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alexclaber

Gain, power and volume - a confusing ménage à trois...

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I found myself writing something relatively coherent about this in the bass guitars forum so I thought I'd move it over here:

In response to Parker's question about his bass being too quiet for his amp to be heard when rehearsing:

"It is very rare for an amp to have insufficient gain to reach full volume even with a very quiet bass. The only way you'll get a 35W bass amp to be heard over a drummer is by sticking the amp in the corner of the room for maximum bass reinforcement, cut back your lows, add midrange, and give the drummer hotrods or very light sticks.

Gain and volume (or more accurately Sound Pressure Level) are two very different things. It works like this:

Movement of strings due to plucking generate peak voltage of, say, 1V in the pickups. This goes to the preamp part of the amp which adds, say, 5x gain, with the knobs at 12 o'clock. Thus the voltage is now 5V. This goes to the power amp section which adds gain of say, another 5x, with the knobs at 12 o'clock. The voltage is now 25V. This power amp is driving an 8 ohm load. 25V into 8 ohms equals (25x25)/8=78W.

Now, say this amp has a maximum output of 100W. This means its maximum voltage output is 28V. Let's say that if you turn the preamp gain to max you get 10x gain. Let's say that if you turn the power amp gain to max you get 10x gain. This means that the amplifier has a maximum gain of 100x. So if you put your 1V signal in the amp will try to put 100V out - BUT IT CAN'T because the maximum voltage output is only 28V.

If you put a preamp or booster in front of the amp you might be able to put 10V into the preamp but you still won't be able to get more than 28V out of the power amp.

If an amp is not loud enough, no amount of louder effects pedals, outboard preamps, pickups, will make it louder.

There is one thing that will make it louder and that is more sensitive speakers, or simply MORE speakers! The more sensitive the speakers the more dB SPL out you will get for the voltage in. So if you were to plug the power amp output of your little 35W combo into a very large efficient speaker cab or two (like a BFM DR280 on top of a Titan 48) then you would actually get enough volume to easily keep up with a drummer. But you wouldn't want to have to move that..."

Alex

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Nice one, Alex. Coherent indeed.

The mention of Gain and Volume brings to mind one of my pet bugbears: the habit that many manufacturers have of mis-labelling the controls on amplifiers.

First let's deal with the worst culprits. It seems to be a commonly held view among certain companies that in selling to musicians, they are ipso facto selling to idiots who are easily impressed by shiny objects and flashing lights, and to whom they can pretty much spin any nonsense they wish. This leads to products on which perfectly normal Signal Level and EQ controls have been given stupid, subjective names like "bite", "heat", "balls" etc. Does anyone really find such descriptions useful? I doubt it. Personally, I just find them embarassing, and I'm sure I'm not the only one who immediately ignores any product with such twaddle printed on its panel.

However, even among those companies who avoid such inanities, there is still a common practice of mis-labelling going on, and one that leads to endless confusion among non-technically minded users. I'm talking about all those amps (including well-respected, high-end products) that have an input level control which, bizarrely, is labelled "Gain", and may well be accompanied a bit further down the panel by another knob marked "Master Volume", "Master Gain" or "Output Level". In nearly every case, such labels are wrong!

And let's not forget that old favourite, "Volume Control". We're all used to saying that, aren't we? Yet even that is a piece of 'creative' labelling - a hangover from the days of domestic wireless sets.

In a typical instrument amp, the first "Volume Control" you find is simply a pot placed in the signal path - just like the one we find in a passive guitar or bass, between the pickups and the jack socket. All it does is act as a [i]potential divider[/i]: a variable resistance that bleeds some of the signal away to earth and allows the rest through to the next amplifying stage. Turn it up full, and all (or nearly all) of the signal gets through. Like a water tap, it's a purely passive device. It can't give out more than is being fed in. In some amps this first pot is positioned directly after the jack input itself, but more commonly these days it is placed after an initial amplifying or buffer stage. Either way, the effect is the same.

Likewise, the "Master Volume" or "Output Level" control is another passive pot, placed at the point where the signal leaves the pre-amp/EQ circuitry and is being fed to the input of the Power Amp.

What it [i]doesn't[/i] do, in either position, is alter the GAIN of the amplifying stage of which it is part ...but that doesn't stop some manufacturers calling it a "Gain" control.

Confused? I'm not surprised!

So, just for the record:
A true Gain control works by modifying the operating conditions of an amplifying device (varying a DC control voltage on an Op-Amp, for example) and in so doing actually determines how much gain that device can apply to whatever signal it is being fed. Genuine Gain controls are usually only found on professional studio equipment, mixing desks and so on. They are not used alone, or instead of passive pots. Both are used together, as they have different roles to perform. They're part of the variety of control options that make such equipment flexible enough to accept and process signals from the widest possible range of sources, and do it efficiently, with the best possible signal-to-noise ratio.

I think that's it. Don't want to hijack Alex's excellent thread. I'll shut up now. Edited by Oxblood

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I don't think you were hijacking it at all. I think it was a useful addition to a very useful thread.

I've seen amps labelled:
Gain / Volume
Gain / Output
Gain / Master
Volume / Output
Volume / Master

I'm pretty sure I've seen Input and Level around the place too.

I just work on the basis that the one on the left controls the preamp stage and the one on the right controls the power amp stage. Edited by The Funk

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[quote name='The Funk' post='39996' date='Aug 1 2007, 01:57 PM']I don't think you were hijacking it at all. I think it was a useful addition to a very useful thread.

I just work on the basis that the one on the left controls the preamp stage and the one on the right controls the power amp stage.[/quote]
+1 On both counts.

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Great thread! [quote name='alexclaber' post='39654' date='Jul 31 2007, 06:23 PM']Now, say this amp has a [b]maximum output of 100W[/b]. [b]This means its maximum voltage output is 28V[/b]. Let's say that if you turn the preamp gain to max you get 10x gain. Let's say that if you turn the power amp gain to max you get 10x gain. This means that the amplifier has a maximum gain of 100x. So if you put your 1V signal in the amp will try to put 100V out - BUT IT CAN'T because the maximum voltage output is only 28V.[/quote]

Could you go through this again, please? dungeddit :)

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Great thread! [quote name='alexclaber' post='39654' date='Jul 31 2007, 06:23 PM']Now, say this amp has a [b]maximum output of 100W[/b]. [b]This means its maximum voltage output is 28V[/b]. Let's say that if you turn the preamp gain to max you get 10x gain. Let's say that if you turn the power amp gain to max you get 10x gain. This means that the amplifier has a maximum gain of 100x. So if you put your 1V signal in the amp will try to put 100V out - BUT IT CAN'T because the maximum voltage output is only 28V.[/quote]

Could you go through this again, please? dungeddit :)

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[quote name='Cantdosleepy' post='49264' date='Aug 23 2007, 12:28 AM']Could you go through this again, please? dungeddit :)[/quote]
Power = voltage squared divided by resistance.

Therefore 100W = 8 Ohms / Voltage squared.

So, voltage squared = 100 x 8 = 800

Square root of 800 is 28.3


That any clearer? [url="http://www.allaboutcircuits.com/vol_1/chpt_2/4.html"]This[/url] may help.

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[quote name='BlueBear' post='51317' date='Aug 27 2007, 03:59 PM']Power = voltage squared divided by resistance.

Therefore 100W = 8 Ohms / Voltage squared.

So, voltage squared = 100 x 8 = 800

Square root of 800 is 28.3
That any clearer? [url="http://www.allaboutcircuits.com/vol_1/chpt_2/4.html"]This[/url] may help.[/quote]

erm now i'm confused
you say voltage squared but where did you get the voltage figure from, the wattage figure was 100

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[quote name='lowhand_mike' post='52903' date='Aug 30 2007, 05:44 PM']erm now i'm confused
you say voltage squared but where did you get the voltage figure from, the wattage figure was 100[/quote]

Wattage (Power?) X Resistance = Voltage Squared, according to the equation above.

So if the Wattage is 100W and the Resistance is 8 ohms, then the Voltage Squared = 8x100 = 800.

So then to get the actual voltage you have to take the square root of that figure, ie. 800.

And the square root of 800 is 28.3 - capisce? Edited by The Funk

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[quote name='The Funk' post='53346' date='Aug 31 2007, 01:42 PM']capisce?[/quote]

Your boat falls over?

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[quote name='alexclaber' post='39654' date='Jul 31 2007, 06:23 PM']There is one thing that will make it louder and that is more sensitive speakers, or simply MORE speakers! The more sensitive the speakers the more dB SPL out you will get for the voltage in. So if you were to plug the power amp output of your little 35W combo into a very large efficient speaker cab or two (like a BFM DR280 on top of a Titan 48) then you would actually get enough volume to easily keep up with a drummer. But you wouldn't want to have to move that..."

Alex[/quote]
I think it is worthwhile pointing out that in general your amp should have a higher wattage than your cab(s). If you have a 100w head and a 400w cab it will work fine but you'll still only perceive the volume to be something close to the rating of the amp. The temptation would then be to crank the amp past its most efficient output level to get more volume. Most amps tend to work best when the output/master volume is between 40% and 60% - any higher and the amp runs out of headroom and will start distorting. Much better to have a 500w head with the output at 50% driving a 300w cab than a 300w amp at 90% driving a 500w cab. Obviously you need to be aware of the output from the amp so you don't blow the speakers!

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[quote name='Oxblood' post='39703' date='Jul 31 2007, 09:26 PM']...It seems to be a commonly held view among certain companies that in selling to musicians, they are ipso facto selling to idiots who are easily impressed by shiny objects and flashing lights...[/quote]

Mmmmmmm flashing lights and shiny knobs. Yup, that's pretty much all I look for when buying gear :)

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[quote name='XB26354' post='195878' date='May 10 2008, 10:42 AM']I think it is worthwhile pointing out that in general your amp should have a higher wattage than your cab(s). If you have a 100w head and a 400w cab it will work fine but you'll still only perceive the volume to be something close to the rating of the amp. The temptation would then be to crank the amp past its most efficient output level to get more volume. Most amps tend to work best when the output/master volume is between 40% and 60% - any higher and the amp runs out of headroom and will start distorting. Much better to have a 500w head with the output at 50% driving a 300w cab than a 300w amp at 90% driving a 500w cab. Obviously you need to be aware of the output from the amp so you don't blow the speakers![/quote]

I think you need to re-read my posts above, you've missed quite a few critical points and thus are pretty much incorrect on all counts.

Alex

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hmmmmmmmm....

as my main instrument is guitar and not bass, maybe this is a little annoying to some, but the gain, volume; gain, output etc make perfect sense to me...

most amplifiers, particularly most modern amplifiers that incorporate such controls have several gain stages... whilst a 'passive pot' is the control, you will hear the gain increase if you turn up the gain or preamp gain on a marshall amp... the reason is (valves) there is a greater signal going into 'control' grid of next preamp tube stage, this alters the bias condition of the tube and will amplify more or less until it reaches clipping.... whilst this is not often clipping with two valve preamp stages, that signal then goes on to another one or two stages before reaching the volume or master volume control.... most of the traditional circuits have the volume or master volume as part of the preamp, that is before the phase inverter and some debate about validity of this...

But yes there is a lot of marketing hype... I don't avoid amps with such labels, they can be a bit silly, but some of them do have quite interesting features on them that act on the circuit in new ways from the amp icons we are familiar with and possibly create new tones...

Just like everything else, you pay your money and you often get what you pay for...

I think the labelling often reflects the goal of the designer... gain and master tend to suggest a preamp distortion created by the gain control and more power amp distortion created by the master volume....

Perhaps the volume and master volume scenario is aimed at the cleaner (less distorted) end of the market? Once you understand what the controls are doing and see through silly marketing, its no biggie and sometimes there are nice surprises... but I get what you are saying with some companies that excel at the marketing and hype - that's entertainment.... or did I mean capitalism?

How should these controls be labelled?

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[quote name='alexclaber' post='204677' date='May 23 2008, 12:40 AM']I think you need to re-read my posts above, you've missed quite a few critical points and thus are pretty much incorrect on all counts.

Alex[/quote]

But aren't drivers supposed to sound better if feeded an amount of power closer to their RMS rating?
In other words, if you have a 4x10" with 150W RMS drivers (600W total), wouldn't they respond better with 200W instead of 50W? Or does it depend on the driver? I find that my Ashdown 4x10" needs to be kind of cranked to start sounding "alive". Same thing for the Ampeg 8x10", there was a gig where I had a Peavey head + Ampeg 8x10" as a backline, and I had to keep the volume really down for several reasons (but REALLY low, most probably under 50W), and my tone was extremely dull. And trying to compensate with EQ was useless, since the cab seemed not to budge from that lifeless tone. Edited by Boneless

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[quote name='Boneless' post='539128' date='Jul 13 2009, 09:53 AM']But aren't drivers supposed to sound better if feeded an amount of power closer to their RMS rating?[/quote]

No. The more power you put into them, the more they distort, from 0.1W to full power and beyond.

[quote name='Boneless' post='539128' date='Jul 13 2009, 09:53 AM']In other words, if you have a 4x10" with 150W RMS drivers (600W total), wouldn't they respond better with 200W instead of 50W? Or does it depend on the driver?[/quote]

Response will be the same if the amp isn't distorting, just the 200W amp will go 6dB louder.

[quote name='Boneless' post='539128' date='Jul 13 2009, 09:53 AM']I find that my Ashdown 4x10" needs to be kind of cranked to start sounding "alive". Same thing for the Ampeg 8x10", there was a gig where I had a Peavey head + Ampeg 8x10" as a backline, and I had to keep the volume really down for several reasons (but REALLY low, most probably under 50W), and my tone was extremely dull. And trying to compensate with EQ was useless, since the cab seemed not to budge from that lifeless tone.[/quote]

In that case you're relying on speaker distortion to get your tone. That isn't a problem, most guitarists do, but many bassists don't.

Alex

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[quote name='Oxblood' post='39703' date='Jul 31 2007, 08:26 PM']In a typical instrument amp, the first "Volume Control" you find is simply a pot placed in the signal path - just like the one we find in a passive guitar or bass, between the pickups and the jack socket. All it does is act as a [i]potential divider[/i]: a variable resistance that bleeds some of the signal away to earth and allows the rest through to the next amplifying stage. Turn it up full, and all (or nearly all) of the signal gets through. Like a water tap, it's a purely passive device. It can't give out more than is being fed in. In some amps this first pot is positioned directly after the jack input itself, but more commonly these days it is placed after an initial amplifying or buffer stage. Either way, the effect is the same.

Likewise, the "Master Volume" or "Output Level" control is another passive pot, placed at the point where the signal leaves the pre-amp/EQ circuitry and is being fed to the input of the Power Amp.[/quote]

I guess they call it a "volume control" because it is a control that changes the volume?

I don't see how else an "output volume" could be achieved - sticking a potentiometer AFTER the power amp would need seriously heavy duty parts, would mess with the amplifier load and would generate a lot of heat.

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[quote name='Protium' post='583636' date='Aug 28 2009, 03:27 PM']I don't see how else an "output volume" could be achieved - sticking a potentiometer AFTER the power amp would need seriously heavy duty parts, would mess with the amplifier load and would generate a lot of heat.[/quote]

I've yet to dig into the details of amplifer design but I presume you could design an amp which is variable gain by taking stages in and out of the signal path or by changing the rail voltage like Class G/H amps do. Definitely easier though to have fixed gain stages and then passive attenuators in front!

My TFPro P3 preamp has an unusual gain knob that actually controls four stages of amplification allowing it to handle mic levels from -70dB to 0dB and line levels from -30dB to +24dB. I have no idea how it works, especially as it's a strange transformerless current sensing (v low input impedance) stage that claims to be immune to cable capacitance issues!

Alex

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[quote name='alexclaber' post='583671' date='Aug 28 2009, 03:52 PM']Definitely easier though to have fixed gain stages and then passive attenuators in front![/quote]

And more importantly, cheaper :)

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[quote name='Boneless' post='539128' date='Jul 13 2009, 09:53 AM']But aren't drivers supposed to sound better if feeded an amount of power closer to their RMS rating?
In other words, if you have a 4x10" with 150W RMS drivers (600W total), wouldn't they respond better with 200W instead of 50W? Or does it depend on the driver? I find that my Ashdown 4x10" needs to be kind of cranked to start sounding "alive". Same thing for the Ampeg 8x10", there was a gig where I had a Peavey head + Ampeg 8x10" as a backline, and I had to keep the volume really down for several reasons (but REALLY low, most probably under 50W), and my tone was extremely dull. And trying to compensate with EQ was useless, since the cab seemed not to budge from that lifeless tone.[/quote]

Two things to consider:-

1. As the SPL drops, your hearing gets progessively less sensitive to very high and very low frequencies - the science is called audiometry. HiFi amps, especially older ones, often have a feature called a loudness control, which progressively boosts extreme treble & bass as you turn the wick down.

2. As signal level increases, loudspeakers progressively distort. Manufacturers deliberately exploit this when they design cabinets, especially guitar and bass cabinets. As you turn the wick up, the sound character changes. Something very similar is built into the design of audio enhancers & exciters (the main difference being that the distortion is designed into the electronics, rather than coming out through the speaker).

Hope this helps.


Andy

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[quote name='Boneless' post='539128' date='Jul 13 2009, 08:53 AM']But aren't drivers supposed to sound better if feeded an amount of power closer to their RMS rating?
In other words, if you have a 4x10" with 150W RMS drivers (600W total), wouldn't they respond better with 200W instead of 50W? Or does it depend on the driver? I find that my Ashdown 4x10" needs to be kind of cranked to start sounding "alive". Same thing for the Ampeg 8x10", there was a gig where I had a Peavey head + Ampeg 8x10" as a backline, and I had to keep the volume really down for several reasons (but REALLY low, most probably under 50W), and my tone was extremely dull. And trying to compensate with EQ was useless, since the cab seemed not to budge from that lifeless tone.[/quote]

Just a quickie about matching speakers and amp power. Consider them like the chassis and engine of a car.

A 10W amp going into 500W speakers would be like running a 10bhp go-kart engine in a 500bhp Le Mans racer. It will hardly move it.

The other way round, a 500W amp into a 10W speaker will, like a 500bhp Le Mans racer engine in a go-kart, blow the thing to pieces at anything above tick-over. 10W into a 10-20W speaker gives enough headroom for safety without too heavy a burden and it will go like a go-kart. 500W into 500-1000W of speakers will fly like a Le Mans racer.

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[quote name='4 Strings' post='638925' date='Oct 28 2009, 11:25 AM']Just a quickie about matching speakers and amp power. Consider them like the chassis and engine of a car.

A 10W amp going into 500W speakers would be like running a 10bhp go-kart engine in a 500bhp Le Mans racer. It will hardly move it.

The other way round, a 500W amp into a 10W speaker will, like a 500bhp Le Mans racer engine in a go-kart, blow the thing to pieces at anything above tick-over. 10W into a 10-20W speaker gives enough headroom for safety without too heavy a burden and it will go like a go-kart. 500W into 500-1000W of speakers will fly like a Le Mans racer.[/quote]

Don't think of it this way, read the rest of the thread. And check what sensitivity, and power ratings mean. And how the watt rating of speakers means absolutely nothing to the sound.

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[quote name='Mr. Foxen' post='639066' date='Oct 28 2009, 01:50 PM']Don't think of it this way, read the rest of the thread. And check what sensitivity, and power ratings mean. And how the watt rating of speakers means absolutely nothing to the sound.[/quote]


I understand about sensitivity etc, that's not what I was saying. Have you ever tried running powerful speakers with a tiny amp? Works just like I said. For volume, multi-speakers, lower impeance (with valves) higher sensitivity etc all contributes.

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[quote name='4 Strings' post='638925' date='Oct 28 2009, 11:25 AM']Just a quickie about matching speakers and amp power. Consider them like the chassis and engine of a car.

A 10W amp going into 500W speakers would be like running a 10bhp go-kart engine in a 500bhp Le Mans racer. It will hardly move it.

The other way round, a 500W amp into a 10W speaker will, like a 500bhp Le Mans racer engine in a go-kart, blow the thing to pieces at anything above tick-over. 10W into a 10-20W speaker gives enough headroom for safety without too heavy a burden and it will go like a go-kart. 500W into 500-1000W of speakers will fly like a Le Mans racer.[/quote]

Almost 100% wrong. (500W into a 10W speaker will blow it up pretty easily, so that's the fraction that is correct). Not a good analogy at all.

Alex

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[quote name='4 Strings' post='639613' date='Oct 28 2009, 11:47 PM']I understand about sensitivity etc, that's not what I was saying. Have you ever tried running powerful speakers with a tiny amp? Works just like I said. For volume, multi-speakers, lower impeance (with valves) higher sensitivity etc all contributes.[/quote]

Yes it works fine. You get much more volume from a low wattage amp into a high powered cab because of the greater speaker area (30w combo amp into a 1x15, and into an 8x10), it sounds fine. No damage was done. Lots more bottom that the crappy low wattage in a crap enclosure combo speaker also.

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