Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Robert Anderson]

Forgive my ignorance - I am new to 70-volt systems.  

Is it true that I can wire an entire 70-volt system in parallel as long as all the sum of the taps does not exceed the wattage of the amplifier?  Or do I still need to try and break it up into series and parallel branches to keep the amp from burning itself out?

Thanks

[Brad Weber]

In a 70V (or more accurately 70.7V) system the speakers, or actually the speaker transformers, are all wired in parallel.  Within reason you may parallel as many speakers as you desire.  This can be one single branch line daisy chained from one speaker to another or you can branch off at various points and have mutliple branch runs as best fits the actual installation.  Whether split at the amp, at a speaker or in a junction box, in a 70.7V system all wiring including multiple branches should be in parallel.

In general you can add speakers whose tap values add up to the output rating of the amplifier, but you do have to be a little careful.  The cabling does have line losses and in long lines this may be a factor.  You also have should verify whether the transformer tap information is indicating the power at the input to the transformer or what is delivered to the speaker, in the latter case there may also be transformer losses to consider that raise the actual power taken from the line.  And be sure to use the 70V rating for the amplifier.  Suffice it to say that you usually do not want to put 490W or 500W of speakers on a 500W 70V amp, but 400-420W of load might be just fine even with fairly long runs.  On the other hand, with a 70V system you can also have a single 4W speaker on a 100W amp without worrying about providing too much power to the speaker.

Also be aware of any volume controls in the runs as they must also be selected to handle the attached loads.

[Ivan Beaver]

The biggest thing to understand about 70V systems is that the transformers used are basically impedance transformers.  A 1W tap on a 70V  line is 4900 ohms-five or take.  10W is 490ohms and 49 ohms is 100watts.

It is a common misconception about using really small wire for 70V applications.  As you can see a 100Watt load has an impedance of 49 ohms.  Often times the loads are higher (lower impedance) and the runs are long.  Often times we use 12 and 14 ga for 70V lines.

[Rick Johnston]

I can't remember the last time I load-tested a constant voltage system (70.7 or 25) and found the speaker load to be equal to the amp's power rating.

Design specs usually call for 20% headroom, and many times I see 50%. That means a 100 watt amp shouldn't have more than 50 speakers (each tapped at one watt) connected to it.

A 100 watt amp may have 100 speakers, each tapped at one watt, but they're only 3dB SPL down in each coverage area if you tap them at 1/2 watt.

The system now has headroom, the amp's running cooler, and you don't have to deal with line loss. You can also expand the system easily by just adding speakers. If you need that extra 3dB of loudness, move up to a 200 watt amp and tap each speaker at 1 watt.

Keep in mind, too, that although a system is called a "70.7-volt constant-voltage system" it will rarely, if ever, reach its rated voltage during normal use. There's some more built-in headroom. (But it needs to test at full load during the sign-off stage.)

Wire size? Typically 14-gauge trunk lines from amp(s) to the field, 16-gauge zone drops from the trunk to each section of the building, and 18-gauge speaker lines within the zones.

(Er ... Have you seen the price of copper lately?!?  The thinner the cheaper!)

For added protection against failure, high-pass the entire system at 100Hz and don't boost the bass on any mixer amp or EQ.

[John Roberts {JR}]

Rick Johnston wrote on Fri, 14 July 2006 00:49

I can't remember the last time I load-tested a constant voltage system (70.7 or 25) and found the speaker load to be equal to the amp's power rating. Design specs usually call for 20% headroom, and many times I see 50%. That means a 100 watt amp shouldn't have more than 50 speakers (each tapped at one watt) connected to it. A 100 watt amp may have 100 speakers, each tapped at one watt, but they're only 3dB SPL down in each coverage area if you tap them at 1/2 watt. The system now has headroom, the amp's running cooler, and you don't have to deal with line loss. You can also expand the system easily by just adding speakers. If you need that extra 3dB of loudness, move up to a 200 watt amp and tap each speaker at 1 watt. Keep in mind, too, that although a system is called a "70.7-volt constant-voltage system" it will rarely, if ever, reach its rated voltage during normal use. There's some more built-in headroom. (But it needs to test at full load during the sign-off stage.) Wire size? Typically 14-gauge trunk lines from amp(s) to the field, 16-gauge zone drops from the trunk to each section of the building, and 18-gauge speaker lines within the zones. (Er ... Have you seen the price of copper lately?!?  The thinner the cheaper!) For added protection against failure, high-pass the entire system at 100Hz and don't boost the bass on any mixer amp or EQ.

Some of the de-rating is to make up for losses in the wiring/transformers.

The advice about HPF and avoiding bass boost is right on because all those transformers hanging on the 70V line can saturate if hit with too much low frequency content.

JR

[Hal Bissinger/COMSYSTEC]

The advice about HPF and avoiding bass boost is right on because all those transformers hanging on the 70V line can saturate if hit with too much low frequency content.

I have had considerable success without HPF and with bass boost by using much larger than necessary direct coupled amps (to eliminate that transformer) and high quality transformers on the speakers. The speaker transformers are critical for decent LF response.

Most speakers like the JBL Control series have large transformers built-in that will give acceptable LF performance but don't expect the $10 8" speaker/transformer package to be good for anything but voice.

If you have to use that type of speaker for music choose one with a good LF response to begin with then use a transfomer with as much iron as you can find. These will normally give the lowest LF response possible.

-Hal

[John Roberts {JR}]

Hal Bissinger/COMSYSTEC wrote on Fri, 14 July 2006 15:12

The advice about HPF and avoiding bass boost is right on because all those transformers hanging on the 70V line can saturate if hit with too much low frequency content. I have had considerable success without HPF and with bass boost by using much larger than necessary direct coupled amps (to eliminate that transformer) and high quality transformers on the speakers. The speaker transformers are critical for decent LF response. Most speakers like the JBL Control series have large transformers built-in that will give acceptable LF performance but don't expect the $10 8" speaker/transformer package to be good for anything but voice. If you have to use that type of speaker for music choose one with a good LF response to begin with then use a transfomer with as much iron as you can find. These will normally give the lowest LF response possible. -Hal

I think most mfrs would love to accommodate you as eliminating the output iron eliminates a significant expense too, but when I was dealing with that market I had to please old line installers who wouldn't even accept auto-formers in place of the fully floating output transformers that are common in 70v amps.

Magnetic saturation is a product of frequency and amplitude so you can get away with some bass boost as long as signal levels remain modest. One of my inventions dealt with a modified tone control circuit for this market that would allow full bass boost as long as signal levels were low but then clamped (just the bass) when it got loud. It was only a simple clamp but since it was operating on just the low bass bandpass it was masked by the full range audio when working.

You can also add a capacitor in one leg of the classic Baxandall tone control to roll off the bass boost adaptively such that the LF cut still looks like a true shelf while LF boost has a bandpass.  

JR
 

[Brad Weber]

JR,

This brings up a question I've had for quite some time.  Why does it seem that nobody has developed an automatic loudness compensation device or DSP algorithm?  I would think this could be implemented in DSP, perhaps even using a room mic like ambient noise analysis.  If we can have the system adjust the level based on the ambient noise, why not also adjust the EQ based on the system loudness or an averaged input level to the device?

Just curious...

[John Roberts {JR}]

Brad Weber wrote on Fri, 14 July 2006 17:42

JR, This brings up a question I've had for quite some time.  Why does it seem that nobody has developed an automatic loudness compensation device or DSP algorithm?  I would think this could be implemented in DSP, perhaps even using a room mic like ambient noise analysis.  If we can have the system adjust the level based on the ambient noise, why not also adjust the EQ based on the system loudness or an averaged input level to the device? Just curious...

A proper loudness contour (ala Fletcher-Munson) could be very nicely done in DSP. I suspect there may be some issues with throwing significant HF and LF boost on playback systems that are not exactly models of high fidelity but in a properly integrated solution the LF saturation characteristic of the system magnetics could be accommodated.

If this was done well, you wouldn't know it was there. I suspect some background music providers might already EQ for anticipated modest playback levels.

If it can't be done already, it's just a matter of time. Larger installs are already resembling a computer system waiting for your plug-ins. It will filter down as DSP becomes cheaper and more pervasive.

JR

[Dave Barker]

DBX has the loudness control feature in the Zone Pro series of DSP boxes for commercial applications.  Works fairly well also.  I believe there are a couple of others that is just the first one that comes to mind and the one we use most.  I believe DBX calls it "Auto Warmth".

[Hal Bissinger/COMSYSTEC]

I'm familiar with Rane DSP boxes. Haven't looked into it but I'm wondering whether you could do a bunch of presets that would adjust the volume and along with each step the EQ as well.

-Hal

[Rick Johnston]

Hal Bissinger/COMSYSTEC wrote

I have had considerable success without HPF and with bass boost by using much larger than necessary direct coupled amps (to eliminate that transformer) and high quality transformers on the speakers.

John Roberts  {JR} wrote

... when I was dealing with that market I had to please old line installers who wouldn't even accept auto-formers in place of the fully floating output transformers that are common in 70v amps.

The output transformer (not autoformer) is another line of defense against failure. Consider the inevitable short to building ground in the field wiring: Eventually someone will be in the ceiling pulling cable, or adding HVAC, or any of a thousand other jobs. Or the guy whose desk sits under a speaker decides to climb up there and cut the wire to the speaker, leaving the loose end to contact the ceiling grid, steel structure, or a chunk of conduit. A transformer-isolated amp will handle that short to ground better than a direct-coupled amp.

It's all in the application, though. A background music/voice paging system doesn't need excessive bass response, so the $10 eight-inch speakers are fine. System costs are very low compared to, say, a high-wattage foreground music EVID system with several full-range speakers each tapped at 16 watts and a couple of 70v subs. In the latter case I'd use an over-powered direct-coupled amp (actually two) as well.

-- RJ

[Iain_Macdonald]

Quote:

This brings up a question I've had for quite some time. Why does it seem that nobody has developed an automatic loudness compensation device or DSP algorithm? I would think this could be implemented in DSP, perhaps even using a room mic like ambient noise analysis. If we can have the system adjust the level based on the ambient noise, why not also adjust the EQ based on the system loudness or an averaged input level to the device?

Hi Brad,

It's been and gone!

Here in the UK when Shuttlesound was a privately owned company. (1987) We manufactured and sold a product called the Inflexor. This did exactly what you wanted. It worked well in a number of applications. The listener still got a "full" sound at low levels. The product was designed by Ben Duncan, and used the Robinson Dadson curves.

Best wishes.

Iain.
London. UK.

[John Roberts {JR}]

Rick Johnston wrote on Mon, 17 July 2006 06:11

The output transformer (not autoformer) is another line of defense against failure. Consider the inevitable short to building ground in the field wiring: Eventually someone will be in the ceiling pulling cable, or adding HVAC, or any of a thousand other jobs. Or the guy whose desk sits under a speaker decides to climb up there and cut the wire to the speaker, leaving the loose end to contact the ceiling grid, steel structure, or a chunk of conduit. A transformer-isolated amp will handle that short to ground better than a direct-coupled amp. It's all in the application, though. A background music/voice paging system doesn't need excessive bass response, so the $10 eight-inch speakers are fine. System costs are very low compared to, say, a high-wattage foreground music EVID system with several full-range speakers each tapped at 16 watts and a couple of 70v subs. In the latter case I'd use an over-powered direct-coupled amp (actually two) as well. -- RJ

I'm familiar with all the rationalizations and since the customer is always right (even when wrong) they got their full transformers.

Considering how many CV systems are in use around the world it seems like a significant expense to burden the ultimate purchaser (rarely the same person who insisted on the transformer). Transformers are larger, heavier, more expensive, less efficient,  worse audio performance (bandwidth and distortion), and so on.

I am well aware of the significant pain and expense of service calls, but it does seem overly generous to spec a more expensive approach to make the system more tolerant of "customer" caused problems. Perhaps the installer is also considering the extra tolerance of sloppy wiring when doing their initial install.

Whatever. It's impossible to know other peoples motives when we so rarely inspect and understand our own.

JR

[Don Boone]

70.7 volt systems were originally a way around the required limits for low voltage (meaning no conduit) system. In olden days anything below 36 volts was considered low voltage. 70.7 volt distribution met that requirement IF the output was balanced, 35.35 volts per side. Thus the requirement for a real transformer.

Don

[John Roberts {JR}]

Don Boone wrote on Mon, 17 July 2006 15:27

70.7 volt systems were originally a way around the required limits for low voltage (meaning no conduit) system. In olden days anything below 36 volts was considered low voltage. 70.7 volt distribution met that requirement IF the output was balanced, 35.35 volts per side. Thus the requirement for a real transformer. Don

I never heard that one, but CV was already a mature technology by the time I started messing with it. FWIW I also don't recall ever seeing a center-tapped 70v (35+35) output winding on anything I was competing against. The output transformers did routinely have taps for lower voltages but nothing close to 35V. One could ground a 25v tap and reduce the peak potential wrt ground but they both wouldn't be below your 36v max. Of course you could probably just use the 25V tap, which is probably why it was there (I also recall it being a requirement for some school installs).

The folks busting my chops for a floating transformer output didn't ask for and were satisfied without a CT.

JR

[Rick Johnston]

I've always understood that in the early days, 100 volts was considered the maximum that a wire could carry in a building without having to be in pipe. 100 volts peak-to-peak means 70.7 volts RMS.

To this day, some local codes don't require pipe while others do.

John Roberts  {JR} wrote on Mon, 17 July 2006 10:39

I'm familiar with all the rationalizations and since the customer is always right (even when wrong) they got their full transformers.

Let's not forget the original benefit of transformers: They step up the voltage to run long distances on smaller wires, then step it back down to run the speakers. The resultant higher impedance also means that more speakers can be added to a distributed CV system versus a low-impedance direct-coupled system.

Regards,
Rick Johnston

[John Roberts {JR}]

Rick Johnston wrote on Sat, 22 July 2006 08:50

=John Roberts  {JR} wrote on Mon, 17 July 2006 10:39]I'm familiar with all the rationalizations and since the customer is always right (even when wrong) they got their full transformers./quote Let's not forget the original benefit of transformers: They step up the voltage to run long distances on smaller wires, then step it back down to run the speakers. The resultant higher impedance also means that more speakers can be added to a distributed CV system versus a low-impedance direct-coupled system. Regards, Rick Johnston

And let's not forget I'm talking about refusal to use 70V auto-formers instead of 70V transformers. The primary difference being isolation from ground or amplifier common. Isolation does provide the benefit of tolerating inadvertent grounding of one of either + or - speaker line. Whether this benefits the customer or installer is a judgement call. Since the decision was strongly influenced by  installers and consultants, transformers it was.

All customers pay more up front to mitigate against shorts occurring during installation or later. FWIW transformer common was always terminated right next to ground on the output barrier strip so it could easily be shunted together.

JR
 

[Hal Bissinger/COMSYSTEC]

Well, in vacuum tube days the output transformer was there by necessity and the secondary common was usually grounded for safety. A primary to secondary short could put anywhere from 400 to 700 volts DC on the speaker wiring otherwise.

Today the isolation properties of an output transformer are a requirement of some electrical codes that date back to vacuum tube days and may be the basis for them being desired by some otherwise.

The transformerless output amps I am familiar with (Crown CT series) will go into protection if one side of the output gets shorted to ground. The same problem exists with a transformer that has its common grounded and the other side becomes grounded.

My concern with a floating output is the possibility that the wiring in a building could be crossed with line voltage, effectively putting that voltage between the speaker wiring and ground. It would go undetected and pose a real safety issue for someone who comes along to service a speaker, the amp or wiring.

Then again I believe there are some codes that take issue with the fact that the amp itself might not be properly grounded in which case a speaker wiring fault could put line voltage on the amp case and controls if one side of the output were grounded to the chassis. So in this case a floating output is reqired even though a hazardous voltage on the output and wiring could exist.

I don't think there is any real "best way" here and I do think, since in may instances an output transformer is an option, performance and economics are the current design factors.

-Hal  

[Rick Johnston]

John Roberts  {JR} wrote

And let's not forget I'm talking about refusal to use 70V auto-formers instead of 70V transformers. The primary difference being isolation from ground or amplifier common. Isolation does provide the benefit of tolerating inadvertent grounding of one of either + or - speaker line. Whether this benefits the customer or installer is a judgement call.

Is there an electrical advantage to using an autoformer instead of a transformer? Both cost about the same AFAIK.

Hal Bissinger/COMSYSTEC wrote

The transformerless output amps I am familiar with (Crown CT series) will go into protection if one side of the output gets shorted to ground.

One of our installations has eleven CTs4200's and six CTs8200's in 15 head ends (one in each building in the complex). No single amp channel has more than 80 watts total load. In three years since the install, only three individual amp cards have failed. Two in the same amp at the same time were due to heat buildup inside the rack. (The air conditioning in that head end's room failed.) The third failure was caused by an errant forklift in a shipping area smashing the PC board in an attenuator -- causing both sides of the amp line to short to each other and to the +24vdc side of the music/page relay line.

I'd say that's a pretty good track record for that line of amps.

Quote:

My concern with a floating output is the possibility that the wiring in a building could be crossed with line voltage, effectively putting that voltage between the speaker wiring and ground. It would go undetected and pose a real safety issue for someone who comes along to service a speaker, the amp or wiring.

A war story: Service call at a manufacturing plant. A very simple 70v system. Biamp CMA-350 with ten 15-watt horns in one building. One 12-gauge PVC jacketed line from that amp ran through underground conduit to another building with six more 15-watt horns. The amp had failed. I put my Simpson meter on the line to check for shorts to building ground and instantly blew the meter's fuse. Replaced it and it instantly blew again. Checked for AC voltage: 118 volts. Come to find out some maintenance guy had opened up one of our junction boxes in the ceiling to connect AC power and a switch leg for a new light -- to our line.

Fortunately, I learned a long time ago to touch only one wire at a time -- even if it's "only" a line-level signal cable.

Regards,
Rick Johnston

[John Roberts {JR}]

Rick Johnston wrote on Sun, 23 July 2006 09:02

John Roberts  {JR} wrote]And let's not forget I'm talking about refusal to use 70V auto-formers instead of 70V transformers. The primary difference being isolation from ground or amplifier common. Isolation does provide the benefit of tolerating inadvertent grounding of one of either + or - speaker line. Whether this benefits the customer or installer is a judgement call. /quote Is there an electrical advantage to using an autoformer instead of a transformer? Both cost about the same AFAIK. Regards, Rick Johnston

Yes.... more efficient (less loss), better freq response, smaller, lighter, less expensive.

JR

[Don Boone]

Rick you might be right about the 100v thing unless telephones
were exempt. Talk battery is 48 volts and ring voltage is 90 volts.
But I heard the 36 volt thing sometime during my education at the
Burn & Learn School of Electronics.

Don

[Rick Johnston]

Don Boone wrote

Rick you might be right about the 100v thing unless telephones were exempt. Talk battery is 48 volts and ring voltage is 90 volts. But I heard the 36 volt thing sometime during my education at the Burn & Learn School of Electronics.

Telco ring voltage maxes at 100 volts @ 20Hz AC on some systems, and I've never heard of a code that says telco lines have to run in conduit. The current is so low that it won't cause a 24-gauge wire to burn, and there are protections at the CO or switch to shut it off if an over-current condition is detected in the system.

Talk battery is typically between 40 and 50 volts DC if the phone is on hook, and it drops to a max of 9vdc if it's off hook. Most phones will work off hook with as little as 4vdc.

The issue I have with the 36-volt number in a 70.7-volt CV distributed sound system is that it's not a real-world scenario. The center tap of a transformer does nothing for the speaker circuit. (In fact, I've never even seen a center tap connected unless it's a school system, where a call switch in a classroom shorts a floating CT to ground to produce a call-in at the head end.)

In your example, there are three wires in the circuit. One is carrying the "hot" or in-phase signal. A second is carrying the "cold" or out-of-phase signal. A third is center-tapped and is referenced to ground. If you measure the voltage across the hot & ground or cold & ground, you'll see a 6dB voltage drop versus the measurement between hot & cold. That's because half of the voltage (-6dB) is being taken to ground by the center tap -- which typically isn't connected to anything in the field.

The real-world measurement of voltage is taken from the hot & cold, where a peak-to-peak reading on a 70.7-volt speaker line will be 100 volts.

Again, though, the vast majority of distributed systems never come close to the full 70-volt potential in day-to-day operation. The highest I've ever measured hit 28 volts on my trusty Simpson meter, and it was crankin' loud!

Regards,
Rick Johnston

[Hal Bissinger/COMSYSTEC]

Again, though, the vast majority of distributed systems never come close to the full 70-volt potential in day-to-day operation. The highest I've ever measured hit 28 volts on my trusty Simpson meter, and it was crankin' loud!

Right you are but you have to remember who writes codes. They take the worst case of an open, unloaded line which could actually have in excess of 70.7 vrms on it, but don't tell them that. If the code says that anything (not just speaker wiring) over 100 volts peak must be in conduit then over 70.7vrms must be in conduit. I don't think they are just picking on us.

The 25 volt system came about the same way. A code somewhere a long time ago (I believe relating to schools) said anything more than something like 50 volts had to be in conduit.

-Hal