Nigel Goodwin

Just a little something to think about? (particularly Audioguru!).

I've just bought a Behringer PA mixer/amplifier, it's rated at 400W RMS per channel, and can be set to either stereo, dual mono (house and monitors), or mono bridged (800W RMS).

It's very small and light (two of the reasons I bought it), and uses a switchmode PSU. It also incorporates Behringers's "patented cool audio technology", which presumably means it's a class 'D' 'Digital' amplifier?.

Being the sort of person I am, I've stuck the scope on it to see what it does - for a start on the outputs there's 440mV 500KHz sinewave, confirming it's a class 'D' switching amplifier.

Now being a 'digital' amplifer it's EXTREMELY important that you don't clip the outputs (just as it's vital when recording a CD), so as such it limits before clipping!.

Now this wouldn't be a problem?, EXCEPT an anlogue amplifier doesn't usually limit like this - and a 400W RMS amplifier will happily provide 800W peak (hence the absurd claims of home and in-car amplifiers). The class 'D' amplifier can't do this, so the 400W amplifier can only supply 400W peak.

This means that an analogue amplifier will sound considerably louder than a similar specified digital one - or even a considerably higher specified digital one.

I consider this means you can't compare the ratings of analogue and digital amplifiers?, and I'll be interested to see how loud it is in practice when I have to do the PA for my daughters band at their next gig.

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Someone Electro

I didnt thogt il ever see nigel start an therd and asking a question. :shock:
Becose you almost anways know almost evrything.

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David Bridgen

If the amplifier can deliver 400W peak then the claim of 400W "r.m.s." is clearly fraudulent. The average power will be 200.

400 watts, only 3dB greater, will not be "considerably" louder.

Nigel Goodwin

No, it CAN deliver 400W RMS, but the constraints of a 'digital' amplifer prevent the voltage swing on the output exceeding that level (it limits before clipping can be reached). So the peak output is no higher than the RMS one.

With an analogue amplifier the peak output voltage swing can go much higher than the RMS one can - with a continuous sinewave for RMS measurement the power supply drops considerably, which limits the RMS power.

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David Bridgen

I don't see it Nigel.
With a given supply rail the peak output voltage will be, ignoring output device constraints, the same value as the rail, and the peak power calculated from that.
With the same rail, the r.m.s. voltage, from which the power is calculated, will be 0.707 of that value. This is true irrespective of the class of operation.

Powers supply sag shouldn't be a problem in a properly designed amplifier and as far as this discussion is concerned is a red herring.

By the way, what do you mean by "limits before clipping can be reached."

Nigel Goodwin

It's vitally important that a digital audio system, either power amplifier or a CD recorder doesn't clip - for example a 16 bit CD recorder would suddenly change from 65535 to zero, as the 16 bit value overflows. With a switching amplifier (class D) it's not quite the same, but if the output stage clips it leaves the relevent transistors turned ON for much longer than the switching time, which could well spell disaster?.

In order to prevent this happening the amplifier includes a 'limiter', if the output is getting too large it will clip the input, keeping the output stage below the clipping point.

400W RMS sinewave into 4 ohms requires 113V p-p, which presumably the amplifier will deliver? - so it's a perfectly accurate 400W RMS amplifier. If you try to drive the amplifier harder, the limiter will kick in, preventing the output exceeding 113V p-p.

If you now drive the amplifier with music, fairly quiet, with loud peaks, and a peak tries to exceed 113V, the limiter will kick in again limiting the output to the same 113V - except now it's limiting on a short peak, rather than a continuous sinewave.

This isn't unique to the Behringer amplifier, it's likely to be common to any similar class D PA amplifier - certainly the others I know about all do the same.

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David Bridgen

Irrelevant. You aren't going to drive the amplifier with a digital signal

Clips what? It's switching on and off.

So it clips the analogue input signal? Nasty.


Ok. And when it is developing/delivering that 400 watts the peak power is 800.

Nigel Goodwin

It's the same principle, the problems of handling analogue signals in a digital fashion.

Clips the drive capability of the output stages, as with the CD recorder example you didn't like, going beyond the available limits - leaving either top or bottom transistors turned permanently ON.

I'm presuming that's what it does, I don't have a circuit.

Point taken - if you wish to be pedantic 8) - but that ISN'T what peak power of an amplifier is about!. Perhaps I should rephrase it for you as peak RMS power? - the amount of power the amplifier can supply in a short period, as opposed to continuous sinewave power.

In any case peak is only 1.414 times RMS, so it would only be 566W, not 800W.

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David Bridgen

Peak voltage is 1.414 r.m.s.
Peak power is twice average.

audioguru

If you turn-up the volume on a 400W RMS analog amplifier, way too high, the output will be a square-wave producing 800W in the load. Both the load's voltage and current are multiplied by 1.414 since they are continuously at the peak.
The class-D amp limits cleanly, probablly with a Fet AGC, so if it is turned-up too high it will sound like a compressed radio station. I prefer clean compression over gross distortion.

Your hearing might be shocked by the analog amp's gross over-drive distortion and make you think it is much louder than only 3dB.
When undistorted as David says, 3dB (twice the power) is only a little louder. Ten times the power sounds twice as loud since your hearing is logarithmic (leaves rustling at a distance, very loud rocket take-off nearby). :lol:

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Uncle $crooge

audioguru

Maybe Nigel is talking about the "music-power" output of an amplifier. It is its instantaneous power capability before its power supply voltage sags.
But maybe the class-D amp has the same thing.
In any event, the difference in volume is very small unless its power transformer is a dinky little thing, like Hee-Fee Steereo Consoles in the '70's. :lol:

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Nigel Goodwin

Yes, I'm aware of that :lol:

I haven't had chance to test it yet (except in my attic, and you can't really compare loud music in a small room!). But from threads on Usenet a number of people are complaining about high power digital amplifiers not sounding as loud as much lower powered analogue ones.

You have to bear in mind that musicians commonly run their amps EXTREMELY loud, and while distortion obviously increases horribly (from a HiFi point of view) it's not a problem with live music (unless it's REALLY over driven).

The same 'problem' exists with valve amplifers against transistor ones, with a 60W valve guitar amplifier often being louder than a 300W transistor - again, you can seriously over drive it without 'objectionable' distortion.

I suspect the apparent less volume of digital amplifiers is simply down to the fact you can't over drive them?, but when I get the chance I'll give it a good test!.

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audioguru

Hi Nigel,
There's the huge difference in volume.
When designing a stadium or PA sound system, I allow a 10dB headroom so that the system can produce occasional peaks of 10 times the power without distortion (it also sells more amps :lol: ). Maybe the AGC on the digital amps also allows a 10dB headroom so that it attenuates the input that much upon the very 1st peak then releases the AGC very slowly.
Musicians (you mean the ones who strum square-wave generators? :lol: ) use-up the headroom and more. So their analog amps produce actually 20 times more power than a same-rated digital amp that has AGC.

Digital amps should strike a happy compromise by using a fast-releasing limiter instead of AGC. Then only the occasional peaks would get squashed instead of all the input for most of the concert.
Try it with the Behringer digital amplifier, scream into the mic then see how long it takes for its AGC or limiter to return the gain to normal. :lol:

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