Signal loss issue

[TonyP]

At our church we have a Soundcraft Spirit LX7 desk of circa 2002/03 vintage.

 

It has a mono output which, by the use of splitter (“Y”) connecting cables we feed the crèche, the induction loop and, more recently, an MP3 recorder. The problem is that, not surprisingly in hindsight, the signal level to any one unit is reduced, presumably by being split across the three units.

 

As far as I can see, with my very limited knowledge, is that we need some sort of amp, signal? pre-?, to boost the signal before being split. Alternatively something very inexpensive that will take the mono signal in, and amplify it, and that has 3 or 4 separate outputs.

 

Temporarily we’ve connected the crèche feed to the headphone socket, as this isn’t used, to reduce the signal loss to the other units.

 

Thoughts on a way forward would be appreciated.

[mackerr]

At our church we have a Soundcraft Spirit LX7 desk of circa 2002/03 vintage.

 

It has a mono output which, by the use of splitter (“Y”) connecting cables we feed the crèche, the induction loop and, more recently, an MP3 recorder. The problem is that, not surprisingly in hindsight, the signal level to any one unit is reduced, presumably by being split across the three units.

 

As far as I can see, with my very limited knowledge, is that we need some sort of amp, signal? pre-?, to boost the signal before being split. Alternatively something very inexpensive that will take the mono signal in, and amplify it, and that has 3 or 4 separate outputs.

 

Temporarily we’ve connected the crèche feed to the headphone socket, as this isn’t used, to reduce the signal loss to the other units.

 

Thoughts on a way forward would be appreciated.

 

If you cannot get an acceptable signal level at the mono output you could use an audio DA like the Aphex 120a.

 

Alternatively you could use one or more of the post fader aux outs to create custom mixes for some of those outputs. Often those feeds don't actually want to be the same as the PA mix.

 

Any passive split will involve a small drop in level, but unless one of the devices has a very low input impedance (or is defective) you should have no problem with a 3 way passive split.

 

Mac

[Simon Lewis]

Tony,

 

As Mac suggests, the desk should be able to drive three devices without a major drop in signal level. Have you checked the wiring to each of the three devices? Could one of them (possibly the MP3 recorder - as this was the last item to be added) have a faulty lead or be wired unbalanced when the other two leads are wired balanced?

 

If it really is the case that the signal is being lost, then one solution would be to use a Behringer ULTRALINK PRO MX882. It costs about £71 and would allow you to split the mono output into ~ 6 outputs, each with its own level control.

 

Simon

[TonyP]

Thanks for these thoughts. Now to show my true ignorance. I really don't know what is meant by balanced in this context. Please enlighten me.

 

The MP3 feed goes into the line in which the MP3 spec says has an impedance of 15k ohms - that sounds fairly high to me.

 

I don't know what the loop amp spec is, I think there's a data sheet at church somewhere.

 

The creche uses a pair of mains powered PC style speakers. Again I don't know their impedance.

 

As we have no means of actually testing the loop, I use the MP3 and the creche speakers as a guide as to whether we have a decent level. When I get chance I'll see what happens without the MP3 in circuit, just the loop and the creche. If the creche level only drops marginally when the loop is connected then it seems to point to an issue/incompatibility with the MP3 recorder.

[TonyP]

OK, I now think I understand by 'balanced', at least in one sense. Essentially the impedance loading of the three units should be similar.

 

The MP3 recorder line input is 15K ohms, reading the booklet for the loop seems to suggest an impedance of around 1M ohms (does that sound right?). The crèche speakers (mains powered PC speakers by the look of it) have no specified impedance but I've seen some comments on the web suggesting PC speakers are designed to match sound cards with an output impedance of c.10k ohms, others that they have an impedance of c.8 ohms. All very confusing.

 

I'm guessing that 'balanced wiring' refers to either balanced loads or to the use of balancing technology in the cable (or thereabouts).

 

This morning, with the crèche feed coming from the headphone output, the people in the crèche could hear things fine, and the recording level on the MP3 was also fine. I've no idea whether the loop is OK.

 

To play safe I'll go in during the week and adopt Mac's suggestion and create a custom mix, voice only, for the recorder and connect it to a spare aux. output. Essentially it's the sermons we record and make available on our web site. Strictly we'd prefer, in an ideal world, all the outputs to have the same mix content. However including the music in the recorder mix is rarely necessary (used for an occasional wedding) and is the one that is the most problematic as the levels needed change from week to week, depending on the musician mix and their 'mood'.

[Pete Alcock]

Hi Tony,

 

Not quite... Balanced refers to a audio feed that has two signal conductors and a shield or ground. The signal on the two conductors is out of phase. The idea is that any interference (hum etc) induced into the cable on a long run is cancelled out at the balance input of the equipment being fed. It is possible to feed a balanced output, say from the XLR outputs of your LX7 into an unbalanced input on your MP3 recorder, creche speaker or loop amp etc. simply by grounding (connecting to the cable shield, ground or earth) the "cold" or "-" of the balanced output.

 

On an XLR the designation is PIN 1 = Ground, PIN 2 = Hot or +, PIN 3 = Cold or -

On a Jack output from your console: SLEEVE - Ground, TIP = HOT or +, RING = Cold or -

 

All your ancilliary equipment (loop amp, MP3 recorder etc) has high impedance inputs (>10k Ohms). You can safely connect four or five of these in parallel to your console outputs. The output impedance of your console is around 150 Ohms and it's good practice to drive it into loads of 10x this figure (>1.5k Ohms). The fact that the input impedances of your ancilliary kit are all different is immaterial, because they're all high so will hardly load the console at all.

 

I do suspect that the problem you initially had was a connector/cabling issue. Now you're experimenting with using auxes etc. might I suggest that you use the mono output of the LX7 for feeding the creche/foyer or whavever, use a post-fade aux for feeding the loop (so you can create an appropriate mix for the hearing aids) and take the record feed from the main desk out or the mix inserts. This way you can keep the recording in stereo.

 

Trust this helps. Pete.

[andy_s]

might be worth investing in one of these which will help non- hearing aid users assess the quality of the feed from the loop.

[TonyP]

Pete, thanks for the explanation. That explains why I see 'stereo' jacks referred to for balanced connections.

 

The recording is only for the sermons so a custom mix will work fine as only 3 channels are used for voice and stereo is irrelevant. Indeed as it is for the whole desk.

 

The crèche and the loop both require the full mix, voice and musicians, so initially at least, I'll probably leave them as they are with the crèche coming from the headphone socket (not otherwise used) and the loop coming from the mono output. I'm wary of using a post-fade mix for the loop as it would need to include the musicians and their levels change from week to week depending on their mood or whatever.

 

Yesterday evening's sermon, co-fed with the loop from the mono mix, had excellent signal level, I almost had to reduce the volume when preparing it for our web site. I use Audacity for that task, an easy to use (and free ) PC package.

 

Andy's link to a loop tester is very interesting. Previously I'd only seen such testers at £300 or so. £73 or so (inc.VAT) is much more reasonable.

 

Lot's of useful information for me to think about here, for which very many thanks.

[Paul TC]

Pete, thanks for the explanation. That explains why I see 'stereo' jacks referred to for balanced connections.

 

 

The crèche and the loop both require the full mix, voice and musicians, so initially at least, I'll probably leave them as they are with the crèche coming from the headphone socket (not otherwise used)

 

Are you just moving the, ( I assume balanced aka "stereo") jack plug for the creche feed to the headphone socket from the other output ? If so you may find the feed to the creche is not what the main mix is as it is not a balanced mono signal but a stereo signal.

[TonyP]

Are you just moving the, ( I assume balanced aka "stereo") jack plug for the crèche feed to the headphone socket from the other output ? If so you may find the feed to the crèche is not what the main mix is as it is not a balanced mono signal but a stereo signal.

That's a very good point.

 

I'm probably going to spend a couple of hours later in the week doing some work on the sound desk. I'll see if I can get two sound sources going together, I usually take a small home radio for one (Radio 3 or Classic FM coming out loud and clear through the main speakers is kinda awesome ;) ), I can use the CD player that's connected to the desk as the other. Mix them up and compare what comes out of the main PA speakers to what's coming out of the crèche speakers.

 

Essentially the crèche and loop both require the full mix same as the main speakers. We'll be happy to use a custom mix for the recorder as we only need for the voice channels for that.

[Simon Lewis]

Tony,

 

sorry not to have responded regarding balanced connections etc. but you have received some good explanations. My thought was that some of your connections to the mono out may be unbalanced and were affecting the level of the signal to the balanced devices. It can get quite complicated!

 

Just one thought - is there a reason for putting a full mix into the loop? If you have a wide dynamic range in the loop feed it tends to trigger the automatic gain control all the time, dropping the loop level. Furthermore, those with hearing loss generally struggle to distinguish speech or sung words when mixed with music (whereas those with normal hearing would find it easy to cope with).

 

Often, a speech rich (or even speech only) mix is better, as they will often pick up enough music as 'bleed' into any open mics. Of course, if what you have now suits your hearing impaired listeners, then that's fine, but many whose listen on loops do not know what can be achieved, and simply wouldn't know how to ask for it.

 

Simon

[TonyP]

Interesting point. One of people is a retired audiologist. She takes a great interest in the 'loop', to her it's far more important than the recording or crèche ;) , I'll ask her for her views, though she's not mentioned the voice/music issue as far as I'm aware. She also regrets that she no longer has equipment to test the loop - but that may be solvable via Andy's link earlier.

[Pete Alcock]

A few years ago when I got involved with church loop optimising, I bought an old hearing aid from a local hearing aid specialist. He cleaned and tested it, and fitted it with an ear-tube and a universal eartip. Now I can hear exactly what the hearing-impaird folks are getting (short of their hearing loss of course).

 

This has been a revelation and I've had many positive comments from members of local congregations on how the loop, "sounded so clear". This is usually because I take most of the music group out of the loop mix and make sure the ministry mics are just lightly driving the compressor in the loop amp.

 

Sure, meters and testers are useful in detrmining field strength etc, but this allows you to hear how it's really sounding. Definitely worth it. I don't go to church without my hearing aid now, and just pray I'll never need it for real.

 

Pete.

[andy_s]

absolutely agree that it's good to hear the sound from the loop, so you can discuss it with the regular users. - for clarity, the link in my earlier post is to a device that enables you to do this, not a signal strength meter.

[TimmyP1955]

With what type of cable (what connectors on each end) do you use to feed the MP3 player into the mixer? And what mixer input(s) do you use? (A lot of folks do it wrongly.)