[sdiy] Roland System 700 VCF

[Oren Leavitt]

Veronica Merryfield wrote:
> Is there a resistor on the output of IC1 that is missing in the schematic?
> 
> The output of the CA3080 (IC1) is fed straight into the non-inverting input of IC2 and is meant to sum the inputs too. As drawn, IC1 output would completely dominate the input to IC2. The other stages show a 15K in the output of a stage so I am guessing the schematic is in error. Or I missed something.
> 
> Vrnc

Looks like IC1 implements voltage-controlled resonance.
The OTA being a high-impedance current-out device is likely just adding 
a little output feedback as a current to the input summing node at IC2.

- Oren

> 
> On 2010-07-10, at 1:37 PM, Stewart Pye wrote:
> 
>> Hi,
>>
>> I've been looking at the System 700 VCF schematics with the thought of designing a circuit based on that but using the SSM2164.
>> Sys700 Schematic here....   http://fa.utfs.org/diy/roland700/703e_a3.jpg
>>
>> It's a 4 pole OTA into integrator design. The output of each integrator is fed back via a 22k resistor to the non inverting input of the OTA for that stage. Does this mean that for the filter to NOT oscillate it needs overall negative feedback from the output to the input? I think this is being done in the resonance control in the Roland circuit...
>>
>> I'd appreciate it if someone could clarify this for me. I may be way off....
>>
>> Regards,
>> Stewart.