# Sticky  70v Distributed Line PA systems, facts, myths and tips. :)



## guest

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

False. 

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 http://assets.peavey.com/download.c....pdf&name=Automatch_II_Transformer_Manual.pdf):

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. )


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## jeffmoss26

Nice writeup.


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## riverpc100

I used to do a ton of PA, intercoms, and sound systems. As far as 25 volt line systems being rare, unless things have changed all school intercoms use 25v. The individual classroom speakers are tapped around 0.25 watts. Then you have the playground speakers that can be tapped higher, common values are 30 watts. Most larger systems have multiple amps for "All" or grouped paging.
On the use of a 4 ohm output to line speakers I've done it a few times. On one install I had 25 outdoors speakers 
(AW-650), with 25v transformers tapped at 15 watts each powered by a Crown amp running in bridge mode.
more than enough volume for a flea market.


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