InterM PA-9xxx 100V amps

[TomHoward]

Afternoon all

We've got a bit of 100V kit, around 30 speakers and horns and some PA-9336 and PA-9348 amps. Gets used on site for sports days mainly.

 

When set up the systems sound typically terrible. Something that's caught my attention is that at their rated outputs the InterM PA-9336 is rated <1% THD but the 9348 jumps up to <5% THD

A comparable Cloud amp is rated at <0.05% THD

 

Are these InterMs known for being terrible? Has anyone had experience of comparing them to anything else and found an improvement?

[dbuckley]

When set up the systems sound typically terrible.

 

What does "terrible" mean?

[Owain]

Are these the ratings (which are actually pretty meaningless in practice) or are you actually measuring the distortion?

 

Public Address horn speakers will never sound lovely but they will sound worse if you drive them too hard - I like to run speakers at one tapping below the max, and keep about 20% spare capacity on the amplifier.

 

Also many public address microphones are pretty poor and many amateur announcers have poor microphone technique.

 

 

[TomHoward]

The mics are out of the rest of our stock, so they're usually Senheiser e845 wireless. I'm not measuring the distortion, it just stuck out to me on the spreadsheet.

 

I think the terrible sound is essentially distortion. Our horns are primarily the DNH music horns, and you just don't seem to get any level out of them for the power, so they are pushed pretty hard to get the level needed. At that point though I can't tell if it's the horn that's the problem or if I should be able to get more reasonable level out of those DNH-MH, or if they're always quiet. It's not something we use very often so I just don't really have any comparisons for it on the same site.

[dbuckley]

OK, if you're getting audible distrotion, I suspect you're actually overloading the amplifier(s) or the transformers.

 

You are high pass filtering to keep the transformers from getting saturated?

 

Can you confirm you have a correct 100V setup, ie that the taps on the speakers are set so that the load is no more than about 80% of the amplifier power output?

 

These are 50W horns, google tells me, so if set to the 50W setting, and you've got 30 of them, you need (obviously!) the thick end of 2KW of amplifier power. And for that I wouldn't use a stack of inter-M's, I'd use a couple of "ordinary" amplifiers (or channels of ordinary amplifiers, like crowns, qsc etc) rated at 1000W into 8R.

[laolu]

Depends on how everything is wired, you would still need a couple of 100V transformers. But then I see 2 problems: (1) PA amps (like the ones you mention) are not very happy with a transformer at their terminals. (2) Transformers would add to the overall THD again.

[Owain]

Depends on how everything is wired, you would still need a couple of 100V transformers. But then I see 2 problems: (1) PA amps (like the ones you mention) are not very happy with a transformer at their terminals.

 

I think the point is that 1000 watts at 100 volt line is going to have a total impedance of around 10 ohms, so an amplifier with an 8 ohm native output can be directly connected without requiring a transformer and not be overloaded.

 

 

In fact 1250 watts at 100 volt line would be a 'perfect' match

 

 

[TomHoward]

It is all within spec - we usually don't run 30 on one job though.

Our are the DNH-30s tapped at 30W. Run 8 on a 360W amp or 12 on a 480W.

They should be high-passed at the amps (it's about 100Hz I think? No external processing but there is a switch on the amp).

It's interesting the idea though that in sufficient quantities they would tend all the way down to 8ohms. It'd be worth looking at, as I could try larger numbers on the low impedance output of one of the existing amps.

[laolu]

I am still not sure whether that works, because you need e.g. the 8 Ohms for your average PA amp to work properly. If you connect the 100V side of the transformer to the amp's output, it is almost like an open circuit to the amp. Unless you parallel 8 or 12 or 30 such transformers and connect that load to the amp. That's what I meant with "Depends on how everything is wired..." If you use a single, long, thin cable (which is one of the advantages of the 100V-technique) to connect all the 8 or 12 or 30 speakers, you won't get much out of them without a transformer. Please correct me if I'm wrong? :blink:

 

Norbert

[timsabre]

I am still not sure whether that works, because you need e.g. the 8 Ohms for your average PA amp to work properly. If you connect the 100V side of the transformer to the amp's output, it is almost like an open circuit to the amp. Unless you parallel 8 or 12 or 30 such transformers and connect that load to the amp. That's what I meant with "Depends on how everything is wired..." If you use a single, long, thin cable (which is one of the advantages of the 100V-technique) to connect all the 8 or 12 or 30 speakers, you won't get much out of them without a transformer. Please correct me if I'm wrong? :blink:

 

He is talking about paralleling a lot of transformers/speakers. So the amp will see a load close to 8 ohms without having a transformer at the amp.

 

You are correct that connecting a single transformer/speaker would not work very well because the load impedance on the amp would be very high, but as you connect more and more 100V speakers then the impedance will get lower and lower to the point where a conventional amp without a transformer output will drive it.

[Owain]

In fact I've got surprisingly loud volume out of a 30 watt (100V) horn when fed directly from a transistor amp with an output in the milliwatts range powered by a PP3 ... I don't quite understand why it works, but it did (with that amp - other amps behave as expected and output into high impedance is next to nothing).

[dbuckley]

You are correct that connecting a single transformer/speaker would not work very well because the load impedance on the amp would be very high...

 

This is where it gets interesting. A single 30W 100V speaker has an impedance of about 330R. With some amplifiers the protection circuits, when faced with a 330R load, will declare it to be an open circuit load, and refuse to supply it. Other amplifiers will just get on with it, and that 30W horn will be driven beautifully.

 

An amp that can deliver 1250W into 8R made up of 100V load, although its working "flat out", as in its delivering the full rated voltage to the load, and the pretty LEDs will be getting to the top of the yellow, but because the load is 8R, the amp is probably rated to supply twice that power into 4R, and thus the amp is only delivering about half the power it can, so its just cruisin'.