[sdiy] string machine technology?

[Richard Wentk]

On 2 Nov 2006, at 07:21, Peter Blackett wrote:

> Dear Rykhaard, and DIy list.
> as mentioned before if you are copying most of the production  
> string machines they use top octave generators and BBD delay ic  
> chips to produce the chorus effect.
> it would be possible to use separate oscilators,to rep[ace the tog  
> ic , but I don't think it is easy to reproduce the chorus effect  
> without the BBD delay ic's.
> [ if it is please let me know ! ]

It's not. The BBD chorus is the key to the sound. The designs I've  
seen use three phase-locked sine LFOs 120 degrees out of phase to  
modulate three BBDs. Three more phase-locked LFOs running at around  
7Hz are added to each mod line to add some extra movement to each  
channel.

You can use two BBD channels instead of three. but that doesn't sound  
as nice.

> But I would be interested to know what the Original String machines  
> built by Ken Freeman [ is that the correct person ? ] used.

If I remember correctly, these used three independent octaver+divider  
circuits to add extra moment. Plus a chorus.

> of course if you have the space you could probably find a late  
> 1970's organ that has a string synth built in at a good price.
> regards

Aside from the chorus, the only thing that makes a string synth  
different to an organ is some high pass filtering and soft attack.

If you can find a circuit for the old Transcendent DPX kit online  
anywhere you'll see a complete design, including the chorus.

It can't be emphasised enough that the chorus unit design is key. The  
BBD delay timing, feedback amount and LFO rates make all the  
difference between a design with a warm smooth sound and something  
with a ringy metallic edge.

Richard