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


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#31 TimR

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Posted 20 May 2010 - 06:27 PM

Layman's guide to the Speaker power v Amplifier power myth exploded with no maths or numbers (or techie terms?)

Generally people mistake loudness for noise and the untrained ear will ignore distortion. Distortion is your enemy. Even comparatively small amounts of distortion, using effects or over-driving pre-amps can be bad if you don't know what you are doing.

Once an amplifier starts distorting the speakers are not going to get any louder no matter how far you turn up the volume knob. Everything will just get noisier and less intelligible, defeating the object!

Published amplifier power is fairly meaningless as it is given as a power with a small amount of distortion. A lot of amps can give out huge amounts of power at huge levels of distortion!

Large amp -> small speakers. The danger is that the user will turn the amp up beyond what the speakers can handle. This will cause distortion in the speakers. The speaker will either overheat or mechanically destroy itself.

Small amp -> large speakers. Again the amp can get turned up beyond the point at which it produces nice clean sound. At this point the amp is distorting and producing much more than the rated power of the amp. Most of the power will be dissapated as heat in the coil and can literally melt the speaker.

So neither is the correct solution:

1. Use your ears not the spec sheets or labels. If you hear distortion - stop and turn down!
2. When buying an amp and cab try before you buy and don't believe the hype.

Edited by TimR, 20 May 2010 - 08:48 PM.

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#32 Mr. Foxen

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Posted 20 May 2010 - 06:43 PM

View PostTimR, on May 20 2010, 07:27 PM, said:

Layman's guide to the Speaker power v Amplifier power myth exploded with no maths or numbers (or techie terms?)

Generally people mistake loudness for noise and the untrained ear will ignore distortion. Distortion is your enemy. Even comparatively small amounts of distortion, using effects or over-driving pre-amps can be bad if you don't know what you are doing.

Once an amplifier starts distorting the speakers are not going to get any louder no matter how far you turn up the volume knob. Everything will just get noisier and less intelligible, defeating the object!

Published amplifier power is fairly meaningless as it is given as a power with a small amount of distortion. A lot of amps can give out huge amounts of power at huge levels of distortion!

Large amp -> small speakers. The danger is that the user will turn the amp up beyond what the speakers can handle. This will cause distortion in the speakers. The speaker will either overheat or mechanically destroy itself.

Small amp -> large speakers. Again the amp can get turned up beyond the point at which it produces nice clean sound. At this point the amp is distorting and producing much more than the rated power of the amp. The power will be dissapated as heat in the coil and literally melt the speaker.

So neither is the correct solution:

1. Use your ears not the spec sheets or labels. If you hear distortion - stop and turn down!
2. Getting huge speakers is genearally a waste of money if you don't have the amp to drive them.
3. Getting a huge amp is generally a waste of money if you don't have (or plan on getting) speakers that it can drive.

Still trying to disagree with the rest of the thread then?

1 is fine.

2 is wrong. Huge speakers are awesome for sensitivity, so you get plenty of volume from not much power

3 is also wrong. A huge amp can drive any speakers.

Also, a distorting amp will only melt speaker coils if it is outputting enough power to do so, even fully distorted, it might not be.

And noisy and unintelligible is awesome. Distorting power amps are awesome. This is why we have power brakes and low powered valve amps. Or really really loud high powered valves amps of Doom.

#33 umph

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Posted 20 May 2010 - 06:49 PM

View PostConan, on Dec 18 2009, 11:52 PM, said:

Anyway, to jump back to the earlier part of the thread (I haven't quite worked out how to do "quotes" yet on here!)...

A better word to use instead of "gain" would be "attenuation", wouldn't it? In other words, zero attenuation equals letting the maximum signal through rather than restricting it in any way...?

Or maybe I'm wrong? :)

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#34 TimR

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Posted 20 May 2010 - 07:24 PM

View PostMr. Foxen, on May 20 2010, 07:43 PM, said:

Still trying to disagree with the rest of the thread then?

1 is fine.

2 is wrong. Huge speakers are awesome for sensitivity, so you get plenty of volume from not much power

3 is also wrong. A huge amp can drive any speakers.

Also, a distorting amp will only melt speaker coils if it is outputting enough power to do so, even fully distorted, it might not be.

And noisy and unintelligible is awesome. Distorting power amps are awesome. This is why we have power brakes and low powered valve amps. Or really really loud high powered valves amps of Doom.

You've basically just agreed with everything I wrote, while saying you disagree? Weird, I'm lost.

To recap:

Power ratings mean nothing.

When you go into a shop and the salesman says you must have speakers bigger than amp, or amp bigger than speakers you will know he is talking rubbish.

I said that distortion is bad if you don't know what you are doing. Using low powered valve amps, power brakes etc sounds like you probably do.

It was intended as straightforward advice for the newbies who are always asking the question. "I've got a 100W power amp what speakers do I need?" and "Why did I fry my 400w speakers with my 150W amp?"

Edited by TimR, 20 May 2010 - 07:25 PM.

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#35 Mr. Foxen

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Posted 20 May 2010 - 07:32 PM

View PostTimR, on May 20 2010, 08:24 PM, said:

You've basically just agreed with everything I wrote, while saying you disagree? Weird, I'm lost.

To recap:

Power ratings mean nothing.

When you go into a shop and the salesman says you must have speakers bigger than amp, or amp bigger than speakers you will know he is talking rubbish.

I said that distortion is bad if you don't know what you are doing. Using low powered valve amps, power brakes etc sounds like you probably do.

It was intended as straightforward advice for the newbies who are always asking the question. "I've got a 100W power amp what speakers do I need?" and "Why did I fry my 400w speakers with my 150W amp?"

Any speakers can go with any amp. There is no relationship between the "wattage" rating of them that needs to be adhered to, or even a guideline. Your points 2 and 3 are what I disagree with.

If you have a 100w power amp you need big speakers if you want to gig.

Quote

At this point the amp is distorting and producing much more than the rated power of the amp. The power will be dissapated as heat in the coil and literally melt the speaker.

Is also not true. People distort their power sections really often without understanding what is happening, they just know it sounds awesome. They are often called guitarists.

Don't make generalisations for the benefit of people who don't understand, they are the reason why they don't understand. Because they are wrong.

#36 TimR

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Posted 20 May 2010 - 08:52 PM

OK I've taken points 2 and 3 out and condensed them into one point as they were basically saying the same thing, but re-reading them I can see how they could be confusing.

Edited by TimR, 20 May 2010 - 08:53 PM.

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#37 johnnylager

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Posted 20 May 2010 - 09:09 PM

http://www.steamingaudio.myzen.co.uk/finnb...ing_and_you.pdf

#38 TimR

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Posted 20 May 2010 - 09:42 PM

View PostMr. Foxen, on May 20 2010, 08:32 PM, said:

Don't make generalisations for the benefit of people who don't understand, they are the reason why they don't understand. Because they are wrong.

Indeed but most of us would rather buy some gear that works as sold than do a degree in electronics and wade through the tons of opinion and misdirection on the web.

After reading that attachment written by an engineer in an attempt to clarify things. We get to the crunch....

Quote

clipping is acceptable provided that the average power over time is lower than the speaker’s limits

The big question is "How do we know what the average power over time is and what the speakers limits are?" If anyone knows a quick easy way to calculate that from the smoke and mirrors leaflet that comes attached to your nice shiny things let me know.
Go careful when running your amp to distortion.

Edited by TimR, 20 May 2010 - 09:43 PM.

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#39 Mr. Foxen

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Posted 20 May 2010 - 09:52 PM

View PostTimR, on May 20 2010, 10:42 PM, said:

Indeed but most of us would rather buy some gear that works as sold than do a degree in electronics and wade through the tons of opinion and misdirection on the web.

This thread is pretty good for it, which is why anything even vague get's pounced on. I think Claber's degree is engineering, but I'd guess electronics figured highly.

View PostTimR, on May 20 2010, 10:42 PM, said:

After reading that attachment written by an engineer in an attempt to clarify things. We get to the crunch....

The big question is "How do we know what the average power over time is and what the speakers limits are?" If anyone knows a quick easy way to calculate that from the smoke and mirrors leaflet that comes attached to your nice shiny things let me know.
Go careful when running your amp to distortion.

That is kind of why that isn't so helpful. The main thing is to remember speakers sound bad before they break. They also get quieter when the heat up, due to their impedance increasing, its called power compression. With bass guitar, its not an issue unless you play full on drone, due to the quieter bits in the dynamic range giving cooling time. Over excursion at the volume peaks is the killer.

#40 TimR

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Posted 20 May 2010 - 10:07 PM

Alex's was Mechanical. Speaker and cabinet design is all mechanical. Crossovers and thermal limits are fairly simple electronic concepts to grasp if you are bright. Mine's Electronics with an Acoustics module but that was in '94. Things have changed a lot since then. The way that engineers resort to numbers, formulas and long words to explain things hasn't and I still get the salesmen giving conflicting advice in shops.
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#41 chris_b

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Posted 20 May 2010 - 10:41 PM

View PostTimR, on May 20 2010, 11:07 PM, said:

....I still get the salesmen giving conflicting advice in shops....
Surely you don't believe salesmen!!
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Posted 13 September 2010 - 06:15 PM

Great thread, but a question i have in respect to the whole premp and power amp sections of a combined all in one amp, is as follows :

I understand that the preamp takes a signal and boosts and shapes it accordingly. If you then, crank the preamp section and various boost and gain pedals, will the power amp section reach full power sooner? Or is there a cap that the preamp maxes the signal out at before it flows to the power amp?

Ive been thinking about this lately and cant find the info i want. When i crank the pre amp at home, a click or two on the power amp has a hell of a lot of gusto. Back off the gain and i then push the power amp harder to achieve the same levels. It makes sense.

I just wanted to clarify whether the amp reached full power well before its designed to if the preamp is cranked. Sorry if this seems rather dense, thought it was the thread to post in though!

#43 Mr. Foxen

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Posted 15 September 2010 - 01:42 AM

Putting more into the preamp section will get more out, it is a multiplier, as is the power amp stage. but both have limits, it is possible to clip the preamp section by putting too much power into it, just as it is possible to clip the power section by trying to get more out of it that it can give. Ideally there is a limiter somewhere along the line, but it is usually left to discretion.

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Posted 16 September 2010 - 06:02 AM

Thanks very much, thats what i thought, i just couldnt find anything to confirm my suspiscions!

#45 jnewmark

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Posted 15 January 2011 - 11:56 PM

One of my favorite combos of all time, and one I use on half of my gigs is the Roland DB 700, which has Gain, Volume, and a Master. I typically run the Gain and Volume as hot as I can, and set the Master at noon. But, I never really understood why that Volume knob is there.
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Posted 14 September 2011 - 03:03 AM

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.

This is why preamps with tubes (like Ampeg SVP Pro or the preamp-stage in my Peavex T-Max)
sound the way they sound when pushing the tube.

The T-Max, for example, has a pre-gain knob for pushing the tube, a post-gain for the tube-sound-
volume, and a master volume for the poweramp. ith pre- and post-gain You choose the sound and
the power of the tube-stage of the preamp. Master will dial the ammount of power sent to Your cabs.

It is not confusing at all.

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#47 51m0n

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Posted 14 September 2011 - 09:16 AM

View PostKong, on Sep 14 2011, 04:03 AM, said:

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.

This is why preamps with tubes (like Ampeg SVP Pro or the preamp-stage in my Peavex T-Max)
sound the way they sound when pushing the tube.

The T-Max, for example, has a pre-gain knob for pushing the tube, a post-gain for the tube-sound-
volume, and a master volume for the poweramp. ith pre- and post-gain You choose the sound and
the power of the tube-stage of the preamp. Master will dial the ammount of power sent to Your cabs.

It is not confusing at all.


With all due respect, thats not actually true. It is not that simple.

If you have a driver that can handle 1000w (ie thats its thermal limit) and you clip the bejesus out of a 50w amp into it it will run all day and never ever have problem.

Clipping is clipping is clipping, pretty much every album released in the last 10 years has some form of clipping (or brickwall limiting, the two are almost identical in terms of waveform) on it, you didnt see your hifi go up in smoke now did you? No. It doesnt sound nice but the transducers can cope because its just not about clipping, its about how much power you are pushing into the transducer. You can push a tube into clipping, it could be a power amp tube or a preamp tube, the result on the wave form is the same.

A simple rule of thumb is if it sounds like its straining/clipping/distorting in a bad way, then it is, back it off. A speaker straining does sound different from an amp clipping, but if you dont know the difference then back off the amp until everything sounds happy, if thats too quiet for your situation you need more power, or more speakers. Its deciding which that can be an issue.
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#48 thunderider

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Posted 09 October 2011 - 07:34 PM

iv got a laney dp 150 head going through a 115 and 118, if i turn up the gain,with the bass on the eq high it distorts even speakers of these size and the red light comes on,altho laney say this has to flicker,my gears on rickenfaker page,but dont seem loud,also the volume on the right seem not to do alot,also talk of volume 2 bassist i admire play heavy rawkus bass,first being lemmy,it suprised me hes sposed to be the loudest bassest yet he running a 100 watt head with 2 412s dont seem to powerfull to me,the other being alan davey he seems to use a mixture of stuff when gigging,but he uses 2 fender bassman 50 heads with a ashdown 115 and a 2112,but they seem to drive these and get some volume outta these!!,i wanna play loud!!
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#49 51m0n

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Posted 11 October 2011 - 09:16 AM

View Postthunderider, on 09 October 2011 - 07:34 PM, said:

iv got a laney dp 150 head going through a 115 and 118, if i turn up the gain,with the bass on the eq high it distorts even speakers of these size and the red light comes on,altho laney say this has to flicker,my gears on rickenfaker page,but dont seem loud,also the volume on the right seem not to do alot,also talk of volume 2 bassist i admire play heavy rawkus bass,first being lemmy,it suprised me hes sposed to be the loudest bassest yet he running a 100 watt head with 2 412s dont seem to powerfull to me,the other being alan davey he seems to use a mixture of stuff when gigging,but he uses 2 fender bassman 50 heads with a ashdown 115 and a 2112,but they seem to drive these and get some volume outta these!!,i wanna play loud!!


Speaker size is not relevant to them distorting, the XMAX is. Typically those older speakers had pretty dismal XMAX compared to todays drivers. Certainly it is very easy to cause them to fart out with excessive bass boost and lowish apparent volumes.

I had a Laney G300 (the big daddy to the series that came after the dp150) back in the day (had HH power amps in it in fact - was pretty well build for the period too). Certainly capable of producing enough oomph in the low end to fart out two 410s.

As for Lemmy, you have noticed he has a massive PA behind him havent you? His onstage volume is probably not so loud in fact, and his tone was never massively bassy, plus he now runs nice shiny new kit, with nice modern drivers in.

Edited by 51m0n, 11 October 2011 - 09:28 AM.

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#50 thunderider

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Posted 11 October 2011 - 04:48 PM

lol of courese ur right about lemmys pa!!
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#51 stefBclef

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Posted 31 May 2012 - 09:48 AM

View PostOxblood, on 31 July 2007 - 07:26 PM, said:

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 potential divider: 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 doesn't 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.


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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Posted 18 September 2012 - 11:38 PM

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

Step one set all tone controls to mid-point

step Two set output (volume control ) to zero

Step three set input gain control to zero

step four set pad switch to passive setting irrespective of whether the instrument is passive or active

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)

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

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)

now turn the output volume control to the desired level

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)

remember you should repeat this every time you change your instrument.

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.
(or bye a better amp)

as i said its not what you call it its how you use it that matters

n just for the record

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

A doubling of output power = an spl level increase of 1db

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

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

basically what I'm saying is if you want louder get more speakers
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#53 Mr. Foxen

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Posted 19 September 2012 - 12:15 AM

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.

#54 Bill Fitzmaurice

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Posted 19 September 2012 - 02:11 AM

View PostKong, on 14 September 2011 - 03:03 AM, said:

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.

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:
http://forum.qscaudi...php?f=29&t=2736

Edited by Bill Fitzmaurice, 19 September 2012 - 02:11 AM.

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#55 stevie

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Posted 20 September 2012 - 11:44 AM

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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#56 Mr. Foxen

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Posted 21 September 2012 - 09:52 PM

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.

#57 subrob

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Posted 31 October 2012 - 01:04 AM

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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#58 Mr. Foxen

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Posted 31 October 2012 - 01:40 AM

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.

#59 51m0n

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Posted 31 October 2012 - 08:22 AM

View Poststevie, on 20 September 2012 - 11:44 AM, said:

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.

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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Posted 17 December 2012 - 01:34 PM

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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