Soundcraft 400B Ground Compensated Direct Outs

[Brian M. Boykin]

I found this mod in an old thread but need some help understanding it.

It's very easy to add direct outs to these. Get some shielded cable, tac at the fader amp's output. Drill and mount isolated 1/4 " stereo jacks. Run the shielded cable to the jack and wire the "tip" to the cable hot. Run a 75 ohm resistor from the "ring" to ground. Tie the jack's ground lug to the input ground, not the buss ground. You now have ground compensated, asymetrical balanced direct outputs!

Jim Williams
Audio Upgrades

1. Does "isolated jacks" mean that the ground lug of the jack is isolated from the chassis/mounting nut?

2. Is 75 ohm resistor run to chassis ground?

3. "Tie the jack's ground lug to the input ground"; What's the input ground, please be more specific.

4. My understanding after googleing this type of output is that it offers the same hum advantage as balanced outputs, I am assuming with a 6db decrease in output since it will be unbalanced. Information I read said it can be plugged directly into a balanced source, true/false?

5. I'm figureing the inverting leg of the input opamp is grounded out with this set up with no ill effects, true false?

6. Do electronically balanced outputs really give more dynamic range as I have read in other threads, as this may change my long term plans for the direct outs? I ask this because I plan to build a new psu and change opamps, so balancing the mains, monitor outputs, and direct outs may be an option down the road.

Thanks,

Brian

[nosebleedaudio]

Not the same as true balanced out, with electronic balanced out you have 6db more headroom.
You can also truly drop the shield to eliminate ground loops.
Dynamic range is a diiferent thing, its your total level capability, noise floor to max out before clipping.
You can raise your output level with some methods and does'nt change your dynamic range at all, for example;
You can add a transformer to the output that has a voltage gain of say 9db, you raise your max. output and also raised your noise floor the exact same level.

[Brian M. Boykin]

So if I use the burr brown drv134, which the spec sheet says increases the level by 6db on the output, I will increase the dynamic range and noise floor by 6db. Is the 6db because it takes a single-ended, ground referenced input and changes it to a floating differential output, the extra 6db comes from the inverted signal. Also, could'nt I also use an opamp in a feedback type circuit to accomplish the same result. If I understand the app. info for the drv134, there is no need to put a resistor at the input or in a feedback loop. Seems a little easier to me.

Thanks,

Brian

[tINY]

Yes the sheild is not tied to the chasis ground - isolated.


The ground for the 75 ohm resistor should be the same ground that the "live" side is referenced to. (not the chasis ground, in this case because you have isolated the sleeve from the chasis). The input ground should be tied to the signal ground - this is the reference for all the audio circuits.

This is a balance output - it is not a differential output. On the receiver end, the negative input is not grounded, it has the same impedance as the "hot" side to ground - any noise picked up along the way is common to the inputs and nulled out by the diffamp.

Can't say about dynamic range, exactly. But it will be close.

The only thing that you loose with the impedance balanced method compared to a differential output is reduction in the emissions of the wire as it is routed to the receiver.




-tINY

[nosebleedaudio]

The reason you have a 6db increase in balanced/unbalanced is this; the unbalanced out is fed into a 2nd op amp's neg or inverting input which now its output is 180 degrees out of phase with the 1st op amp, doubling the output level which is a 6db increase.
Looking at a schematic would make more sense.
Or another way to look at it is, as op amp1 is going high at the same time op amp 2 is going low. Giving twice the ouput swing...
As for noise, its more around 3 db increase, have checked that before, but my memory could be off.
I will say this that every mic pre I can remember that has electronic balanced out, GML & Bryston for example has more noise than a single stage pre like a Hardy, API, JM-130. They only have one op amp stage, the trade off is a little higher distortion at higher gain.
Its a compromise...

[Brian M. Boykin]

tINY,

So I would tie the ring into the ground reference point of the opamp comming off the fader; same reference point as the "live" or hot signal which leads to tip. This is why Jim says to use a shielded cable. Still lost on the input ground, were is it. It sounds like it is the point where the input ground reference enters the pcb. Yes/No.

Thanks

Brian

I had time to think about this some more and if this was just regular unbalanced outputs the amps signal output would go to tip and the sleeve would get the audio ground, not the chassis ground. So is this just basically that with the ring tied to the audio ground with a 75ohm resistor? Can I just attatch the resistor between the ring lug and the ground lug?

[Jim Williams]

The input ground is isolated from the main audio ground in the input module by a 10 ohm resistor. This keeps ground currents running the right way and avoids looping in the module. The input ground is where you connect the sleeve wire to. The isolated Neutrik or Switchcraft jacks prevent the sleeve from connecting to the metal chassis.

Jim Williams
Audio Upgrades

[Brian M. Boykin]

Thanks Jim,

So does the ring with the 75ohm resistor go to chassis ground or to the same audio input ground as the sleeve.

On another note, you're going to see the master module for this console soon, I want to change the resistors to metal film, should I do that before you see it or after? I'm assuming thats not included.

Thanks,

Brian

[Jim Williams]

Originally Posted by Brian M. Boykin

So does the ring with the 75ohm resistor go to chassis ground or to the same audio input ground as the sleeve.
Thanks,
Brian

To the same audio ground as the sleeve. Resistors are extra. I use Dale RN60's in there unless you want Caddocks or Vishay.

Jim Williams
Audio Upgrades

[Brian M. Boykin]

Thanks again Jim,

You'll be getting a call soon.

Brian

[Brian M. Boykin]

Ok, I have two more questions. First, I noticed the output pin of the tl071 has a 75ohm resistor just off it. Since the point in all this is to match the impedience of the hot and cold signal, shouldn't I tac the hot line off the 75ohm resistor just after pin 6 of the tlo71? Second, I ordered 1/4 watt 1% 75ohm resistors for the ring by accident. Does it matter that they're 1/4 watt instead of 1/2watt?

Thanks,

Brian

[Jim Williams]

1. Yes

2. No

Jim Williams
Audio Upgrades

[funk5]

Hello Brian, if you're still out there. I'm curious if you ever did this direct out mod for the 400b? Or anyone else that has done this mod? Does it leave eq, fader, etc. in tact? Wow that's an old thread!

[biam]

Originally Posted by funk5

Hello Brian, if you're still out there. I'm curious if you ever did this direct out mod for the 400b? Or anyone else that has done this mod? Does it leave eq, fader, etc. in tact? Wow that's an old thread!

I'm curious too!
Just bought a 400B and want some direct out also.

[Matti]

This from AD 145 Pico, I´m making direct outs post fader for that and ask if the
same applies here also. I will tap from the red marked point and was thinking of getting unbalanced direct outs but this would be better ;-)

Thanks

Matti

[Matt Syson]

Hi
You need a 75 Ohm taken from the point marked with red and another 75 Ohm to take the 'ring' (cold signal) to audio ground near IC7. What chip is IC7? A TLO71 is not suitable as a 'line driver' or output device. NE5534 With compensation cap) would be better as would a host of other more 'fancy' chips that are rated to drive 600 Ohms or sometimes less. A TLO71/2 is unhappy driving much less than about 2K5 and you already have the panpot and other junk in there.
I would say that the sleeve (or pin 1 if XLR) should go to chassis so that any incoming interference picked up on the cable screen goes to ground and not circuitry 'audio ground' inside the box where it could reappear and get amplified.
Matt S

[Matti]

Folks at Audio Developments said to change the ic to tl071, it has tl061 now.
The converter I´m mainly using has 10kohm line inputs so it would work somehow.
I´m totally lost here with suggested NE5534 , how to do this compensation cap circuit and what could be the fancier ones with low current draw?

Thanks and sorry for this being more complicated than I anticipated

Matti

[Matt Syson]

Hi
OK the input of your convertor may be 10K but there is also the panpot circuit and the 'feedback' resistor.
The worst thing will be the capacitance of the cable which will add extra 'load' at high frequencies.
The TLO71 is better than a TLO61 (but most things are as the TLO61 has 'low power' as it's merit, not audio quality).
A NE5534 is not 'stable' at low gains so you need typically a 22pF capacitor between pins 5 and 8 to stop it oscillating. Other chips could be used but the 5534 is cheap and plentiful and MUCH better than a TLO71 IN THIS APPLICATION.
Matt S

[Matti]

Thanks, I´ll do that - the 22pf cap polarity? I hope the mixer has juice for these
mods even with battery driven although will be used mainly with mains supply.
It is a field mixer after all...

Cheers

Matti

[Jim Williams]

That circuit does not have input or output blocking caps. Input bias current should be taken into account. You could install a 2~10 uf cap (film prefered) between the fader wiper and the input resistor to pin 3 of the fader opamp. That will allow you to use the 5534 or other bipolar input opamp. I suggest the AD8597, ADA 4898-1, BB OPA1610 or LME49710. All are much improved over the 5534.

If you leave out the caps and just want to shove a better chip in, use a fet input device with low offsets. You can use the BB OPA827 or the new AD4627-1. Those will drive 600 ohm loads as well. Most of these new parts are SOIC so you need an adaptor for them.

If you are in need of low power consumption, use the Linear Tech LT1351. It's 10 nv noise, 200v/us slew rate and 260 ua current consumption.

Jim Williams
Audio Upgrades

[Matti]

Ordered samples of OPA827, need one more - 6 total.
So I just put these in with the adapters and should be happy?

Thanks

Matti

P.S. didn´t notice your update, can I put LT1351 there without any futher mods

Thanks again!

[Matti]

Originally Posted by Jim Williams

If you leave out the caps and just want to shove a better chip in, use a fet input device with low offsets. You can use the BB OPA827 or the new AD4627-1. Those will drive 600 ohm loads as well. Most of these new parts are SOIC so you need an adaptor for them.

If you are in need of low power consumption, use the Linear Tech LT1351. It's 10 nv noise, 200v/us slew rate and 260 ua current consumption.

Jim Williams
Audio Upgrades

Jim, I don´t want be irritating but do I read this right, I can use LT1351 without
the caps also? That would be optimal for me being quite awkward - or whats the word- with electronics

Thanks

Matti

[Matt Syson]

Hi Matti
You need to ask yourself how far you are going with this.
If your channels are full of TLO61 or TLO62 chips then putting an expensive device as Jim suggests into the post fader buffer does not make a lot of sense. You should really be thinking of changing all of them and the low power LT1351 should to be a good candidate although it could be tedious and expensive if it is not in a DIP package.
It is getting a bit like putting a Formula 1 car engine into your Ford 'runaround'.
The fact these desks are intended to be battery operated put some limitations on the design paricularly with earlier chips. If you still want to use this possibility but get a serious improvement you will need to do some work.
Matt S

[Matti]

Yes I realise that, I just got confused after the manufacturer saying TL 071
and it is not the way to go... Anyways ordred NE5534 also and having the OPA827samples coming this has not been too expencive yet. My trouble is that in practice
I´ve never done any electronics other than making my cables.

Best, have a nice weekend

Matti

[Matti]

Follow up:
Chose to have the direct outs unbalanced with 4 channels NE5534 and 2 with LT1351 I got samples for.
About this balanced sceme, I´ll have to look at it some other time

Thanks

Matti

[wullong]

Originally Posted by Brian M. Boykin

I found this mod in an old thread but need some help understanding it.



1. Does "isolated jacks" mean that the ground lug of the jack is isolated from the chassis/mounting nut?

2. Is 75 ohm resistor run to chassis ground?

3. "Tie the jack's ground lug to the input ground"; What's the input ground, please be more specific.

4. My understanding after googleing this type of output is that it offers the same hum advantage as balanced outputs, I am assuming with a 6db decrease in output since it will be unbalanced. Information I read said it can be plugged directly into a balanced source, true/false?

5. I'm figureing the inverting leg of the input opamp is grounded out with this set up with no ill effects, true false?

6. Do electronically balanced outputs really give more dynamic range as I have read in other threads, as this may change my long term plans for the direct outs? I ask this because I plan to build a new psu and change opamps, so balancing the mains, monitor outputs, and direct outs may be an option down the road.

Thanks,

Brian

Hi everyone,

Ok I know this post is old, but I've just bought a Soundcraft 400b, and willing to get directs out I naturally found this post. Seeing a large blank space under the IN part of each strip on the back bay of the console, I fisrt thought : "e, that's will be so easy, almost made on purpose by the manufacturer.." but yes, when I opened the real panel, a lot of stuff right behind...
yes I know, this was naive hope.
So could someones tell how they practically did, and where exactly they drilled and located their direct out connector ?
I'm sorry for the dumb question, but what is the procedure to unplug some internal connectors in order to pull plates out and have a better access ?
I'm quite isolated geographicaly, and it would be fine if I could prevent having to take it in the car to a tech, after having stupidly messed with the inside..
thank you all in advance !

[wullong]

(ok actually I wanted to quote the Jim William part, explaining his procedure)

[Matt Syson]

Hi
Item 6, An electronically balanced output CAN give increased HEADROOM in many circumstances as it can provide (theoretically) double the signal capability of an 'unbalanced' output.
This increase may not always be realised in practice if the RECEIVING gear can't handle the increased level. There may be a small penalty in increased noise but this should be a small fraction of a dB for any well designed system. It is possible that the dynamic range is extended but it is not necessarily by 6dB.
Item 1. It is now regarded good practice to ground the sleeve of jacks firmly to the metal panel to reduce the possibility of RF being dragged inside the chassis of the gear, taking it around the 'outside' instead. Similarly with pin 1 of an XLR.
The other items are subject to the exact situation both inside the desk, and to a fair extent what is going on outside with the other gear.
Item 4. Simply putting a 75 ohm resistor in the 'cold' lead does not necessarily improve matters to the same extent as a true floating balanced or a 'ground compensating' output scheme.
There is an awful lot of 'half information' and much 'hype' about the whole issue of 'balancing' and as there are so many permutations available some of which fail seriously if misapplied it pays to read up on the possibilities to get a decent understanding if you want 'repeatable' results.
Matt S

[wullong]

Thank you Matt for the input.

Could having done the actual mod proposed by Jim Williams or any other good one on this precise model shime in here with the details on how the did exactly ? (even with some pictures if it's too boring to write a novel about it)
It would very helpfull for me ! thanks in avance

[wullong]

ok, as my purpose is to feed some 16 track Reel to reel RCA unbalanced inputs with these direct outputs, can I keep going on with this design (because yes, in the case I would want to feed something balanced someday), or do I have to wire a simple mono jack output from the fader's amp end ?
thanks