'm pretty sure you could get workable results by feeding the grounds in that schematic with a 4.5v voltage divider, and sending +9v/0v to the +V/-V connections...


drew
toothpastefordinner.com

[drew]

Oh- PS- the schmitt triggers in the Psycho LFO are outputting square waves, which means they are probably squirting out 15Vpp signals as they're running from 0v/-15v... then the signal level of those is going to be dependent on the "range" control, and on the schematic they're mixed with 560k input resistors so when "range" is at maximum (100k), the maximum gain in the mixer is going to be 4* (100k/560k) which works out to be 0.57... so the signal at the output of the first opamp is going to be swinging between 0v and (-15)(0.57) = about -9v. The second opamp is going to invert it with respect to ground which leaves you with a signal that swings between +9v and 0v.


Someone please correct me if I am wrong here, I just like picking things apart... Isn't there a +1 in that gain calculation somewhere? It should get you close to where you need to be, at any rate.


drew
toothpastefordinner.com

[R.G.]

The Psycho does have some problems with +9V only.

Here's the scoop. It is customized for the +/-15V environment of the analog synth. The four schmitt oscillators work fine from 0V and +9V as is, just put 0V on the logic ground pin, +9 on the +Vcc pin, and you're set.

The special case Ken alludes to on +/-V is a problem. What we want in a +9V world is an LFO that moves around somewhere between -V and +9V, and has a bottom or top that it doesn't go beyond somewhere in that range.

CMOS outputs combined in a passive mixer this way make a voltage that is always within the 0V to +9V range, and may have a mixing reference resistor or none at all. As shown, the resistors that join from the outputs of the CMOS pull the mixed voltage up to +9 when they're all high, down to 0 When they're all low, and have an average value of about middle of the range. If you take a single resistor from the junction of the 560K's to any voltage within the 0V to +9V range, it both referrs the LFO output to that voltage and scales it by the resistor values. For instance, if you tie a 180K resistor from +4.5V to the junction of the 560K's, you'll get a waveform at that junction of about 4.5V peak to peak, and average of 4.5V.

The inverting mixer can still be used, you just have to tie the non-inverting input to a reference voltage. This would nominally be 4.5V, but could be something else to move the LFO output up or down. The glide generator would still work as shown.

It's not the right voltage for a Small Stone. Very few things are, as the Small Stone LFO puts out a current. The Iabc input for OTA's are one or two diodes above the negative power supply rail.

R.G.

[drew]

Would it be possible to use a V-I converter (these are one or two transistors, don't have the schematic nearby - they are complex for precision circuits like VCOs but basic V-I conversion is pretty easy, as I recall) to use a regular LFO to inject current into the OTAs? Or is this what the small stone's LFO is doing - putting out a voltage and converting it at the end to put it into the OTAs.


drew

[moosapotamus]

For a real random circuit I'd try the one taken from the Roland SH-09: http://members.tripod.com/urekarm/synth/sh09_shn.pdf

Thanks Drew. That looks cool, too. Guess I'd need to add a clock generator with a speed control and tweak it to run on 9V.

But then, as R.G. mentions, there's the "current" thing...

The Psycho does have some problems with +9V only.

Here's the scoop. . .

It's not the right voltage for a Small Stone. Very few things are, as the Small Stone LFO puts out a current. The Iabc input for OTA's are one or two diodes above the negative power supply rail.

That's great, R.G.! Thanks for the helpful info on adapting the Psycho to 9V. I know I can find a use for it in some other stompbox.

Regarding the Small Stone, tho, would a voltage-to-current converter be a workable solution to grafting other LFOs (that have more control/waveform options) into the SS?

I happen to be stuck on the Small Stone, at the moment. I like the character of the sound a lot. The vibrato mod is cool. The phasefilter effect is awesome! I'd just like to be able to wrangle some more out of it.

So, I'm interested in attacking the LFO, maybe adding a squarewave option (with PWM would be nice), a pseudo random option, and so on... I'm not holding out a lot of hope for figuring out how to do envelope control of speed or intensity, but I'd be all ears if anyone has any ideas. :mrgreen:

Any other ideas about modding or replacing the LFO section in the SS with something more interesting?

Thanks all!
~ Charlie

[cedric]

so who will do this add-on on my MoogerFooger.
i LOOOVE the thing, but i hate when the LFO is engaged.
it's sooooo boring. sweeping uuuuuup and doooown....

*yawn*

- ced

[moosapotamus]

so who will do this add-on on my MoogerFooger?

ced - I think the Mooger is different. As opposed to the Small Stone which requires current output from the LFO (not much more than ~1mA, I think?), the Mooger can handle a basic control voltage LFO. So actually, no need to mod the mooger. You can plug anything you want into the LFO/CV input jack on the back... anything from a simple squarewave oscillator to a multi-step analog sequencer... Seek-Wah on steroids!

Still looking for ideas specifically for the Small Stone LFO. :mrgreen:

Thx!
~ Charlie

[puretube]

a series-resistor from LFO-out to Iabc-in will convert voltage to current...




//www.puretube.com

[moosapotamus]

puretube... you rock!

So, is it kind of like calculating the optimum value current limiting ( :idea: doh!) resistor to use with an LED? If my LFO has a 10V swing, then I should use a 10K resistor to limit the current output to 1mA, no?

Thanks!
~ Charlie

[puretube]

yes, calculate your 10V minus about 0.7V for above mentioned diode-voltage-drop (or is it 2 drops?), divided by the R-value to remain under 1mA.
For a 9V circuit, 10kohm is a nice value to be safe.

Note: it would be nice if your LFO`s output-level at it`s lowest point is exactly this one (or 2) diode-drops....



//www.puretube.com

[moosapotamus]

yes

~ Charlie