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#214793 - 01/24/15 11:01 PM 70 V pa system  
dougwells  Offline

Joined: May 2003
Posts: 1,138
kamloops BC Canada
I been looking at a simple install that needs troubleshooting they are running a home stereo type 8 ohm output stereo amplifier on a 70 v system the speakers and volume controls are all 70 v style. I think they need a proper 70v pa amp or bypass all the transformers and install proper 8 ohm stereo volume controls.

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#214799 - 01/25/15 03:20 AM Re: 70 V pa system [Re: dougwells]  
mxslick  Offline
Joined: Oct 2004
Posts: 803
Atomic City, ID USA
The best option would be a proper 70v amplifier, OR a driver transformer which will allow the regular amplifier to drive the system successfully. I strongly advise against trying to change out the volume controls to an "8 ohm" type...connecting multiple speakers to keep the total connected impedance to a reasonable level (MUST stay between 4-16 ohms total connected or the amp can burn out) is very difficult to do.

QSC makes a 70v driver transformer: 70v up to 300 watts
and 70v up to 600 watts

Peavey also makes one: 70v up to 200 watts.

And here's a bunch of info on 70v distributed audio systems: (I originally posted this on another forum) :

There are several myths and misconceptions of what a "70v Distributed Line" PA system is, how it is wired and set up, and what it is capable of. Well these next posts will hopefully answer most of the common (and some uncommon) questions in regards to those systems.

First up, what it is and why it was invented:

70V distributed line systems were invented to allow for the easier transmission of sound over a large area, with a savings in wiring and complexity over conventional (Low Impedance or "Low Z" systems, the 4 and 8 ohm nominal systems used in hi fidelity and other single-speaker per channel) sound systems.

The system is driven with a transformer at the amp end and each speaker has a transformer with wattage taps. This allows you to do two things:

1: Drive a very large number of speakers by simply wiring all of them in parallel without worrying about impedance matching; and

2: By selecting the desired wattage taps at each speaker, controlling the overall level of each speaker to allow for better distribution of the sound levels. (Thus the common use of the term "70v distributed system".)

In theory, if you have a drive transformer large enough (with the corresponding power rated amplifier driving it) there is no real limit on the number of speakers you can connect to a 70v system, the only limit is that the total of all of the wattage taps MUST NOT exceed the amplifier/driver transformer ratings.

For example, a 400w rated amplifier, with the proper drive transformer, can drive 400 1-watt speakers, 800 1/2-watt speakers, 1600 1/4-watt speakers, etc. And you can connect ANY combination of wattage of speaker to the system, but the TOTAL wattage must not exceed the rated power of the amplifier.

These systems are also done in 25v (very rare) and 100v (and higher) versions. The 100v and higher systems are usually done at extremely large venues like motor speedways, large convention centers, etc. The higher voltage allows for greater distances for a given size of wire, and of course greater power for a given size of wire. (Same as the wattage capacity of a conductor is doubled going from 120 to 240 volts.)

The savings in wire (both by allowing smaller conductors, and not requiring individual speakers to be "home run" to the amplifier [or a distribution splice block] for series/parallel connections to get to a 4 or 8 ohm impedance) is considerable and in many cases offsets the slight increase in costs due to the transformers used at the amp and speakers.

Now to address the first myth: 70v systems are frequently run over telephone lines and are perfectly acceptable.

"Frequently run over telephone lines" is the sign of a hack install. Yes, the 70v system allows for smaller conductors in a given installation, but using telephone lines results in wasted power heating up the wire. Simple electrical laws make it so.
First off, using simple math, the current in a 70v system can still reach several amperes depending the the total wattage of the connected speakers. Easily well over what a 24ga wire can handle, plus you STILL have to factor in for voltage could easily have the speakers at the amp end driving at ear-bleed level and have barely a whisper from the ones at the far end.

Second myth: 70v systems are intrinsically safe and you can't get shocked by one.

Ask any experienced installer and they will tell you it is not so. Even though the amplifier's drive transformer results in what you could call an isolated or SDS type of system, capacitive coupling to the amplifier can and will allow a very nasty shock to occur. (And see my next post about using an amplifier without a driver transformer.)

Further, a short in a 70v system will allow enough current to flow (up to the rated power of the amplifier) which can result in burned conductors, transformers or amplifier failure which can and has caused serious fires.

Third myth: You can use any speaker on the 70v system without a transformer on the speaker itself.

False (and true..see the last sentence.). You may get away with it for a while on some paging systems (where the announcements are short like "Bill, call on line 2") but a standard speaker without a transformer to limit the voltage and power applied to it WILL eventually fail. In extreme cases the voice coils have caught fire. ALL speakers used in a 70v distributed line system MUST have the transformer and it MUST be connected to a tap not to exceed the speaker's power rating.

You can buy (and cheaply) transformers to allow ANY speaker to be used in a distributed line system, juts make sure it has taps rated up to the speaker's power handling.

Fourth myth: 70v distributed systems cannot handle high-fidelity signals (low bass down to 20hz and high end to 20khz.)

Absolutely false. 70v distributed systems are VERY capable of full-range (20-20khz) audio signals..BUT the transformers get pretty pricey. (And big.) Both the driver transformer at the amp and the ones at the speakers MUST be of high quality and made for full range signals. Since the most common use of such systems is background music, paging and outdoor announcing, the most common (and least expensive) speakers are purposely limited in frequency range to save money.

Next post: Is it possible to drive a 70v (or 100v) distributed system without a driver transformer at the amplifer?

Stupid should be painful.

#214800 - 01/25/15 03:22 AM Re: 70 V pa system [Re: mxslick]  
mxslick  Offline
Joined: Oct 2004
Posts: 803
Atomic City, ID USA
Part two:

Myth: ALL amplifiers MUST have a driver transformer to power a 70v distributed line system.


As long as the amp can handle the load impedance, transformers are not necessary at a certain output.

Real-life example:

I use QSC's Digital Cinema Amp (DCA) series for my home screening room, which is similar in electrical design to their CX series.

I have a part-time gig right now announcing motocross races. Our P.A. system uses "70v Distributed Line" speaker horns (One location has eight horns at 30 watts each, the other has five at 30w and 4 at 70w connected).

The race club originally bought a PA amp with built-in 70v output rated at 300w. That amp didn't last ten minutes at the location with 8 horns. (240w total load) Cheap piece of crap.

Well, I had to get the show going, so I pulled one of my DCA 2422's from my rack and took it out to the track to see if it could drive those 8 horns....

The dc line resistance of the system was 2.4 ohms....(actual impedance would be higher or lower depending on frequency) so it was borderline for what the amp should be able to handle, plus the DCA amps aren't really intended to drive 70v systems directly.

The end result? No surprise to me really, but that amp drove those horns to near ear-splitting levels, more than enough to overpower 30 motocross bikes in a very large outdoor area.

Why did that work? Here's the answer based on this formula (Courtesy of Peavy

V= (sq root of)(PxZ) Where:

V= Output voltage
P= Amplifier Rated RMS Power in watts
Z= Amplifier Rated Load Impedance in Ohms

So, in the case of a single channel of the DCA 2422 we get:

700(P) x 4(Z) = 2800
Sq root of 2800 = 52.92 (V) (rounded off)

So I was technically under-driving the speaker array quite a bit.

Using the EIA power rating we get:

825 x 4 = 3300
Sq root of 3300 = 57.45v

For fun (and I haven't tried it yet) let's see what the bridged mode gets:

2400 x 4 = 9600
Sq root of 9600 = 97.98v (A bit too high for a 70v line, can burn up speaker transformers or drivers, and may cause the amp to shut down. Would probably work well for a 100v system though.)

But let's try this if my speaker array presents a nominal 8 ohm load to the amplifier: (Bridged mode)

1500 x 8 = 12,000 (very interesting..)
Sq root of 12,000 = 109.55v

Note that as the impedance goes UP, so does the voltage. This is consistent with Ohm's Law and the standard formula for power.

Now to get as close as possible to my ideal 70v line, let's see what a QSC 3022 can do:

**All below are single channel mode: **

900 x 4 = 3600 (RMS rating)
Sq root of 3600 = 60.00v (not bad but still about 12-15% low)

So let's try the EIA rating:

1050 x 4 = 4200 (EIA rating)
Sq root of 4200 = 64.81v

How about a QSC 3422 at EIA:

1250 x 4 = 5000 (EIA rating)
Sq root of 5000 = 70.71v **We have a winner!!**

So, in my application, and to drive the array without a transformer , a QSC 3422 in single channel mode would be the best bet without a transformer. (Or any other comparable amplifier rated at 1250w at 4 ohms.)

A couple of caveats:

1: Many amplifers cannot reliably drive a low-impedance load that the average 70v array would present...the amp may overheat, shut down in protect mode or fail altogether. If a given amplifier is rated to handle 2-ohm loads (most of DCA's can) they will probably work. If the amp cannot handle a 2-ohm load, then a driver transformer is a must for reliable performance.
2: The 70v array is no longer isolated from the amp's ground by the driver transformer, so the risk of shock is higher. By the same token, the amp's output stage is no longer protected from seeing a direct short (or fault to ground, which WILL cause most modern amps to fail) by the driver transformer.
3: There is a possible loss of efficiency due to the mismatch of amp output impedance and the array's impedance.
4: A lot of amplifier power is required even with a small array load. In my example, the ideal amp must be rated at 1250 watts to feed a 320 watt array. That could result in a much increased up-front cost for a heavier amplifier.

So while it CAN work, there are some trade-offs involved. It is always best to get an amplifier designed for a 70v system, or get the proper driver transformer. (On that, Peavey cautions that the amplifier's RMS power rating should not exceed 110% of the rating of the transformer to avoid the risk of transformer fire. )

Stupid should be painful.

#214801 - 01/25/15 05:18 AM Re: 70 V pa system [Re: dougwells]  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA


Thanks so much for the interesting and informative Posts!!!

cheers thumbs

--Scott (EE)

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#214802 - 01/25/15 06:13 AM Re: 70 V pa system [Re: dougwells]  
dougwells  Offline

Joined: May 2003
Posts: 1,138
kamloops BC Canada
I did find that today on another forum today after I posted here . But i didnt read the whole thread smile

Last edited by dougwells; 01/25/15 06:35 AM.

#214803 - 01/25/15 06:29 AM Re: 70 V pa system [Re: dougwells]  
dougwells  Offline

Joined: May 2003
Posts: 1,138
kamloops BC Canada
yes someone recently installed a yamaha R-S500 amp and it is shutting down. I did not turn the amp on as wouldnt want to be the one that blew it up smile

the have both right and left channels feeding some existing 70v volume controls.

I would like to find a diagram how 4 of these controls can operate 4 areas of the restaurant. I did mention the amplifier was non compatible. this is a mono wired system.

I see in the specs it can do 2 ohms.

Last edited by dougwells; 01/25/15 06:34 AM.

#214805 - 01/25/15 06:37 PM Re: 70 V pa system [Re: dougwells]  
mxslick  Offline
Joined: Oct 2004
Posts: 803
Atomic City, ID USA
Originally Posted by dougwells
yes someone recently installed a yamaha R-S500 amp and it is shutting down. I did not turn the amp on as wouldnt want to be the one that blew it up smile

the have both right and left channels feeding some existing 70v volume controls.

I would like to find a diagram how 4 of these controls can operate 4 areas of the restaurant. I did mention the amplifier was non compatible. this is a mono wired system.

I see in the specs it can do 2 ohms.

Ok, some serious issues here:

1: Are both right and left channels' outputs tied together into one mono line? If so, that is a HUGE no-no..they are lucky the amp hasn't already blown up. You can NEVER, EVER tie two or more amplifier outputs together!! Your first step is to disconnect one channel from the system.

2: Since the output of each channel is only 75 watts it is still the wrong rating to run a 70v line directly. It may work, but it will be stressing the amp and speaker's transformers unacceptably.

If they want to keep the Yamaha, your best (and least expensive) bet is to get two of the Peavy transformers I linked to in my first post and use one transformer on each channel of the receiver to drive the system. Since you mentioned four controls, are there four "home runs" to the receiver location? If so, get two transformers, one for each channel, and hook two runs to each transformer.

If there is only ONE "home run" to the receiver location, use one transformer and connect it to ONLY ONE CHANNEL of the amplifier. Leave the other channel disconnected.

If the controls are AT the receiver location that makes this a lot easier...get two transformers, and connect one transformer to each channel of the receiver. Then connect TWO of the volume controls to the OUTPUT of each transformer. DO NOT tie the outputs of the transformers is unnecessary and can cause the transformer(s) or receiver to fail.

I would like to find a diagram how 4 of these controls can operate 4 areas of the restaurant.

Remember what I posted about how the system works...think of each volume control (and area of speakers) simply as receptacles (or loads if you prefer) on a branch circuit. Each volume control is like a switch (or dimmer actually) controlling a set of speakers in each area. The single mono feed you mentioned is the branch circuit.

Try not to over-think this too much....sound systems can be complicated, but the setup of a 70v system is, by design, simple even when a large number of speakers or control zones are involved. smile

Last edited by mxslick; 01/25/15 06:43 PM.

Stupid should be painful.

#214806 - 01/25/15 07:01 PM Re: 70 V pa system [Re: mxslick]  
dougwells  Offline

Joined: May 2003
Posts: 1,138
kamloops BC Canada
Thanks very much mxslick. I found it much easier reading the reply's here instead of posting at the other forum. As they were not prepared to pay me yet they want a quote and i did a basic troubleshoot of the system I have more info to give them now.

I think what happened here was a background music supplier put in a stereo set top box and thought they could provide a stereo signal.

#214807 - 01/25/15 10:23 PM Re: 70 V pa system [Re: dougwells]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,085
The Muzak (TM) box my wife gave me was mono. Maybe they do stereo but not at her place. It also had 70v and regular 8 ohm outputs.
The 70v was bad but the 8 ohm still worked. My neighbor is using it now. I coupled the stereo line level input from an MP3 player through a ground loop filter (pair of transformers) to a "2 for one" RCA cable and it seems fine. I was concerned about tying to outputs together directly too.

Greg Fretwell

#214808 - 01/25/15 10:50 PM Re: 70 V pa system [Re: dougwells]  
dougwells  Offline

Joined: May 2003
Posts: 1,138
kamloops BC Canada
I am estimating no more than 15 or so speakers on 4 home runs. I was wondering if this would work Thanks.

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