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Watts aren’t all equal. How to measure an Amplifier’s “Grunt”?

S

strummer

pfm Member
Something I’ve never understood. Why amplifier output is only ever quoted in Watts?

I’ve heard 11 watt Valve amps produce low end grunt that a 150w SS can’t for example.

I hear people say it’s because X amp “delivers more Current”.

If Current is the answer, why do we never see Amps quoted?
 
One way is to see its RMS (assuming honestly measured) power into 8, 4 and 2 ohms loads. If it is a linear voltage source, it will double down into halving impedances. This is why a Krell KSA-50 sounds bigger than its 50W rating suggests.
 
strummer said:
I’ve heard 11 watt Valve amps produce low end grunt that a 150w SS can’t for example.
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The high output impedance of the valve amp reacts with the speaker impedance's resonance to create a bass peak. Unless this is intentional it is a form of distortion. Nothing to do with grunt.

James said:
This is why a Krell KSA-50 sounds bigger than its 50W rating suggests.
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Krell consistently under-specified their amplifiers, so that they appeared to be able to neatly double power into halved impedances.
A KSA-50 would have been something like 60-70W in 8 Ohms, not 50.
 
This depends critically on the PSU as well as the actual amp design. Testing with continuous sinewaves tends to cause some PSUs to drop their output voltage within the first fraction of a second. The steady RMS level may then be well below the initial level. That might seem a bad idea. But allows the designer to tweak the amp to get max output into high impedance loads for *music* whilst helping to protect the amp from being required to survive sustained levels into low impedance loads.

The reality is that much of well-recorded music has a high peak/mean ratio, so the above can mean an amp that can deliver far higher peak powers for *music* than a mere 2x the sustained RMS.

Hence the best way to test genuine ability is to measure the 'tone burst' power available along with RMS. Alas, since no such test has been widely adopted, no-one seems to bother.
 
An amplifier has an output impedance that is frequency dependant and as such can deliver its power with varying efficiency depending upon the load impedance.

So a speaker with an average quoted impedance of 8 ohms might dip below 2 ohms at low frequency and the amplifier would need an ability to deliver a relatively higher current into this load in order to deliver the same power at lower freq than say at 2khz.

Because a valve amplifier generally depends on an output transformer to deliver its power it suffers badly especially in the bass.

Take a look at:

http://sound.whsites.net/valves/valve-trans.html#s21
 
Well, watts are indeed watts, what varies is how amplifiers behave when they run out of clean watts (clip). Many valve amps (better quality ones) clip in a way some seem to find inaudible or, even more often, in a way they actually prefer.
 
Speakers are a reactive load and thats somewhat different from the restive load that is used to measure amplifier Watts. In a resistive load current is proportional to the voltage i.e double the Volts and the Amps also double. Not so in a reactive load where the voltage and current can get out of step (phase). A consequence of this is that as the phase angle increases less power gets into the speaker and more power is wasted heating the output devices in the amplifier. Another problem is that this phase angle and also the speakers impedance changes with frequency.

An amplifier that can cope with the above under all circumstances will be heavier and more expensive to build so compromises are made. I have always said that you should choose an amp to suit your speakers just as you would choose an engine size to suit your car and its proposed use. My ex 6L V12 Jag was effortless to drive and didn't notice hills going up nor down in cruise mode unilke my wifes little car that you have to use the brakes going downhill whilst in cruise mode.

Cheers,

DV
 
The 2x 165w my Audionet amp delivers is much more powerful than the Devialet 220 and 250, it went back to back with a Vitus R100 and Mcintosh MA9000 and came out on top. It has 1500va (2x700va) transformers for the mono amp modules and 2x50va transformers for the preamp and digital sections.
 
strummer said:
Some interesting replies, thanks.






As per the last line of my OP, why is this never quoted?
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It is sometimes but those amps tend to be powerful, heavy and expensive. My own amps are rated at 150A burst into a 1 Ohm load and that equates to 22500W bang!!! In order to get that there are 10 pairs of 16A output devices per channel and 80,000μF of top grade storage capacitors. So size, weight and costs go up.

Cheers,

DV
 
Darth Vader said:
It is sometimes but those amps tend to be powerful, heavy and expensive. My own amps are rated at 150A burst into a 1 Ohm load and that equates to 22500W bang!!! In order to get that there are 10 pairs of 16A output devices per channel and 80,000μF of top grade storage capacitors. So size, weight and costs go up.

Cheers,

DV
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Never get tired of this.
 
Cooky,

Those amps run the Death Star, not speakers. Think you can blow up a planet with some wimpy 3.5-watt SET? Good luck with that, man.

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Joe
 
Arkless Electronics said:
Watts are Watts, current delivery is nowhere near as important as people think and nor are huge power supplies.
As ALWAYS in hi fi there is a huge chasm between "what I read somewhere" and reality...
We've been through all this before and last time, in detail, only a few months back...
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But these are not just any watts, these are hifi watts. The rules are different:
Class A watts are hotter and faster.
Valve watts are fatter and glowier.
Huge power supply watts are huger.
:D
 
Well, watts are indeed watts, but perception is perception and many things influence perception.

Joe
 
Werner said:
The high output impedance of the valve amp reacts with the speaker impedance's resonance to create a bass peak. Unless this is intentional it is a form of distortion. Nothing to do with grunt.
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Years ago I owned a big pair of Tannoy FSM. I tried all sorts of amps on them, ranging up to a 1000w Crown K2. Of course they all went loud, but I wasn’t getting the bass I was expecting from those 4x15”. Just deafening treble.
Then I tried a Black Cat Valve amp. Can’t remember what valves were in it, or whether it was SET or push pull etc.. all I know is it gripped the Tannoys & gave a deep, powerful sound with no nasty treble.

Maybe an amp with High Output Impedance is what I’m looking for? But again, I never see that quoted.
 
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