Problems with Aux feeds on Mackie SR24-4

Jay Ashworth

Well-Known Member
I'm feeding Aux 1 to a modulator that drives our backstage monitor, and that's been working ok.

But for this new show, I need stage monitors, and when I tried to patch 2 and/or 3 to the stage amp through the snake returns, I had all kinds of whacky problems: hum, noise, "unplug the plug and bridge tip and ring with my finger and audio comes out the speaker, even though I'm not touching anything else", and the aux masters don't actually seem to *cut* anything; turn them to infinity-cut, and there's still sound.

Fried aux board in the mixer?

Everything else I've touched seems to work perfectly.

Tried the PFL switch in both directions, no change.
 
I would ring out the snake cable and patch cords with an ohm meter. It sounds like the circuit between the console and the amp is not balanced. Also, take a hard look at the specs for the amplifier you are driving. Some amplifiers are notorious for having un-balanced inputs, even on xLR connectors. If the whole system is not balanced, hums and buzzes will happen. If the amp is unbalanced, the solution is a transformer at the amp's input.
 
Wow. An SR24 is still going.

Get a hotspot/guitar amp/pc speakers of some sort and see if you have sound coming out of those outputs. Start there. That console is very old... and not really well built. It really could be anything. If those auxes were not used much they could have some serious dirt in the pots/outputs and have some real corrosion.
 
A guitar amp and speaker is precisely what I'm using as a test sink, yes. That's what provided the results mentioned above. I don't have the proper adapters handy to test A and B against that Main out, and when I did try moving them to a Sub out, I didn't get anything at all. The house was built in 2001; the board is probably original equipment.

Is there an easy, reliable way to test an XLR snake run with something other than audio that I'm not thinking of right now?
 
A guitar amp and speaker is precisely what I'm using as a test sink, yes. That's what provided the results mentioned above. I don't have the proper adapters handy to test A and B against that Main out, and when I did try moving them to a Sub out, I didn't get anything at all. The house was built in 2001; the board is probably original equipment.

Is there an easy, reliable way to test an XLR snake run with something other than audio that I'm not thinking of right now?

Wait, what? You have clean signal coming out of the board and all of the auxes are working correctly?

Best way to test a snake is to take cable tester or better yet a meter, run an XLR through the house, and test continuity one by one while someone on the other end advances the "test" cable. You can test for resistance and pure continuity.
 
I can see that I'll get opens, though it may be hard to figure out what's open -- but I see no way to test for crosses.

Audio from Main out XLR goes through D cleanly to the amp, but audio from Aux Out 2/3 and Sub Out 1/5 TRS is down-level, sometimes buzzy, and the Aux masters don't do much (not nothing, but maybe a couple dB end to end).

On D, we run the channel and master gains on the monitor amp at 2-3; on the A/B auxes, we needed to run them both close to 10; at least 20dB difference.
 
I can see that I'll get opens, though it may be hard to figure out what's open -- but I see no way to test for crosses.

Audio from Main out XLR goes through D cleanly to the amp, but audio from Aux Out 2/3 and Sub Out 1/5 TRS is down-level, sometimes buzzy, and the Aux masters don't do much (not nothing, but maybe a couple dB end to end).

On D, we run the channel and master gains on the monitor amp at 2-3; on the A/B auxes, we needed to run them both close to 10; at least 20dB difference.

D? What D? What A/B? Those snake channels? Are your returns balanced? Have you confirmed your board is outputting correctly on all auxes?

Sent from my Nexus 7 using Tapatalk
 
A guitar amp and speaker is precisely what I'm using as a test sink, yes. That's what provided the results mentioned above.
Are you running into an instrument input for that amp or does it have an actual line level input?

Just to make sure, you are using the 1/4" TRS "AUX SENDS" on the rear panel of the mixer, correct? Just wanted to be sure you are using "return" in relation to the snake cabling and not to the Aux Return connections on the mixer.

Your console's aux outputs are labeled as balanced/unbalanced on the rear panel, however the mixer schematic seems to show them as unbalanced with the ring connection of the TRS jack simply tied to the sleeve connection with a resistor. So they may be a bit noisy over a long run. And the 1/4" TRS Aux and Subgroup outputs on your mixer are a 6dB lower level than the XLR Main and Mono outputs

When you tried the subgroup outputs, did you make sure you had active signal routed to those subgroups and the subgroup faders up?

You might want to wring out your snake just to make sure nothing got mislabeled on either end and that Return 'X' on one end is indeed Return 'X' on the other.
 
Are you running into an instrument input for that amp or does it have an actual line level input?

It's an XLR front panel input (this is a little 4-input power mixer). I have no manual for that, so I don't know what level it's rated for; I'm assuming the input is wide range.

Just to make sure, you are using the 1/4" TRS "AUX SENDS" on the rear panel of the mixer, correct? Just wanted to be sure you are using "return" in relation to the snake cabling and not to the Aux Return connections on the mixer.

Correct. The returns I was talking about were the A/B/C/D returns on the snake, which is a 16x4 with the box built into the stage wall. Fan cable at the booth end; A/B are TRS plugs, and C/D are XLR3F.

Your console's aux outputs are labeled as balanced/unbalanced on the rear panel, however the mixer schematic seems to show them as unbalanced with the ring connection of the TRS jack simply tied to the sleeve connection with a resistor. So they may be a bit noisy over a long run. And the 1/4" TRS Aux and Subgroup outputs on your mixer are a 6dB lower level than the XLR Main and Mono outputs.

I had missed the level difference, but I'm a bit irritated with them that they lied about TRS; Mackie has traditionally been better about stuff like that, IME.

When you tried the subgroup outputs, did you make sure you had active signal routed to those subgroups and the subgroup faders up?

Yeah; the 1-2 assign was down on the relevant source, both with and without L-R, and I had the 1/5 fader up. Tried it with the L-R assign on and off, though I did not expect it to matter.

You might want to wring out your snake just to make sure nothing got mislabeled on either end and that Return 'X' on one end is indeed Return 'X' on the other.

Well, the signals show up on the expected wire (and not on the others), and it's been used this way in the past, so I would expect more rat-induced opens than mislabeling as my trouble.

[ For the record, I've been doing sound one way and another for 25 years or so, so the Really Stupid mistakes I hope not to have been making. ]
 
[ For the record, I've been doing sound one way and another for 25 years or so, so the Really Stupid mistakes I hope not to have been making. ]
I've been involved in audio for over 30 years and it's still sometimes the basic but easy to overlook things that bite me.

For example, now that I think about it the TRS outputs on your mixer probably are balanced in that the internal resistor between ring and sleeve is likely there to make them appear impedance balanced, i.e. both tip and ring having the same impedance to ground, so they should provide the EMI noise resistance a balanced output can provide. However, only the tip is apparently driven and since that means they do not have signal symmetry as apparently do the XLR outputs, there is thus 6dB less output. That is also reflected in the specified maximum output levels of +22dBu for the 1/4" outputs versus +28dBu for the XLR outputs.

It might be worth opening up the console just to clean everything and to make sure any internal connections are properly seated.
 
Mackie consoles tend to have two problems when they get older. One is that the insert jacks gets dirty, so they interrupt the signal when nothing is plugged in. The second is that there are dozens of multi-conductor jumpers between circuit modules, along with ground buss jumpers. All of those things need to be methodically cleaned using Caig DeoxIT, and then treated with Caig Gold. Radio Shack carries them, at robbery prices.

If an input channel or output buss goes dead, it is likely the insert jack for that function. I suspect your sub output has an open insert jack. It can be cleaned by spraying it from the outside, and exercising it with a plug.

Since the aux sends don't have inserts, I suspect a bad jumper or ground. The symptoms can include weird crosstalk and noise. They can be accessed by removing the console back. Do one plug at a time to ensure it gets put back together correctly. The ground jumpers are fat, green wire with slip on crimps. Yes, they get dirty, too.

By the way, this isn't an indictment on Mackie. I suspect that many analog consoles suffer the same issues when they get old. I have certainly had to clean the inserts on Soundcrafts. Pollution and oxidation don't discriminate on electronic connections.
 
Yeah; I ain't takin' it personal, even though I didn't spec this board.

The Subs were a red-herring, merely my attempt to work around the Auxes. Does this *have* sub inserts, BTW? I wasn't looking for those -- and, really, can't see why you'd need them; you just patch between the sub and the external effect, then onto the destination, no?

But the Auxes are a different matter. Remember: the problem is that (at least Aux 1) works fine, it's just that the Aux Send Master is ignored -- or mostly so. I get an ok signal from Aux 1 to the RF mod that feeds backstage. Aux 2 is the same thing: master send has no effect on the level, but the channel sends are working as expected.

That sounds to me more like someone spilled a coke and it got between some traces on the Aux board or something, no?

Ignoring the board, though, for a moment, how, exactly, does one test a snake? I mean comprehensively: shorts, crosses and opens, in a channel and between channels? Is there any good way to do that?

Or is it just the "known good devices at each end" method, and that's all there is?b
 
Ignoring the board, though, for a moment, how, exactly, does one test a snake? I mean comprehensively: shorts, crosses and opens, in a channel and between channels? Is there any good way to do that?

Or is it just the "known good devices at each end" method, and that's all there is?b

Like I said.... Run an XLR through the house as your return... and go through it pin by pin. Plug into the first jack, test for resistance on each pin to itself on the return XLR... advance to the next... etc. Its a slow process.

How I do it? Plug pink into the snake and SMAART at the other end. As we advance through the snake we can see phase issues and fun stuff like that.

One of the shops here is town takes their snake heads, loops it through itself however many times they can, then throws smaart through it to bench test it... but that can only be done if the snake is in one location.
 
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Not sure if I have the requisite 100 ft cable around for that. Or, alas, the time. I may have to grab a couple gender benders and use a mic run, I guess. Thanks anyway, though.
 
What I did instead was to use the appropriate adapters to reverse a mic run, and use it as my return.

Worked perfectly on the first try.

So, clearly, the returns are bad, and I need to squawk them.

Thanks everyone for your help.
 
[ looks ] Yeah, those absolutely look like the toys I need to own; thanks for the pointer.

I assume people don't worry about inter-pair crosses in snakes because the pairs are individually shielded?
 

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