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alexclaber

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

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[quote name='Oxblood' timestamp='1185910019' post='39703']
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.
[/quote]


Hiya, thank you for this info, there is some good stuff on this thread. Can I ask, what are acceptable names for these two pots then? My amp is labelled "Input Level" and "Output Level" which I thought sounded about right when I read your post and started to understand what these two controls actually do. However, later posts suggest that these are not correct either.

Anyway, I would like to find out about how to SET these knobs so I would like to get the names right first before I start a thread about them! Thanks.

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[font="Times New Roman"]i have just read some of the posts in this thread, I am now of the opinion that most ppl have very little idea about the things they post. As for the names used to describe the various pots, i have to say dose it really matter and (although i didn't read all the posts) non of the posts recognised that the tone shaping stage has a strong effect on the gain (or attenuation) of the gain. Anyway that's not really important, basically to set up an amp for input level and out put level [/font]

[font="Times New Roman"]Step one set all tone controls to mid-point[/font]

[font="Times New Roman"]step Two set output (volume control ) to zero[/font]

[font="Times New Roman"]Step three set input gain control to zero[/font]

[font="Times New Roman"]step four set pad switch to passive setting irrespective of whether the instrument is passive or active[/font]

[font="Times New Roman"]step five plug in instrument and set volume to full and tone to mid point (if instrument has multiple pickups remember to chose the hottest pickup)[/font]

[font="Times New Roman"]step 6 turn on amp (assuming the amp has a clip indicator) play a scale as you do this slowly turn the input gain control till the clip light starts to come on[/font]

[font="Times New Roman"]repeat with higher and lower scales till you find a good average position (if the clip indicator comes on before the mid point you may need to engage the pad switch)[/font]

[font="Times New Roman"]now turn the output volume control to the desired level[/font]

[font="Times New Roman"]slowly adjust the tone controls (a small amount at a time) checking the clip indicator as you go and increasing or decreasing the gain control (a little at a time) [/font]

[font="Times New Roman"]remember you should repeat this every time you change your instrument.[/font]

[font="Times New Roman"]if your amp dose not have a clip indicator then you have to use your ears and listen for clipping (distortion) and adjust back from this point just a little.[/font]
[font="Times New Roman"](or bye a better amp)[/font]

[font="Times New Roman"]as i said its not what you call it its how you use it that matters[/font]

[font="Times New Roman"]n just for the record[/font]

[font="Times New Roman"]A 3db increase in sound pressure = a doubling of the perceived sound but only in the main part of the audible frequency range i.e. extreme bass and treble frequency's are perceived as less loud even at the same spl as mid range frequency's[/font]

[font="Times New Roman"]A doubling of output power = an spl level increase of 1db[/font]

[font="Times New Roman"]i.e. a speaker with an spl of 93db at 1w measured from a distance of 1m will double in perceived volume when the out put of the amp reaches 8w to double the perceived output again would require 64w to double it again would require 500w giving an output of 99db a 1000w amp would only increase the output to 100db[/font]

[font="Times New Roman"]doubling the number of speakers is a the way to go as this increases the sound output by 3db and 4 speakers would increase it by 6db[/font]

[font="Times New Roman"]basically what I'm saying is if you want louder get more speakers[/font]

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Fails at step one because you don't know the mid point of your tone controls.

3db isn't doubling of perceived sound, 10db is doubling perceived, 3db is what you get from doubling power or speakers.

Distortion is an integral part of many tones.

Doubling power is 3db increase.

What you call it does matter if you want to actually be able to talk about what is going on.

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[quote name='Kong' timestamp='1315969397' post='1372737']
There's a big difference between Pre-Amp - distortion and power-amp - distortion aka clipping.

If You drive Your power-amp into clipping You will destroy Your speaker.

This is why it is dangerous using a small amp (let's say 100 watt) into a big cab (let's say 500 watt).
If YAou have Your preamp maxed out, it will start clipping. If You don't hear this, the speaker will
get killed by thermal issues.

Pre-amp - distortion, carefully amplified with the power-amp, will add harmonics to Your sound,
compress it a bit and makes it richer.

[/quote]Clipping is clipping, no matter where in the signal chain it occurs. That includes in the drivers; one reason why guitar drivers have a very small xmax is so they'll clip at relatively low signal levels. And if clipping hurt drivers guitar players would swap them out after every set. Only tweeters are inherently at risk with clipped signals, due to the higher percentage of harmonics, and that's one reason why they aren't used in guitar amps. Required reading:
[url="http://forum.qscaudio.com/forum/viewtopic.php?f=29&t=2736"]http://forum.qscaudi...php?f=29&t=2736[/url] Edited by Bill Fitzmaurice

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Overdriving the power amp can destroy speakers by pumping more power into the speaker voice coils than they are designed for. Using a distortion pedal - or overdriving the preamp - won't. So clipping is not just clipping.

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Overdriving an amp is a bit irrelevant to it overpowering speaker coils. If it puts out more than the speakers can take, they can't take it, whether or not the signal is distorted is pretty much irrelevant., since the speakers will be distorting anyway, so it will still sound distorted. distortion pre power amp can contribute to speaker damage, since that distortion covers the speakers distorting, which is the warning to back off your volume.

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I've just enough knowledge to be dangerous on this subject. Where we've talked about eq and clipping, when winding up the bass pot, which might be a low shelf at 80Hz, say, why does this have a more pronounced effect on the available headroom than wildly spinning up the mid and treble controls?

(pretty sure I know the answer to this - energy under the wave - but i'm interested in the checksum)

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Lots more power needed to make lows is basically it. The bulk of the wave is the bass, the mid and treble are just along for the ride and put kinks in it.

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[quote name='stevie' timestamp='1348141490' post='1809859']
Overdriving the power amp can destroy speakers by pumping more power into the speaker voice coils than they are designed for. Using a distortion pedal - or overdriving the preamp - won't. So clipping is not just clipping.
[/quote]

No. This is a myth, its just not the case.

You are confusing overdriving amps and pushing a speaker beyond its ability to handle either the heat build up or more likely the excursion the signal into it is asking it to produce.

Clipping is clipping is clipping in fact.

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I'm going to dare to open this can of worms again and ask if someone can explain the controls on my MB Fusion.

I have A and B Gain and A and B Master. The B channel is called "boost" in the literature, I think it engages the valves to provide a dirty overdrive as opposed to the clean A channel. If I turn the Gain B up the OD gets more pronounced, Gain A just gets louder as far as I can tell.

I keep calling it Gain as that's what it says on the box, from an earlier post should it be Pre-amp Volume?

My question is really do I run the Gain at full and then control the volume with the Master controls, or is there a particular ratio between Gain and Master.

I suppose that the B Gain is to taste as it were, more or less OD?

The manual says leave both Master controls at 3 o'clock and fiddle with the Gain controls until the required level is reached. It'll happily run at quiet practice levels or ear bleedingly loud and anything in between but I don't want to break it so I'm asking for help. Better that than the alternative :)

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On the MB Fusion you effectively have two independent channels - A and B - selected using the button on the front panel, or the footswitch. The two channels don't interact with each other - controls marked "A" have no effect when channel "B" is selected and vice versa. It gives you the option of two different sounds, selectable at the flick of a switch.

You shouldn't be able to "break anything" on the MB Fusion no matter what control settings you use, so you can experiment with the controls as much as you like. The only time you need to be cautious is when using lots of volume through speakers. If its starts to sound horribly distorted then back off any of the volume or gain controls to reduce the volume and start again.

When setting up, treat it as two separate amps - amp A and amp B. To setup amp A, the A/B switch should be off (out) then use only the controls marked "A". To setup amp B push the A/B switch in then use only the controls marked "B". The tone controls affect BOTH amp A and B settings.

Setting the Masters at 3 o'clock should allow you to hear what is happening at the input stage as you adjust Gain A or B. The 3 o'clock setting is a sort of "average" setting to use as a starting point. Without hearing how loud this it's difficult to say exactly how to go about setting this up. But start at this Master setting, select channel A then bring up "Gain A" until you like what you're hearing.

If "Gain A" has to be set very high to get the desired volume and is starting to distort a little, then you might want to turn UP "Master A" and turn DOWN "Gain A" to get the right balance of volume and distortion (or lack of distortion). Fine tune the two controls to get what you like.

Then do the same for channel B - bearing in mind that you might get some warm valve distortion earlier since this is what it's meant to do.

Once you have both channels sounding the way you want, leave "Gain A" and "Gain B" as they are. Now only adjust "Master A" and "Master B" to get the correct relative volumes from each channel so that when you switch from one to the other there isn't a huge change in overall volume (unless that's the effect you want).

That's how I would go about setting it up.

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Cheers chap.

I suspect that this is hijacking the OP somewhat, but are those Gain and Master controls a ratio or something? If it sounds nice at home will it still sound the same but louder once it gets into rehearsal or rocking it out at the Dog 'n' Duck if I leave the Gain where it is and just turn the Master up?

If that makes sense :)

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+1 to what Mr. Foxen says, unfortunately.

Stuff doesn't even sound the same when you leave all the controls untouched but play in a different room sometimes !

But it'll give you a starting point for setting it up at a gig or rehearsal.

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[quote name='SteveO' timestamp='1211436921' post='204037']
Mmmmmmm flashing lights and shiny knobs. Yup, that's pretty much all I look for when buying gear ;)
[/quote]

What else is there when you are standing at the back bored?

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[quote name='SteveO' timestamp='1211436921' post='204037']
Mmmmmmm flashing lights and shiny knobs. Yup, that's pretty much all I look for when buying gear ;)
[/quote]

Agreed and if you are worried about your speaker not being up to the job, buy a bigger speaker :rolleyes:

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[quote name='4 Strings' timestamp='1256729132' post='638925']
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]

Nice analogy mate!

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As musicians, we are used to calling the attenuation on the input "gain" and the output "master" or "volume" - we've been calling them that for 40 years, we all know what they mean so lets keep on calling them that.

There are good technical reasons for this, but they really dont matter since theres no need to redefine a language that works. Edited by Mikey R

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Rather than start a new topic, thought I'd post this in here. Ive been reading through some threads in Amps and Cabs, and there seems to be a trend.

In the late 90s, I used to play with quite a loud rock rap band. We had a hard hitting drummer and the guitarist used a 4 x 12. Because of the hip hop influence, most of our songs were driven by the rhythm section so were kinda bass heavy.

All of this, and I kept up with my old (new at the time) Trace Elliot GP7SM 150 watt combo. This had the slotted front port and the smaller (10 inch maybe?) driver, so was probably far less efficient than a modern Neo loaded cab. Since I wasnt using an extension speaker, this was probably putting out around 80 watts.

So why, as bassplayers, are we feeling the need for a half kilowatt stack for the low end? Are we competing with guitarists with terrible scoopy tone? Do we just like the headroom? Edited by Mikey R

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Lower tuned cabs, instead of a high tuned one for a midbass hump, now the trend is actual low end extension.

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[quote name='Mikey R' timestamp='1367648271' post='2067863']
So why, as bassplayers, are we feeling the need for a half kilowatt stack for the low end?
[/quote]To paraphrase Sir Edmund Hillary, "because they're there". In 1965 I bought a 50 watt Fender Bassman because it was what was available. If I could have bought a 500w ten pound Class D micro-amp instead, I would have.

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Apologies for re opening an old thread again. Im a newbie to heads and cabs so just to clarify it wouldn't matter if I had a 300w head driving a 300w or a 1000w cab it wouldn't damage it or be detrimental to the sound in anyway?
Also what has been baffling me is how is the power distributed when you daisy chain 2 8ohm cabs together to run at the amps 4ohm recommendation. In other words how is the wattage distributed between the two cabs?

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