Glide circuit of the ProphetV (PART 2)

[Harry Bissell]

Roel Das wrote:

> Few more questions; what is the function of Q1? Which transistor do you use
> and why?

Q1 is a simple current source (current mirror). You input a positive voltage
from 0 to some positive
voltage and it sources a corresponding current to the OTA. The two transistor
version in
the proV schematic is far superior. Temperature makes the single tranny drift,
and what might make a nice long glide at one temp might completly cut off the
OTA a little later
The two tranny version (especially with two trannies in one package...)
temperature cancels out, and the one tranny biases the other at exactly the
right point for the input voltage range to get to zero volts.  The Pro5 uses a
dual transistor in one package... the better the thermal and electrical matching
the better it works... The ProOne used two trannies (2N4250) with the packages
held together with a clip to keep the temperatures equal...

How much money you got ??? use better parts.... If you want all your glides to
match each other perfectly... keep ALL the glide trannies at the same
temperature....

The OTA normally doesn't cut off until the current into Gm input is "zero" which
happens (in voltage terms) one diode drop above the NEGATIVE rail, which is damn
hard to run a CV to... this tranny (or preferred trannies...) makes the control
work over a more usable range.

>
> How do you find the values of the res. and caps? By simulation? or
> calculation?

In this case by... looking at the schematics... The two resistors need to be the
same value,
and I wanted another resistor much smaller... so with the 4.7K from the ProV
print,
I'd need to use like a 47 ohm... so I scaled it up. Things can get funny if you
use too low or too high value resistors... Practical resistor values for most
DIY semm to go from hundreds of ohms to low megohms. Too low and the current
gets so high things burn up...
op-amps cant deliver etc... too high and things like tiny noises, stray
capacitance, PCB leakage (across the surface from dirt moisture etc...) tend to
screw things up... So usually
we try to scale things for reasonable currents.

M1 the meter was for me to monitor the current into the OTA Gm input. If it is
big (1mA is big here...) the thing should be full on... and glide the fastest.
If it is real low (1uA) then the glide should be long... The 10K in series with
the tranny gives just over 1mA.

I used to (in industrial controls) think 1mA was a very small current... till I
got more into DIY where 1mA is big... depends on what you are doing.

The cap was just right off the schematic....

The thing to do is... COPY success. Everyone here does it. If someone has a
piece of a circuit that works, steal that piece and reuse it. I like the EFM
site, because everything has been pre-stolen so I don't have to find the
originals and re-steal them !!! I knew the ProphetV glide worked (in my P5) so
when I needed a glide circuit... POP!

The Two tranny mirror is much better... use it. The one tranny is good only if
you are going to twist the knobs and get the sound by ear...

Glide times depend on what you do musically... I suck at keyboard playing (my
computer recorded CD covers up this fault beautifully: shameless plug...) so I
like LONG glides like
10-20 seconds... the kind you hit the low key... then the top key... then leave
the stage and you are in the limo (VW Microbus....) before the glide tops
out....

OTOH good keyboard players use subtle glide times... low milliseconds. You
probably should go from about 1mS (no glide) to whatever you like or find
musically pleasing...

> And which glide times should I go for, what is the usual value?
>
> By the way, I'll leave the expo glide out... Don't want to much trouble....
> :-)

Expo glide sucks anyway (flamebait anyone???) like BBDs do ;^)

H^) harry

>
>
> Thanks
> Roel
>
> > Roel Das wrote:
> >
> > > Wat is M1 in fig Glide2? A matched transistor pair?
> > >
> > > Thanks again
> > >
> > > Roel
> > >
> > > ----- Oorspronkelijk bericht -----
> > > Van: "Harry Bissell" <harrybissell at prodigy.net>
> > > Aan: <synth-diy at mailhost.bpa.nl>
> > > CC: <jhaible at debitel.net>
> > > Verzonden: donderdag 17 februari 2000 4:00
> > > Onderwerp: Re: Glide circuit of the ProphetV (PART 2)
> > >
> > > > Hey Hey JH:
> > > >
> > > > Now I remember... The trick was to remove the diodes and substitute a
> > > > resistor
> > > > across the inputs of a substantially lower value. I think the diodes
> do
> > > > "clamp" the
> > > > feedback, so the differential arcoss the inputs is .7volts, and
> > > > reasonably constant
> > > > until the input and output are within the diode drop.
> > > >
> > > > Replacing this with a resistor of small value will limit the
> > > > differential voltage
> > > > (its a divider) but the difference will be a proportion, instead of a
> > > > constant.
> > > > Eventually, in either case the DC level will be reached.
> > > >
> > > > My model of course doesn't have any nasty offsets etc. But it does
> show
> > > > the idea of
> > > > the linear/expo operation. This is using the LM13700 model from
> National
> > > > Semiconductor...
> > > >
> > > > And a "perfect" op-amp. ;^)
> > > >
> > > > Damn I had to think for an hour about this one... I remembered it was
> > > > "so easy" but
> > > > I never did it... 'cause I don't like expo glide anyway.. LOL!!!
> > > >
> > > > H^)  harry
> > > >
> > > > Harry Bissell wrote:
> > > >
> > > > > Hey JH:
> > > > >
> > > > > thanks for the info. I'm not sure I agree about the diode
> function...
> > > maybe.
> > > > > There was a mod that sequential wrote for "expo glide... I'll try to
> > > find it an
> > > > > make this more accurate. Its been a long time and I don't remember
> it
> > > > > perfectly...
> > > > >
> > > > > H^)
> > > > >
> > > > > jhaible wrote:
> > > > >
> > > > > > > > And, when you use the 13700, you don't need the buffer,
> because
> > > you can
> > > > > > use
> > > > > > > > the one on the chip right?
> > > > > > >
> > > > > > > No... the darlington buffer can only "pull up" and has a passive
> > > > > > pull-down. so
> > > > > > > it can source lots of current, but sink is fixed... this would
> cause
> > > > > > asymmetry
> > > > > > > in the glide.
> > > > > >
> > > > > > No. Not if you add a small enough resistor from the buffer's
> output to
> > > the
> > > > > > negative supply. An emitter follower has low output impedance
> > > regardless of
> > > > > > polarity,
> > > > > > as long as you don't steal *all* the current from the transistor.
> > > > > >
> > > > > > > The diodes clamp this and the resistor limits the voltage to the
> > > clamp. as
> > > > > > a
> > > > > > > side effect this makes the ramp linear for large steps (input
> > > differential
> > > > > > is
> > > > > > > clamped at .7 volts) and sort of exponential for small steps. In
> > > practice
> > > > > > you
> > > > > > > get a linear glide.
> > > > > >
> > > > > > The diodes are just for protection. They don't have anything to do
> > > with
> > > > > > linear
> > > > > > or expo glide. (when they start to conduct, the OTA's input stage
> is
> > > long
> > > > > > saturated.)
> > > > > >
> > > > > > > If you remove the diodes and resistor and ground the negative
> input
> > > you
> > > > > > will get
> > > > > > > exponential glide
> > > > > >
> > > > > > No. That's the case if you remove the diodes and use small
> resistors
> > > to GND
> > > > > > at the OTA inputs.
> > > > > >
> > > > > > (and problems of offset voltage which you must trim out for DC
> > > > > > > accuracy)
> > > > > >
> > > > > > Yes. That's why expo glide with OTAs is a bad idea.
> > > > > >
> > > > > > > The feedback assures DC accuracy.
> > > > > >
> > > > > > Yes. What you see at the output is the offset of the input, i.e.
> > > small.
> > > > > > In the expo case, the offset voltage is amplified according to the
> > > chosen
> > > > > > amount of negative feedback. Small feedback -> high offset,
> > > > > > strong feedback -> glide becomes more and more linear for larger
> > > > > > intervalls.
> > > > > >
> > > > > > JH.
> > >
> >
> > --------------------------------------------------------------------------
> --
> > > ----
> > >
> >
> > --------------------------------------------------------------------------
> --
> > > ----
> >
> >