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[doho]

Our church's sound system has an Alesis Matica 500 ampilifier hooked up to the speaker system which is setup with 70 volt transformers on each speaker. Originally the system had a Bogen CT100 PA amp using the 70 output to drive the speakers. A sound company installed the Matica 500 driven by a Alesis M-EQ-230 equalizer and a Audio Centron ACM-12 Mixer. This setup has been operating for at least 10 years. Is it acceptible to have the 8 ohm amp driving the 70 volt speaker system?
Thanks for any advice. Don

[John Roberts {JR}]

If it's performance has been acceptable for 10 years I would guess so.

Driving a 70V system with lower voltage will just result in lower nominal power from the transformer taps to the speakers. This can be somewhat mitigated for by using hotter taps.

If you are getting acceptable volume from the system, and by definition it appears reliable, why fix it if it ain't broke?

JR

[Ivan Beaver]

While it is not the "proper" way to do 70V, it will work fine-as long as you are getting enough volume out of the system.  Some of the big misconceptions about 70V are 2 fold. The first is there is 70V on the speaker line, just like 120Volt AC power.  What it actually means is that when the line has 70VRMS on it, the output power applied to each speaker is whatever the tap on the speaker is rated at.  The other is a basic lack of knowledge of how the system works.  It works by the fact that the speakers are actually usually 8 ohms each.  When hooked to the transformer the efective impedance applied to the line is much higher.  If tapped at 1 watt the impedance is 4900 ohms. 10watts 490 ohms etc.

When applied to a standard solid state amp the high impedance is not a problem.  If the amp were of the tube variety, it would have failed long ago.  Tube amps like shorts and solid state like opens.

[Tom Young]

When driving a 70V ldspkr ssytem, in most cases you want an amp designed for 70V opertaion or an amp with an external 70V transformer.

Amplifiers that provide 625 watts (or greater) per channel can drive a 70V system with no transformer.

Obviously, despite not obeying the above rules your amp has survived.

If you are concerned you can simply remove the transformer on each of two loudspeakers and this will then be a better match to the amp and will have no consequences that are common with cheaper 70V transformers.

[Norbert Ruf]

Tom Young wrote on Sun, 01 May 2005 00:12

If you are concerned you can simply remove the transformer on each of two loudspeakers and this will then be a better match to the amp and will have no consequences that are common with cheaper 70V transformers.

Agreed. But you may then need "thicker" cables running from your amp to the speakers. That is actually the only advantage of the 70-Volt technique: that you can use cables of smaller gauge. Which saves $$, when the distance amp-speaker is loooooong.


Norbert

[John Roberts {JR}]

laoluu wrote on Wed, 04 May 2005 08:10

Tom Young wrote on Sun, 01 May 2005 00:12] If you are concerned you can simply remove the transformer on each of two loudspeakers and this will then be a better match to the amp and will have no consequences that are common with cheaper 70V transformers. laoluu wrote Agreed. But you may then need "thicker" cables running from your amp to the speakers. That is actually the only advantage of the 70-Volt technique: that you can use cables of smaller gauge. Which saves $$, when the distance amp-speaker is loooooong. Norbert

Reduced wire loss is only one of several benefits.

The original poster never mentioned how many speakers were involved. In constant voltage systems you can easily manage multiple speakers. In low voltage distribution you quickly run into loading issues with even a few in parallel.

Perhaps his application was addressing the latter issue and not specifically wire loss, although he receives that benefit too. With enough speakers and adjusting the taps to compensate for lower drive voltage, one could load the amplifier reasonably well.

JR

[Dan Nelson]

Hi Ya'll,  Just a couple of thoughts here.  First 70 volt systems are great for getting even coverage over large areas  99% of all mega store/ mall systems are 70 volt. Generally there are a large number of speakers.  Of course we all know about 8 ohms stuff. My suggestion would be to change over the side, amp or speakers that fit your application least.
I would be very cautious about removing the transformers. This is because many 70 volt systems are low power often less than 10 watts per speaker.  At least verify that your amp wont smoke those guys the first time you hook them up.  Another thought is that I have seen appplications where a 8 ohm amp has been hooked to a matching transformer, to correctly drive 70 volt stuff. Sorry I do not have any info about brands or suppliers fo these.  

[Norbert Ruf]

A not too expensive piece of equipment is this TOA P924:

http://www.toaelectronics.com/amp0002.asp

We use the 240 Watt version for exactly the described purpose: vocal announcements to cover a large area. Not much good for music. I also made the experience that I have to high-pass the input or the fuse will blow. Doesn't seem to like any bass...  

Norbert

[John Roberts {JR}]

laoluu wrote on Thu, 05 May 2005 22:17

A not too expensive piece of equipment is this TOA P924: http://www.toaelectronics.com/amp0002.asp We use the 240 Watt version for exactly the described purpose: vocal announcements to cover a large area. Not much good for music. I also made the experience that I have to high-pass the input or the fuse will blow. Doesn't seem to like any bass...   Norbert

WRT the bass, an issue with any transformer coupled signal chain is LF cut-off. If there isn't enough LF flux capability (iron core & copper turns... weight and cost) it will saturate when asked to reproduce higher levels of LF content.

If the problem is only an undersized input transformer, like in a cheap DI, it may just sound a little puny. When it's an output step-up transformer connected to a power amplifier.. saturating looks like presenting a short circuit to the power amp. Blowing fuses is not the worst thing that could happen.  

One of my earlier patents was a tone control circuit for use in this market that clamps just the bass boost. You get all the boost you command at low levels, but at higher levels it clamps just the bass boost... actually sounded quite good since it passed the HF and flat input signal unmolested.

JR

[doho]

Thanks everyone for your advice. I should have mentioned that the 70 volt speaker system is composed of two 35 watt tower speakers and two 10 watt side speakers in the main sanctuary and three remote speakers. The longest remote wire run is 125 feet using 18 gauge speaker wire. The sanctuary is 40' X 70' all wood with massive wood arches and wood pews, creating terrible acoustics and feed back. There is carpet on the floor. There are 8 active microphones feed into the mixer.

When I hooked up the Alesis 500 watt with bridged output, the system volume control was almost maxed out to get an acceptable volume level. Yet with a Radio Shack 20 watt 70 volt amp, the volume was about 1/3 from max. I hooked up the mixer and amp in my shop with an 8 ohm speaker load and the Matica amp had plenty of adjustment left. Anyway, I ordered a Bogen HTA-125 amplifier to replace the Matica 500. Hopefully this is a good choice.

Thanks again for your comments.
Don

[John Roberts {JR}]

doho wrote on Sun, 08 May 2005 11:10

Thanks everyone for your advice. I should have mentioned that the 70 volt speaker system is composed of two 35 watt tower speakers and two 10 watt side speakers in the main sanctuary and three remote speakers. The longest remote wire run is 125 feet using 18 gauge speaker wire. The sanctuary is 40' X 70' all wood with massive wood arches and wood pews, creating terrible acoustics and feed back. There is carpet on the floor. There are 8 active microphones feed into the mixer. When I hooked up the Alesis 500 watt with bridged output, the system volume control was almost maxed out to get an acceptable volume level. Yet with a Radio Shack 20 watt 70 volt amp, the volume was about 1/3 from max. I hooked up the mixer and amp in my shop with an 8 ohm speaker load and the Matica amp had plenty of adjustment left. Anyway, I ordered a Bogen HTA-125 amplifier to replace the Matica 500. Hopefully this is a good choice. Thanks again for your comments. Don

I will assume you don't have flexibility with how the speaker step down transformers are tapped. Your basic problem is that the 35W tap is based on 70V nominal. If you could push up the tap you would get more volume from the lower than 70V line.

The next cheapest solution is to purchase an outboard 70V step-up transformer. I know PV sold these for years (automatch?) and they were pretty inexpensive.

Of course a true 70V amp should work just fine.

Good luck.

JR

[doho]

The taps on the 35 watt speakers are set at 15 watts and the side speakers are set at 5 watts. Rather than try to make a mismatched system work, I thought it best to match the 70 volt speaker system with a 70 volt amp. Then, if any other problems develop, at least I know the main components are matched.
Don

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