[sdiy] Ultrarough draft of my hot-rodding of the Music Easel ringmod

[Scott Gravenhorst]

"David G. Dixon" <dixon at interchange.ubc.ca> wrote:
>Following on Grant Richter's comments on this thread:
>
>This may be a stupid question, but does anyone use XR2240 or ICL8038 to
>build actual VCOs for modular synths, and if not, why not? 

Out of the box, if you use the datasheet circuit, neither part has very good linearity
with respect to a pitch CV.  Thomas Henry (I think) has a novel circuit using a 8038 from
just a negative supply instead of bipolar.  He put it in the feedback loop of an opamp to
linearize it's response to pitch CV and it's supposed to work pretty well, but the
circuit is _linear_ CV, not expo.  As a side benefit, this circuit allows the IC to run
cool because it runs at half the normal voltage.

> Is it just
>nostalgia for circuits with lots of parts to solder, or is there a deeper
>reason?

The reason these are attractive is they have lots of waveforms, all from one IC.  If they
had accurate expo CV inputs, they'd be good stuff.  Many of us have used these for LFOs
instead of VCOs because of the CV issue and lots of waveforms benefit LFOs.

>David G. Dixon
>Professor
>Department of Materials Engineering
>University of British Columbia
>309-6350 Stores Road
>Vancouver, B.C.  V6T 1Z4
>Canada
> 
>Tel 1-604-822-3679
>Fax 1-604-822-3619
> 
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>-----Original Message-----
>From: synth-diy-bounces at dropmix.xs4all.nl
>[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Grant Richter
>Sent: Thursday, November 27, 2008 3:53 AM
>To: Aaron Lanterman
>Cc: sdiy DIY
>Subject: Re: [sdiy] Ultrarough draft of my hot-rodding of the Music Easel
>ringmod
>
>> Questions:
>>
>> * Any other cool mods I should add?
>
>Yes, with all due respect professor, here are a few fun ideas:
>
>1. Use an LM837 or other modern low noise chip. They have the same  
>4.5nV/Hz rating as the 5532s but would reduce current draw by like 50  
>ma.
>
>2. Bring out IC1a pin 1 to a front panel jack through a 1K. This is a  
>highly distorted signal, or a free "fuzzed" version of the MODIN signal.
>
>3. Along the same lines you could AC couple the bottom of R22 to a  
>panel jack, and the top of R19 same way which will give you the  
>positive and negative half-wave rectified distorted signal.
>
>4. Make the SYMTR trimmer a combination of trimmer and front panel  
>control. The you can have a fixed trimmed carrier null, but also be  
>able to unbalance the carrier in a controlled way like Don does in  
>the bottom section of the 285.
>
>5. IC5B is doing nothing exciting (no offense) so why not make it a  
>floating microphone preamp normalized to signal in. You need some  
>kind of preamp on a ring mod for microphone or instrument pickups.
>
>6. Add and XR2206 or ICL8038 for an on board crude sine wave carrier  
>source. With an onboard carrier and onboard preamp you have a useful  
>stand alone module. A ringmod is pretty much useless without a  
>carrier source and preamp. (IMHO)
>
>>
>> * For my inverting buffers, I use 68K on the input and feedback,  
>> since that's what Buchla used to buffer the signal input. Based on  
>> the various bits of theory discussed on this list, to compensate  
>> for input currents, I should put 68K || 68K from the + terminal to  
>> ground. I've done so. But, I notice Buchla's choice of + to ground  
>> is much lower - namely 4.7K (although that is shared between  
>> several op amps). What's going on there?
>
>Those resistors are used to compensate for the error introduce by the  
>bias current of the inputs. If the bias current is 5 microamps like a  
>5532 they matter, but if the bias current is in the nanoamps there is  
>hardly any error. And for a JFET inputs, you can skip them altogether  
>since the picoamp bias introduces a 2 microvolt error.
>
>>
>> * If I want a noninverting buffer, I'm guessing it's bad form to  
>> just plug the input from the jack straight into the - terminal of  
>> an op amp in case someone plugs something bad in. So I put in a 1K  
>> please-don't-blow-up-the-op amp resistor. I also put 100K from the  
>> - to the + terminal, and + terminal to ground, because it seemed  
>> like modules should have some noninfinite input impedance because,  
>> well, they just all seem to do. Is any of this sensible?
>
>Perfect, bam, just right, the 1K keeps out the static lightning  
>bolts, and the 100k keeps them from floating with no input. Voltage  
>drop is only 1% which is nothing in dB terms.
>
>The only times I have seen a resistor in the feedback loop of a unity  
>gain follower is for a high frequency unit where you are compensating  
>a rogue pole created by parasitic capacitance at the input pin. The  
>LM318 is an example, without the 10K and 4.7pf in parallel the thing  
>oscillates like the dickens.
>
>I have never seen them used for op-amps with a GBP less than 10 MHz.  
>Audio op-amps have GBP of around 3 MHz and aren't bothered by a stray  
>5 pf.
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-- ScottG
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-- Scott Gravenhorst
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