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output preamp & attenuator coloring box for eurorack modular to balanced +4db
output preamp & attenuator coloring box for eurorack modular to balanced +4db
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Old 10th August 2019, 09:01 AM   #1
Gitaarwerk is offline Gitaarwerk  Netherlands
diyAudio Member
Join Date: Dec 2014
Default output preamp & attenuator coloring box for eurorack modular to balanced +4db


First of all, I'm a little hesistant to start this project, because i have no knowledge about electronics, and the level on this forum is pretty high. Bear with me please :-)

i'll give it a shot... I've been reading a lot lately on how I want to do this, but its not easy. The idea may seem like it, but there's a lot involved (so i read). Here, I hope any of you can point me into the right direction (not expecting full solutions). Be honest if this is a bad idea, if else, hopefully I can build my first project :-)

Impedance matching
Ok, so, the idea is, a eurorack system is build of modules that have quite a hot output. The impedance lies around 1K. To convert it to line level which can go into my daw, it should be around 10K. Many of the outputs will be -10dB, but obviously, I prefer it balanced to +4dB.

Choice to use transformer
While doing this, I could choose to have an active build (like an active DI) or do it passively with a transformer. By reading, an active output is often preferred with a passieve circuit and vice versa. Since I want to convert it to balanced, and not step it down to mic level, I might be wrong about this assumption to do it with a transformer.

Signal coloration
So, lets assume this output transformer is a good idea, and I balance the output signal with this. In comes the idea of transformer saturation. I would like to color the signal to get a more signature sound out of the synth.
So, my idea is to to pick 2 types of output transformers to get this done. So basically an idea like the Shadow Hills mastering compressor to choose a tranny type (metal, nickel,..). The idea here is to use a cheaper telephone transformer for early saturation, and a good audio transformer like jensen etc for less/no coloration. Transformers and coloration looks easy (hey, just drive the tranny), but its a big debated topic and transformers are if im correct quite complex beasts.

Transformer drive circuit
So, to drive the transformers, I would need a circuit (often built with opamps if im not mistaken) to raise the signal. I was thinking of using the echoplex preamp. It's nice and warm. I could add a cleaner pre as well for choice..and choice of bypassing the drive circuit. I was reading about a zobel network, that I most likely need that as well, but not understand yet why.

So, basic flow could be this; (double for stereo)

eurorack output
-> preamp (a/b with switch, drive will be stepped for just a few dB)
-> transformer (a/b with switch)
-> balanced output with passive stepped attenuator with something like Vishay resistors
-> output

So my questions;

- is this in general, a good/bad idea?
- if I take a 10k ohm 1:1 audio transformer set, will this get me there? Or do I need to step down and do impedance matching?
- I am probably missing a lot, what are the things I need to specifically be aware of?
- Do I need to take a step back and just learn a lot more about this subject?


Last edited by Gitaarwerk; 10th August 2019 at 09:03 AM.
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