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Author Topic: Basic amp question....  (Read 367 times)

Mark Wilkinson

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Basic amp question....
« on: February 20, 2020, 02:54:04 pm »

....that mainly applies to subs.

How do we know when an amp has run out of current?

My understanding is that amp's clip lights tell us when voltage is being clipped.  And the various QSC models I've measured all confirm that, under load or no load.

But when driving a low impedance sub or subs down in 2 ohm range...., before a clip light ever blinks.....,
how do I know the amp hasn't run out of current?

I guess I could tell if the voltage rails sag....but who ever sees that?

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Ivan Beaver

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Re: Basic amp question....
« Reply #1 on: February 20, 2020, 03:00:38 pm »

....that mainly applies to subs.

How do we know when an amp has run out of current?

My understanding is that amp's clip lights tell us when voltage is being clipped.  And the various QSC models I've measured all confirm that, under load or no load.

But when driving a low impedance sub or subs down in 2 ohm range...., before a clip light ever blinks.....,
how do I know the amp hasn't run out of current?

I guess I could tell if the voltage rails sag....but who ever sees that?
Unless the amp has specific indicators, you can't tell-except possibly sonically. 

But then, is that compression the amp running out of current or the power compression of the drivers?

In many (I don't know about all) DSP amplifiers, the DSP indicators are before the power amp.  So the clip/limit lights are for the output of the DSP, not the amplifier itself.  They "assume" the amp is doing what it is supposed to do.
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Mark Wilkinson

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Re: Basic amp question....
« Reply #2 on: February 20, 2020, 03:21:52 pm »

Unless the amp has specific indicators, you can't tell-except possibly sonically. 

But then, is that compression the amp running out of current or the power compression of the drivers?

In many (I don't know about all) DSP amplifiers, the DSP indicators are before the power amp.  So the clip/limit lights are for the output of the DSP, not the amplifier itself.  They "assume" the amp is doing what it is supposed to do.

That's what I've been thinking has to be going on.
And yeah, how can we sonically separate power compression from amp current shortfall?  Unless maybe by comparing different amps on the exact same sub(s) to neutralize the compression variable.

Have you seen any amps that have specific indicators for voltage or current output sag?
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Chris Grimshaw

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Re: Basic amp question....
« Reply #3 on: February 20, 2020, 04:50:30 pm »

If the PSU rails are sagging, a good clip indicator will spot that the loudspeaker output is getting close to the (sagging) rail voltage, and still illuminate the red lights.

... or have I missed something?

Power compression has a time constant of seconds, while saggy PSU rails will be back up shortly.
... Unless the toroidal transformers get hot and undergo their own power compression. Uh oh.

Chris
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David Sturzenbecher

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Re: Basic amp question....
« Reply #4 on: February 20, 2020, 05:34:16 pm »



In many (I don't know about all) DSP amplifiers, the DSP indicators are before the power amp.  So the clip/limit lights are for the output of the DSP, not the amplifier itself.  They "assume" the amp is doing what it is supposed to do.

I know at least when it comes to output meters, most amps are not like this (meters are indicators no??). The meters show the actual voltage the amp is outputting.  The easy tell is mute a single woofer channel in a big line array or point source box.   The muted channels still show small amounts of activity from the woofers acting as mics.

Crown amps used to have the input output comparator And if the output signal was different from the input signal in a significant way, it would light up.  Pretty sure this was under the hood in the iTechHD line.
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Frank Koenig

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Re: Basic amp question....
« Reply #5 on: February 21, 2020, 11:05:08 am »

Look at the output on an oscilloscope. Clipping is pretty easy to recognize on a scope -- the usually rounded peaks get flat tops around a certain voltage. If that voltage is at or slightly below the open-circuit voltage limit of the amplifier then the amplifier is voltage limited into that load.  If clipping occurs at a substantially lower voltage then it is the result of running out of current. A rough measurement for sure, but with a little experience it likely will tell you what you want to know.

As for clip light behavior, it depends entirely on the design. I would hope that premium amps would indicate clipping due to all causes, but cheap amps might have cut corners there. Amps with built in limiters and monitoring software give a more detailed view, including limiter gain reduction history, which is pretty cool.

--Frank
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Re: Basic amp question....
« Reply #5 on: February 21, 2020, 11:05:08 am »


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