Interesting...I was assuming the poor-quality sine was a result of
using diodes rather then the more common differential transistor
pair circuit. But by adding the trimpots to improve the diode sine,
it's now looking more like a bug in the original design the two
operating points may be set incorrectly (I can make my Aries sines
look exactly like Doepfer by mis-adjusting them!).
If the triangle level is consistent, maybe all we need to do is
tweak the two resistor values to optimum points as you suggest. If
the level is not consistent, then trim pots are needed instead.
--- In Doepfer_a100@yahoogroups.com, "Tim Stinchcombe" <timothy@...>
wrote:
>
> Hi Dieter,
>
> Without doubt adding a separate PCB would be the easiest way by
far
> of modifying the module. I am more than happy for you to do this
if
> you think it is worthwhile, as I hardly think I can make
any 'claims'
> on the circuit, as all I've done is copy it from several sources
> (Chamberlin's book and Barry Klein's book, both probably sourced
from
> Electronotes, where it appears at least half a dozen times)!
>
> Unfortunately your assertion that it needs 'no power' is not
entirely
> true - I mentioned at the bottom of the page that I had omitted
two
> trimpots, and one of these *is* across the rails. I didn't include
> them originally as I didn't fully understand the implications of
the
> component value selections: however I have just updated the page
and
> now included them. Not fully understanding the circuit as yet, it
is
> not clear to me what spread of values will work, but simulations
show
> that altering them could easily affect the range of adjustment
> available. For the circuit to work at all the triangle must swing
by
> more than the JFET pinch-off voltage, and the circuit needs to be
> adjusted to just the right spot, hence the 'shape trim'. At the
> moment I'm using a 2N3819: with Vp approx -8V, I've had to
slighlty
> amplify the triangle to get the circuit to work. However with a
> BF245B, it should run directly from the triangle available in the
A-
> 110 (I hope to get a few of these soon, to find out!).
>
> I have the original paper on order from my local library, and when
I
> get it in a few weeks, hopefully it will explain how the circuit
> works, and in particular whether the 150ohm resistors will need
> tailoring for the Idss of a BF245B and the slightly lower tri
swing
> of the A-110 (the other references to the circuit seem to be based
on
> a +/-10V triangle).
>
> Such a PCB would make modifying the module pretty easy: remove
R42,
> R52, C9, D1, D2; input to the PCB from one end of where R42 was,
> output back to 'O4' to the other; straps for the power and ground
> rails are the biggest headache! Setting the 'symmetry' trimpot
(which
> nullifies the effect of any residual DC in the triangle) can
probably
> be done well enough by eye using a scope; getting the 'roundness'
> right - the 'shape' pot - seems a little trickier (but possibly do-
> able by ear for those able to distinguish presence/absence of
> harmonics).
>
> Incidentally I also wondered whether the component values for the
> existing set-up could be better 'optimised': after a handful of
> simulations, and then discovering the other circuit, I was
convinced
> that that path wasn't worth pursuing further!
>
> Regards,
> Tim
>
>
> --- In Doepfer_a100@yahoogroups.com, <hardware@> wrote:
> >
> > Tim,
> >
> > excellent work. Sounds very interesting - especially as the
circuit
> > obviously does not need a power supply. Maybe we could built a
> small pc
> > board that can be added to the A-110 (provided that we get the
> permission
> > from you). Only three wires are needed (tri input, sine output,
> GND).
> > Another solution would be a modification of the small output
board
Show quoted textHide quoted text
> of the
> > A-110 (adding the additional parts to this board).
> >
> > Best wishes
> > Dieter Doepfer
>