- Jul 7, 2022
- Northern Virginia, USA
There's no telling from here. You can set DSP to monitor average power and limit intelligently when it gets too great. Perhaps someone had that setting on a low power, or it was a default.I'm starting to regret asking......
Does none of the info on the DIY-audio thread ring true?
Two things -The sinewave is reproduced by the amp but as the level is increased the amplifier heavily limits the signal. You don't get anywhere near the potential full output of the amp.
As the title suggested, I don't understand why a sine wave can't be used to test modern, high-power, class d amplifiers.
Some of the large PA amps I've recently looked at have dsp build-in which appear to clamp down any waveform that looks like a continuous sine wave.
Would someone be able to explain the reasoning behind this? Is it the Class D amp or the SMPS that is the issue?
Considering the waveforms of some modern music, does this not affect the audio quality? Some synth music is very sinewave based. How does the DSP differentiate?
So if you can't use a sine wave, how could I perform a frequency sweep of an amplifier? I have one particular amp that has built-in dsp and speaker presets, I'd like to see the crossover point and such.
Thanks in advance for your help.
Thanks very much for the explanation ChrisG.In times gone by, you could connect an amplifier to a resistive load bank, run up a sine wave with a frequency of your choosing, measure the output voltage while watching a 'scope for clipping, and work out the power output.
The amplifiers that did well under this test regime were often large and heavy, because they were designed to pass this sort of testing.
With the demands of:
- More power
- Lighter weight
- More efficiency
- Smaller size (ie, fitting more amps in a chassis or rack)
from the live sound world, some changes started to appear.
All electrical signals have a crest factor, typically measured in dB. This denotes the difference between the peak and average power level in a given signal.
A square wave has a crest factor of 0dB, since the signal spends all of its time at +/-100%.
A sine wave has a crest factor of 3dB.
Pink noise is 12dB
Music can be anything from 40dB to 6dB - the latter being for heavily-compressed program material.
In most cases, it makes very little sense to optimise an amplifier to be able to hit full power, continuously, for hours at a time: that does not happen with real music.
Instead, many manufacturers choose to produce amplifiers which will put out large voltage swings for a few cycles (which is often all that's necessary), but sacrifice the long-duration sine tests to get there.
I have a rack of Powersoft T-series amplifiers. The T604, in some ways, is disappointing: according to Production Partner's rather extensive testing, the 6KW-(total)-rated 4-channel amplifier will only produce 4x 380w into 4ohm, all channels driven.
That's with a 1kHz sine wave, after a minute of testing.
However, that same amplifier will also put out 4x 2634w unclipped peaks, when driven with pink noise.
It's worth noting that, after considering crest factors etc, that the sine wave would actually present more heating power to a resistive load: 4x 380w RMS, vs 4x 324w RMS.
Other tests yield results that are in-between: https://www-production--partner-de...._sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
Now, it is possible to make a class D amplifier that will produce large amounts of sine-wave-power. It's expensive, and almost always unnecessary. I'd much rather have an amplifier capable of 500w average and 5kw peaks, instead of an amplifier that'll put out 1kw sine waves for hours at a time.
It's also worth briefly noting that some genres of music will often present relatively low crest factor signals, particularly in the bass, which can be troublesome for these "squishy" amps. The operators that run those PA systems will often choose the old-school amplifiers, like the Crown MA5002VZ.
Finally, to respond to DVDdoug's comments about speaker ratings: there are standards in place for this stuff, but you have to be careful: to use a tweeter-example, a 100w test signal with a 3.5kHz 12dB/octave highpass filter does not mean your tweeter can take 100w. It means it can only take the highpassed portion of that signal, which is probably about 5w.
Hi, amplifers like the lab gruppen PLM's:
Or the Crown I-tech range:
Try sticking a sine wave into an amp like this, it's not happy. I'm just trying to understand the technical reason why.