Adding a sidecar mixer to my JH428B. Bouncing some ideas around.

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Rob Flinn

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Joined
Jun 3, 2004
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Between Sussex, UK & Aude, France.
Hello folks,

I love my MCI Jh428b, but sometimes wish it had more channels for stereo reverb returns etc.  As luck would have it a friend of mine has given me a sidecar of 12 channels that were originally added into his JH538.  On his console they were installed into the panel that is in front of the patchbays on the right hand side of the console.  Since this sidecar appears to have been custom made for the console it is exactly the width that will fit in the hole in the JH538 which is 38 patch points wide.  The power was tapped from the back plain & the bus signals were fed straight into the aux & stereo buses on the JH538.  My console is only 28 points wide, therefore I can't install it in the same place like it was on the JH538.  The sidecar channels are just that, there are no power supply, mic amps or summing, but they are designed with a JH5xx  in mind in that they have faders operating vca's so that it is possible to use them with grouping on MCI desks (my 428b had automation at one point)

I have a rack unit with all my reverbs & FX that sits on the floor just in front on the patchbay section facing left, so I can get to it easily. 

My question(s):-

I was thinking that perhaps that I could mount this sidecar on some slide out rails in the top  4u of the fx rack.    If I did this would I have ay issues connecting this up to the power & buses of the console.  I could use an edac  connector to connect it up which would make it easy t remove it from the system without having t cut cables etc etc.  The thing that I am wondering is whether the extra bus length using a 3 or 4 metre cable will compromise that system as I've read that long bus lengths are not good and add noise.  If this is the case could I add some local summing amps  & then feed them into the JH428 buses, or would this compromise the system in the same way ?  Or is there another better way to do this ??

Any ideas/suggestions would be appreciated.
 
Rob Flinn said:
My question(s):-

I was thinking that perhaps that I could mount this sidecar on some slide out rails in the top  4u of the fx rack.    If I did this would I have ay issues connecting this up to the power & buses of the console.  I could use an edac  connector to connect it up which would make it easy t remove it from the system without having t cut cables etc etc.  The thing that I am wondering is whether the extra bus length using a 3 or 4 metre cable will compromise that system as I've read that long bus lengths are not good and add noise.
Certainly. You would have to extend the bus via shielded cable, which would increase the bus capacitance, which will in turn tax the stability margin. But the worst is the increase in longitudinal noise - the sum of all the return currents along the 0V reference (some call it audio "ground") creates a voltage that is applied to the whole bus extension.
  If this is the case could I add some local summing amps  & then feed them into the JH428 buses, or would this compromise the system in the same way ?
That's the best option IMO, but in order to make it work, you must make sure that you have a good differential stage receiving the signal from the partial summing amps before reinjecting in the bus. I don't remember if the bus are balanced on the JH400 . In that case you may not need additional receivers, just a good balanced wiring.
 
Thanks for the response Abbey

The buses in the JH428 are not balanced.  Could a system that used local summing feeding a balanced line driver (maybe a THATS chip),  feeding a balanced line receiver (maybe a THATS chip again)in the JH428, which then feeds into the bus, be a better solution ? 

That way the bus is kept the same length in the MCI & at least the signal between the sidecar & MCI will be as noiseless as possible.
 
Rob Flinn said:
Thanks for the response Abbey

The buses in the JH428 are not balanced.  Could a system that used local summing feeding a balanced line driver (maybe a THATS chip),  feeding a balanced line receiver (maybe a THATS chip again)in the JH428, which then feeds into the bus, be a better solution ? 

That way the bus is kept the same length in the MCI & at least the signal between the sidecar & MCI will be as noiseless as possible.
That would be an excellent solution. You don't really need to use a balanced line driver, though.
 
If I wasn't using a balanced line driver/receiver combination, areyou saying that  you think it is still worth using an opamp to feed the MCI bus locally to isolate it from length of cable from the sidecar summing amp, which would be adding extra capacitance.?
 
Rob Flinn said:
If I wasn't using a balanced line driver/receiver combination, areyou saying that  you think it is still worth using an opamp to feed the MCI bus locally to isolate it from length of cable from the sidecar summing amp, which would be adding extra capacitance.?
Yes. A fully balanced connection (balanced drive - balanced receiver) is not needed for short distances. Half-balanced is excellent. Easiest implementation is unbalanced output - differential receiver, but balanced driver - unbalanced receiver works well also.
Just take the analogy with transformers; you don't need two transformers to establish a connevction and benefit from galvanic isolation. Only one is necessary.
 
Hi Robb: I ran console checkout at the MCI factory for nearly two years. You will need to have another set of power supplies to power the side car; the console power supplies will not be able to power the additional modules.

Are the modules 500 series or from either a 416 or 428 board? The 500 I.O. modules draw a bit more power than the 400 series. The main difference between the 416's & the 428's is the paint job & part no.
Dave Harrison came up with the design & idea for track assign switches, when he was chief engineer for King Records in Cincinnatti, Oh. Jeep Harned (President of MCI) was paying him a liscence fee on every console. Harrison wanted Jeep to manufacture what he dubbed a super console; had 24 opamps on the I.O. module. Jeep refused & Dave started building his own console, on the sly & showed up at AES with it.
Hence the rename & paint job change.
 
Hello Bill,

I know you from the MCI forum.

The modules/sidecar in question was custom made for my friends 538 in Japan where his console came from. They are not made by MCI.  See pic.  Are you sure I would need extra audio Power supplies.  They used the original 400 series supplies on the JH440 ?  They supplies are supposed to be capable of supplying 3A, & MCI literature that I have states that the JH428 draws 1.3A  The sidecar needs just +24v for lamps and relays, & I intend to build a dedicated PSU for that as the JH428B relay supply has a much higher current draw than the one used for the audio.
 

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Excuse the mess.  I am intending to install this in the top 5U of the rack in front of the right had corner of the console.  It will be on rails, so I can slide it out of the way, because space is of a premium in the room I currently am set up in.
 

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Rob Flinn said:
Hello Bill,

I know you from the MCI forum.

The modules/sidecar in question was custom made for my friends 538 in Japan where his console came from. They are not made by MCI.  See pic.  Are you sure I would need extra audio Power supplies.  They used the original 400 series supplies on the JH440 ?  They supplies are supposed to be capable of supplying 3A, & MCI literature that I have states that the JH428 draws 1.3A  io.
There are advantages in running separate PSU's for the side-car, particularly it eliminates one major cause of longitudinal noise build-up, as long as they are clean and floating so there is no ground loop. The cost of a suitable PSU is not prohibitive IMO. You may even use smps; I have successfully used the ones made by Sunpower. They need only minimal filtering for LN applications.
 
Hello Abbeyrd,

I drew this diagram based on my understanding of the unbalanced to balanced configuration that you suggested.  I'm not sure that I have got the hook up quite right where it feeds into the THATS chip. Is that what you were suggesting ?  The that's chip would need a 10k resistor on the output to feed into the MCI bus.
 

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Rob Flinn said:
Hello Abbeyrd,

I drew this diagram based on my understanding of the unbalanced to balanced configuration that you suggested.  I'm not sure that I have got the hook up quite right where it feeds into the THATS chip. Is that what you were suggesting ?  The that's chip would need a 10k resistor on the output to feed into the MCI bus.
That's exactly it. I would suggest you put the 100uF cap at the sidecar side with a resistor to ground (10-100k).
 
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