Hard and Soft reply [sdiy] Photodiode Ladder Filter?

[Magnus Danielson]

Dear Grant,

Grant Richter skrev:
>> 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.

You can still build a ladder with them. It is a fairly simple design 
method anyway.

> 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).

I think many forget this finer detail.

However, even if the control-current needs to be suppressed like in the 
traditional diode/transistor setups, it is a very straightforward way of 
building a multistage thing with few components.

> 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.

The set of components at hand used to be far from perfect, so correcting 
for imperfections comes at the cost of more components. The arrival of 
OTAs is a minor revolution in itself, not to speak of op-amps as such.

> 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.

I have been saying about the same thing for ages. Many of our "classics" 
is really just different approaches to cheap enought design. Some of the 
corners cut have provided the basis for sound-scapes that artists used. 
An engineer can create the "perfect" design but it may be totally 
uninteresting since it does not have the character the artist needs.

I think there's a place for both. Dirty designs for when that is called 
for and clean designs for other uses. A modular should contain both.

> 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)

Well, in the case of a photodiode ladder filter, I think the case is 
really about "what if?" and we are certainly not talking about 
perfection but rather, will it be (scientifically) good enough and have 
an interesting characteristic?

There is only one way to really know: Build one and report back.

Using vactrols is certainly not the way to get precision, but it was a 
handy way to get something useful built in the days when OTAs where not 
the way to go.

Talking about precision... one of my counters uses a LM13600 and some 
2N3906 in the interpolating core to produce 4 ps timing resolution. I 
was quite supprised to find them there... but it is really the core! ECL 
and other funky stuff surrounding it.

Cheers,
Magnus