Amp speaker capacity

[The JC]

Hi all,

 

I have 2x QSC CX108V amps that are feeding all of my backstage show relay speakers. My predecessors seem to have installed some unbranded 8” speakers with an 8ohm, 5W, 10W, 15W, 20W and 30W selector dial on the back.

 

There have been several issues with some speakers being outrageously loud and others pitifully quiet regardless of volume controls on the amp, so I’m starting the diagnosis with calculating how many speakers each of the amps can actually have and at what impedance. How do I go about calculating this? My gut is telling me that these amps are being stretched and also that people are turning the selector dials assuming that it’s just volume (Will be mounting the speakers higher up to stop them from doing this ?)

 

Cheers!

[sandall]

Your amps are 70v output; your speakers are almost certainly 100v, so a "10w" tap in effectively a "7w" one, or to put it another way your total allowable load per channel is 70w, not 100w. Unless you know which channel feeds which speakers, I would start by turning off all but one of the channels, & check the tappings of the speakers that are making a noise (e.g. 7 speakers at 10w each is a full load). If any have been turned to 8-ohm they may have damaged the amp. Once you have them all at a sensible level I would suggest either removing the knob & covering the shaft with a cap, or taking off the lock-nut & hiding the switch inside the box.

 

E2A: Even when the speakers have proper volume switches & you've used a mole-wrench to tighten the lock-nuts, there will always be someone with sufficient strength in their fingers to twist the knob beyond its end-stop

Edited March 2, 2021 by sandall

[sunray]

Hi all,

 

I have 2x QSC CX108V amps that are feeding all of my backstage show relay speakers. My predecessors seem to have installed some unbranded 8" speakers with an 8ohm, 5W, 10W, 15W, 20W and 30W selector dial on the back.

 

There have been several issues with some speakers being outrageously loud and others pitifully quiet regardless of volume controls on the amp, so I'm starting the diagnosis with calculating how many speakers each of the amps can actually have and at what impedance. How do I go about calculating this? My gut is telling me that these amps are being stretched and also that people are turning the selector dials assuming that it's just volume (Will be mounting the speakers higher up to stop them from doing this )

 

Cheers!

Your amps are 70v output; your speakers are almost certainly 100v, so a "10w" tap in effectively a "7w" one, or to put it another way your total allowable load per channel is 70w, not 100w. Unless you know which channel feeds which speakers, I would start by turning off all but one of the channels, & check the tappings of the speakers that are making a noise (e.g. 7 speakers at 10w each is a full load). If any have been turned to 8-ohm they may have damaged the amp. Once you have them all at a sensible level I would suggest either removing the knob & covering the shaft with a cap, or taking off the lock-nut & hiding the switch inside the box.

 

E2A: Even when the speakers have proper volume switches & you've used a mole-wrench to tighten the lock-nuts, there will always be someone with sufficient strength in their fingers to twist the knob beyond its end-stop

Minefield time.

For a start I don't know the product so a very quick www search tells me it's 8 independant channels which appear to be 70V 100W each. without any low impedance ability.

 

Assuming the speakers are rated for 100V, full power will be half at 70V ie a speaker set to 10W will present a 5W load to a 70V source and as such you can load each channel with 200W of speakers. I'll very happily do the calculations on here if you wish but may be easier via phone, I've PM a phone number.

This is assuming they are 100V speakers.

 

Are all channels being fed by the same signal or are they zoned or even switchable sources at multichannel speakers?

Edited March 2, 2021 by sunray

[sandall]

Quite right on the power halving - I don't do speaker calculations much these days !!

[sunray]

Quite right on the power halving - I don't do speaker calculations much these days !!

No worries, I know people who do this stuff on a regular basis who can't get their head round it.

We installed a lecture theatre system with 6x 15W ceiling speakers and had to make arrangements to link them into the buildings EVAC, were allocated 50W. I suggested taking over 3 of the speakers and wired them to a relay controlled by the EVAC.

 

The alarm engineer commissioned the connexion and testing declared it to be 24W.

My boss was there and measured with his 'calibrated' impedance meter as more like 45W, wanting to raise an issue with the discrepancy he left the room to make a phone call. Using my meter displaying impedance rather than power it was near enough to the 333Ohms expected and I immediately asked the EVAC engineer if they were running 70V?

Straight away I wired all 6 speakers to the relay, he retested as 48W and signed off... My boss was still unconvinced.

As it happens my boss at the time was ex BBC, masters degree, AV consultant and still highly respected in the AV industry to this day, interviews in publications such as AV Magazine, ISCVE trainer etc. On another job the finance director discovered that deleting the 'T' off a ceiling speaker reduced the price by 25%, they were purchased and installed by our installers and not being able to find any way to tap them at 10W simply wired them in parallel. While commissioning the system with the same boss we were trying to drive 100V 120W into 12x 8ohm speakers, I suggested rewiring in series but he was convinced it wouldn't work... It did. TBH it was the only field that he didn't seem to have a grip on.

 

Oh and the Finance Director objected to 4 hours of overtime to rewire the speakers being allocated to the purchasing budget, saying it should have been done properly...

 

[Junior8]

E2A: Even when the speakers have proper volume switches & you've used a mole-wrench to tighten the lock-nuts, there will always be someone with sufficient strength in their fingers to twist the knob beyond its end-stop

 

I once tried to explain to someone new to supplying the Education field that he would not credit what idiots could do to equipment - it turned out after a minute of two of confusion he thought I was talking about the kids. Clark & Smith knew better!

 

 

 

[sunray]

E2A: Even when the speakers have proper volume switches & you've used a mole-wrench to tighten the lock-nuts, there will always be someone with sufficient strength in their fingers to twist the knob beyond its end-stop

 

I once tried to explain to someone new to supplying the Education field that he would not credit what idiots could do to equipment - it turned out after a minute of two of confusion he thought I was talking about the kids. Clark & Smith knew better!

 

 

 

I think anyone working on school AV knows too.

[alistermorton]

I once tried to explain to someone new to supplying the Education field that he would not credit what idiots could do to equipment - it turned out after a minute of two of confusion he thought I was talking about the kids. Clark & Smith knew better!

 

Fond memories from my schooldays of "built like a tank" C&S radios on trolleys, record players in laminated cases and ferrograph based tape decks - all with key locks to keep prying fingers out.

Edited March 3, 2021 by alistermorton

[J Pearce]

Off topic, BUT - It all started going wrong when schools stopped buying Coomber. Bomb proof, and fixable - every part available as a spare and easily swapped out, and spec'd per badminton court of coverage.

 

When I worked in schools I worked on spec'ing kit so that ears would blow before drivers, then adding a limiter if levels regularly got silly. Adding a sub to the dance studios actually really helped with volume as they could feel the beat and stopped pushing it as loud.

[Junior8]

I once tried to explain to someone new to supplying the Education field that he would not credit what idiots could do to equipment - it turned out after a minute of two of confusion he thought I was talking about the kids. Clark & Smith knew better!

 

Fond memories from my schooldays of "built like a tank" C&S radios on trolleys, record players in laminated cases and ferrograph based tape decks - all with key locks to keep prying fingers out.

 

Anybody of tender years who wonders what the heck we are on about click here In the past many users bought for ruggedness first - for example the majority of showmen and a lot of PA contractors bought the Vortexion 'Wimbledon' that was built like a tank and did a simple job reliably. I bet in some corner of a village hall somewhere one is still happily working away.

Edited March 3, 2021 by Junior8

[sunray]

Off topic, BUT - It all started going wrong when schools stopped buying Coomber. Bomb proof, and fixable - every part available as a spare and easily swapped out, and spec'd per badminton court of coverage.

 

When I worked in schools I worked on spec'ing kit so that ears would blow before drivers, then adding a limiter if levels regularly got silly. Adding a sub to the dance studios actually really helped with volume as they could feel the beat and stopped pushing it as loud.

You sound far too young...

 

AFAIC Coomber was a backward step, C&S was bomb proof. My school had an amplifier and the wooden sleeve showed significant signs of damage and repair, caused by a ww2 bomb going off next to the old school in a 'controlled explosion' which took out the wall of the store room. We had loads of C&S stuff which simply didn't go wrong and when I think of the abuse it got, quite frankly I'm amazed.

 

Moving on from leaving school in '72 I followed in dads hobby of PA which led me on to disco kit etc and starting a business in '77 due to the crazy number of bookings for jubilee year and soon we started getting involved with schools sports days/fetes/stage shows which led on to repairs. Broken Coomber kit,especially those powered speakers with a cassette sitting in the top, came at us from everywhere and I soon built up a stock of parts. Mid 80's things changed in my life and passed the school repairs on to another. To my mind Coomber came no where near the quality or reliability of C&S. For that matter useability too, the C&S 10W record players would fill a hall full with 600 with clean crisp audio with ease, which is more than can be said for the Coomber apparently rated at 25W.

Edited March 3, 2021 by sunray

[Junior8]

For that matter useability too, the C&S 10W record players would fill a hall full with 600 with clean crisp audio with ease.

 

I know this thread has strayed off topic too far but the above is a fact needs to be stressed. This type of equipment looked archaic when it was new was robust and simple but it did the job - and that wasn't to deafen people or allow manipulation of the source or take multiple inputs or attempt the highest quality - it was simply to let people hear comfortably. There was a time when 15W was seen as quite adequate for most purposes if this kind and with decent equipment it was.

[sandall]

Anybody of tender years who wonders what the heck we are on about click here.

Despite being of rather less tender years (certainly less than Sunray), I have no recollection of C&S, though I do recall meeting a theatre control-room equipped with a Vortexion amp, record-player & Wearite tape deck (& an imaginative way of running wind loops), so the C&S history made a great read. On the other hand, mending Coombers was a useful sideline for a few years.

Edited March 3, 2021 by sandall

[Junior8]

It probably meant that your local authority was one of the few that did not deal with them. There were a few who only specified Major for lighting and I don't think C&S were used much in Lancs. On the subject of age I think if you still mentally sometimes think of a stage sound system as the Panotrope, even if you don't say the word, it gives some indication of maturity! Edited March 3, 2021 by Junior8

[sunray]

The trouble is; it got to the point it wasn't a sideline, it was taking over my life, what with repairing Bose loudspeakers and Bremi power supplies...

Anybody of tender years who wonders what the heck we are on about click here.

Despite being of rather less tender years (certainly less than Sunray), I have no recollection of C&S, though I do recall meeting a theatre control-room equipped with a Vortexion amp, record-player & Wearite tape deck (& an imaginative way of running wind loops), so the C&S history made a great read. On the other hand, mending Coombers was a useful sideline for a few years.

 

Looking at the link brings so much back to me...

However the bomb damaged amplifier I mentioned before, we had two which I remember as AM/FM receiver/amplifiers but the top being wooden like the AM only version. I'm thinking we might have had one of each, however reading more of the text it seems some of the products were manufacturered indifferent formats.

Example; we had 2 pairs of the loudspeakers top left of page 6; about 2ft square, 70V transformer [C&S used 70V (across their range I believe) and not 100V for safety], volume control bottom right and as mentioned we had 2 versions, one used an 8" speaker and the other a 6" speaker on a sub baffle to fill the 8" sized opening. Additionally one pair had clips so they could be joined back to back to ease transportation. I remember them sounding very different.

Edited March 3, 2021 by sunray