Soundcraft M12 direct outs - What's the verdict?

  • Thread starter Thread starter Phantasm
  • Start date Start date
P

Phantasm

New member
I've searched, and i see conflicting posts.

Are the direct outs balanced or unbalanced?

Can someone who actually owns the board give me an answer? It seems like a lot of you own them. :)
 
From the manual:

MIX & SUB OUTPUTS
The MIX and SUB outputs are on 3-pole ‘A’ gauge jack sockets, wired as shown, and
incorporate impedance balancing, allowing long cable runs to balanced amplifiers and
other equipment.

AUX OUTPUTS
The Aux outputs are on 3-pole ‘A’ gauge jack sockets, wired as shown on the left, and
are balanced, allowing long cable runs to balanced amplifiers and other equipment.

CHANNEL DIRECT OUTPUTS
The Direct outputs are on 3-pole ‘A’ gauge jack sockets, wired as shown on the left, and are unbalanced.

The diagram on the left however shows balanced so this must be a mistake in the manual meaning they are balanced not unbalanced.
 
Middleman said:


The diagram on the left however shows balanced so this must be a mistake in the manual meaning they are balanced not unbalanced.
Click to expand...

Yeah, i thought i had remembered that someone who actually owned one said that the direct outs were actually balanced, and that it was a misprint. I wasn't sure though, so I was trying to see if i was dreaming... :)
 
Someone did in fact say this, Philip??? I emailed Soundcraft and was told that the direct outs were balanced too, but when I asked for clarification--as the manual stated otherwise--I never heard from them again. I've since emailed two additional times and have not heard a response. Weird.
 
There are two ways to test it.

The easiest way would be to take an insert cable that is TRS to dual TS. Try each TS end into an unbalanced input and see if they are both sending a signal. If not then it is unbalanced.

If you don't have one of those cables then plug a balanced cable into the direct out with bare cable on the other end. Plug an ubalanced cable into an input on something else with a bare cable on the other end.

Then tie the Sleave on the two bare wires to each other. Alternately tie the Tip and Ring on the balanced cable to the Tip on the unblanced cable. If the unbalanced device gets a signal off of the balanced Tip and the Ring then it is sending a balanced signal. If you get a signal off the Tip but not the Ring then it is not balanced. You don't have to strip the ends on the cables if you have some wire or alligator clips to connect the different contacts.

Does that make sense?
 
Out of curiosity, what would be the purpose of having balanced direct outs if the insert points are already balanced? I'm thinking the inserts would be used for processors while the direct outs would simply be used for recording devices and such.
 
esun127 said:
Out of curiosity, what would be the purpose of having balanced direct outs if the insert points are already balanced? I'm thinking the inserts would be used for processors while the direct outs would simply be used for recording devices and such.
Click to expand...

Inserts are rarely balanced. They usually use a TRS to Dual TS cable for unbalanced send/receive.

Even if the inserts were balanced that has nothing to do with balancing the direct outs. Most pro recorders and sound cards have balanced inputs so balanced DO's makes sense.
 
Tex- your "plug in a TRS and see if there's a signal on both tip and ring" won't tell you much, since Soundcraft uses "impedance balanced" outputs on most outputs of this board (and the Ghost, for that matter). What that means is that they drive the tip, and then connect the ring to ground through a resistance that is equivalent to the output impedance of their driver. They do *not* drive a true balanced signal out, in the sense that tip goes positive and ring goes negative.

However, the "impedance balanced" outputs do offer essentially the same noise rejection that a true balanced output does: noise induced on the ring sees the same impedance that noise induced on the tip sees, so the voltages are essentially identical: and a true differential _receiver_ will reject it as efficiently as if it was actually driven differentially. The only difference is that you get 6dB lower output...

This trick works in many ways. In fact, you can make any single-ended output behave as an "impedance balanced" output by using a shielded pair, driving the tip, and connecting the ring to the shield ground through a resistor of say 50 ohms.

So the bottom line is that it really doesn't matter if the DOs are balanced or not: even if they aren't, you can fake it just as easily as Soundcraft does by adding a resistor in your connector... It's a good old circuit designer's cost reduction trick, and it works very well.
 
Just checked the manuals online, and the direct out is definitely unbalanced. They show a diagram of using a TRS cable to connect to it, and that might be causing some confusion- but they also show shorting the ring to the shield in the connector.

It's supposed to be a 75Ohm output, if their block diagram is to be believed. So if you'd like to fake up an impedance-balanced connection to it, you could connect the ring to the shield inside the plug at the board end with a 75-ohm resistor. Whatever is on the far end would see a reasonably impedance balanced signal, and you'd have minimum noise (compared to simply going single-ended).

Or, you could simply short ring to shield, and have *almost* the same level of performance. For my money: do it the simple way, unless and until you have a noise problem- and then muck with it, doing the impedance balancing thing (or more likely, putting a transformer in there to get a _real_ balanced differential output).

I just realized that I probably sold Soundcraft a bit short in that last post: impedance balancing really isn't a trivial exercise. The hard thing about doing impedance balancing after the fact (on somebody else's product that you just bought) is that it is very difficult to determine the actual output impedance of a driver. For any active driver, its output resistance as a function of frequency will be a much more complex function than a simple resistor to ground.

When the manufacturer does it, they have the circuit design right in front of them- so they can use a more complex circuit (probably an RC pi section or two) to more closely model the output impedance across all the frequencies of interest. I'll pretty much guarantee that Soundcraft doesn't just use a simple resistor. The closer the impedance match of that balancing circuit to the driver circuit, the better the noise rejection will be at the receiving end.

Anyway, a simple resistor won't be perfect, but it might be close enough for after-the-fact use. Sorry if that caused any confusion.
 
I'm not sure what all of this mean since I've never had a mixer before. I've thought of buying a soundcraft m8. Should I use balanced or unbalanced cables to connect the board to the soundcard (it accepts both balanced and unbalanced)?
 
The rule is to use balanced whereever it is easy to do- as in this case, since both ends offer the balanced option. It is preferable to unbalanced because it should offer somewhat better noise rejection.

The other rule is to not lose any sleep over it. If you only have unbalanced connections (like on the insert points on the board), you use them. Every room I've ever worked in has had a combination of balanced and unbalanced interconnect, and has transistioned back and forth between them as a matter of convenience. It's only when you start to run into intractable noise problems (ground loops and the like) that you have to start worrying about _forcing_ balanced interconnect, by using transformers and the like.
 
Soundcraft M12 Inserts

The manual states "Insert Point (Alternative Direct Send) is unbalanced ". It also states that the send may be tapped off as an alternative pre-fade, pre E-Q direct output if required, using a lead with tip and ring shorted together so that the signal path is not interrupted ".

I'm sure this was explained on previous posts, just figgured the manual description may help.

Andy
 
You must log in or register to reply here.
Back
Top