Sound source options

[Brian]

I'd not bother trusting the headphone socket.

 

We have come across situations when running dual mono tracks out of a laptop...

 

This has been my experience as well, and I have found no way to stop the unintended mixing of channels...

 

It seems quite a few people have seen it on a number of different brands of laptop and versions of OS. What is interesting is how little real information about what is going on 'under the hood' is out there.

[timsabre]

A lot of PC audio drivers seem to have "enhancement" built in to the headphones output. My HP laptop does it, in this case it is some sort of dodgy 3D processing or something and it also has an awful AGC type thing enabled. You can turn it off with a registry hack but Windows keeps turning it back on.

I have a Tascam US 2x2 for this sort of thing which is a handy little box.

 

[sandall]

I don't recall ever coming across any bleeding or processing, & I always sum L & R to mono, unless I know it's a split-track recording (I suspect the chances of the OP meeting time-code on 1 channel are fairly remote!). The UCA-202 is a very useful little tool, though loading up-to-date ASIO drivers is a bit fiddly & restricts you to using a single USB port. I use one for recording, but have never bothered for playback in the field - for live situations the less connectors between source & speakers the better (I'm sure there's a "Law" somewhere that states that the chances of something going wrong are proportional to the square of the number of interconnections).

 

However I am a bit curious as to why so many posters insist that transformers are essential. It is many years since I have been troubled by buzz on laptop headphone outputs, & I don't recall ever having hum problems, - DI's for balancing on long lines, yes, but for less than a metre of what will still be unbalanced ??

[Brian]

However I am a bit curious as to why so many posters insist that transformers are essential...

 

Several scenarios...

 

1) Laptop with 3-pole mains adapter. This means the 0V of the adapter is very likely to be connected to mains earth and hence the sleeve of the output jack is also earthed. In a simple set-up this will cause no problem but if the 'connected-to' equipment gets its mains from somewhere different to the laptop then you can get an earth loop and hence hum.

 

2) Laptop with 2-pole mains adapter. To make such an adapter pass the EMC rules some manufacturers connect the 0V output back to 'somewhere' on the mains side via a capacitor. The idea is that the 'somewhere' will be a low impedance at RF frequencies and the capacitor will filter any RF on the output back to this point. The problem is that the capacitor will also pass any HF crap forwards onto the 0V line and thence onto your audio output's 0V.

 

I can demonstrate the second scenario on the TV downstairs. The TV has a 2-pole mains connector. The external speakers have an external 2-pole PSU. When I first connected them up the speakers had a slight hum. I could solve it by introducing the DI box between the two or by randomly swapping 180 degrees either one of the 2-pole mains connectors.

[Andrew C]

However I am a bit curious as to why so many posters insist that transformers are essential. It is many years since I have been troubled by buzz on laptop headphone outputs, & I don't recall ever having hum problems,

You should buy a lottery ticket; luck favours you! http://www.blue-room.org.uk/public/style_emoticons/default/biggrin.gif

[sandall]

My "luck" may be helped by always making sure that all audio sources derive their power from the same bit of distro as the mixer :).

 

I do usually carry a pair of transformers with me, but mostly because a lot of video guys insist on taking a low-level feed into the mic input of their cameras but don't know how to turn the phantom power off :(.

[timsabre]

Same distro does not help. Some laptops just buzz without transformer di for the reasons Brian gives.

[sandall]

Maybe, as Andrew C says, I've been lucky, but I regularly use my own HP (3 pole mains), Dell (3-pole & 2-pole), Toshiba (2-pole) & Acer (2-pole) laptops, plus a slew of Macs (various connectors) & assorted customers' own laptops, & don't ever recall a problem with hum (dodgy 3.5mm connectors are a different story altogether!!).

[sunray]

Maybe, as Andrew C says, I've been lucky, but I regularly use my own HP (3 pole mains), Dell (3-pole & 2-pole), Toshiba (2-pole) & Acer (2-pole) laptops, plus a slew of Macs (various connectors) & assorted customers' own laptops, & don't ever recall a problem with hum (dodgy 3.5mm connectors are a different story altogether!!).

All I can say is I don't know how you have got away with it, at one time I automatically used this sort of device: https://cpc.farnell.com/av-link/201-086uk/ground-loop-isolator-rca/dp/AV26879?st=rca female but rewired to make them bespoke. I have found that they are required less frequently now than in the past.

[jamesperrett]

However I am a bit curious as to why so many posters insist that transformers are essential. It is many years since I have been troubled by buzz on laptop headphone outputs, & I don't recall ever having hum problems, - DI's for balancing on long lines, yes, but for less than a metre of what will still be unbalanced ??

 

Are you always feeding the laptops into the same type of input with the same cables? You might get away with this if you are sending the laptop output into a balanced input with the ground from the laptop going to the - input rather than the ground on the receiving device.

[Jivemaster]

Over several years of laptop DJing I've had lots of issues if the system was changed, all solved by isolation. For me, isolation by transformer is the cheap and effective way of killing interference/noise. Maplin and ChinaPLC would not produce "in lead" assemblies of transformers as hum blockers earth loop breakers if there wasn't a market by the million.

 

Search OEP in RS, or http://www.oep.co.uk/

 

Sometimes a cheap mixer will have transformer balanced outs and these may work well the input being grounded or not with the mixer and isolated from the grounded amplifier.

[sandall]

Are you always feeding the laptops into the same type of input with the same cables? You might get away with this if you are sending the laptop output into a balanced input with the ground from the laptop going to the - input rather than the ground on the receiving device.

Nope. I have a bag of leads, made up from cheap 3.5mm - 3.5mm leads cut in half & cut-off leads from dead headphones, all feeding plastic Rean mono jack-plugs, with a small (100R - 470R) resistor in one leg, in case a line output objects to L & R being shorted together. Purists might suggest it should really be a pair of matched 2% resistors, one in each leg, but when feeding a 10k or 20k input impedance the difference in level between the legs is negligible. In any case it's quite hard soldering 2 resistors into a jack plug :(. Mixers are a variety of Soundcraft, Mackie & Behringer. As I'm using mono jacks the inputs are always unbalanced.

[Brian]

I'd rather spend £80 on an external USB box with transformer balanced outputs than risk a compromised show.

[Jivemaster]

Where my laptop feeds someone else's system it's transformer isolated.

 

If my laptop feeds my system exactly as last time it is taken out as it last worked.

[sandall]

It's all to do with Darwin. We survive as a species because we try to avoid things that might bite us. I use laptops, etc in controlled environments, with gear I've either brought with me or met before. I've never been bitten, so I don't expect to be (though I never travel without a couple of DIs, so I always have a work-round in case I ever do meet a rogue laptop).

 

On the other hand, having had PSU connectors (Soundcraft Spirit) & regulators (Behringer) fail on me in the past at embarrassing times I do carry a small back-up mixer for critical gigs. I used to always carry a spare amp as well, but now limit that to taking a battery-powered back-up system when working outside where there is only a single source of mains. It's all a balancing act - with the spare amp it's a balance between the very low risk of failure against the much higher risk of damaging my back lugging it into the venue. In the case of laptops it's a balance between the possible advantages of isolation against the risks involved in adding extra things that could themselves bite you (even a simple pair of transformers adds an extra 8 connectors to the signal path), & the extra faff & time (of which there is never enough) involved.