Hard and Soft reply [sdiy] Photodiode Ladder Filter?

[Grant Richter]

> Isn't the wiard Boogie basically that, using vactrols for the ladder?


This post contains both factual technical data and personal artistic  
opinion.

I can answer that: Yes the Boogie is the filter Moog would have  
shipped if Don Buchla had worked for them.

But, Vactrols are optically isolated, so no ladder is needed.

The only time you need a ladder design is when the transconductor has  
no CV rejection which transistors and diodes do not.

How to address this design problem?

Make two parallel paths of the poor performance transconductors.

Inject the signal differentially at the input (usually bottom in  
drawings). That is a positive phase on one leg and negative phase on  
the other.

Use a differential amplifier at the top to extract the differential  
signal and reject the control voltage which is equal in amplitude,  
but appears common mode, which differential amplifiers reject (CMRR).

Filters like transistor and diode ladders were designed to cost. That  
is, it was the cheapest circuit they could use that fit the design  
description. But the designers knew that as filter designs go, they  
had appreciable technical defects.

Then everybody got trained to associate the sound of cheap, low  
quality distorted noisy filter designs with synthesizers, to the  
point that when ARP came out with a clear fidelity filter, everybody  
hated it.

But if "Flashlight" was played on a high fidelity synth, it wouldn't  
be as dirty and sexy and fun. So in this case, the lack of perfection  
in the filters, perfectly suited the artistic application of sweaty  
dance floor music and getting the Foxes "shaking that thang" um, um.

More power to scientific perfection.
More power to artistic imperfection.
They both have their place on this list.

I think this list needs to clarify which side is under discussion at  
what moment.
Is the goal of the thread topic the perfection of stability from an  
objective instrument standpoint? (Hard Science)
Or is the thread topic about the subjective musical character of a  
circuit which possesses some technical flaws, but sounds cool. (Soft  
Artfulness)

If we could just work that out, these repeated squabbles would be  
greatly reduced. (IMHO)


On Dec 13, 2008, at 11:37 AM, Romeo Fahl wrote:

> I
>
> On Sat, Dec 13, 2008 at 5:46 AM, Scott Gravenhorst  
> <music.maker at gte.net> wrote:
>> Crazy thought I had, maybe not so crazy?
>>
>> I was researching IR photodiodes to be used in a non-synth project  
>> when I thought - what if photodiodes controlled by LEDs were used  
>> to create a diode ladder filter of some kind - is this even  
>> possible and/or has it been done?
>>
>> What I've read about photodiodes is that in photoconductive mode,  
>> they are usually reverse biased - would this present a problem in  
>> something like a filter?
>>
>> I'm not a filter giant, certainly not a ladder filter giant, I'm  
>> just asking...
>>
>> -- ScottG
>> _____________________________________________________________________ 
>> ___
>> -- Scott Gravenhorst
>> -- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/ 
>> FPGA_synth/
>> -- FatMan: home1.gte.net/res0658s/fatman/
>> -- NonFatMan: home1.gte.net/res0658s/electronics/
>> -- When the going gets tough, the tough use the command line.
>>
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