How to measure load impedance of a PA amp? - diyAudio
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Old 20th February 2011, 01:13 PM   #1
Jay is offline Jay  Indonesia
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Default How to measure load impedance of a PA amp?

I have a 120W PA amplifier for ceiling speakers. It uses output transformer with center tap of the primary winding connected to B+. I haven't measured the voltage but I guess it is 24V.

The secondary winding has 0/25V/50V/75V/100V taps.

I don't understand what is happening when I connect the amplifier to an ordinary speaker (without step down transformer). I have tried connecting my 6-ohm speaker to the 25V and 75V without knowing the difference. There was nothing wrong, but either way the amp got hot as if in class-A. Where did the hot temp come from? What was happening? What should I need to know?

Question 2: how can I make use of the output transformer, for a high quality amp? Is it possible to build something like the Zeus Amplifier? The -3dB pole for the amp is at 40Hz and that is acceptable for me.

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Old 20th February 2011, 01:47 PM   #2
AndrewT is offline AndrewT  Scotland
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I have never used a high voltage SS amplifier.
So this is a bit of guesswork.
Using the 100V tap and 120W you can find the recommended minimum impedance that can be hooked up to the 100V tap. Rmin = V^2 / P = 100^2 / 120 >= 83ohms.
You can attach 10 speakers each with an effective impedance of 830ohms on the 100V tapping. These will be described as 12W 100V speakers. All 10 add up to 120W.

Using the 25V tapping the Rmin = 25^2 / 120 = 5.2ohms.
Again attaching 10 speakers each of 52ohm impedance to draw a maximum of 120W from the amplifier. These speakers will be described as 12W 25V speakers.

You should be able to attach an 8ohm speaker to the 25V tapping, but not a 4ohm speaker. Change to the 50V tapping and you need to use 4times the impedance of speaker, i.e. >=21ohms. Double the voltage again to 100V and 4times the last impedance and you are back to >=83ohms

your 6ohm speaker should be OK since it is higher than 5.2ohm when attached to the 25V tapping.
A 4 to 8ohm speaker is likely to overheat the amplifier if you drive it hard.

Maybe this old amplifier needs some tender loving care, i.e. maintenance.
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Last edited by AndrewT; 20th February 2011 at 01:50 PM.
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Old 20th February 2011, 03:09 PM   #3
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That amplifier output transformer is a step up transformer and the standard PA system relies on each of the speakers being connected via a step down transformer mounted on each speaker.....Go research 100V line PA systems.

Anyway, the amplifier is usually a standard audio power amplifier driving the primary winding of an output transformer - most such amplifiers will drive a standard 4/8 ohm load directly conditional that the output transformer primary winding is disconnected.

Eric.
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Old 20th February 2011, 08:06 PM   #4
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These type of amplifiers were used to drive many speakers such as in drive-in cinemas. The amplier output was stepped up to 100 V.

Each speaker in the chain was fitted with a step down transformer from 100 V. The reason they used this was so that the voltdrop over long distances were avoided. It used a similar principle of electricity distribution systems.
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Old 21st February 2011, 12:42 AM   #5
djk is offline djk
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"Anyway, the amplifier is usually a standard audio power amplifier driving the primary winding of an output transformer - most such amplifiers will drive a standard 4/8 ohm load directly conditional that the output transformer primary winding is disconnected."

We missed the part about:

"It uses output transformer with center tap of the primary winding connected to B+."
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Old 21st February 2011, 07:34 AM   #6
ontoaba is offline ontoaba  Indonesia
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Build like zeus with 25V, 50V, 70V, and 100V outputs? also to drive that horns and ceilings?
May be the result will not better for driving those speakers, your amp is good choice for that job.
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Old 21st February 2011, 09:09 AM   #7
Jay is offline Jay  Indonesia
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Thanks for the answers guys...

AndrewT,

I think the amp is in good condition. I have read about PA amps, including some datasheets from TOA, but none has explained about the use of the amp for ordinary hi-fi speaker. What I can find from googling is people telling to not to use the amp for ordinary speaker, with no technical reasons/explanations. Some even think that the high voltage will blow your speaker!

Djk,

I think Eric said something about disconnecting the primary of the output transformer. If what he meant was disconnecting from the B+, then it is a good hope for me. Meaning that the transformer can be used like an interstage transformer. Iím hoping that disconnecting the center tap from B+ and connecting the speaker to 0-25V or 0-50V and the amplifier to the primary will be equivalent to using 1:1 or 1:2 audio transformer. But even so, of course I need to modify the output stage a little to make it work.

Ontoaba and everyone,

The point is I want to make a standard high end amp (not for those ceiling and horn application) using components I have at hands. I have come to a conclusion that the original circuit must go to the trash bin. The output transformer (also the input transformer!) is the one that I want to make useful. No PA amp which uses output transformer is high end. But tube amps and Zeus-like amp are acceptable. So who knows this output transformer can be useful? I have 8 of these heavy monoblocks!
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Old 22nd February 2011, 01:56 AM   #8
ontoaba is offline ontoaba  Indonesia
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For high end, use current driving, like tubes. With voltage driving the sound will be similar with your TOA, it has metal can effect ("kemlontang" in java).
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