Dmitry,

thank you for the detailed report. That was interesting for me too.
Unfortunately the OP470 is very expensive (~ Euro 15 here in Germany) and
would increase the price of the modules clearly.

Best wishes
Dieter Doepfer

> -----Ursprüngliche Nachricht-----
> Von: Doepfer_a100@yahoogroups.com
> [mailto: Doepfer_a100@yahoogroups.com ]Im Auftrag von therpocc
> Gesendet: Dienstag, 26. Juni 2012 13:19
> An: Doepfer_a100@yahoogroups.com
> Betreff: 1 Reducing the A-138a/b noise with OP470.
>
>
> 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!)
> 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
>
>
>
>
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