More stuff on the 303...

[pharding at mail.ndirect.co.uk]

Hi Tony,
             Just thought i would share some thoughts on the 303 
with you.I've never built any 303 bits myself although i've put 
together a modular synth.....[there's a picture of it on Paul 
Maddox's Modulus web site..]

I've got 2 303's in the studio right now[jammy git or what!]mine's a 
bog standard 303 with sockets added by Kenton for gate,cv,slide ,etc 
and the other one belongs to a friend and has an older kenton midi
retrofit.This is where the 303 sits on top of a steel case which 
presumably contains all their electronics +PSU,i've never has cause 
to open it.....

I dont want to put you off in your 303 studies and i'll try not to 
write a thesis now but i've come to the conclusion that it's the 
sequencer part of the 303 that gives it the biggest part of it's 
characteristic sound that we all love.IN fact i'd go as far as saying 
that both 303's sound worse over midi compared to when they are 
programmed with their own sequencer.With my 303 connected via gate /cv 
it takes longer to program a decent pattern than using the 
internal/infernal? sequencer.All 303 clones suffer from this as you 
have to program the slides and accents via midi controllers.The 
kenton retrofit 303 has a switch on the front panel to allow midi 
velocity to control filter cutoff,yet whilst this is amusing for a 
bit the resulting patterns are never quite as satisfying as those 
programmed on the 303's own sequencer.   As an experiment i've 
connected my modular up to the 303's own gate/cv outs and now my 
modular can sound quite like a 303 as all the slides and rests are 
transmitted too ......irrespective of what modular components i use[ie 
i can change over the filter module to a different sounding LP filter 
or even change VCO from an ASM-1 vco to a LM13700 vco ]the resulting 
sound still has that certain,303ness to it.

Both my 303's internal sequencers also have this other quirk which 
may be to do with the way they follow the incoming Sync 24 
pulses,i'm not sure but what happens is that no 2 bars are absolutely 
the same timing wise.I proved this when trying to sample a 303 
pattern and i could visually see the difference in the waveform on my 
akai..The timing is only a tiny bit out[real small amounts]but i fell 
that this certainly adds to the all important groove of the pattern 
where as over midi you get a more static pattern.[you could say that 
this staticness applies to all digital synths though].
 The accents are well important too,[so it's vital to 
incorporate an accent feature on your design.]especially when feeding 
the 303 in to that all important distortion unit,if that's you're 
thing.The accents get flattened out slightly when distorted and this 
compression makes the sound even more immense.  

> 
> The filter resonance is not only dependent on the res pot, but also on
> the actual value of the 5V3 power supply. If it is only a little higher,
> the gain of the diff amp becomes large enough to cause the filter to
> self oscillate with the res pot fully clockwise. The 5V3 is set up to
> feed the dac, and hence to give 1v/oct, so I suppose most 303s will have
> fairly similar power supplies.
  IIRC the resonance or cutoff? pot is dual ganged.what is the 2nd 
element in the pot doing? 
> The mixer is greatly affected by output load. When loaded with a 22k
> input impedance from my bench amp, it clips on negative swings, and the
> positive is affected in an unusual way. This generates some nice
> distortion. This may well be a cause of that TB sound. However, the
> amount of distortion is dependent on the level of signal from the volume
> pot. I am experimenting with additional overdrive circuitry to enhance
> the sound. But I can't decide whether to put it before the VCA or after
> it.

What would be really cool is for you have 2 VCA's one as normal then 
feed into a distortion circuit then another VCA post distortion set 
up as an expander gate.Maybe have 2 separate outs,1 clean sounding 
and one distorted?

One problem with Basstations and other 303 clones is that they have 
too many knobs.In theory the more adjustments the better but in 
practice i find that all that happens is that the 303 sound that you 
want goes "off the boil" too quickly whereas on the 303 all the 
parameter ranges are usable.

 The more I look at the waveforms, the less I am convinced that the
> BA662 actually affects the sound... but more tests have yet to be done.

I doubt if the VCA is that important to the overall sound which is 
also a combination of all the above things,i guess that as with all 
analogue synths it's the imperfections that make them sound so great!

I hope this may be of some use in your research

Good luck
Paul Harding.