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Topic: Continuously monitor Power Amp Input or Output to see if it is maxing out (Read 472 times) previous topic - next topic
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Continuously monitor Power Amp Input or Output to see if it is maxing out

Is there a way to monitor either the input or output voltage of a power amp to get an idea if it is overloading? I know overloading isn't quite so "digital", but I can get an idea if my power amp, that is driving a sub on a home cinema, is too small.

I am thinking of a gadget designed for the job, or maybe of making something based on a USB ADC with some sort of software, or a multi-meter that continuously monitors a signal,

It needs to be reasonably easy to use.

Re: Continuously monitor Power Amp Input or Output to see if it is maxing out

Reply #1 –
Quote
I am thinking of a gadget designed for the job, or maybe of making something based on a USB ADC with some sort of software, or a multi-meter that continuously monitors a signal,
There is a circuit called a peak detector, which at its simplest is a diode and a capacitor.  The capacitor charges-up to the peak voltage and the diode prevents it from discharging.    You can measure the DC voltage and calculate peak power.   Or, once you know the approximate maximum voltage you can get, you can just monitor and mentally compare with the maximum.

As you may know, there is a ~0.7V drop across the diode, and the RMS is 0.707 times the peak.  So make those adjustments when calculating power.   Or if you make a full-wave version, that's a ~1.4V drop.

When testing for the maximum, don't run continuous test-tones or you might burn-out the woofer!  (Run quick bursts.)

I actually ordered some parts to build one (in a box with banana jacks, etc.) but I haven't started it yet.  I added 2 resistors - A ~10 Ohm resistor in series with the diode limits the current so the capacitor doesn't "short out" the amplifier when charging.    And a high-value resistor in parallel with the capacitor to slowly discharge it (depending on the RC time constant).    I think I ordered a 5W, 10 Ohm resistor.   It doesn't have to handle the full amplifier power because current only flows when the capacitor is charging.   (I didn't do any "real calculations".)     

I apologize for describing a circuit with words...  I might have a schematic on another computer and If I can find it later, I'll see if I can attach it.

You can also make an Active Peak Detector with an op-amp, and the diode drop is automatically compensated for by the feedback.   (Of course you need a power supply, and a voltage divider and maybe some over-voltage protection for "speaker voltages".)     Then once you are using op-amps you can add a comparator to light an LED, etc.

...I've made several sound activated (line level) lighting effects using peak detectors and Arduinos.   One of those effects is a "Giant VU meter", but it's not calibrated to anything...  It continuously self-calibrates to the recent peaks for lots of "meter action" no matter the overall loudness.

I've thought about making an audio power meter with an Arduino but to be really useful it should be auto-ranging, and I have to decide on the number of channels, and I'm probably never going to build it.   I can solder-up & assemble the passive peak detector in a couple of hours and there's no programming so I WILL get-around to that someday...

Re: Continuously monitor Power Amp Input or Output to see if it is maxing out

Reply #2 –
Quote from: DVDdoug on
There is a circuit called a peak detector, which at its simplest is a diode and a capacitor.  The capacitor charges-up to the peak voltage and the diode prevents it from discharging.    You can measure the DC voltage and calculate peak power.   Or, once you know the approximate maximum voltage you can get, you can just monitor and mentally compare with the maximum.

That's ingenious! TBH my knowledge of electronics is pretty limited. I guess the diode just has to have a maximum voltage greater than the amp (so 100V/1A would easily cover it). But what sort of capacitor? Also given the peak may only last a millisecond or so, will the capacitor actually charge in that time? And won't the capacitor drain pretty quickly anyway (even if the parallel resistor is removed) - ideally I need it to last for minutes rather than seconds as I don't want to have to keep an eye on it all the way through a movie.

To be honest I had hope for a more off the shelf solution, one that more continuously monitors things so I get more than one data point. But I may well give the above a go!

Re: Continuously monitor Power Amp Input or Output to see if it is maxing out

Reply #3 –
Whatever circuit you use, there can be an impact on the noise and THD figures.  A simple diode-and-capacitor (plus a resistor to gradually discharge the capacitor) peak voltage monitor arrangement will be stealing current from the load in a non-linear manner and needs to be buffered.

Typically, power amps with built-in clipping detectors (they light a LED just before the onset of clipping) compare the output to the speakers with a proportion of the output stage's voltage rails, because the main cause of clipping is when the amp runs out of voltage headroom.  The clipping circuit only cuts in when there is an imminent problem, so distortion hardly matters at that point.

It's not an easy thing to do as an add-on, you can get signal level meters sure, but how do you then calibrate what full scale should be?
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