MIDIO128 for external mutes on analog console

[Doodah]

Hi,

First post - What an amazing website!

I'm trying to find a solution for midi muting channels on my analog mixing console (36/8/2). Each channel has a pin which just needs shorting to ground to activate the mute so I'm pretty sure it's a fairly simple job for MIDIO128 controlling relays. I need to control 46 relays in all but don't require any digital inputs. Is it OK to build a MIDIO128 with no DIN boards and two DOUT boards? If so, should I be disabling the DINs somehow - either in the firmware or terminating somewhere on the CORE or both? Just want to get everything clear in my mind before ordering parts. Thanks.

[stryd_one]

Hi mate, welcome aboard :) Yeh TK and the mods have a heck of a place here, it's pretty impressive eh!

Sure, you don't have to use the DIN's... You simply don't install the module, and then reconfigure your application as required.

[jimhenry]

Confirmed on it's OK to use MIDIIO128 with only DOUT or only DIN.

SmashTV's latest version of DOUT board allows the use of driver chips as an alternative to the current limiting resistors.  You might be able to use those instead of relays or they will at least simplify driving the relays if needed.

[Doodah]

Thanks. That's enough info. for me to go ahead and get the parts together. Not too worried about the hardware side of things but the programming aspects are a little scary! I definitely intend to use the SmashTV DOUT boards but I'm under the impression that I'd still require the relays (46 to be exact). If anyone knows of another way of shorting these pins to ground then I'd be interested to hear.

[stryd_one]

There were some switches we looked at in the MBFX thread recently....

[doc]

Hi,

I'm pretty sure, you won't need relays !

The outputs of the ULN2803 are switching a "ground" signal. So you just need to connect the Outs of the driver to your mute switch of the analog desk.

Just give it a try with maybe one ULN2803. Such an amount of relays is surely not useful if you can avoid it.

And ... don't worry about the programming. It is well documented in the source files. In your case you just have to change several numbers and remarks. Thats all.

greets

Doc

[smashtv]

I'm trying to find a solution for midi muting channels on my analog mixing console (36/8/2). Each channel has a pin which just needs shorting to ground to activate the mute so I'm pretty sure it's a fairly simple job for MIDIO128 controlling relays.

Heya Doodah!

Just to be sure: when you say that shorting the ground activates the mute, In your console is this setting a VCA state, or is it simply shorting signal to ground in a convenient area of the circuit?

If it's muting by shorting signal and ground I suggest relays to keep the MIDI stuff completely isolated from the console, since muting through a DOUT/ULN combo without relays will at least tie the ground rails of the MIDIbox and console together, and at worst add a diode and some other not freindly parts to your ground path.

If it's a VCA mute then no problems, ditch the relays, but be sure to add some very large caps as bypass on your +5/Ground lines, like 10,000mF at 35v (or more on the voltage).  And I would also add a bypass cap to every connector on the MIDIbox side (to keep the shared ground cleaner).

If you do have to go with the relays you should consider "low signal" or even RF type relays, or you might see some contact issues as signal loss after a few years use.  I strongly suggest shopping the surplus houses for your relays, unless you are wealthy.  ;)

Best

Smash

[Doodah]

Good news! The external mutes set a VCA state so it looks like it's byebye relays. Thanks for the heads up - this should save a lot of work. Smash, please excuse the daft question but when you mention bypass caps on the +5/Ground lines, I think you mean in the MIDIbox (not the +5v logic supply on the console)? If so, where would be the best place in the circuit to add them? I'm guessing before / in place of C5 on the Core schematic? Also what would be a realistic value for the smaller caps on each connector? Sorry - electronics not one of my strengths.

[jimhenry]

If you have a +5V logic supply on the analog console that can source a few hundred milliamps for the MIDIbox, it would probably be better to run the MIDIbox on the same power as the analog console.  Hopefully Smash will smash in here as he is a much better source of advice on these types of things than I am.  But if you do run separate power for the MIDIbox, it is that power that Smash was suggesting putting the caps on to get a very low noise power supply.

[smashtv]

Good news! The external mutes set a VCA state so it looks like it's byebye relays. Thanks for the heads up - this should save a lot of work.

Very cool.  ;D

Smash, please excuse the daft question but when you mention bypass caps on the +5/Ground lines,

hehe no such thing as a daft question, especially when you are talking about bypass voodoo.  There are daft answers though, and I'll do my best not to give you any..... ;)

I think you mean in the MIDIbox (not the +5v logic supply on the console)?

correct

If so, where would be the best place in the circuit to add them? I'm guessing before / in place of C5 on the Core schematic?

This all depends on if you will be using the console logic supply or an separate supply....If your console supply is beefy enough to run the MIDIbox also, you will want to lose almost everything in the Core power section, and put that big cap across +5 and gnd as close to where the power comes in to the Core as possible.  In other words, if this were my build I would lose J1, BR1, C5, C6, and IC3, Then I would bring in the 5v logic+gnd from the console on J2, and wire the big cap directly to the pads for J2.

Looking at the notes from the last broadcast meter bridge I built many years ago, on a clean linear supply one 10,000mfd @ 15v across the power rails, and the usual .01mfd caps everywhere were enough to keep the 500+ LED contraption from noising up the ground rail....the big cap is kind of an indirect way of lowering the noise floor, since a lot of self generated digital noise on the power rails it due to fast changes in current requirements vs. trace/path impedance. This is minimized by the cap helping the power supply meet demand from the circuit at the point of load.

The car stereo guys use the same technique these days, calling it a "stiffening" cap.  ;)

Also what would be a realistic value for the smaller caps on each connector?

The usual .01 mfd should do fine, I would build it first, listen & test, then add these only if there is a noise issue.

If you do end up going with a separate supply, you might consider losing the ULN drivers, and driving optoisolators in place of the usual DOUT/LED configuration....This would keep everything isolated like the relay method, but without the relay coil noise.  :)

So I can get a little more specific.....what is the make/model of the console?  I'm hoping we can dig up a schematic and get a more accurate idea of what's a good idea and what is a bad one... ;)

Best

Smash

[Doodah]

The desk is a Soundcraft B800 which is an old broadcast desk. If you Google it, the first result should be the page on Soundcraft's website which has pdf manuals, tech specs etc. There are 3 identical PSU's which I thought I could tap in to for the 5V but it seems they only supply +48V and +/-17v. Dropping the voltage to the +5 which I have measured on the ext. mute pins must be done somewhere in the desk itself. This thing is not particularly easy to work on - even with all the channels removed, so I think I'm swinging towards using a separate power supply for the MIDIbox and the excellent suggestion of using optoisolators in place of the usual LED's. If I understand correctly, I will be keeping the resistors but perhaps a different value than 220R, depending on the opto used? Any suggestions for a good combination? Can't wait to get started on this!

[smashtv]

If I understand correctly, I will be keeping the resistors but perhaps a different value than 220R, depending on the opto used?

Yes.

Any suggestions for a good combination? Can't wait to get started on this!

I am partial to the 6n138, but that's only because I have many tubes of them, and they are closer in spec to the current requirement of a normal LED than most optos.  :)

I would say knock up a tester proto for this part of the circuit before anything else, that way we can be sure there are not gotyas like fluttering vcas etc.

Take a look at http://www.jaycar.com.au/images_uploaded/optocoup.pdf for all the hints about dual darlington optos.... ;)

Upon a closer look, the 138 might not be the right tool for the job, unless you have are planning for an inverter in there somewhere (would have to have the MIDIbox open the mutes to use the console, needs to be only muting when the MIDIbox is telling it to be quiet....the 638's are more suited to switching power than a ground rail due to that dual darlington design)....So what is needed is a simpler device, with an efficient diode (20ma current draw or less for the LED) and a receiver side that is not so uppity and would be happy to switch a ground without involving the positive rail.......unfortunately I don't have a number to go with that spec, so it's your turn to do some digging and let me know if you find some candidates.  :)

Best

Smash

PS before I forget:

Here are some notes about switching audio with an opto:

http://www.silonex.com/audiohm/softswitch.html, But we don't need to be as quiet as the ideas shown there.... Just an interesting link for future use in MIOS effect control vactrol stylee.... :)

[stryd_one]

Nice linkage thx smash!

Hey illogik did you catch that one?

[Doodah]

Thanks Smash,

Is this up to the task? Specifically the SFH618-2. Seems about as simple as you can get and if I'm not mistaken, draws virtually no current:

www.isocom.com/datasheets/db92370.pdf

Just finalizing the parts list. Expect an order any second  ;D

[smashtv]

Thanks Smash,

Is this up to the task? Specifically the SFH618-2. Seems about as simple as you can get and if I'm not mistaken, draws virtually no current:

www.isocom.com/datasheets/db92370.pdf

Just finalizing the parts list. Expect an order any second  ;D

That one looks like what you need...

A similar part should be available from just about any company that makes optoisolators, just in case these specific ones are hard to get your hands on or are priced like gold....  ;)

Hopefully I'll get enough pending orders knocked off the list this weekend to get yours shipped fairly quick....

Best

Smash