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Soundcraft EPM Balanced outputs - electronics


TomHoward

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Apologies for a very specific post but I'm trying to understand something;

 

I was messing about earlier trying to cure a noisy input on an output from a Soundcraft EPM12 into an ATEM Mini analogue unbalanced input. There is a ton of digital noise on the input and it sounds terrible.

 

The ATEM is unbalanced stereo input on 3.5mm line level and the EPM12 is on XLR.

 

I was trying different cable wiring, trying with -ve shorted to earth, -ve left floating so only pins 1&3 attached, to see if any made a difference to this static.

In doing this, one time I wired it up wrong, with ground on pin 1, and signal on pin 3 (so basically an unbalanced but out of phase signal).

 

Our of the EPM this gives absolutely no sound, which was odd as once I'd worked out what I'd done I would have expected signal, but out of phase. This made me experiment with shorting +ve and -ve together, which gave me signal as opposed to the silence I would have expected - shorting pins 2&3 on the output XLR doesn't seem to effect the output.

 

The manual says the outputs are electronically balanced. Are these cleverer than transformer balanced, and will avoid a short between +ve and -ve (but this still doesn't explain a lack of signal between ground and -ve), or is it possible the outputs from the Soundcraft EPM series are unbalanced?

 

I tried two different EPM desks and an EFX desk and got the same results. I was going to put a scope on it to see what's going on but before that I wondered if I was missing something.

Edited by TomHoward
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Apologies for a very specific post but I'm trying to understand something;

 

I was messing about earlier trying to cure a noisy input on an output from a Soundcraft EPM12 into an ATEM Mini analogue unbalanced input. There is a ton of digital noise on the input and it sounds terrible.

 

The ATEM is unbalanced stereo input on 3.5mm line level and the EPM12 is on XLR.

 

I was trying different cable wiring, trying with -ve shorted to earth, -ve left floating so only pins 1&3 attached, to see if any made a difference to this static.

In doing this, one time I wired it up wrong, with ground on pin 1, and signal on pin 3 (so basically an unbalanced but out of phase signal).

 

Our of the EPM this gives absolutely no sound, which was odd as once I'd worked out what I'd done I would have expected signal, but out of phase. This made me experiment with shorting +ve and -ve together, which gave me signal as opposed to the silence I would have expected - shorting pins 2&3 on the output XLR doesn't seem to effect the output.

 

The manual says the outputs are electronically balanced. Are these cleverer than transformer balanced, and will avoid a short between +ve and -ve (but this still doesn't explain a lack of signal between ground and -ve), or is it possible the outputs from the Soundcraft EPM series are unbalanced?

 

I tried two different EPM desks and an EFX desk and got the same results. I was going to put a scope on it to see what's going on but before that I wondered if I was missing something.

 

Your output is electronically balanced, which means pins 2 & 3 have the same impedance to ground, it does not mean there is signal on both pins. You only have signal on pin 2 and should wire that as the hot into your unbalanced input on the ATEM.

 

Mac

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Is there not an inverted signal on pin 3 then? How does the balancing work at the other end, or is it not as effective as transformer balanced?

 

Sorry, I've got it. The matching impedance to ground basically means one leg is signal (and induced noise) and the other side is just inverted induced noise, as opposed to symmetrical transformer balancing. Well that's completely passed me by all these years, and I'd always assumed electronically balanced mean the signal was inverted with an op amp rather than there just being signal on one leg.

 

https://www.soundonsound.com/sound-advice/q-how-impedance-balancing-audio-different-normal-balancing

 

Still doesn't explain why the line inputs on the Atem mini sound so terrible, mind, but it did keep me guessing for a bit longer.

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Still doesn't explain why the line inputs on the Atem mini sound so terrible,

 

I've found the ATEM inputs to be awkward, too. They are very easy to overload and don't match up well with mixer outputs. We struggled to get one working satisfactorily with a QU16. With the ATEM inputs set to line level, we had to drive the desk really hard to be audible. With the ATEM set to mic level, the slightest touch led to distortion.

 

I suspect they've been designed to take similar mics to some SLRs, or consumer-grade podcast mics rather than anything professional.

 

That, unfortunately, is just the start of the long list of problems we've had trying to get ATEM minis to interface with other equipment. Unless your setup literally matches the photos on their website you can expect to encounter issues. And their technical support is utterly useless.

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We've got two that have had typically hard use due to a new-found enthusiasm for streaming, I can't get a clean input on either of mine and I get digital noise carried across from the HDMI inputs if I use a grounded computer, it's only if I use a laptop on it's battery that I get a clean sound. It was particularly bad as there was a cheap unshielded unbalanced Phono-Minijack cable in the mix, but now I've heard it I can't get rid of it. We stream from OBS (there's a mismatch between the input audio levels the ATEM shows in software and the levels it sends to OBS) and I'm going to try just adding a USB audio interface separately to take balanced inputs and see if I can get the sync right to match the delay of the ATEM. I'm trying to find if the delay is consistent or if it wanders.

 

The weird thing we've had on ours is that I think on two different inputs (1 on one and 3 on the other) I think the EDID has died. We have inputs that just flat out refuse to work, while the others do with the same camera, and a camera of laptop doesn't see it's connected to anything and detect the display. They will only work on those inputs when fed via a splitter or something like a SDI-HDMI or HDBaseT box where the output device is seeing something else for the EDID.

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There is a quick explanation of 'Impedance Balancing' here; https://www.soundonsound.com/glossary/balanced-wiring

 

and a useful 'connection guide' in the mixer manual here (P44); https://www.soundcraft.com/en/product_documents/efxepm-ug-20101130-pdf

 

I don't mean to teach grandma to suck eggs, but I assuming you definitely have the Atem inputs set to line (it can only be done in software I think).

 

I'm also guessing that you have HDMI sources connected so there will be lots of digital 'shash' floating around. My go-to problem solver in this situation, which I come across a lot using Macs, external monitors and audio devices is one of these; https://cpc.farnell.com/pulse/pls00548/ground-loop-isolator-3-5mm-p-s/dp/AV25542?st=loop%20isolator

 

Alternatively, you may find that your whole system is, in effect, class 2, so adding an earth to the 'screen' side of things may help.

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Sorry, I've got it. The matching impedance to ground basically means one leg is signal (and induced noise) and the other side is just inverted induced noise, as opposed to symmetrical transformer balancing.

 

Slight correction... the other side is just induced noise.... not inverted noise. The whole point of twisted pair is to ensure that the same noise signal is picked up on both cores. The differential input then cancels any common mode (noise) signals. Unfortunately, a belief that some combination of inverted and non inverted signal is doing the noise cancellation has arisen,

This also explains why a balanced signal doesn't have to be symmetrical or even on two wires. The identical impedance to ground helps ensure that the same noise signal is induced.

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My go-to problem solver in this situation, which I come across a lot using Macs, external monitors and audio devices is one of these; https://cpc.farnell.com/pulse/pls00548/ground-loop-isolator-3-5mm-p-s/dp/AV25542?st=loop%20isolator

 

I'd be doing as P.K. suggests and try a GLI (as he linked to above, GLIs are also available with phonos or 6.3mm jacks).

 

A combination of HDMI/USB/seperate audio links/Class 2/Class 1 seems to be a 'magnet' for noise, be it ground loops or other random digital noise. Can be a nightmare to find and eliminate. Don't get me started on my way over-the-top home TV/AV/PC/Amp/VCR/Laserdisc/turntable/STB setup :blink:

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Apologies for a very specific post but I'm trying to understand something;

 

I was messing about earlier trying to cure a noisy input on an output from a Soundcraft EPM12 into an ATEM Mini analogue unbalanced input. There is a ton of digital noise on the input and it sounds terrible.

 

The ATEM is unbalanced stereo input on 3.5mm line level and the EPM12 is on XLR.

 

I was trying different cable wiring, trying with -ve shorted to earth, -ve left floating so only pins 1&3 attached, to see if any made a difference to this static.

In doing this, one time I wired it up wrong, with ground on pin 1, and signal on pin 3 (so basically an unbalanced but out of phase signal).

 

Our of the EPM this gives absolutely no sound, which was odd as once I'd worked out what I'd done I would have expected signal, but out of phase. This made me experiment with shorting +ve and -ve together, which gave me signal as opposed to the silence I would have expected - shorting pins 2&3 on the output XLR doesn't seem to effect the output.

 

The manual says the outputs are electronically balanced. Are these cleverer than transformer balanced, and will avoid a short between +ve and -ve (but this still doesn't explain a lack of signal between ground and -ve), or is it possible the outputs from the Soundcraft EPM series are unbalanced?

 

I tried two different EPM desks and an EFX desk and got the same results. I was going to put a scope on it to see what's going on but before that I wondered if I was missing something.

The soundcraft SX and F1 ranges ARE NOT BALANCED OUTPUT I don't have personal experience of the innards of the EPM range but I believe them to be the same, I've certainly had the same interfacing issues. The unbalanced output is connected to pin 2 via a 75 ohm resistor. Pin 3 is 'ground compensated' by being connected to ground with a 75 ohm resistor. Thereby presenting a balanced impedance on both wires ... sort of... but absolutely no signal on pin 3 other than hum/buzz if the gain is turned up on the next stage. My only clean solution ever to interfacing to a 3.5mm jack has been to use isolating transformers I've found these to be pretty good:

https://cpc.farnell....oks-BIGBOOK2019

 

In fact I automatically use transformers on all o/p's if more than one output is connected to another device.

 

I've adapted several of those transformers over the years as the metal cans split in half with very careful tapping after cutting the heatshrink and then changing your cables to suit length and connectors.

Edited by sunray
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Slight correction... the other side is just induced noise.... not inverted noise. The whole point of twisted pair is to ensure that the same noise signal is picked up on both cores. The differential input then cancels any common mode (noise) signals.

Of course yes sorry, it would carry a copy of induced noise to be inverted, not inverted noise.

 

I don't mean to teach grandma to suck eggs, but I assuming you definitely have the Atem inputs set to line (it can only be done in software I think).

 

I'm also guessing that you have HDMI sources connected so there will be lots of digital 'shash' floating around. My go-to problem solver in this situation, which I come across a lot using Macs, external monitors and audio devices is one of these; https://cpc.farnell.com/pulse/pls00548/ground-loop-isolator-3-5mm-p-s/dp/AV25542?st=loop%20isolator

It's digital shash for sure, as the beating and tone changes with the images on the ATEM. I was feeding it with BBC News for testing and with the white screen graphics it increases significantly.

 

Happily a couple of days ago I had ordered a couple of days ago a couple for ground loop isolators from Kenable to try: https://www.kenable.co.uk/en/audio-cables/audio-adapters/8073-35mm-stereo-ground-loop-isolator-female-to-male-earthing-for-audio-008073-5015972172537.html

Which have improved things. There's still a bit of digital beating but it's much more acceptable.

 

So the last question is, if it's impedance balanced, and if the wiring is done at the output stage (ie at the XLR connector, and the cable itself is single core and screen) is there an advantage to shorting pins 1&3 or does it make no difference?

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Slight correction... the other side is just induced noise.... not inverted noise. The whole point of twisted pair is to ensure that the same noise signal is picked up on both cores. The differential input then cancels any common mode (noise) signals.

Of course yes sorry, it would carry a copy of induced noise to be inverted, not inverted noise.

 

I don't mean to teach grandma to suck eggs, but I assuming you definitely have the Atem inputs set to line (it can only be done in software I think).

 

I'm also guessing that you have HDMI sources connected so there will be lots of digital 'shash' floating around. My go-to problem solver in this situation, which I come across a lot using Macs, external monitors and audio devices is one of these; https://cpc.farnell....loop%20isolator

It's digital shash for sure, as the beating and tone changes with the images on the ATEM. I was feeding it with BBC News for testing and with the white screen graphics it increases significantly.

 

Happily a couple of days ago I had ordered a couple of days ago a couple for ground loop isolators from Kenable to try: https://www.kenable....5972172537.html

Which have improved things. There's still a bit of digital beating but it's much more acceptable.

 

So the last question is, if it's impedance balanced, and if the wiring is done at the output stage (ie at the XLR connector, and the cable itself is single core and screen) is there an advantage to shorting pins 1&3 or does it make no difference?

 

Just ignore pin 3, it could still come back to bite you.

 

There is a very slight chance the arrangement with the 2x XLR to 3.5mm jack between mixer and transformer could still be creating problems, which is why I commented about modifying the transformer and changing the cable arrangement in my previous post.

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https://photos.google.com/u/1/album/AF1QipOF06DIybHihHgDFVLD_ip9CGbgoTTtu52eHFBs

My sketch.

The bold black is the mixer and unchangeable, Ideally you require the top half shown in red, orange, green & BLUE

 

 

With the adapter arrangement you will be creating the lower half with the PURPLE.

 

There is a hig chance it will be fine but there is just a tiny chance noise could be created in the loop shown with the arrow, I will say this was an arrangement I used to front end a halls system for several yearswithout any issue... until I found myself with the mixer in a different location and extended the XLR part of the system with 20m mic cables and I aquired terrible, no really really terrible, 50Hz mains noise. I was able to get round it by adding XLR - XLR transformers at the mixer.

 

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I've put up 3 more pics of the real thing which has been feeding into a mini hifi in the kitchen with phono plugs for many years

https://photos.googl...GbgoTTtu52eHFBs

 

Originally it was fed from a Glensound mixer but now from the lounge hifi with a similar unit at the other end.

 

In this instance wired directly with CAT5, I've sketched roughly how a pair of twin screened would be wired with the screen isolated at the transformer. The cables will be connected to 2 & 3 in the XLR's, also shown how the 3.5mm cable is likely to be wired.

 

All I hope now is the pics are visible, could someone let me know please.

Edited by sunray
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