Balanced - unbalanced

[Jordan]

This is probably an old subject but I have done quite a bit of reading on the subject and cant find the answer.

 

I am driving our monitors using an old amp BK electronics mxf600 http://www.bkelec.com/Pro/Amplifiers/Mxf600.htm I am using the aux sends from a Behringer ub2442fx desk which are unbalanced. I have always assumed the amp inputs are unbalanced (I have just emailed the supplier to try and get confirmation ) as it has jack and XLR inputs and the markings on the back say "Pin 1 - 0 volt, Pin 2 signal" with no mention of pin 3. With a continuity tester pin 1 has continuity with the chassis. To this end I have always connected it TS jack - TS jack with signal on tip and shield on sleeve. This has always resulted in a little bit of low feq hum (probably ground loop). This isnt noticable when playing but is just enough to be anoying when everything is quiet.

 

I have recently started running the signal through an Apex dbqzero 30 band graphic (balanced in/out). I refer to the Rane note on sound system interconection when making up cables for things like this and so made a jack - XLR cable to feed the apex from the desk with tip to pin2, sleeve to pin3 and shield to pin 1 but not connected at the jack end. To feed the amp I retained the jack - jack cable so that if necessary I could go straight into the desk and used an off the shelf XLR - jack adaptor with pin 3 disconected. This resulted in the same hum only louder.

 

I tried the jack cable straight into the apex trs socket (coupling ring and sleeve I think), this resulted in the original level of hum.

 

I tried a balanced XLR cable from the apex into the amps XLR, this gave the same hum but with added hiss.

 

I tried an off the shelf XLR - jack cable with pins 1 and 3 coupled, this gave the original hum.

 

What I would like to try is making an XLR - jack cable with only the signal connected at the jack end then signal to pin 2 and shield to pin 1 at the XLR.

 

Is this a good idea or could it possibly be detrimental to the equipment? Should I also couple the shield to pin3? Am I just stuck with hum?

 

I should also say that I have connected the output of the apex to another amp with balanced input and this has no hum and that the mxf600 doesnt hum with no input present.

 

Sorry to have rambled on a bit.

[jamesperrett]

It sounds like you need to use an isolating transformer on the signal input of the power amp as the amp's internal grounding is making it susceptible to ground loops.

 

One other thought... do you know what sort of circuit is used on the balanced output of the Apex? There is a slim chance that connecting pin 2 on the Apex to pin 2 on the amp and pin 3 on the Apex to pin 1 on the amp might just work.

 

Cheers

 

James.

[Jordan]

It sounds like you need to use an isolating transformer on the signal input of the power amp as the amp's internal grounding is making it susceptible to ground loops.

 

One other thought... do you know what sort of circuit is used on the balanced output of the Apex? There is a slim chance that connecting pin 2 on the Apex to pin 2 on the amp and pin 3 on the Apex to pin 1 on the amp might just work.

 

Cheers

 

James.

 

Thanks James, if I cant fix it with cabling then I will get an isolating transformer. I dont know what sort of circuit the apex has but I am willing to give your sugestion a try if it is safe to do so.

 

I suppose I should ask is it safe to just try different wiring configurations in the hunt for a solution or is there a risk of frying something? I imagine I should always only connect pin 2 to pin 2.

[Sound Man]

Does the BK amplifier have an earth lift switch on the back?

 

If so, that would solve the hum issue.

 

David

[Jordan]

Does the BK amplifier have an earth lift switch on the back?

 

If so, that would solve the hum issue.

 

David

 

Unfortunately not.

[Sound Man]

Does the BK amplifier have an earth lift switch on the back?

 

If so, that would solve the hum issue.

 

David

 

Unfortunately not.

Then something like this unit is what you require.

 

The internal wiring of the amplifier can be altered to solve this hum issue but you would need to know what you were doing.

 

David

[djandydee]

The amp in question uses a pair of BK electronics OMP modules at 300 watts into 4 ohms. The modules input is on solder pins with a screen and a hot - no cold so unbalanced.

 

I had 6 of them at a time and was very pleased with the sound quality....very low noise, low THD and as linear as I needed at the time. I mouned them into RS 3U rack cases and changed the heatsinks. The inputs were wired to female panel mount XLRs with pin 2 hot and pin 3 and 1 screen.

 

Eventually I used a small balancing transformers which was hacked out of some piece of redundent BBC kit. Wired to pins 2 and 3 with 1 screen as standard.

[Jordan]

The amp in question uses a pair of BK electronics OMP modules at 300 watts into 4 ohms. The modules input is on solder pins with a screen and a hot - no cold so unbalanced.

 

I had 6 of them at a time and was very pleased with the sound quality....very low noise, low THD and as linear as I needed at the time. I mouned them into RS 3U rack cases and changed the heatsinks. The inputs were wired to female panel mount XLRs with pin 2 hot and pin 3 and 1 screen.

 

Eventually I used a small balancing transformers which was hacked out of some piece of redundent BBC kit. Wired to pins 2 and 3 with 1 screen as standard.

 

Cheers, any ideas where I might source balancing transformers that would do the job. Sounds like a mod I could tackle myself. I would certainly like to have a go.

[tolley1466]

Is this an install or does the problem persist from venue to venue?

[Jordan]

Is this an install or does the problem persist from venue to venue?

 

Venue to venue.

[KevinE]

The BK (mainly aimed at mobile discos) are floating unbalanced input via XLR and 1/4in jack, (I use one as a workshop amp), the OMP modules are floating unbalanced too but come with a note saying the OV input is best tied to ground. The BK XLR inputs use metal connectors, are you getting a hum loop via an earthed-shell XLR lead somehow? As you already mention, the BK XLRs arent conventionally wired.

 

The cheapest way of making the BK into balanced input would be (IMHO) to get a budget stereo balun box (or a 1:1 DI box) and strip the transformers out of that, it'll probably cheaper than buying 2 component pro balun transformers from somewhere like Canford or Neutrik.

[jamesperrett]

I suppose I should ask is it safe to just try different wiring configurations in the hunt for a solution or is there a risk of frying something?

 

If the manufacturer says that it is safe to connect pin 3 on the output to ground then you should be OK to experiment.

 

Cheers

 

James.

[Jordan]

I suppose I should ask is it safe to just try different wiring configurations in the hunt for a solution or is there a risk of frying something?

 

If the manufacturer says that it is safe to connect pin 3 on the output to ground then you should be OK to experiment.

 

Cheers

 

James.

 

There is nothing in the documentation to say that but I suppose I have already done it by trying the XLR - jack cable with pins 1 and 3 coupled. That had no apparent ill effects, so I think I will have a play around tomorrow. Thanks

 

 

A concurrent post has been automatically merged from this point on.

 

The BK (mainly aimed at mobile discos) are floating unbalanced input via XLR and 1/4in jack, (I use one as a workshop amp), the OMP modules are floating unbalanced too but come with a note saying the OV input is best tied to ground. The BK XLR inputs use metal connectors, are you getting a hum loop via an earthed-shell XLR lead somehow? As you already mention, the BK XLRs arent conventionally wired.

 

The cheapest way of making the BK into balanced input would be (IMHO) to get a budget stereo balun box (or a 1:1 DI box) and strip the transformers out of that, it'll probably cheaper than buying 2 component pro balun transformers from somewhere like Canford or Neutrik.

 

Yeah, thats why I was asking if it would be ok to disconect the cable shield from the amp end of the cable. Am I on the right track with that?

 

Cheers for the tip regarding the transformers, I will have a look at that.

[dbuckley]

The problem with transformers is that quality is a function of price.

 

The cheapest decent line level audio tranny I know of is the Neutrik NTL1 1:1 which in the "under the fifty quid" price category. It also doesn't allow the hottest of signal levels, but is adequate for sane use.

 

You can get something from your local emporium for under a fiver, and it will work "OK"; I use this class of audio transformer for uncritical things like the stage relay feed, which goes to 100V line amps and box speakers, but its not a quality component. However, it does isolate grounds with 100% effectiveness of a more expensive transformer, and it is of adequate quality for some jobs.

[Sound Man]

These audio transformers from Rapid Electronics work well. I put a pair of them into my amplifer to give it a balanced input.

 

I would recommend putting them into the optional mu-metal can to avoid hum pick-up.

 

David