[sdiy] Oi vay. The trials and troublations of sequencer design. ;)

[Tim Stinchcombe]

Hi Ryk,
	I didn't follow exactly what you are trying to do, but for a
semitone, i.e. 1/12V drop, across the 120ohm resistors requires a current of
(1/12)/120=0.7mA. This is then 'programmed' via R1+R2 at amp A1:

R1+R2=(15-Vp)/0.0007,

where Vp is voltage at A1 non-inverting input. So for R3=4.7k, R4=10k (as
the book) Vp=10V, requiring R1+R2=7.1k, and swapping R3 and R4 gives Vp=5,
so R1+R2=14.3k (so if this is what you've done, you *will* be at one end of
R2 and still will not have what you want, so increase R1!).

R5 just sets the 'reference' voltage, given when the lowest key is pressed,
and can be adjusted to suit (according to the current prgorammed above).

Tim

__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk


> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl 
> [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of 
> Rykhaard D.A.M.I.A.N.
> Sent: 11 December 2005 01:03
> To: synth-diy
> Subject: [sdiy] Oi vay. The trials and troublations of 
> sequencer design. ;)
> 
> 
> Well - after 10 years of development (with only a 9 year, 11 
> month, 30 day and 20 hr. break in between ;) LOL ) of my 
> sequencer, some progress has finally been made. :)
>  
> Using the 'keyboard interface' constant current source, to 
> drive my 12 step rotary switch, from Barry Klein's book - I 
> spent well over 2 hours trying to figure out WHY, I was 
> getting a continuous 10.21VDC across my trimmer; my tuning 
> control and at the beginning as well as end of the 12 x 121R 
> 1% resistors, around the 12 step rotary switch. I tried 
> switching the trimmer for a pot; switching pots; switching 
> the 2N3906.  I KNEW the LF353 dual opamp was running fine. 
> 
> Almost 3 hours into bug searching, looking at Barry's 
> schematic - I realized I didn't have the Tuning pot, GROUNDED.
> 
> Ummm.  DUH?  :D  LOL
> 
> I'm still having a minor trouble though.  After switching the 
> 10k/4k7 resistor divider to the 1st opamp, to give 5V from 
> 15V instead of 10V, I found that I STILL have to have the 
> Tuning pot, as well as the Trimmer, set to NO resistance, to 
> get what I originally THOUGHT was 1 octave, between steps 1 and 12. 
> 
> Running my LFO (driving the ADSRs to the VCF and VCA), so 
> that I could switch the rotary TWO steps at a time (to go my 
> Whole tone - I figured), I found that THEN - I was going by 
> semitone! So originally - my 12 step rotary was giving me 1/4 
> scale tuning!  Haha. :D 
> 
> Now - I just have to figure out the proper resistance in the 
> entire chain, to give me 1/12th of a V/octave per step, 
> rather than a 1/24th. ;)
> 
> Finally though - once again after 10 years - development of 
> my sequencer (named Woody ;) - 10 years ago) has begun. :) 
> 
> Once he's up and running, the full schematic will be posted 
> to the webpage.
> 
> But - for anyone that has Barry's book out there - looking at 
> the single bus keyboard interface - could you possibly point 
> out ANYthing that I'm doing wrong - with minimum resistance, 
> giving me a 1/4 octave tuning scale rather than a normal 
> equal temperment scale??  (Increasing resistance on the 
> trimmer, from 0 resistance DECREASES the amount of voltage 
> between steps, rather than increasing.  I'm on a break for 
> the moment - just stumped. :( )  Help?  :) 
> 
> 
> -- 
> Take care,
> Warmth and Peace,
> Ryk
> 
> http://deathlehem.bravehost.com/damian.html - D.A.M.I.A.N.;s 
> webpage - Dec 0805 updated 
>