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Relative levels fading a little


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Not sure that this is an actual problem but seemed very apparent at the time and appears to have cleared up based on some rerouting so just looking for some opinions.  I'm just curious to find out if it a potential cable issue or just the way  the balanced line in's handle this as it was the mix of balanced and unbalance inputs.  Worst case I hope it's not the start of a fault

 

This is what I was doing had a zoom RT-223 drum machine hooked up to i/p 15 using a unbalanced 1/4" jack cable, output from zoom is line and unbalanced I think.  Had a Blackstar HT20 (Emulator out line balanced) hooked up to i/p 14 using a balanced 1/4" TRS.  Auto trimmed both RT 14dB, HT 10dB set the faders to about -3dB and -1dB respectively and started to jam along with the drum machine.  After about 2/3 minutes got a very noticeable drop off in drum level (hadn't touched a thing) so watched on deep tweek to see if the auto tracking algorithm was changing trim.  No change I could see, switched off trim tracking to see if this had an effect nothing changed on the level .

 

Continued to play along and level seemed to go back to original, again other than turning off trim track hadn't touched a thing.  So after a few more repeats of drop off then recover some time later decided to reboot.  Same thing happened again only the amp had decided to copy the fault and level dropped off on this too, then came back.    

 

This is what I did moved drum to i/p 12 re trimed and set-up level and left HT20 in i/p 14 unchanged.  Level issues so far no problems and it's been running like this for a few hours.

 

So do the balanced line in's not like unbalanced line connections and somehow it effects all (13-16) i.e is it a common multiplexed ADC or is there something weird going on.

 

Opinions?

 

 

 

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I'd be tempted to try again with an alternative cable, BUT one which is TRS (as least at the M20d end) with no connection to the ring.

 

Depending upon the internal wiring of the TS output on your zoom, it's possible a balanced TRS cable will insert into the zoom without causing issue (you can always peer into it with a light to see if it has three cross bar connects inside or just two).

 

The way a balanced input works, the ring (balance -ve) should not be connected to ground/shield. In case you didn't know (or for anyone else reading) the balance line (ring in a TRS cable and pin 3 in an XLR) carries no level of it's own. As the signal passes through the +ve signal line in the cable, any interference is generated almost identically in the balance wire. At the end of the line, the +ve and -ve lines are passed through a process during which the equal interference generated in the +ve and -ve lines cancel each other out leaving just the original source signal. Connecting the shield/ground to the -ve can create significant problems.

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Same thing happened again only the amp had decided to copy the fault and level dropped off on this too, then came back.

This sounds like it could be a power issue. How do you know it was the mixer in the first place? Could have been the amp all the time.

 

The way a balanced input works, the ring (balance -ve) should not be connected to ground/shield. In case you didn't know (or for anyone else reading) the balance line (ring in a TRS cable and pin 3 in an XLR) carries no level of it's own. As the signal passes through the +ve signal line in the cable, any interference is generated almost identically in the balance wire. At the end of the line, the +ve and -ve lines are passed through a process during which the equal interference generated in the +ve and -ve lines cancel each other out leaving just the original source signal. Connecting the shield/ground to the -ve can create significant problems.

Well, technically both lines carry signal in a true balanced line. The "hot" line (generally pin 2 or the tip) runs the signal. The "cold" line (generally pin 3 or the ring) runs an inverted copy of the same signal. At the receiving end, the cold is inverted and added to the hot. The result is 2x signal level with common-mode noise and hum cancelled.

 

If the source signal is unbalanced, you would typically ground the cold line at the input or leave it unconnected. There wouldn't be any advantage to connecting it because the source isn't balanced.

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Changed cable to full balanced TRS both ends, no reoccurrence of the level fade.  So changed cable back to original unbalance 1/4" Jack no level fade but did pick up a low level buzz/noise, didn't sound like 50Hz.   Ran it for a good while and though everything was fine, came back after a switch off and a few hours away, tried again with the full TRS cable and back cam the level fade. 

 

So as Rick suggested could be power, could be the amp DI input both are on the same surge protection strip so shouldn't have grounding issues.  If it is the amp why would it be affecting the drum machine level (don't think the output is balanced). Maybe the original issue is having the contact with the -ring and shield within the M20 which is what would happen if a mono unbalanced cable was inserted.  Full TRS appeared initially to solve it now I'm not so sure.  

 

Just don't like random things happening I know the keyboard player has a unbalanced 1/4" so could this reappear if I use this set-up at the next gig, need all the other 1-12 for the mics and other things so these line inputs are important.  May just get a mono to TRS cable for the keyboard but still don't appear to have worked out why the level appears to move.

 

 

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