patchell patchell at silcom.com
Tue Jul 4 17:18:31 CEST 2000

Paul Maddox wrote:

> Dear All,
>  Ok couple of questions regarding LFOs..
>   Im developing a polysynth (with the interest of a certain synth
> manufacturer) and I intend to make it
> 8 voice poly. Now couple of questions about LFOs,
>   1) Should I have 8 seperate LFOs or one global? I dont want both.

    This is an engineering decision.  Having 8 LFO's will increase the price of
the product, however, if you want to do number two (no pun intended), 8 is going
to be mandatory.

>   2) Midi Syncable LFO, Ive been advised that this is something they would
> like to see,
>         someone explain to me what it is, Im guessing that with each
> successive midi clock received the LFO is
>         stepped through its waveform. am I right? and if so how do you tell
> it how many steps it takes to do a full
>         cycle, ie 1 beat, one bar, 8bars??

    I don't really know for sure what it is, but in the past, I have synced
LFO's to the note on GATE.  This will insure that the LFO starts from the same
place every time.  For instance, if you are using the LFO to produce vibrato,
you might want the tone oscilator to start on pitch every time, and then vary.

>    3) I would like the LFO to be voltage controlled (its going to be
> programmable) and have a sine wave (if the
>         above point is true then it doesnt matter as its going to have to be
> a lookuptable anyway).
>   oh, I also want it done with just 2 opamps (ie ONE TL072), I need this
> thing to be compact if Ive got to build
> 8 voice cards. Trying to find an easy way to do 3 EGs under CV/DIGITAL
> control. All with as few chips as
> possible..

    It is posible to make an VC oscilator with just two opamps, in fact, two
opamps and one transistor, but it will have a linear freq response.  Look in a
National Semiconductor Data sheet for the LM324.  There is a single supply VCO
in there that will work just fine with the TL072 (maybe with some slight mods),
but, I have never tried to add sync to this oscilator, although, just using a
fet to short out the cap genearally always works.

>    Thanks for any help,
>   Paul Maddox
> ________________________________________
> Modulus Synthesiser Website;-
>     Http://www.noxons.ision.co.uk/


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