Soundcraft Mixbuss mod

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Arno

Well-known member
Joined
Oct 24, 2005
Messages
454
Location
Belgium
Hi everyone (or some)

Thought I'd share this:

Recently I built a new mixbuss for a soundcraft 1600. No passive summing + gain amp, because the console had 80+ channels (two linked soundcrafts) and that's probably way too much for passive mixing  ;D

So I made a virtual gnd amp with two discrete opamps and transformer (sowter 9825a) output.
The console had an empty slot next to the master section, so it seemed like an obvious place to put the new mixbuss.
Got a 34pin connector for power and input, cut some traces on the stock buss, routed via switches to be able to AB between stock and discrete at any time.
Future mod might be: balanced output from seperate tx winding to bypass fader amp and differential output, straight to protools or 2track.

The difference was amazing. I would advise everyone that's using a "budget" console to do this. It's easy, and it sounds great. Harsh higher mids (2K-3K) were a bit attenuated and sounded "softer/nicer" but the low mids where much "bigger".
Yeah I hate this kind of description too but how else can I explain.
The producer/mixer who owns/works on this desk said he finally lost that blanket-over-the-speaker-sound he's been hearing for years.

I tested JH990, diy 990, diy Melcor and OPA604 (ab'd them all in mono)
JH990 and diy 990 sounded pretty much the same (I couldn't tell the difference) they were both miles ahead of the opa604.
The melcor was, for some reason, worse than the stock 5532
When they arrive, I might test 2520's against the 990's.

We tried pushing the buss completely over it's limits (rising protools levels). The stock buss got fuzzy and saturated much earlier than the 990, which held dynamics nicely, right till it was seriously distorted.

So, DO try this at home  :D
 
Sounds like a cool mod!
Did you compare 990 and 604 and Melcor in the same circuit?
Did you add resistors to ground to make virtual ground?
 
I'll try to make a picture somewhere next week.

//Did you compare 990 and 604 and Melcor in the same circuit?

yup, I put the 604 on a small pcb with 5 pins

//Did you add resistors to ground to make virtual ground?

Well I could do that cause there's 2 pins available for audio ground on the ribbon connector but that would involve putting two resistors in 80+ consoles
Soundcraft didn't do it, and the buss sounds great now, very quiet !
Isn't that mostly a noise-issue ?
 
radiance said:
Do you mean a ground summing bus?
;D Probably!
I'm a bit tongue-tied with all this terminology... Just been looking at different mixer's schematics, and was wandering if Arno totally re-did the whole summing topology, or just made a new summing amp. I have this amek mixer and it sums channel outputs and then sums channel grounds - is that what virtual ground stands for?
 
jackies said:
I have this amek mixer and it sums channel outputs and then sums channel grounds - is that what virtual ground stands for?


a virtual ground (or virtual earth) is a node of the circuit that is maintained at a steady reference potential, without being connected directly to the reference potential.

I think this term is mostly used when for example you have 24V  and want to make a  +12V  0V  -12 PSU of it. The 0V will be the virtual ground.
But in theory you're right I guess, since the ground summing bus is also not connected to ground thus having it's own reference potential.

I started a thread with questions about this once...

http://www.groupdiy.com/index.php?topic=27351.0

Arno said:
Isn't that mostly a noise-issue ?

Yes...if your desk is quiet I should not bother..
 
I though there were only two types of mixbusses:

*Passive

*And active, which is always referred to as virtual ground.
But the mixbuss works the same when the gnd potential on the opamp is not the same as every channel
So it might not be "true virtual eart" but it is by no means passive

;D


I did pay attention to make sure that I used the same audio ground, which was used to ground all channels, not the chassis or whatever gnd buss is available
 

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