posted 07-15-2014 12:46 PM                    

quote:

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Some amplifiers can drive a 70 volt line directly. QSC also makes low ohm to 70 volt output transformers you can use on the output of their amplifiers as well. You really ought to consult with them on this. My first instinct would be to say try one out and see how it goes. The amp should protect itself if it doesn't like the load.

P.S., How on earth did you end up in Mooreland?

Mark

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Mark, I have tried it out without the 70v transformer and the amp doesn't mind the load at all. (That was the uses in the OP I mentioned.) I am sure it is not quite as efficient as using the proper transformer though.

BUT:

1: The proper transformers would have to be minimum of 400 watts rated for my uses, and those can be found, but good ones are spendy and weigh half the weight of the amplifier (important in this case since this is a "portable" system, and has to be hand carried everywhere);
2: Driving a 400 watt transformer with an amplifier capable of more than 110% of the transformer's rating is risky; (The 2422 is rated at 700w into 4 ohms /825w EIA 1khz at 1%THD);
3: and as long as the amp can handle the load impedance, transformers are not necessary at a certain output 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 x 4 = 2800
Sq root of 2800 = 52.92 (rounded off)

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

Using the EIA rating we get:

825 x 4 = 3300
Sq root of 3300 = 57.45

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 = 97.98 (A bit too high for a 70v line, can burn up speaker transformers or drivers, and may cause the amp to shut down.)

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 = 109.55

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 = 60.00 (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 = 64.81

How about a 3422 at EIA:

1250 x 4 = 5000 (EIA rating)
Sq root of 5000 = 70.71 **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.)

Now the reality is, if I could rely on every racetrack I go to having a fixed number of PA speakers in good working order then I would benefit from installing 70v driver transformers at the head of each speaker array and then most any amp capable of driving the total 70v array wattage will do the job nicely. It would then be a simple thing for my DCA to connect to every system.

But that is not the case. The one exception will be the local track where I am installing the P.A. myself and have control over how many speakers are installed and how they are connected. For that system I will be installing a driver transformer.

But, it was nice to know that for the other local venue (that I have no control over) that my DCA can handle the system just fine.

posted 07-15-2014 01:56 PM                    

quote: Scott Norwood

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My limited knowledge of 70v systems (mostly from old Altec surround systems and modern Muzak-like systems) is that they require a transformer at the amplifer end (or an amp with a built-in transformer) and at each loudspeaker. The installer then adds up the number of watts tapped for use at each speaker and that value indicates the number of watts needed for the amplifier that drives the system.

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quote:

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So, if it is possible (with the right amplifier) to drive a 70v system without a transformer at the amplifier end, I am still confused as to how one would determine the power requirements. Or is this just a case of intentionally throwing more amplifier power than would ever be necessary at the problem?

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posted 07-17-2014 01:05 AM                    

quote: Louis Bornwasser

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Tube amplifiers were much preferred back on drive in systems, which is about what you are building here.

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Ugh, that brings back some horrible memories from my RCA Service days.

I had a drive-in in So Cali in my "kit" that had the old hybrid "Quad-Eight" brand amplifiers. IIRC they had IC preamps, transistorized driver and tube output stages.

When those amps were properly cared for (i.e. keep GOOD tubes in them) not only did they sound great but they were pretty rugged and easily pushed out more than rated power. They weighed in at around 45lbs each (they were mono BTW) and were not overly efficient in regards to power draw.

But when you have a location with a so-called "ham radio expert", a super cheap owner, and a box full of old tubes.....

Right before I told RCA where to stuff it, I had one last service call at that location. They had managed to kill so many of those amplifiers that they had two screens down. I went there to pick up all six (!) of the dead amps and brought them back to the shop for repair.

I got 5 of the six working again, and with fresh tubes we had on hand they each put out so much clean power I kept overloading our dummy loads. The sixth one had an apparent short in the output transformer, because it only worked for about 20 minutes before giving up in a cloud of nasty smelling smoke.

Now my boss, the regional manager, was a bit of a mouse and I had told him (as well as the owner and "ham boy") that I would only warranty the repair IF and only IF they bought NEW tubes for the amps. I pushed my boss into ordering the fresh tubes and billing the theatre so I could bring them back with new tubes, but he didn't have the sack to make it happen. I then told the location that I would not be back with the amps until they had the new tubes waiting for me. Anyone want to guess what happened?

A week or so after I fixed the amps, the call came in to bring them back. I drove all the way out to the theatre, cornered "ham boy" and asked to see the new tubes before I lugged those monsters back upstairs to the booth. He assured me he had them and even carried one of the amps up for me. (Such a helpful old coot he was.)

Well, as I am racking the amps back in I am still waiting for the new tubes to appear. I am finally ready to do the testing, and ham boy wanders over to the "parts cabinet of doom" and says "Well, let's see what tubes we have here that will work." He grabs the same old box of crap tubes out and by looking at the glass magically decides which ones are good.

I go to the phone and call my mouse of a RM and tell him that they did NOT get new tubes as I requested, and that I am now voiding the warranty on ALL of the five working amps. A heated discussion ensues. (Which resumed in his office 2 hours later.)

Well, to get to the point: All five of the repaired amps eventually burned up - again. The first one went literally a few minutes after power-up, the second and third before I even had my tools packed and could leave. The last two held on for a few weeks.

It was about a month after this incident that I left RCA....because this location had called and was crying that now they had 4 screens down with no sound because of failed amplifiers. After being paged 7 times in a row (this was all before cell phones and a page meant you had to either be at home or track down a pay phone) I simply drove to the office, turned in my pager and all the company-issued test equipment and told the RM I quit.

All that said, Louis is right on one thing..in many ways the old tube amps would be great for this kind of work..but who wants to lug around 80+ lbs of just amplifiers and risk broken tubes?