Know a way to measure amp output?


Does anyone know a way to tell what your amp is putting out at any given time while playing? I want to identify clipping.

I am envisioning some meter I can put between my speaker and the amp terminals. No, this is not a permanent setup, but just to satisfy some curiosity about how much power I really am using as related to the volume and or musical content I am playing.

I have a nice Fluke multimeter… maybe someone can tell me how to use it for my geek need.
I have a PrimaLuna tube integrated now and one of the nice things about tube amplification, is that it is not as easy to hear clipping. With transistor amps, I usually can tel right away… I know I can tell but I was never sure how far into clipping I was… but I deinitly can hear it. Tubes are a bit tougher unless you really crank it.

Bruce in Philly


You really need a scope.


Couldn’t you design a small, cheap product, say with two leads … or something… that sensed a flat topped wave? say a particular frequency content, and lit a lamp? Something we could install temporarily to learn our systems… and then remove?

Doesn’t sound complex.

Bruce in Philly


Radio Shack used to sell audio power meters under their Realistic brand name. Model numbers include APM-100 (analog), APM-200 (analog plus LEDs), APM-300 (LEDs), APM-500 (LEDs). They were connected between your speaker output terminals and the speakers. Unfortunately, they used spring clips for the speaker wire, so you could not use cables with spades or bananas. Also, I doubt the they would handle anything larger the 18 gauge wire.

There are some for sale on eBay, but make sure that it is in good working order and I wouldn’t pay more than $40. For a temporary hookup, you could use some cheap 18 gauge zip cord.


Yeah, Paul is right, you need to see the wave form.


I thought there may be some tricks other than seeing the waveform. A squared-off wave contains some pretty high frequencies… so I read a long time ago… something about the laws of wave physics. No? If so, you can just “listen” for their presence or characteristics and conclude a squared-off waveform.

Bruce in Philly


Yes indeed, a theoretically perfect square wave reaches infinity high frequency (infinite bandwidth) and also contains only odd-integer frequencies. This what gives them a hollow sound (the sound of a clarinet contains a lot of square wave).

And is really hard on tweeters.

I’m not sure most of us can hear distortion unless it is truly bad because a musical signal is so complex.

Are you concerned your amp is clipping or just curious?


The guys are right, if you want to see if there is clipping you need a scope. You can, however, use your multimeter to get a ‘feel’ about whether clipping is likely. Set it to AC volts on a 5v or less range. With music playing at normal volume put the probes between the terminals of one cable (at amp or speaker). Note the sort of average voltage you are getting. Now convert that to a power value by multiplying it by itself and then dividing by your nominal speaker impedance. Most program material has peaks which are around 13db above the average so if you multiply the power you calculated by 20 and the result is less than the rated amp power you are unlikely to hear clipping.

It is a bit rough and ready. You can buy USB scope units for PC or tablet which cost less than $50 if you want to be sure.


Curiosity and neurosis…

Coupla items… I’ve been traumatized and scarred when young… I burned out tweeters. I built these speakers when I was a teenager… wow they were good (at the time)! I since learned, as you pointed out, tweeters burn out more from under powered amps clipping than high power amps.

But also, I want to get the max out of my system and my neurosis tells me if I am clipping, I am not getting the most purity. Also, I don’t want to be stressing my components for longer life. I use a PrimaLuna HP Integrated with KT150 power tubes… I love this amp and configuration and want to preserve its life. It is also a step down in power from my prior amps… VTL Deluxe 300s… with 16 6550 tubes!!! The PLs put out about 100 watts into 8 ohm with the KT150s, the VTLs about 300 watts no load spec’d.

Plus just intellectual curiosity… It is a tough decision when buying an amp… how much power do you really need or want? Over the years, I came to the conclusion that you generally want a high watt, high current amp. These amps just work with whatever speakers or system changes you make in the future. Plus, on those (notso) rare occasions when the Pilsner Urqells get the best of you and you start crankin’ it up… well bad things can happen with under-powered amps.

George Martin in his book “All You Need is Ears” advised watching out for volume creep… Ever enjoy your system for an hour or so, then leave the room to do something… comback and hit PLAY and … yikes … that is LOUD! Why did your perception change? Anywho… knowing the volume that your system starts clipping is a handy thing to know… kinda like bumper stops.

Bruce in Philly


Makes perfect sense, well explained.