100-volt line attenuator

[Mike Stranks]

Greetings!

I need some advice from the electronics gurus, please...

I'm currently upgrading a sound system in a private school chapel/village church.

There's a stepped attenuator/volume control in the back pew, which is intended for a responsible adult to be able to adjust the overall volume. However, irresponsible children have a tendency to turn it to the minimum as they depart, leaving older church attenders frequently subsequently scratching their heads as to why the system 'won't work'!

The upgrade includes locating all volume controls in a cupboard at the rear of the church, so the stepped control is to be removed. However, when I went to do this today I discovered that the input cable has only three conductors - the contractor seems to have used standard 10-amp (ish) 3-core mains cable...

What do I do with the three termination points of the cable when I by-pass the control - what should be connected to what?

Thanks in anticipation.

Mike

[Hugh Robjohns]

Without knowing exactly what is installed I can only give general guidance...

But a 100V line attenuator is basically a transformer with multiple output taps, and a switch to select the required tap, thus changing the voltage on the line and the acoustic power output from the downstream speakers.

As it's a transformer there are two input connections (to each end of the primary), and two output connections (from one end of the secondary and the switch wiper).

Many 100V line attenuators also include a remote operated bypass relay (typically energised by 24V DC), so that emergency announcements can be at full volume regardless of attenuator setting). That would be accessed via two more terminals if used.

So... that's the normal arrangement. You'll need to examine how your installation has been wired in reality... but using standard mains cable is not usual for the job: cheap, available, and adequately specified. You'll probably find one core is either cut back or screwed into a spare terminal.

Or maybe it's arranged as a go/return with a common cold side?

Photos and more details would help with the remote analysis...

[Mike Stranks]

Thanks Hugh

Yes; I was wondering about a common 'cold' line...

I'll try and pop over again later and take a photo or two... (forgot my 'phone earlier... )

The amp is locked in the vestry so I can't check source wiring without a keyholder being present.

M

[BWC]

There's a stepped attenuator/volume control in the back pew, which is intended for a responsible adult to be able to adjust the overall volume.

Sorry,

However, irresponsible children...

Well, of course... Ah, thanks for that! Waiting with interest for the pics, and Hugh's analysis. Carry on...

[Mike Stranks]

Pics...

[Hugh Robjohns]

That looks like a go/return with commoned cold side arrangement to me.

So you now need to check what's at the other end of the cable.... presumably amp out on brown, speakers on blue (or maybe the other way round), and shared return of green/yellow.

However, from the point of view of relocating the attenuator the rewiring is pretty simple.

Just make sure you don't locate it anywhere near something with a big mains transformer!

[Hugh Robjohns]

That looks like a go/return with commoned cold side arrangement to me. I'm assuming that short blue wire link is doing the commoning.

Are there any terminal markings on or near the terminals or elsewhere in the box? Can't see anything in the photo.

So you now need to check what's at the other end of the cable.... presumably amp out on brown, speakers on blue (or maybe the other way round), and shared return of green/yellow. ...

But beware... a logical coding coding scheme might not apply!

However, from the point of view of relocating the attenuator the rewiring is pretty simple.

Just make sure you don't locate it anywhere near something with a big mains transformer!

[Mike Stranks]

Are there any terminal markings on or near the terminals or elsewhere in the box? Can't see anything in the photo.

Nope! I did look hard/check carefully

However, from the point of view of relocating the attenuator, the rewiring is pretty simple.

Just make sure you don't locate it anywhere near something with a big mains transformer!

Ah! I didn't make myself clear in my original post... imprecise and vague words... This attenuator/box remains in situ but with an internal by-pass by connecting wires together and the new controls - aka 'mixer' in a cupboard on the west wall. The whole scheme is designed to be invisible and thus avoid the tiresome Anglican business of getting a 'faculty' - like Planning Permission!

Thanks, as always, for your careful and detailed explanation, Hugh.

[James Perrett]

This attenuator/box remains in situ but with an internal by-pass by connecting wires together and the new controls - aka 'mixer' in a cupboard on the west wall.

Forgive me if I'm teaching you to suck eggs, but you will still need a tapped transformer somewhere in the system if you want to be able to adjust the level. If you just want a fixed level then you can use the transformer in the existing box hard wired to the desired setting but you in a 100V line system you need a transformer somewhere between the amp and speaker.

[Hugh Robjohns]

This attenuator/box remains in situ but with an internal by-pass by connecting wires together...

Ah... that's getting more complicated, and possibly less than practical. The transformer is probably still required somewhere in the amp-speaker wiring, especially if you want to retain a degree of attenuation (fixed or adjustable).

If you want to bypass it completely (after which it can be removed), I'd strongly advise figuring out what's on the other end of that cable. It may well be that you can disconnect it and drive the speakers directly from the amp (assuming it has the appropriate 100V output transformer built-in).

The whole scheme is designed to be invisible and thus avoid the tiresome Anglican business of getting a 'faculty' - like Planning Permission!

Ah yes... very familiar with that tedious problem, having been dragged into a few church installations myself.

[Mike Stranks]

Thanks for comments and 'take-care' alerts - all welcome.

I can certainly investigate what's happening at the amplifier; just couldn't do it today as I didn't have a keyholder with me.

The other problem is that the speakers are high up on the walls - definitely a ladder-job and possibly even a scaffolding tower. Normally, I'd be checking out what they are and what tappings are being used. Just not possible, and Mrs S has me on a suspended sentence regarding ladder-work!

Maybe I'll just leave the attenuator in situ, set on max, and remove the knob from the spigot to make turning it difficult/impossible.

BTW, the Mixer-amp is an Inter-M PAM 120A - a good, reliable workhorse used in very many churches IME. It has 100V and 70V outputs...

Thanks again!

[ef37a]

Hi Mike, when you say "bypass the transformer VC" do you mean that you want to simply run that speaker at the maximum line level? If so then AFAICT you just need to loop out the brown/blue to whatever pair is feeding the speaker.

I suspect the Y/G wire was connected simply because the contractor had used 3 core mains cable. Not usual for 100V installations and 10 amps is well overkill for a 120 watt system! There is no safety implication since the case is all plastic and there is no earth lug for the M3.5 screws. 100V lines are in any case usually earthed by a centre tap on the output transformer.

If you want to retain individual control of that speaker simply lift out the assembly and transfer it further 'upstream' and fit a blanking plate to that back box (looks like a standard 13A double?)

Dave.

[Mike Stranks]

Hi Mike, when you say "bypass the transformer VC" do you mean that you want to simply run that speaker at the maximum line level? If so then AFAICT you just need to loop out the brown/blue to whatever pair is feeding the speaker.

I suspect the Y/G wire was connected simply because the contractor had used 3 core mains cable. Not usual for 100V installations and 10 amps is well overkill for a 120 watt system! There is no safety implication since the case is all plastic and there is no earth lug for the M3.5 screws. 100V lines are in any case usually earthed by a centre tap on the output transformer.

If you want to retain individual control of that speaker simply lift out the assembly and transfer it further 'upstream' and fit a blanking plate to that back box (looks like a standard 13A double?)

Dave.

Thanks Dave

Good to hear from you!

My intention was simply to make the attenuator ineffective by taking it out of circuit. The diagrams I'd seen online for such attenuators had a + and - in and out.... Simple thinks I - just use a mini two-pole connector to connect the respective +s and -s. Err, no! Not in this case...

Yes; my intention is simply to take the attenuator' out of circuit. The redundant box/control will be left in place....

[ef37a]

Hi Mike, when you say "bypass the transformer VC" do you mean that you want to simply run that speaker at the maximum line level? If so then AFAICT you just need to loop out the brown/blue to whatever pair is feeding the speaker.

I suspect the Y/G wire was connected simply because the contractor had used 3 core mains cable. Not usual for 100V installations and 10 amps is well overkill for a 120 watt system! There is no safety implication since the case is all plastic and there is no earth lug for the M3.5 screws. 100V lines are in any case usually earthed by a centre tap on the output transformer.

If you want to retain individual control of that speaker simply lift out the assembly and transfer it further 'upstream' and fit a blanking plate to that back box (looks like a standard 13A double?)

Dave.

Thanks Dave

Good to hear from you!

My intention was simply to make the attenuator ineffective by taking it out of circuit. The diagrams I'd seen online for such attenuators had a + and - in and out.... Simple thinks I - just use a mini two-pole connector to connect the respective +s and -s. Err, no! Not in this case...

Yes; my intention is simply to take the attenuator' out of circuit. The redundant box/control will be left in place....

Been an AWFUL long time since I did this stuff! I have been looking for a site with a wiring diagram but little joy. The best I can see is that they use what is in effect an 'auto transformer'? Therefore there will be just two wires in and two out to the speaker, one being common. Is that 3core the only wire to the box? Does a cable go to the speaker and then down to the VC?

Dave.

[Hugh Robjohns]

Without knowing what's on the other end of that cable, this is all just potentially perilous speculation....

[ef37a]

Without knowing what's on the other end of that cable, this is all just potentially perilous speculation....

Not that perilous Hugh? Worse case is a short on the 100V line and I am sure Mike is not that daft! In any case, 100V line amps are very robust in my experience, even the transistorised jobs.

Dave.

[Folderol]

In a situation like this, I'd do as Dave suggests, but cut off the incoming 3core leaving a bit of the sleeving on the wires. That way it's easy to ensure you wire or the extended cable the same way.

[Drew Stephenson]

Can you just set the attenuator to max and then just bolt another box over the top? An old ice-cream tub would do the job, it just has to take the temptation to twiddle away.

[ef37a]

Can you just set the attenuator to max and then just bolt another box over the top? An old ice-cream tub would do the job, it just has to take the temptation to twiddle away.

OR! Drill a hole in the knob skirt and tap in a 3mm roll pin.

Dave.

[Mike Stranks]

Can you just set the attenuator to max and then just bolt another box over the top? An old ice-cream tub would do the job, it just has to take the temptation to twiddle away.

Nail... head, methinks! Although I'll be slightly more sophisticated and build a wooden cover... Outline plans are already being drafted...

Thanks all... I'm done now; you can stand down!

Mike