[sdiy] AM8060 - Jupiter 6 SVF

[rob at emulatorarchive.com]

Hi
The IR3109 in SMD form has been replicated using LM13700 SMD chips in 
the Analog Rennaissance 80017A replacement for the Juno106.
In particular the expo response was carefully analysed, which is 
obviously an important charcteristic. Other clones such as the Synth 
Restore version probably use the same approach.

If you go back to the introduction of the IR3109 in the Jupiter 4 mid 
production in 1981, you can see it replaces 4x BA662 OTA chips in the 
original 1978 version. The external circuitry is similar in both 
filters, the IR3109 is a set of matched 4x BA662 chips on one die. The 
BA662 OTA filter in the Jupiter 4 is based on the CA3080 version in the 
Roland 700, and you can see the 4 pole OTA filter in Rolands mono synths 
from 1978 - 1981. The Oakley COTA filter is a replica of this design 
using LM13700 chips. It would be interesting to know how similar in 
sound the COTA and SH-1 filters are.

The resonance feedback loop is usally via a BA662 on the polyphonic 
designs, but in mono synths it is a simple potentiometer with two diodes 
acting as voltage limiters preventing feedback runaway. The JP-6 SVF 
makes use of this diode voltage limiter in the 2nd 2-pole OTA feedback 
stage. Roland certainly had problems with clipping at high resonance in 
Unison mode with the IR3109 and introduced the IR3R05 into the MKS80 
REV5 to address this.

So of course its possible to use LM13700 or BA6110 OTA chips to build 
the JP-6 SVF. And I have a BA6110 version on the way.
Will they sound and respond the same? With care around the expo CV 
circuits for frequency and resonance, probably.

Regards - Rob
www.amsynths.co.uk


Paul Dhillon Weber wrote:
> Has the IR3109 been sufficiently documented/reversed engineered to support
> replication with OTAs (LM13700) in a manner similar to analysis surrounding,
> say, the unique sounding SSM2040? I am thinking of the various discrete
> approaches by Schmitz and MOTM and lately, Hearn.
>
> That would be interesting.
>
> Paul 
>
>
> On 6/5/10 12:05 PM, "Aaron Lanterman" <lanterma at ece.gatech.edu> wrote:
>
>   
>> On May 29, 2010, at 1:38 PM, rob at emulatorarchive.com wrote:
>>
>>     
>>> Hi SDIYers,
>>> I have just completed a recreation of the 4-pole State Variable Filter from
>>> the Roland Jupiter 6 using a Boss Phaser pedal as the source of components.
>>> The details are on this web page, along with pictures and samples of the
>>> completed FracRac module. The filter sounds a lot better than the old phaser,
>>> and the 3 modes of BP, HP and LP all with self oscillating resonance makes
>>> for a versatile and great sounding filter.
>>>
>>> http://www.amsynths.co.uk/Filter_Bank2/AM8060/am8060.html
>>>       
>> Nice work!
>>
>> I bet it wouldn't be too hard to replace the custom IR3109 Roland chips with
>> LM13700s and some usual expo converters... I might do it once I get through
>> the 10 unfinished projects I have going on... ;)
>>
>> - Aaron
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