100 Volt Line amplifier

[miwin]

Hi

Excuse my ignorance, but wanted to ask a question about 100 Volt line amps!

I have previously taken the output of a regular 2 x 50 Watt amplifier and stepped it up to a nominal 100 Volt line by using a line transformer. This has worked fine.

What I am thinking of doing, however, is to use an amplifier that is powered by 100 Volts+, which would then provide the advantages (I assume) of:
1 More Power! (currently looking at a 2 x 900 Watt amp -
WONDOM | STORE )
2 No Transformers. This means I can invest more money in the amp, as I
am saving the cost of the transformers.

I would obviously need to have a bigger power supply (100 Volt output, 2kW!), but I dont seem to be able to find one off the shelf that I could use. I have previously used Meanwell enclosed PSU, but the highest output I can find is 48 Volts.

Am I trying to do a dumb thing? Hopefully not, but looking for any guidance.

Thanks in advance

Mike

 

[6L6]

What's the specific application?

 

[miwin]

Thanks for replying 6L6

I am looking at a 100 Volt Line Public Address system.

Previously, I have stepped up using transformers, and then each speaker has its own step down transformer. I was hoping I could eliminate the step up transformer by the amplifier output already being up at approx 100 Volts.

As i said, I may be asking a dumb question!

Mike

 

[Pano]

There have been direct out 70/100V amps around for decades, so you are one the right path. I don't know the topology used, but I"m sure you can find schematics are examples.
Sometime, tho, it's just easier to use a transformer. 🙂

 

[miwin]

Thanks for the reply Pano

Good to know that I am barking up the right tree!

I thought that by removing the transformers, it removes a significant cost, plus gives an increased power output.

The advantages I could see:
1 Increased Power
2 Reduced Costs
3 Lighter - no heavy toroids

Disadvantages:
1 More expensive amp needed
2 More powerful PSU, and probably more expensive (if I can find one that
does 120V DC)

Mike

 

[wg_ski]

A 100 volt output would require 140 volt supply, in bridge mode. There’s that little matter of the square root of two. In reality, you need 5 to 10 volts more, but you can always drive with a little less voltage. If you used a 100 (or maybe 110) volt supply, you would drive a 70 volt line. It’s just a different tap on the transformers used at the speakers. if you ran an amp board with two of those 48 volt regulated supplies in series (making a +/-48 supply) it’s probably close enough to “70 volts” output to call it good. The wattage would be most likely limited by the power supplies - if you used two 300 watt supplies, you would end up with somewhere in the neighborhood of 300 to 600 watts, depending on how fast and how aggressive the current limiting is.

 

[ICG]

A 100 volt output would require 140 volt supply, in bridge mode. There’s that little matter of the square root of two. In reality, you need 5 to 10 volts more, but you can always drive with a little less voltage.

No, you don't. A 100V PA system uses a 'soft' audio step up transformator and most of the times do not reach actually 100V. The main reason for the step-up and step-down transformators are to avoid the losses of the very long speaker wires at low impedances, which require (relatively) high current. That means, the serial resistance and therefore losses are very low and the speakers are fitted with a step down transformator, but generally are all connected in parallel to the 100V rail, allowing the 'soft' transformator to drop the voltage if the load becomes too high. To adapt to the actual needed power, the speaker audio transformers usually have different taps, like 25V, 50V and 100V.

Generally, you can use almost any amplifier for such a 100V system if you fit it with a 100V transformer and there are a huge variety of power ranges in such amps, usually starting at ~50W up to ~600W but in general if you need that much power, it's preferred to divide it to several lines and use multiple amplifiers because that's a lot cheaper and gives a very much preferred redundancy/high fault tolerance, if one line drops, the others continue to work, very important for emergency messages and warning/alarm systems. The 'soft' (relatively high resistant) transformers on both sides (amp and each speaker), which just drop in voltage, also serve failproof reasons, a shorted speaker i.e. causes a drop in voltage/loudness but the other speakers usually still work anyway, just not as loud anymore.

The needed audio transformers at the amp and the speakers and the resulting influence on the impedance don't make class D amps the preferred amp type in ideal circumstances though, because of the interaction of the impedance and the output filter. In many PA situations that does not make much difference though since the most PA systems in buildings, stadiums etc. are quite limited in the frequency range anyway.

So, if you need more power, it is pretty much preferred to rather use multiple amplifiers instead one large, powerful one. For fault/redundancy issues that's pretty much preferrable except if you absolutely have to use one and only one very long line and don't have any alternative. If the 100V PA sound system does not serve any emergency purposes, you're pretty much free how you design it. If it does, however, the liability is a very serious issue and I'd really, REALLY let that do a professional - no matter how much you could 'save' by doing it yourself.

 

[bimo]

You can BTL (Bridge Tie Load) amplifier with 77VDC dual PSU (77-0-77).
Total output transistor depend on load impedance, usually 3 sets of each amplifier is enough.

 

[bimo]

No, you don't. A 100V PA system uses a 'soft' audio step up transformator and most of the times do not reach actually 100V.

100V line distribution mean is voltage output maximum = 100V rms.

 

[JMFahey]

I am looking at a 100 Volt Line Public Address system.

Previously, I have stepped up using transformers, and then each speaker has its own step down transformer. I was hoping I could eliminate the step up transformer by the amplifier output already being up at approx 100 Volts.

Ok, but please be more specific.
At least real power needed and amount of speakers.

 

[miwin]

Thanks for all the replies, all very helpful!
My use is where I will have 2 speaker circuits, each with up to 20 speakers, each tapped at 2.5 Watts = up to 50 Watts per circuit. I have to allow a at least 25% headroom, plus occasionally some of the speakers may be tapped at 5 Watts, so realistically I would need 2 x 100 Watts.
In the event of needing more power, I would look at using a dedicated external PA.
From your help and my research yesterday, it looks like what I was trying to do would be uneconomic, making the 100 Volt Toroid solution better.
Costs for toroid solution
amp (2 x 100 watts) £ 30
Toroids x 2 (£50 ea) £100
PSU £ 60
Total £190

The cost of the amp that I was looking at using is £150 on its own, not even allowing for the larger power supply that I would need!

At least I know that technically I was right (other than needing more than 100 Volts DC for a 100 Volt line system), and it would work, but economically not worth my while!

Mike

 

[JMFahey]

Yes, looking at your actual needs, "using 900W amps just to save on trasformers" does not work in an economic sense.
Specially because you will need a monster very expensive supply for it.

And why toroids as 4/8 ohm to 100V line converters?
I guess some PA specific shop or supplier should have cheaper EI versions.

At least for reference browse Parts Express site, I bet they offer something similar.
Then get a UK supplier 🙂

 

[steveu]

Without using a transformer nor a bridged output pair, you need a 300VDC power supply. Because the peak output is always a few volts below the power supply and 100V rms is 283 Volts peal-peak. Today using high voltage MOSFETS and IGBTs, you could make such an amplifier. I know that amplifiers that run directly from the power line, ie no power transformer do exist and that would give you such an amp, but of course there are safety issues and speakers would require class-1 wiring just like electrical power lines.
However except for a stadium, massive power amplifiers are rarely a good solution. In most cases, using an array of amps is much more reliable and unable to destroy expensive speakers.

 

[ricortes]

At one time I had ~similar dream of just using rectified AC line power. Why I mention it is because I would like it if someone can remember the chip I am talking about. I don't think it was specifically called an audio IC but went by something ~high voltage driver. It came in several grades i.e. 120V, 160V, ??? and came with a spec sheet that showed an audio amp that had the IC outputs tied directly to a couple of MOSFETS or Bipolar power transistors.

I ordered all the parts, I could only find one that had the IC in stock, and they changed the order, shipped me the lower cost 120V chip at the same price as the higher voltage one I ordered, and I quit in disappointment. 🙁