Amp Volume Factors

[sws1]

All things being equal (same power tubes, OT, speakers, etc.), would 2 amps sound the same in terms of volume if they had a different preamp?

Specifically, for simple amps which only have 1 preamp tube and a PI before the power tubes, would I get more volume by putting 2 preamp tubes before the PI?

I would think that once the amp is up into the top end of its volume range, the signal is very compressed, so adding another preamp tube is just going to raise the signal going into the power tubes, ultimately compressing them quicker, right? So in essence, you end up with more compression but not necessarily more volume.

Ultimately, what raised this question is that I have 2 18 watt amplifiers that sound very different in terms of loudness (marshall 18 watt and DR Z MAZ 18). I switched the speakers and realized that the speakers were a bit source of the volume difference. But I also noticed that the MAZ has 2 preamp tubes before the PI. Hence, it just got me thinking.

And exactly what is "18 watts" a measure of? plate dissipation? Probably not.

Hoping an amp expert can explain.

 

[Gabriel E.]

I think you're mostly correct regarding compression.

A big thing here is "apparent loudness". While two amps may put out the same volume in decibels, the human ear perceives certain frequencies to be louder than others. Emphasize these frequencies and you'll think the amp is louder.

Try turning down the bass and turning up the treble and mid on an amp and it will sound louder.

 

[aleclee]

Speakers will make a huge diff in loudness. A 3dB swing in speaker sensitivity could theoretically make up for a 100% difference in amp power.

 

[Mark Kane]

You can build a circuit with two preamp tubes that is louder than three preamp tubes. The number of tubes is not the deciding factor.

 

[sws1]

I guess another way of asking the question is...

If the output section of 2 amps are exactly the same, they are considered to have the same wattage. However, if the signal is boosted higher by the preamp BEFORE getting to the power amp then it will be louder.

But this should only happen to a point, because a higher input signal in to the power amp just means that you will hit peak volume sooner on the dial since an output can only go so far before the alternating signal gets distorted.

So, the perceived volume differences between 2 amps claiming to have the same output (when run into distortion) is more due to the frequencies present, and which ones are being compressed?

Simple way of asking... why is the MAZ 18 Jnr so DAMN loud compared to other 18 watt amps?

 

[John Phillips]

It's much more to do with the power supply design than the preamp. You're quite right that once the preamp has driven the power stage to the point of clipping, no amount of extra gain will really make the amp louder (only more distorted).

But the perception of volume has a lot to do with peak power as well as continuous - and if the power supply has a big 'capacity', it will deliver much higher peak power than one with less (in rough terms). I think you will find that the Maz 18 has a very over-spec power supply, with a heavy duty transformer and bigger filter caps than are really needed. I'm not totally sure because it's an amp I'm not familiar with, but this effect is normal. There are other factors as well, but mostly related to the ratio of peak to continuous power the amp will deliver.

Alec is also absolutely right about speaker sensitivity - and it's not theoretical. 3dB greater speaker efficiency really is equivalent to doubling the amp's power (though not the volume). And Gabriel is right about 'voicing' too - the human ear does not have a uniform response, and the same amount of power won't sound the same volume if it is in different frequency ranges (one of the reasons you need so much more powerful bass amps).

BTW, "18W" is the maximum continuous sine-wave electrical power output (if it's described honestly by the manufacturer), and needs to be measured at a specific frequency and amount of allowable distortion really (eg 1kHz, 1%), into the correct load impedance, and is quoted in RMS. Despite the difference in volume, I'd guess that both the Marshall and the Maz are 18W RMS.

IMO, other measurements have a certain BS factor... although it's less important for how loud an amp will 'sound' for single-note stuff (peak is probably more useful). This is the basis for at least one large manufacturer rating their small amps more in terms of peak than RMS, but when I tried one it really couldn't handle continuous rhythm work at the volume I expected from the 'power' rating, no matter how loud it sounded for soloing.

 

[sws1]

Great explanation.

So plate dissipation has nothing to do with power output of the amp. (e.g., 12 watts for each EL84). Different power measurement.

 

[John Phillips]

Plate dissipation does have something to do with it - it limits the amount of power you can get in any mode of operation. But there's nothing to stop you designing an amp which produces less. In fact, the maximum for two EL84s in Class AB is about 22W (which is why Mesa's small 2-EL84 amps are this power). 18W is just a little more conservative, and allows the tubes to be run slightly hotter - necessary in the Marshall which is cathode-biased (you can't get a cathode-biased amp as close to 'limiting' as you can with fixed bias), I'm not sure about the Maz.

 

[nek]

Maz output tubes are cathode biased colder than a Marshall or Matchless. More like the original Vox AC15.