Elliptic Filters and SSM 2045 fun and frolics

Dave's Synth account jupiter4 at bt-sys.bt.co.uk
Mon Apr 22 11:34:00 CEST 1996


Hi  all,


Well I had a major sort out this weekend in my studio.

I know have a "proper" workshop area built at one end.

I have had some bits to build for a while so I thought 
Sunday afternoon would be an Ideal time to start building.

Project 1)

The Maplin MAX293 Elliptic Filter.

This is a nice little pcb kit from Maplins.
The Filter basically has a strange response,
more a sort of comb filter really.

Well to control it you have to send a clock signal in and this will
select the Corner frequency?!?! The Maplin data sheet doesn't
really describe the Chip or the Circuit in any great detail.
You have to pick your own values for some of the components too.

The big question : What does it sound like?




configuration :

I had  system 100M 110 VCO/VCF/VCA running from a sequencer.

VCA output to Elliptic Filter Input.

The Elliptic Filter Clock Input was from a Farnell Function Generator.
The Function Generator Can be controlled by Voltage. It also has sweep
function. I mainly controlled Function gen Freq from the follwing : LFO, VCO and EG.


Well when the clock frequency is stable it doesn't really sound too
different (This depends on the clock frequency tho, lo freq will 
modulate slightly).

When you sweep the clock freq via the Function Gen the effect is like
Phasing/Flanging depending on the speed of sweep. (Which you would expect from
a comb filter I suppose...). Anyways thats about all I had time for with that one.
Interesting little filter, which I may experiment more with.
Oh yeah the corner freq I selected with external components to 15Khz so the 
sweep starts there. You can select lower (or higher, up to about 50Khz) Corner Freqs.


Project2 :

This was a bit more straight forward. It's the Maplin SSM 2045 Voicing system.
This consists of 2 and 4 Pole LPF and Independant  twin VCA's

This again is a sort of building block PCB where you have a couple of options to
figure out. Simple enough, on/off board Q control and dual/independant VCA control.
There is no Filter Cutoff Control. This is done via CV only... I'm sure this 
shouldn't be too difficult to do tho, when I buy the Data Sheet for the SSM2045.
I haven't tested the VCA section yet, only the Filter. The 2 and 4 Pole filter
is controlled by the same Fc and Q control, I think this a limitation of the Chip tho.

Couple of specs of board.Freq Range 10Hz to 25 Khz. Dual PSU +/- 15V required.
Fc control Range +/-14V. Signal Input 150mV. (You can bung in higher ones and get distortion
tho...).

Config :

System 100M 110 VCO/VCF/VCA module running from sequencer.
Low O/P from VCA (@100mVp-p) to Filter Sig I/P
Filter 2 and 4 Pole O/Ps direct to Mixer (Hot Hot... ouch my Ears...) Note I did not have a
VCA controlling the Filter Sig output, SLAP! Screaming Resonance!!!! SINE... mmmm...
Wind the Q control back round to off. Phew... 

Control Input :
>From Sequencer Channel Works best on 0-3V range nice and burbley, especially with res up...
Also tried it out as VCO. Q all the way up, no Input and CV's from seq with a bit of
Portamento, nice....

On to EG's. Hmmm  Strange the control is not that cool. That must be due to the control
range being +/-14V and I don't have any Voltage Changer thingys... It seemed to work best on
Inverted Env Gens, Q up and lots of thips and thwacks etc...

Onto LFO - Fc modulation. This was NICE. The Sys 100M can switch Voltage level to *1 and 1/10
only (I definately am building an Atennuator Module soon). This gave me a good idea
of the FULL range of this little baby. I was getting some nice Bass Drums with Ramp
Waveform.

Anyways, that was a fun weekend. I nearly deafened myself with the SSM Resonance, my eyes are
still watering now thinking about it. I reckon I will buy another couple of the SSM2045 from
Maplin if they still do them. I have a SSM2044 module still to build that I bought from
them last year and they promptly discontinued it :(   Hopefully I'll have the SSM2044 
built soon, this is a better spec'ed kit with 4 CV inputs to Fc, I/P signal Level
control (luxury..) Filter control Control (yet more luxury) external resonance pot (joy , 
dribble), single supply  ahhhhhhh yeeeess....

Anyway,

I must dash now.

CHeers, Dave...






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