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

[Stewart Pye]

If you're using the 907 schematic from J. Haible's website, reducing the 
summing amp feedback resistor to 12k and adding a non inverting amp 
stage with low value resistors will reduce the noise of that stage, 
though if there is heaps of noise generated before that you won't notice 
much difference.

I'm not sure if lower noise op amps for the GIC will make a difference. 
You could try it.

Regards,
Stewart.


On 5/7/2014 6:35 PM, 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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