Amplifier volume/gain structure comparisons - an attempt to demystify

[12bass]

Hey Everyone,

Some time ago there was a considerable amount of discussion about how people perceived that the Shuttle 6.0 was not as loud as the Little Mark II. Upon further investigation, it became apparent that these two amplifiers have drastically different amounts of gain at similar knob settings. So, while the Little Mark II was nearly maxed out at 1/2 rotation, the Shuttle 6.0 was not even close to reaching full volume.

Interestingly, my Eden WT-500 came from the factory with a C-taper master volume control. The C-taper means that the output volume potentiometer increases in an anti-logarithmic fashion, with the implication being that the output gets loud very quickly and the bulk of the master volume adjustment is crunched into the first 1/3rd of the knob's rotation. This gives the impression that the amp has a lot of power, but in reality there's little extra gain available after about 1/2 rotation of the master volume control. I suspect amplifiers like the Little Mark II use a similar setup.

Rather than focus on the specific amplifiers above, I thought it might be worthwhile to discuss this issue in more general terms, in the hope that it might help clarify some aspects of gain structure. An important point to take away is that, due to discrepancies in how amplifier gain structures are set up, comparing two or more amplifiers with similar knob positions does not give a fair comparison of their output potential.

[KJung]

While the LMII still outperforms the Shuttle 6 when both are pushed to their maximum 4ohm volume IME, you are correct that you cannot use your eyes when adjusting the maximum volume, due to different taper designs.

The only way to truly compare performance of two amps is to compare max volume.... i.e., the volume the amp can achieve when pushed to compression or limiting. Even that gets difficult, since clipping a preamp and also speakers exceeding xmax can sound VERY much like an amp going into safety limiting.

The issue with the Shuttle preamp is that the clip lights continue to confuse people. Having a clip light on a tube pre makes little sense, and should be ignored. And, the Shuttle power amp clip light indicates 'tube emulation circuitry' kicking in, not maximum clean output, so that should be ignored also.

Most amps, (GK, MB, Eden, Mesa, etc., etc.) have a typical linear type taper on the input and master volume (the terms are counterintuitive, since a 'linear' pot actually results in that big, early volume bump, since db's are non-linear).

Amps like Glock, TecAmp, EA use some form of audiotaper (i.e., a non-linear pot that results in the volume increase of the log DB scale being 'linear'...i.e., every throughout the full range of pot travel), and those knobs need to be cranked (i.e., my Puma does not even start to open up until the master is at about noon or 1 o'clock).

Never use your eyes on a master volume control when comparing the output of amps.

[jnewmark]

Far be it for me to demystify this topic, but I've always wondered why, if gain structures are so different from amp to amp, why manufacturers continue to put numbers on those dials, as if, the higher number will give you the most gain ?

[KJung]

Originally Posted by jnewmark

Far be it for me to demystify this topic, but I've always wondered why, if gain structures are so different from amp to amp, why manufacturers continue to put numbers on those dials, as if, the higher number will give you the most gain ?

The numbers are just indication of level within a given amp. They are useful for the amp user to quickly remember the input gain setting for different basses. They are useless (as is the physical position of the knob) for comparing settings across different amp manufacturers.

[12bass]

Originally Posted by KJung

... Most amps, (GK, MB, Eden, Mesa, etc., etc.) have a typical linear type taper on the input and master volume (the terms are counterintuitive, since a 'linear' pot actually results in that big, early volume bump, since db's are non-linear).

IIRC, the Eden's input volume is linear taper, while the master volume is anti-logarithmic, which is even louder in the first part of the rotation. Actually, I replaced the master pot with an regular logarithmic taper, but I think that linear would work better for my taste.

One of the reasons why manufacturers do this is so that their amplifiers appear louder than competitors in the showroom. If it's this loud at "2", just imagine how deaf I'd be if I turned it to "11"!...

[jnewmark]

It's even more confusing when you us a pre/power amp rig, where you have a pre gain, a master volume, and attenuators on the power amp. I once asked a tech guy from Crown how to set these controls properly. He told me the last thing to set was the power amp. Get the preamp where you want it, then set the power amp. But, in order to hear how to set the pre and master, you have to have some nominal setting on the power amp to actually hear what the preamp sounds like. I asked him where I should set the power amp to begin with, and he said I should'nt set the power amp at all until I had the preamp set where I want it ! Talk about confusing.

[billfitzmaurice]

Originally Posted by 12bass

One of the reasons why manufacturers do this is so that their amplifiers appear louder than competitors in the showroom. ..

If there's another reason I can't come up with it.

[KJung]

Originally Posted by 12bass

IIRC, the Eden's input volume is linear taper, while the master volume is anti-logarithmic, which is even louder in the first part of the rotation. Actually, I replaced the master pot with an regular logarithmic taper, but I think that linear would work better for my taste.

One of the reasons why manufacturers do this is so that their amplifiers appear louder than competitors in the showroom. If it's this loud at "2", just imagine how deaf I'd be if I turned it to "11"!...

+1 Eden is notorious for this, as is GK. It makes it seem like the amps are super powerful to the kid at Guitar Center.

As long as you know this is the case, I don't really think there is a need to change the pot... just realize that 'it is what it is'. That being said, I do like the longer taper of an audio taper type pot, and it makes doing subtle volume adjustments very easy compared to the 'jump' of the GK and Eden pots. The Genz and MB tapers seems to fall in the middle of the pack of other amps... not full audio for sure, but still with enough play to be fine for most.

[BbbyBld]

One of the reasons why manufacturers do this is so that their amplifiers appear louder than competitors in the showroom. If it's this loud at "2", just imagine how deaf I'd be if I turned it to "11"!...

It seems to work. I've probably seen about a million posts on this board that say something similar to this, as if the clock scale around the volume know represents the output power level !

[12bass]

Changing to an audio (log) taper on the master volume makes it so that there's a lot of subtle volume adjustment up to 12:00, then proportionally fairly large increases after that. I think that linear taper would sound more... uh... linear.

[KJung]

Originally Posted by 12bass

Changing to an audio (log) taper on the master volume makes it so that there's a lot of subtle volume adjustment up to 12:00, then proportionally fairly large increases after that. I think that linear taper would sound more... uh... linear.

I'm over my head on the technical terms, but my understanding is that a linear taper is the least linear sounding, since db's are based on a non-linear scale. So, a linear taper will actually sound 'non linear', since it does not take into account the non-linear nature of how humans perceive volume increases (as taken into account by the non-linear db scale).

Is this correct (I love this topic, since, as posted above, the vast majority of people use their eyes on the master volume to determine if an amp 'kicks a** or not')

[KJung]

Originally Posted by 12bass

Yes, logarithmic taper is supposed to be closer to how we perceive volume changes. However, sometimes the adjustment range of linear potentiometers sounds better. And with linear pots, 1/2 rotation means 1/2 resistance.

Over my head there However, at my 'layman's understanding' level, I agree with your entire premise. Don't use your eyes and the physical setting of the master volume control to determine amp performance.


I also appreciate the much more exact functioning of the more 'audio taper' type controls, even though it takes a little bit of 'relearning' to feel OK about an amp putting out less than full volume even with the master at 3 o'clock (like the EA amps, the TecAmp amps, and the Glock amps, etc.).

And your comment about limiting in the WT800 thread is key also.... different designs can make the 'maximum volume' of different amps behave much differently (i.e., the TC RH450 just keeps getting louder but will eventually grind and 'give' a bit due to the 'tube amp emulation limiting', while, for example, the Markbass limiting circuit keeps the amp sounding the same all the way through its rated specs, but then starts to significantly compress the signal when that limit is reached... to my ears anyway). GK has yet another design where there really doesn't seem to be any safety limiting... they will just keep getting louder and then eventually shut down when really pushed beyond their limits (IME again).

[12bass]

Yes, logarithmic taper is supposed to be closer to how we perceive volume changes. However, sometimes the adjustment range of linear potentiometers sounds better. And with linear pots, 1/2 rotation means 1/2 resistance. Anti-logarithmic is called "reverse log".

Here's an illustration of the various potentiometer tapers:

[billfitzmaurice]

Originally Posted by 12bass

I think that linear taper would sound more... uh... linear.

Exactly the opposite, as the sensitivity of the ear isn't linear, it's logarithmic.

[KJung]

Originally Posted by billfitzmaurice

Exactly the opposite, as the sensitivity of the ear isn't linear, it's logarithmic.

+1 Bill, again over my head, but I think the plot he put up with 'resistance to voltage' does not correlate exactly with increase in perceived db's (which is on a log scale). Is that correct? I assume voltage is a linear scale, which would not be useful in determining the impact of these pots on actual perceived volume.

I think that might be where the confusion comes in.

I believe the 'audio taper' is the design that results in the most linear impact on db's (i.e., in yin yangs the log db scale resulting in a linear (straight line) correlation between knob position and perceived volume).

[12bass]

Originally Posted by billfitzmaurice

Exactly the opposite, as the sensitivity of the ear isn't linear, it's logarithmic.

Yes, I realize that... but as described above, the replacement log master volume doesn't sound linear to me. IIRC, I used a high quality dual from Mouser. Originally it was reverse log. As it is (using log), the gain increases sound disproportionately large after 12:00. From my experience, I think that linear taper would represent the "happy medium".

[billfitzmaurice]

Originally Posted by 12bass

the replacement log master volume doesn't sound linear to me

If it doesn't then something else is causing that to happen. Audiophiles think nothing of paying out $1k or more for stepped ladder pots loaded with 1% precision resistors just to be sure that the taper is perfectly logarithmic... within 1%, that is.

I assume voltage is a linear scale, which would not be useful in determining the impact of these pots on actual perceived volume.

True. Doubling of voltage translates to 6dB. But it takes 10dB to result in doubling of volume.

[agedhorse]

It's clarification time again...

First of all, to correct for accuracy, we (typically) use a full audio taper (normal log slope) for gain, an audio log cheated linear taper for volume and linear taper for master volume.

Why do we choose what we do?

It's to provide the right feel and adjustability, the right width of scale for the most commonly used operating portion of the control. Most users prefer smooth adjustability over a wider range of the control and find rapid increases or decreases of level to be undesireable within the normal operating range. The reason why the log functionis used is because in a voltage divider circuit (typical use), the power is the square of the voltage so to linearize the power (loudness) equation, there needs to be a log function inserted in the gain/volume adjustment.

Now it's been presented simplisticly here that there is an audio/log taper, linear taper and a reverse log taper. That's only partially true. Each generalized taper class has multiple slopes between the classes, you can have a taper that's between full log and linear for example. Another way to alter the feel is to cheat the curve by altering the loading conditions of the pot as it changes position. A good designer uses all of these design techniques to develop a product that feels good to the widest range of players.

Of course, as it has been pointed out, some manufacturers do use these tapers to generate a feeling of "big power" by choosing a taper to make the volume increase rapidly at first but the tradeoff to this is that for most users this makes the amplifiers much more difficult to "fine adjust" and in some cases can appear to feel granular when adjusting.

To answer another question that I know will be asked, if audio responds to audio or log curves, why use a reverse log pot, which seems to be the opposite of what would be desireable... this would be true for use as a traditional volume pot but there are pots in amps that adjust other transfer functions like frequency which is also a log function but depending on the circuit topology might require the change to track in a reverse log slope to linearize. This is somewhat common in tone control and timing networks linearizing frequency with knob position.

[rpsands]

The stuff early in the thread needs to be stickied somewhere. In fact, maybe this whole thread could be stuck in the FAQ.

For the record, I've owned an MB500, F1, and Shuttle 6, and didn't notice any serious volume difference between the three (in terms of maximum) except that the Shuttle 6's preamp made sounds I do not care for when turned up to super loud.

Once excursion limited power handling gets totally demystified and people stop thinking their 500 watt speakers can take 500 watts without regard to db's of bass eq, gain staging will be the final frontier of bass amplification. Perhaps then folks will stop with the "My 500 watt amp isn't as loud as amp company Y's 500 watts!"

[agedhorse]

Originally Posted by rpsands

Once excursion limited power handling gets totally demystified and people stop thinking their 500 watt speakers can take 500 watts without regard to db's of bass eq, gain staging will be the final frontier of bass amplification. Perhaps then folks will stop with the "My 500 watt amp isn't as loud as amp company Y's 500 watts!"

... and pigs will begin to fly