Eurorack Synth Master Output Module

[Brian Roth]

Damn...I'm over thinking this. I don't know much about Eurorack synths but in my "semi retired/old age" <g> I've become more interested in synths.

Going back to your post #1 you mentioned ".....but the impedance is rather high..." What does that mean in the context of the "Eurorack Universe"? Is the typical protocol 10K input impedance? 100K? 1000000 Megs? <g> Signal source impedance...50 Ohms, 600 Ohms, 100000 megs?

I'm trying to learn Real World info on this "new experience" as an aging tech guy.

Are there any published specs?

Bri

 

[PermO]

I'm also not really knowledgeable on this subject, I have a simple Moog monosynth.

Here's some info;

https://intellijel.com/support/eurorack-101/
The signals in general are 1V per octave, and all these patches are not "audio" as we know it, also there a huge mishmash of brands and modules, and patches can be split and combined so if there's really a standard for input and output impedance I don't know, I think the signal flow of a full system will look like the super dense brain map Boij posted in the brewery

All these systems use a master output module to get a sound from it, now these vary hugely in quality, some are just some pots and resistors, completely passive.

For me it was "the great unknown" that made me decide to go with a 10M input, I know these chips sound really good, so let's just use that.

The guy I build it for is also on this forum, maybe he can chime in with some additional info.

 

[PermO]

This IS the stuff for old farts, as long as you stick with the dresscode

 

[Brian Roth]

1 Volt per octave is the control signal that adjusts the frequency of tone generators, filter centers, etc. Audio signals are a different path.

"Classic" analog modular synths have three (maybe more?) signals that are routed.

1. Actual audio.

2. Control voltages which change oscillator frequencies, amplifier gains, center frequencies of filters, and various Partridges in Pear Trees <g>. That would be the 1 Volt per Octave universe.

3. Trigger (on/off) pulses which trigger events, like attack and decay timing into a controller driving a VCA,

Back to item #1 above. There must(?) be some sort of agreement on fundamentals such as "typical" signal levels. Phono cartridge levels? +8 dBm Broadcast levels? 240 VAC power wiring levels <LOL>? One clue would be the supply rails in the rack.

Likewise...is this a "matched" audio system...ie 600 Ohm source and 600 Ohm load (unlikely!) or the more typical/modern low Z source into a bridging load (my uneducated guess).

I need to find spec sheets for some Euororack modules to learn more.

Bri

 

[PermO]

I've heard 10K input and 100K output is typical, but that seems a bit weird to me as the output impedance is much higher over the input impedance... not a good thing ?

 

[Brian Roth]

I've heard 10K input and 100K output is typical, but that seems a bit weird to me as the output impedance is much higher over the input impedance... not a good thing ?

That makes NO SENSE! 100K output impedance????? Do you have any links to spec sheets for any Eurorack modules ("factory" or DIY)?

I ASSume the audio outputs come from an opamp. Unless the modules have a huge series resistor between the output opamp and the "patch world", that makes no sense.

Bri

 

[Brian Roth]

I would hope the Eurorack World would be similar to "modern" recording desk designs....very low output impedances and bridging inputs.

Bri

 

[PermO]

Well there's all kinds of building blocks in the form of mixers, combiners, dividers, stacking patchcables, anything goes.
FX modules are fed with audio in a eurorack system, also all these audio paths are single ended, also a reason for me to give it some proper low impedance balanced outputs as you rarely see these in eurorack.

I'm also browsing the web but I can't find anything meaningfull.

One thing I'm sure of, that 10M input will take anything...

 

[Brian Roth]

Yep....10 Megs is quite high Z! As I recall, a Fender tube amp input is 1 Meg.

Bri

 

[Brian Roth]

I've found some schemos......I would think 100K input impedance for anything (audio/control/trigger) would work?

Bri

 

[PermO]

Probably.. ?

The thing we do notice now, with the high input impedance and the big film caps on the inputs this thing really comes alive !
The first time we hooked it up we had this "oh wow, this is a different animal" experience, it really opens up the sound and there's so much more detail and small nuance differences in timbre. Also the low end is just slamming !

Same goes for the headphone output, I never expected it to sound this good.

Just based on what we heard, this thing rocks !

 

[PermO]

Next time it will be a pair of 12AU7 driving a pair of big Edcors in push-pull

 

[Brian Roth]

Experiment...see if 1 Meg for R1 makes it "less magic" or "no change".

Bri

 

[PermO]

The way it is crammed together on a piece of prefboard I would rather not do that

I do have a instrument version with OPA2134 input with four input resistors on a switch, 10M, 2M, 1M and 250K ... they all sound fine. Switching to a lower impedance does not suck the life out in this case.

I'm quite used to the sound of 55xx as these are everywhere... I kinda like OPA2134, maybe you should get a pair and try.

 

[Brian Roth]

@PermO....in post #8 "as built", C7 input cap in the schemo says "0.1 uF -3.3 uF FILM" What value did you end up with?

Bri

 

[PermO]

I've used a pair of my personal favorites, big 3.3uF ERO polycarbonate caps.

 

[Brian Roth]

I've used a pair of my personal favorites, big 3.3uF ERO polycarbonate caps.

OK....I looked around a bit and found several comments re. Eurorack I/O impedances. Nothing "official". Suggested input Z is 100K. With your cap, corner freq -3 dB calcs to under 0.5 Hz with a 100K resistor.

Bri

 

[KA-Electonics.com]

I haven't done a "deep dive" about what 1646 parameters degrade with a "non-zero Ohm" driving source, but it seems to be the "quality" of the output CMRR is reduced. I hope Wayne will see this and comment.

In my few 1646 applications, I just took THAT's spec sheet at face value vs. trying to second guess. <g>

Bri

https://groupdiy.com/threads/drivin...-line-outs-line-ins.85873/page-2#post-1121241