Laptop soundcard hum, Art DTI and loss of bass

[dirkenstein]

This weekend I was trying to get a friends's Vestax pcv-275 mixer to talk to both an in-house sound system and my powered w-audio speakers at a very small private venue. He was mixing using Traktor and a native instruments Audio 4 DJ USB interface.

 

The connectons to the vestax mixer were on phono leads to the mixer and ostensibly balanced 1/4 inch TRS outs from the mixer converted to XLR for all the other audio components (built in Citronc system, not further identified). My speakers were looped through this on balanced XLR passive splits.

 

I'm not convinced the mixer itself actually has balanced outputs, as it doesn't list this as a feature anywhere in the manual.

 

This set-up worked OK, but the cheap aftermarket PSU on his laptop was injecting significant noise into the audio device chain. You couldn't run off battery simply because the battery in the laptop was very. very dead.

 

The easiest thing was to try to isolate the mixer and audio 4 dj interfacefrom the balanced part of the system by using an ART DTI isolator/ground lift box. However, the result still injected noise if either of the balanced feeds from the mixer was connected on channel 1 of the DTI. If you only used channel 2 on the DTI it worked with no noise, but removed almost all of the bass (even though you used the same feed originally tested with channel 1 on the DTI, so I don't think it could have been that the outputs were somehow split bass/treble rather than left/right from the mixer).

 

Any ideas what happened here? I didn't get a chance to try to investigate this further, unfortunately. Is the DTI box likely defective somehow or does this behaviour make sense in some cases of crude balanced/unbalaced conversions? Any Vestax mixers known to develop a bass/treble cut fault on only one stereo channel?

 

Thanks,

 

Dirk

[grizzly]

I'm not convinced the mixer itself actually has balanced outputs, as it doesn't list this as a feature anywhere in the manual.

 

You're quite correct.

See here, bottom of page 2: PCV-275 specs.

[wmcalin]

Might be worth trying a DI box.....

 

Regards

 

Will

[tinnitus_man]

This weekend I was trying to get a friends's Vestax pcv-275 mixer to talk to both an in-house sound system and my powered w-audio speakers at a very small private venue. He was mixing using Traktor and a native instruments Audio 4 DJ USB interface.

 

The connectons to the vestax mixer were on phono leads to the mixer and ostensibly balanced 1/4 inch TRS outs from the mixer converted to XLR for all the other audio components (built in Citronc system, not further identified). My speakers were looped through this on balanced XLR passive splits.

 

I'm not convinced the mixer itself actually has balanced outputs, as it doesn't list this as a feature anywhere in the manual.

 

This set-up worked OK, but the cheap aftermarket PSU on his laptop was injecting significant noise into the audio device chain. You couldn't run off battery simply because the battery in the laptop was very. very dead.

 

The easiest thing was to try to isolate the mixer and audio 4 dj interfacefrom the balanced part of the system by using an ART DTI isolator/ground lift box. However, the result still injected noise if either of the balanced feeds from the mixer was connected on channel 1 of the DTI. If you only used channel 2 on the DTI it worked with no noise, but removed almost all of the bass (even though you used the same feed originally tested with channel 1 on the DTI, so I don't think it could have been that the outputs were somehow split bass/treble rather than left/right from the mixer).

 

Any ideas what happened here? I didn't get a chance to try to investigate this further, unfortunately. Is the DTI box likely defective somehow or does this behaviour make sense in some cases of crude balanced/unbalaced conversions? Any Vestax mixers known to develop a bass/treble cut fault on only one stereo channel?

 

Thanks,

 

Dirk

 

Hi Dirk

 

I have had people with the same problem as you had with the laptop. Any laptop will make a hum through pretty much anything that it's connected to when it is connected to mains. The PSU on the laptop is a switched mode transformer which is NOT relative to a ground point, it only has mains going into it so the output - the connector that goes into your laptop could potentially have anything 0v to 240v mains riding on top of the DC voltage coming out of it! The voltage that rides on the output is at a very very high impedance so if you touch it with your multimeter you may not even see the voltage appear on the meter. Don't worry you won't get a shock off it as it is at a high impedance. The possible cause of loss of bass is no ground at all! But what I always tell anyone regardless of what audio gear they have connected to their laptop is to put a line isolation transformer in between the two. The line isolation transformer has no connection between it's output and input, it uses a transformer which basically converts your audio signal into a magnetic field and then back into an audio signal again less any voltages that could be present. An example of this is http://www.connect-audio.co.uk/proddetail.php?prod=MONFGA30 I always recommend a line isolation transformer over say a DI box that really only uses electronics to do the job. You can find these transformers all over the net this is just an example.

 

Hope this helps If you have any other questions email me sales@ultimateproaudio.co.uk I can give advise and even sell you sound and lighting gear if you want! Have a look at our website - www.ultimateproaudio.co.uk We deal in sound and lighting for professionals, DJ's Public, Schools etc.

 

Stuart

[mervaka]

some of you guys really should read more carefully. the OP said he has an ART DTI, which is an isolator box.

[tinnitus_man]

The ART DTI it very much a balanced isolator box using transformers! I have always told people to go with the simple option http://www.connect-audio.co.uk/proddetail.php?prod=MONFGA30 as to many inputs and outputs can become a nightmare - Never has a complaint in all my born days about noise issues with these.