When is a Watt, not a Watt

[dincz]

[quote name='Japhet' timestamp='1446458986' post='2899428']
why is it that a 200 Watt valve amp outperforms a 600Watt class D?[/quote]

Their rated powers are measured differently.

[r16ktx]

[quote name='JPJ' timestamp='1446481715' post='2899699']
I have nothing to add here other than to say its threads like this that keep me coming back to this site time and again. Some excellent knowledge imparted here. Thanks chaps (and chapesses)
[/quote]
+1 from me also, thanks for all the time and patience.

[Chienmortbb]

[quote name='dincz' timestamp='1446486503' post='2899773']
Their rated powers are measured differently.
[/quote]No it is not that simple. Lets take say a 400W SS and a 200W Valve amp.

If we measure them both at 200W into an 8 ohm resistor,* they may well both have a distortion of under 1%. There is no standard for distortion for MI use but 1% is the figure I used when I decided on the power unit for my amp. If they are well designed the output will be almost identical.

Push them harder and the SS amp will start to sound nasty and the valve amp will "compress and distort" in a more musical way. TC electronics add this effect to their amps to claim much higher perceived power than the real power of their amps. So you will get a little more usable power from the valve amp and it sounds louder. Louder in the same way the adverts sound louder than the programmes on TV.

Assuming the valve amp has 4 ohm tap on the output transformer, we connect both to a 4 ohm speaker. Now the Valve amp will again output something over 200W but the Solid State amp will be outputting 400W, This would be almost 3dB in power terms and as as 1 db is a just noticeable change in level, 3 dB is obvious but not earth shattering.

Of course what is earth shattering is the weight of the 200W valve amp.

* a resistor is used because loudspeakers are coils of indeterminate values and the impedance (ac resistance) at each frequency differs from driver to driver, cabinet to cabinet.

Edited November 2, 2015 by Chienmortbb

[Bill Fitzmaurice]

[quote name='dincz' timestamp='1446486503' post='2899773']
Their rated powers are measured differently.
[/quote]No, a watt is a watt. A valve amp and SS amp of the same power will both deliver the same voltage and current into a given load. A valve amp can sound louder than an SS amp even though the delivered voltage and current from the valve amp is less because of the alteration of the waveform by the natural compression that valves impart. This begs the question as to whether you can duplicate the effect with SS by using a compressor, and the answer is yes.

[dincz]

[quote name='Bill Fitzmaurice' timestamp='1446492683' post='2899827']
No, a watt is a watt. A valve amp and SS amp of the same power will both deliver the same voltage and current into a given load.[/quote]

Yes, see post #11. They will, but for how long? What I was getting at is the difference between continuous power ratings and burst power ratings. Burst is more commonly used to rate modern SMPS/class D amps.

I'm not suggesting it's the only difference but highlighting the impossibility of predicting perceived volume based on power ratings measured in completely different ways.

Edited November 3, 2015 by dincz

[Nicko]

Well, its taken some time to catch up on that lot. Lots of interesting stuff above, but I'm not sure anyone's really nailed the answer though.

Ultimately a Watt is a measure of rate of energy (Joules per second). If you put 1 watt into a speaker the speaker dissipates the energy. Remember you cant destroy energy. The main outputs of the speaker are heat (due to resistance of the coil) and sound. Given that the heating effect is completely associated with the speaker resistance I'm still lost as to why 1 watt from a MB amp is not the same as 1 watt from a TC through the same speaker. I think there's a possibility that power can be wasted producing sound we cant hear (too high frequency). This would be amp specific. So my conclusion is if Markbass claim to be louder for a given wattage gives us the following possibilities:
[list]
[*]the competitors are overstating their claimed output or markbass are underclaiming
[*]Markbass are telling porkies
[*]Markbass frequency response is different so that we can actually perceive a different volume
[*]theres a bigger difference between the RMS wattage claimed and the peak wattage achieved by markbass compared to others. .
[/list]

[alexclaber]

There are basically two different issues, perceived loudness and actual power - frequency response and distortion will affect your perception of loudness.

There are real differences in power output for a given rating, because there's a lot of variance in how amps are rated nowadays. The biggest variance is how long amps can sustain their full 'rms' power for. That's why for a given power rating, heavyweight amps will tend to have more oomph than similarly priced lightweight amps, so you get full power for a whole big bass note rather than just the start of it.

[r16ktx]

[quote name='alexclaber' timestamp='1446645824' post='2901073']
... heavyweight amps will tend to have more oomph than similarly priced lightweight amps, so you get full power for a whole big bass note rather than just the start of it.
[/quote]

Is this important perceptually? I.e. although it is measurable with some device is it measurable by the average ear, or given that the ear in question has an above average concern with acoustics, an above average ear?

[alexclaber]

[quote name='r16ktx' timestamp='1446646588' post='2901086']Is this important perceptually? I.e. although it is measurable with some device is it measurable by the average ear, or given that the ear in question has an above average concern with acoustics, an above average ear?[/quote]

It's only important if it bothers you! But it does explain why some bassists find lightweight amps frustrating.

This article that I read years ago was what first got me thinking about it: http://www.meyersound.com/support/papers/amp_power.htm

[Bill Fitzmaurice]

[quote name='alexclaber' timestamp='1446651008' post='2901157']
This article that I read years ago was what first got me thinking about it: [url="http://www.meyersound.com/support/papers/amp_power.htm"]http://www.meyersoun...s/amp_power.htm[/url]
[/quote]Referencing the link, here's the issue:
[i]Recently Meyer Sound measured a well-known dual 18” subwoofer system that came with a power amplifier. The amplifier’s power supply rail when it was not being driven sat at 160 volts. Using this rail voltage, we could calculate the instantaneous peak power for a 4 ohm (resistive) load to be: [/i][i] E[sup]2[/sup]/R = 160[sup]2[/sup]/4 = 6,400 watts per channel[/i]

That's not how one measures amp output. The correct method is to feed the amp pink noise, turn up the volume until THD reaches the design spec into the specified load impedance, then measure the output voltage. Anything else is smoke and mirrors.

[deepbass5]

Can you guys Bill/ Alex explain this with ref to qouted or more commonly non quoted Slew rates and in rush currents. Established companies were once proud to quote this which i believe is the ability of an amp to deliver that instand power when the string is pucked. Transformers used must have an affect on this re the old school large transformer / mosfet amp to those used on class D. or can both have good response.

[Chienmortbb]

I hope I will do.

Inrush current is the extremely high instantaneous current that happens at switch on. It is determined by the power supply design and/or technology. It wan't make a jot of difference to your sound but, if not controlled may affect the reliability.

Slew rate is the speed at which an amplifier can change over time. Usually measured in volts/uS (micro seconds).. As far as I know, most modern amps have an adequate slew rate for bass.

Early op amps like the 741 used in many early SS amps had a slew rate typically of 0.5V/uS. Generally considered too low for audio. Industry standard Audio opamps like NE5532 have a slew rate of 9V/uS. Of course the slew rate of a complete bass amp is more important than an individual component, and is rarely quoted.

Edited November 6, 2015 by Chienmortbb