[sdiy] Sources of noise and how to deal with them?

[mark verbos]

I can't speak on the 914, but my friend has both versions of the 907 and we marvel at how quiet they are.


Mark




On May 7, 2014, at 4:35 AM, David Ingebretsen wrote:

> I've just finished (well, sort of) a Moog 914 fixed filter bank clone. I
> will admit right off that I have gained a real appreciation for the fact
> that these magic vintage designs have problems, are not necessarily the best
> designs, and have their own quirks, issues, and unwanted behavior which I
> will call "charm". I did it as a learning experience and to make one that is
> more euro rack friendly. As much as I love my YUSynth module, it's hard to
> shoe horn it into a Frac or Euro rack.
> 
> That said, I did it two ways, one with real inductors thanks to Carsten
> Tonnesman and one with simulated inductors. The real inductor version was
> picking up so much hum from my power supply, I had to put it about 2 feet
> from my cabinet. I abandoned that option, at least for now.
> 
> For the simulated inductors, I looked at several different methods but I
> decided to use an Antoniou GIC in the Moog 914 fixed filter bank clone. I
> studied what Yves Usson and Jurgen Haible did and decided I liked the idea
> of a simulated inductor more than an active filter. 
> 
> I have two options for the output, a simple op-amp circuit and the Moog
> three transistor output stage. Both have to have a gain of about 220 due to
> the "passive" filtering stages. 
> 
> The problem is even with all the input attenuators to the filter cells shut
> down and no input, there is some very apparent noise and just a little 60Hz
> at higher output gain levels using either output stage. Jurgen warned "There
> will be some noise - just like the original had some noise, even with all
> the channel potentiometers set to zero." He was right apparently.
> 
> I will admit part of the problem might be because I turn the input/output
> pots pretty much all the way up because the Moog signal is much lower than
> in the other equipment I use. But even so, it seems the signal to noise
> ratio in the 914 is not nearly as good as I am used to or expected.
> 
> I'm wondering a couple of things:
> 
> 1. Has anyone heard an original 914? (even a 907) If so, was there
> noticeable or objectionable noise? 
> 2. Does the Antoniou topology have an inherent noise problem that is somehow
> worse than using real inductors? I used a resistor network, as did Jurgen,
> in the GIC and the same 4558 op-amp. Would a different op-amp be better?
> 3. What are the typical sources for noise in a circuit like this? Are there
> some design/build tips that would help minimize the noise? 
> 4. Jurgen mentioned the noise was because the attenuators come before the
> filter stage, why does that matter?
> 
> Really, if noise is part of the "charm" of this filter, then I embrace the
> noise, I cherish the noise, I will try in vain to ignore the noise :) But,
> if the noise is not inherent in this type of design, I might go back to
> square one and adopt a Salen-Key type of topology like Yves Usson did. I
> compared my YUSynth 914 clone against the simulated inductor clone and the
> YUSynth 914 is amazingly quiet. Kudos to you Yves, brilliant module.
> 
> David
> 
> 
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