Laptop audio feed problem and loop system

[Tomo]

I still don't get how it will work ok at 9v but not at under 15v on a phantom power unless the current push is different, could anyone answer this for me?

<{POST_SNAPBACK}>

Either:

There is circuitry inside it to boost the voltage from the 9v PP3 battery up to whatever it requires internally.

This will be different to the circuitry used when being run from phantom power.

 

Or:

It uses two 9v batteries which gives 18v. (Not looked at the spec)

[dominicgross]

just a thought about the hiss: this is due to a poor noise to signal ratio from the laptop: not much you can do without getting a different sound card...

 

BUT: using a cheep gate, with the threshold set to the right level will happily cut out the hiss when no 'real' signal is being output...

[J Pearce]

It will most likely use either a voltage regulator to hold down the 48 volts or a transformer to up the 9V or down the 48V.

 

Looking at the fact that it can cope with variable voltage it will probably use a voltage regulator to hold down the 48V.

 

I could however be totally wrong.....

[mac.calder]

Just a thought... To get rid of the click as the jack is slid in, have you thought about something like this usb external audio device... of course this only works if you only have the same laptop used all the time (cos it will probably need drivers)

[Simon Lewis]

Two things...

 

1) A passive DI may provide better isolation between your laptop and the PA. If all else fails, I have had success with USB preamps for recording sermons and playing back material from PCs.

 

Otherwise, read the other posts on getting laptops and PAs to talk with each other.

 

 

2) No, the loop cable should not run alongside the multicore. Tell your installation company to do it properly;-) If you are an Anglican Church, let your Diocesan Technical Adviser know that the install was not carried out correctly.

 

The loop does not radiate RF, (as some posts suggest) but a varying magnetic field. This can be picked up by a variety of devices, and if there is noise / screeching present it is quite possible that you have an electromagnetic feedback loop. WHen you plug the laptop jack in, the usual click generates a pulse from the loop which is a high enough signal to leak into the input cabling and to then cause feedback. The loop's limiter will probably stop it running away, which is why it isn't continuous.

 

You need the laptop interface sorting out (why is it unshielded anyway?!) and the loop cable routing correctly.

 

Hope this helps!

 

Simon

[SceneMaster]

Ok, well I think we have to go along the DI route anyway as we need to turn the signal into a balance feed. However for the connection noise problem maybe it would be worth installing a mute switch for the laptop feed in the pulpit. If so can any one recommend any small switches which would be suitable for cutting/muting the stereo output on the cable. Which is going to be installed into the current PC box. With regards to the gate could anyone suggest a cheap/decent model (links please)?

 

Thanks,

[Simon Lewis]

You still need to address the issue of the loop and multicore cable routing. You could still end up with various strange problems occuring.

 

Is the loop fed from a mixer aux output or from the line output of an integrated mixer amp?

 

Simon

[SceneMaster]

The loop out put is from an aux output on either the amp or the graphic I am not sure which way it was done probably though from the graphic. I don't think the loop is causing too much problem though but I could be wrong.

 

However I need to know whether buying a DI box is the correct thing to do for the laptop output I would have thought so as at the moment it is an unbalanced feed running more than 30m and this needs to be balanced. Also is the idea of fitting a mute switch in the pulpit for the laptop feed a good idea? The switch would be inline with the cable coming out of the pc connection box already installed.

 

With regards to rerouting the multi-core this is impossible as there is no other route for it as it is a small church the loop cable also has to run along this route. Surely if the signals are balanced they shouldn’t be too greatly effected by it?

 

I will see if I can find a suitable mute switch and post it here.

 

Thanks,

[david.elsbury]

I think, though not 100% sure, that for the mute switch any toggle switch should be fine. However, I'd set it up so that instead of simply breaking the line between laptop and input, it shorts the output to ground. This should help stop any clicks or pops when you mute it.

 

Gate wise, any cheap but decent compressor should have a gate function, e.g the Alesis 3630 or something from behringer. However, consider not a gate, but something called a "downward expander". A gate is either on or off, (you will notice it acting) however a downward expander is a more gradual fade-off.

Have you ensured that the gain structure of your system is correct? As in livesound, set the laptop as hot as practicable (without clipping or distorting anything though) and then the input may not need to be as high, so there is less preamp noise from the mixer.

 

David

[dbuckley]

The fundamental problem which has been brushed up to by a couple of contributors is that the speaker output on laptops are inherently noisy, and will probaly be unsuitable for your use, no matter how many DI boxes you use!

 

You need to use an external USB audio output device. Even the cheapest moeny can buy (which I think is an Eiderol something or other) is so much better than the PC speaker output will be.

 

This wont directly solve the balanced / unbalanced problem, unless the USB box is balanced out, which several of the more expensive models are. But it may be sufficiently better that you dont need to use a DI. I'm sure I'm not the only person on here who has shoved unbalanced signals down snakes with acceptable results...

[bassman]

Just had it from Ampetronic that it is considered to be "...asking for trouble..." to run a loop directly alongside an audio multicore. For all but short runs a gap of at least 250mm should be kept between the two, even with balanced lines. I would have words with your install company...

[J Pearce]

My church's loop system runs in the floor round the edges of the building and our multi runs straight down the middle, direct line from booth to stage. However as I have previously mentioned we have interference with electric guitars and when unplugging unbalanced devices.

 

Our install was poorly done, they got the multi and the loop right but we had to rewire the booth as unbalanced cables had been used when we specced a fully balanced system, and all the auxes were set up wrong. The speakers supplied and fitted were in the wrong place and too small for the music application specced (they were only designed for speech), and the lighting bars ended up 2m too low.

 

It seems to be the same situation as with schools there are a lot of con-men out there who profess to specialize in schools and churches, then give them a system poorly designed around their specs.

 

.....Rant over.....

[Simon Lewis]

The loop out put is from an aux output on either the amp or the graphic I am not sure which way it was done probably though from the graphic. I don't think the loop is causing too much problem though but I could be wrong.

 

It isn't going to help...! The quote from Ampetronic should highlight the issue, if you don't believe me

 

The way it's been set up is for everything that goes through the audio amp or graphic also goes through the loop. This is OK for simple installs, but as soon as it gets more complex (or has musical instruments etc.) you need to create a separate mix from a spare desk aux send.

 

Regarding laptop sound, can you borrow a couple of passive DIs to try out? The USB pre is going to cost more, but will give a better solution. However, I've used DIs with laptops before with reasonable success.

 

 

Simon

[Alec]

As others have said, a USB interface with balanced outputs will give you the best result, but cost £100-150.

 

In the absence of this, a DI box should certainly improve things. I'm a bit wary of the Behringer DI20. I bought one and exchanged it because it was so noisy and the replacement was as bad - and we're talking *really* noisy. Their DI100, however, is a little more expensive and mono-only, but an order or magnitude better.

[SceneMaster]

Ok, I went down to the church to have a play with the laptop and the loop system. The multi-core although runs along the same route as the loop cable it has be cable tied to the a heating pipe so it is about 10-20cm away from it for most of it’s length only at both ends do the two cables come into direct contact with each other.

 

I did a test with several mics and there were no feedback problems and no interference with them with the loop amp running. The loop amp is in the same rack as the mixer and graphic ect. With the laptop problem, the hiss is reduced to a expectable level by turning the mixer volume down to nearly zero on it’s channel and putting the laptop output on near full (as suggested here, thanks guys) The sound is great quality and plenty loud enough and the hiss in the down time is minimal. This just leaves the connection squeal problem to sort this I will put a mute switch inline with the cable. Could anyone recommend switch for this? I presume a double pole double throw would the right way to go. As in a pole for each of the signals of the stereo and the double throw so the bottom throw shorts the signals the ground of the feed to the mixer to ground (would this work)?

 

As with regards to DIing the laptop I will do this as I don’t really want an unbalanced feed running down our multi. I presume shorting the signal in the mute switch won’t cause too much problem for the DI or the mixer. Will the DI20 do and not add noise to the signal for this situation, it is only going to be used for the PC and surely it will improve the current situation regardless?

 

In the absence of this, a DI box should certainly improve things.  I'm a bit wary of the Behringer DI20.  I bought one and exchanged it because it was so noisy and the replacement was as bad - and we're talking *really* noisy.  Their DI100, however, is a little more expensive and mono-only, but an order or magnitude better.

<{POST_SNAPBACK}>

 

This would suggest not but others say they are ok and would fine for this application? :D I don't have the funding for much better unless there is a better two channel DI for under £30 it needs to be two channel for the stereo feed.

 

Any advice is greatly appreciated, :D