Hi, everyone.
I just found extremely simple way to dramatically improve signal/noise ratio of a modular system. Here i assume using of A-138 inevitable as it's common utility component for creating complex, fat patches and also is common module to pass sound to next system.
We all use mixers all time — to mix several oscillators, mix CV (with linear mixers), to mix gate signals and get simple sequencer. Mixer also as do VCAs or filters, contains amplifier. In case of mixers we are dealing with operational amplifier working at little gain or even unity gain.
Unfortunately, complex components such as OpAmps are noisy things. Moreover, mixers often may work with wide dynamic range. All of this leads to problems with noisy output of the whole system. Did you experienced a situation where you've made some very pleasant sound utilizing your lovely custom VCOs, softened it with Lo-Pass filter, set up some modulations, but it all can be easily ruined by VCA-Mixer chain? You try to increase volume, but either mixer's input is overloading or thermal noise becomes clearly audible. That's why we need modules to be the same quality level and that's why some critical components must be precise and expensive.
I've done investigations about OpAmps noise characteristics and done some experiments with my modular. The following is what i've found.
The OpAmps used in Doepfer modules most of time is:
LM324, TL062, TL064, TL084.
Look at the characteristics taken from ST and AD datasheets:
TL062/064 (Low-power version)
Equivalent Input Noise Voltage: 42 nv/sqr(Hz)
Channel Separation: 120 dB
LM324 (Old design
Equivalent Input Noise Voltage: 40 nv/sqr(Hz)
Channel Separation: 120 dB
TL072/074/082/084 (Low-noise
Equivalent Input Noise Voltage: 15 nv/sqr(Hz)
Channel Separation: 120 dB
We can see that LM324 is actually not the best existing amplifier for audio purpose however there's much worse models usually used only in power aplications.
Now let's take a look on one AD's modern component. OP470 is expensive operational amplifier dedicated especially for audio. It has the almost best electrical characteristics and it has supply and input conditions that perfectly fit for modular synthesizer purposes: ±18v supply, rail-to-rail input, large gain, hi-liniarity, etc. So, it fits as direct replacement for OpAmps used in certain models.
It even encloses only in ceramic package.
AD OP470
Equivalent Input Noise Voltage: 5 nv/sqr(Hz)
Channel Separation: 125—155 dB
The A-138 mixer has extremely simple schematics: four passive attenuators built around quad OpAmp and one more attenuator for master level. That's all. But it can be simply overloaded and is really noisy component.
I've got trial amount of 6pcs OP470AY from China at price of 5$ each and replaced standard OpAmp on the A-138 with it and guess what? The amplification chain noise just almost disappear and even dynamic range has increased (distortions are gone away)! So, i got possibility to increase both input and output level. I did not measure s/n ratio gain before and after replacement because it was clearly audible, as i said before. I beleive, it's about 40-60 dB. My test setup was like A110Sine->A-106-6LP2 (Closed, no reso.)->A130->A138
A-138 costs about 50 Euro, so you need to add just $5—15 to get high-quality OpAmp and you'll get a much better mixer.
I also got similar improvements replacing OpAmp on the A-116, but has no significant result on Dark Energy, A-130, A-119 and none on any of my filters. (alas, these module type is really noisy!) [DAS WURDE VOM ORIGINAL POSTER SELBST SPÄTER KORRIGIERT, DAS PROBLEM BEIM 116 LAG WOANDERS] But i believe OP470 must fit to simple mixer-like modules used for audio such as panners, x-faders, matrix mixer, maybe to some pre-amps or VCAs. Also many modules such as oscillators have output buffers or input pre-amps at there's non-zero chance to make it better. I'll return to my investigations later when i receive Hi-end VCO and VC Panner modules.
It would be highly appreciated if somebody who will try to improve his own mixer following this post, he will take the opportunity to test other modules, even non-Doepfer. The replacement is simple done with screwdriver (or chip-removing tool) because every IC on Doepfer modules is placed thru DIP-panel.
You can also try to install other OpAmps, like TL-082 (but changes would not be dramatical) or maybe another AD, THAT, etc production.
Also if any of the Doepfer representatives will read it, maybe you also so some research about this matter? It may be interesting to add additional items to catalog and it takes nothing. Also, maybe you will produce some "low-noise hi-end multimode filter" using this simple idea?
Best regards,
Dmitry Shtatnov
kay,hab`s gesehen.Manchmal hilft es schon einfach etwas genauer hinzuschauen 
bin da echt n noob 
