Balanced input on Soundcraft 2400

After doing a lot of work, I just tried the first channel strip (and parts of the patchbay) that I converted to balanced I/O's (the 2400 originally came unbalanced). All that buzz that I was struggling with (trying to ground everything 'just so', simply up and disappeared. Amazing, yet predictable. It sounds the same as before except that low-level buzz that was maybe 6 or 10 dB higher than the hiss is now imperceptible. All I hear is hiss at about the same level as before. I used 'THAT' balanced input receivers.

What did the mod consist of? Was it just an IC sub, or did you have to cut traces? I'm curious because I'm involved in a thread on another forum with a guy who's having buzz issues with his 2400.

Cheers

Kris

Awesome.

D.

It is an IC and board mod - lots of cut traces, and a lot of new PC boards. It's actually quite a lot of work. Also, new tie line plugs - I'm using 120-pin EDAC connectors in place of the 56-pin EDAC's it came with.

It involved all new PC boards in the patchbay (each and every one of them!), rewiring the tape I/O a bit (so the tape I/O's are balanced into the patchbay), and balanced line receiver IC's on little daughterboards on each module. There are two THAT balanced line receivers on each board. I have not done the master module I/O yet - this is still a work-in-progress.

I don't know if this is a 'little' mod to try to get some buzz out - grounding work would be far easier. But I wanted to rebuild the patchbay anyways since it had a lot of PC board issues, so I decided to rebuild the patchbay balanced, then add balanced inputs to all the cards as I go.

I would guess that doing the balancing mods, including PC board layout, would be about 150 hours of work, and probably $2000 of parts and PC boards.

-Dale

I've been thinking of making my insert points balanced using the new that's chips, because of slight, but annoying hum issues which I suspect would be better if this part of my desk were balanced.

That sounds interesting. And a loooot of work.

Wouldn't it be easier to use the 56pin Edac to run the unbalanced wires (send and return) to an external box behind or under the desk and do the balancing /debalancing in this box?
So you don't need to buy 160pin edacs and can use what you already have.
You could use a seperate PSU for all the THAT's and its possible to unmod it very fast.

Maybe you'll have some spare time then for some other mods. :grin:


I (maybe) want to add these mods to my soundcrafts: (at the moment) Hopefully I'll never find much time for doing this

1. Putting two 2400 together to one big 2400. (the first a 4800 maybe) :green:
Connecting the Aux and Busses to both desks. Modifying the master channel with discrete opamps.

2. Modifying the routing.
before: Right Output is connected to even channels, Left to odd channels
after: connecting the Left Output to ALL Bus-Switches (1-24) of the left desk. connecting the Right Outputs to ALL Bus-Switches (1-24) of the right desk. So I can route to both desks from every channel. (hopefully) :wink:

3. VU Led Illumination for the second desk. White or purple leds for the "ON" LEDs? :wink:

4. Making a complete new masterchannel with Analyzer, Peakmeter and G-SSL of course. :green:

5. Adding a flying fader automation.
Idea 1: Buying 3x BCF2000. Installing the fader to the desk. Using a fixed 10k Resistor instead of the channel fader. Control the volume with midi from Cubase/Logic/ProTools...

Idea2: Buying 3x BCF2000 again. Buy dual fader with linear taper and log taper. So you can really change the volume at the channel fader.

6. Making a new PSU. Make new channels. (by using the matrix and aux... from the existing channels)
Either digital Channels with rotary encoder (one more BCF2000) :wink:
(I think I need only one "digital channel" because its easier to select each channel in the sequencer for adjustments.
New analog channels with my preferred channel dynamics and eq. (dynamics and eq on 160x100mm pcb)

7. After 5 years - selling everything and get an old SSL4000 for cheap. :wink:

I thought about that, except that I needed to replace the patchbay boards anyways. These boards were so patched-up :green: due to 22 years of hard work and resoldering that I was tired of re-patching the patches. So I decided to make new patch cards. Changing to a double-sided board should stabilize the jacks significantly and improve the reliability of the bay. Then I thought, why not make the cards work with either balanced or unbalanced inputs? After all, it's just a few more traces to run. I made it compatible with the existing cabling where I could.

Also, doing it this way runs all of my outboard gear to the patchbay balanced - I never loose the balanced signal until it reaches the channel strip like on the TS12. Also, it allows easy expansion of the patchbay area with more patch points without having to debalance before hitting the patchbay - just add another surplus 19" patch bay. My tie lines are all filled up with compressors and effects - my pres are on a separate patchbay that sits beside the console. Yea, it is a lot of work....

The tape returns I rewired to use the SYNC and the edge ground pins as the negative (signal -). I have no need for the SYNC jacks with a hard-drive recorder. So those were ok with the 56-pin EDACs. I am using the existing two 56-pin EDACS (from the original tie-lines) to feed the 'SYNC' jacks - they are just another 24 tie-lines except I'll probably run them to my DA88 rack - my tape returns are from my hard drive recorder. The two 'extra' holes are where I'm putting in the 120-pin EDAC's. I used ribbon cable for everything. I'm going to be cutting mount plates for the 120-pin EDACs at work this week.

I liked your site, it's quite good. I rather like the 2400. I find I use it always in a mixdown mode - mostly the channel strips are in line in mode and I use the patchbay and my mic pre rack for recording. The group side I use in monitor mode and connect effects returns to it, mostly.

I've also been replacing capacitors and op-amps in the board, and modifying the power supply to cope with the added load. It seems well under the limit so far. Only the output transistors get warm - the transformer and all other parts stay pretty cool.

-Dale

Hey Dale,
one of my soundcrafts have the GND connected over the ribbon cable - the other one, probably a newer version, has bigger ground wires at each channel... something like Eddie Ciletti did with a 200b http://www.tangible-technology.com/articles/200b.html
I thought about modding the other desk, too. Do you have any experiences with that? Or do you own one of the newer desks?

What about using ground lift on all the gear and using plastic washers at every piece of equipment to isolate the chassis from other equipment. So that the protective earth is only connected to the chassis and the ground reference is the mixing desk?

Did you replace the audio/coupling capacitors with panasonic FC's or something like this? Or did you tried to bypass the electrolytics with 100nF or so? (ssl style)

Double sided pcbs for the patchbay is a very good idea! I have to think about this! I already organized some TT phone connectors.
Maybe I'll put the patchbay of the second desk to the first desk - this way I can use all equipment at both desks. + free hum because of longer cable runs. :green:

Yes, I replaced all of the coupling caps with FC's, 220uF. I also did the power supply decoupling caps with 47uF FC's, but I did not bypass them with film caps. I also added a 10pF capacitor from op-amp out to op-amp inverting input on the output of the LF EQ output - the one that drives the cable. There is an issue that is almost there with the TL072 - a slight oscillation at 6 MHz, but if you put in any other op-amp, it oscillates full-steam. Adding the cap makes that problem go away. The inductance of the output capacitor causes some issues there. The other 'mod' is I put a 0.1uF decoupling cap under each op-amp IC, because I have been changing op-amps to LM833's - it makes the desk a lot quieter that way. There are a few spots I have tried other op-amps too.

Only caution with the LM833's is that they are sensitive in the mic pre to blow-ups. The mic-pre really should have some zener diode clamps to prevent the op-amp from blowing up if you plug in a mic with phantom on. TL072's don't blow up so easily. But when they do, usually they seem to take out the two power filtering resistors - and generate lots of smoke. The 833 just quit working.

I've thought about working on the grounding, but what clinched it for me was adding an Antares auto-tuner that buzzed a lot - and even isolating it and working a lot on grounding didn't get rid of it all.

I just finished my first 120-pin ELCO to 2x50 pin IDC cables for the new tie-lines. It took about about two hours to solder all the pins - 96 of them. Anyways, that looks like you managed to snag a lot of TT jacks there. I've been just re-using mine after cleaning them. There are a few that I change out because of bad connections.