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#1
02-29-2012, 04:25 AM
 Registered User Join Date: Sep 2008 Location: Queens, NY
Another wattage question

If you have a 400watt head, and have your volume at the halfway point, does that mean that your feeding your cabinets 200watts? And if at the 1/4 mark 100 watts? Seems reasonable, but not sure if it's true. I just wanted a rule of thumb when it comes to powering speakers with a more powerful head other than "use your ears".
#2
02-29-2012, 04:33 AM
 Registered User Exar went out of business, so... Join Date: Oct 2005 Location: PDX, OR
No, it's completely not true. Your amp is capable of putting out its full wattage at any setting of the vol knob other than "off". Basically the amp is like a multiplication sign, and the vol knob is like a minus sign.

10 x 10 = 100.
10 x 10 - 50 = 50. (seems to cut output in half, right?)
100 x 10 - 50 = 950. Whoah!!! - 50 is not cutting the output in half anymore... In fact the output is much higher!

The wattage depends almost entirely on the signal you feed into the amp.
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Last edited by bongomania : 02-29-2012 at 04:42 AM.
#3
02-29-2012, 04:50 AM
 Registered User Join Date: Sep 2008 Location: Queens, NY
That is way too cerebral for 6:30am . I'll have to let that sink in for a bit before I can ask anything else. So pretty much "use your ears" for now. Is there any way to determine how much wattage is actually getting to your cabs?
#4
02-29-2012, 06:06 AM
 Registered User Join Date: Oct 2007
Quote:
 Originally Posted by Truktek2 If you have a 400watt head, and have your volume at the halfway point, does that mean that your feeding your cabinets 200watts? And if at the 1/4 mark 100 watts? Seems reasonable, but not sure if it's true. I just wanted a rule of thumb when it comes to powering speakers with a more powerful head other than "use your ears".
The position numbers of the control are only for reference when someone wants to set the control and remember- there's no correlation of numbers and actual power, partially because the tone control settings change the output power along with the volume control. Even a control that shows attenuation isn't usually very accurate.
#5
02-29-2012, 06:22 AM
 Registered User Join Date: Mar 2000
Does too!
Mine goes to 12!
#6
02-29-2012, 06:25 AM
 Registered User Owner, Bill Fitzmaurice Loudspeaker Design Join Date: Sep 2004 Location: New Hampshire
Quote:
 Originally Posted by Truktek2 Is there any way to determine how much wattage is actually getting to your cabs?
No. Watts cannot be measured, because they vary with the speaker impedance, and impedance varies with frequency. This is but one of many reasons why watts should not be used as a measuring tool.
The maximum output of an amp is best defined by voltage swing, and that can be measured.
As for the volume control position, google 'gain structure'.
#7
02-29-2012, 07:34 AM
 Registered User Join Date: Sep 2008 Location: Queens, NY
Thanks everyone for all the info. It's starting to make sense a little bit at a time, but it's obvious I have a lot of reading to do. Google here I come. If anyone has any good websites on theory I would love to hear about it.
#8
02-29-2012, 07:37 AM
 Registered User Join Date: Oct 2007 Location: Ottawa, Ont
Quote:
 Originally Posted by billfitzmaurice No. Watts cannot be measured, because they vary with the speaker impedance, and impedance varies with frequency. This is but one of many reasons why watts should not be used as a measuring tool. The maximum output of an amp is best defined by voltage swing, and that can be measured. As for the volume control position, google 'gain structure'.
Watts can most definately be measured.

But taking a single measurement at a single frequency really doesn't say much.
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Last edited by Jonyak : 02-29-2012 at 11:09 AM.
#9
02-29-2012, 08:18 AM
 Registered User Owner, Bill Fitzmaurice Loudspeaker Design Join Date: Sep 2004 Location: New Hampshire
Quote:
 Originally Posted by Jonyak Whats can most definately be measured.
Not without going through a complicated process. There is no meter that you can simply connect across the amp output terminals that will give a power readout. That's why limiters used to protect speaker systems aren't set to limit the power output of an amp, they're set to limit voltage swing.
#10
02-29-2012, 11:03 AM
 Registered User Join Date: Oct 2007 Location: Ottawa, Ont
Quote:
 Originally Posted by billfitzmaurice Not without going through a complicated process. There is no meter that you can simply connect across the amp output terminals that will give a power readout. That's why limiters used to protect speaker systems aren't set to limit the power output of an amp, they're set to limit voltage swing.
There are such things as power meters, but they generally need to use a matched load to properly read the power.

I use them everyday.
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#11
02-29-2012, 11:42 AM
 Registered User Join Date: Jun 2011 Location: DC
Bongo, I'm pretty sure you're wrong about your example. When you turn the volume knob all the way down, you get "0", not "900" as in your third equation.\

In your example, turning the volume down -99 would yield a "901", and then turning it down 1 more (to 0) would cut it completely off. But that's not how it works. Volume knobs have different tapers, but they all start from 0 and go UP from nothing, either in a linear (in which 50% would be half way between the range of available values) and logarithmic (where the increase gets bigger, the farther up you are on the knob).

Anyway, the short answer is no, you cannot predictably measure how many watts you are putting out in any easy, convenient way. Unless the amp has a built in power meter that is calibrated to show what the power level is at at any given time.

Suffice to say, watts do a lot more than most people think. Typical speaker sensitivities are between about 90 and 105 db/w at 1 meter (at 1000hz). That's pretty loud, like hearing a note somewhere in the top few frets of the high E on a guitar, at a volume that would probably be uncomfortably loud without earplugs. It takes more power to produce lower notes at the same volume, but even then watts go farther than you might think.

But all that is somewhat irrelevant anyway, except for your personal curiousity. As long as you can pay attention to how your gear is sounding and behaving and can tell when too much is too much and you should back off, you'll be fine and the actual number of watts won't matter.
#12
02-29-2012, 12:16 PM
 Registered User Owner, Bill Fitzmaurice Loudspeaker Design Join Date: Sep 2004 Location: New Hampshire
Quote:
 Originally Posted by Jonyak There are such things as power meters.
#13
02-29-2012, 12:35 PM
 Registered User Join Date: Oct 2007 Location: Ottawa, Ont
Quote:
 Originally Posted by billfitzmaurice None that will read power into a complex load.
I'm not going to argue this any further after this.

I work in RF, a speaker is no more complex of a load than an antenna system. We use power meters to measure output power of transmitters all the time. For the sensor to do it, they become the load.

If you can't do that there couplers, which can be used to measure the power with the load in line.
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Last edited by Jonyak : 02-29-2012 at 12:38 PM.
#14
02-29-2012, 12:37 PM
 Registered User Exar went out of business, so... Join Date: Oct 2005 Location: PDX, OR
Quote:
 Originally Posted by jungleheat When you turn the volume knob all the way down, you get "0", not "900" as in your third equation.
I specified that at the very beginning.
Quote:
 Originally Posted by jungleheat In your example, turning the volume down -99 would yield a "901", and then turning it down 1 more (to 0) would cut it completely off. But that's not how it works. Volume knobs have different tapers, but they all start from 0 and go UP from nothing, either in a linear (in which 50% would be half way between the range of available values) and logarithmic (where the increase gets bigger, the farther up you are on the knob).
That's because literal maths were not in any way the point of my illustration. The function of an amplifier is multiplicative, and the function of a volume knob is subtractive. You can subtract whatever literal number you want, it still does not annul the logical point that it is always possible to feed in a larger input.

You might take it a step further and argue that if it was a 100 W amp for instance, it could never reach 950 W as per my example--and you'd be right, again if that was the point. It's not. There is no setting of the volume knob (aside from "off", as I said) that inhibits the amp from producing its full wattage potential, whatever that potential may be. That is what the OP needed to hear.

For example, when driving an amp into hard clipping it is possible for the amp to output as much as twice its rated wattage. So let's say the input signal level required to do that is "20", and the volume knob has a range of "100", and it's a nominally 100 W amp.
(20 x 10) - 99 = 101 watts.

Again, the point of this is not to demonstrate accurate real-world calculations, the point is to show that the volume knob is in no way ever an indicator of wattage.
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#15
02-29-2012, 12:48 PM
 Registered User Owner, Bill Fitzmaurice Loudspeaker Design Join Date: Sep 2004 Location: New Hampshire
Quote:
 Originally Posted by Jonyak I'm not going to argue this any further after this. I work in RF, a speaker is no more complex of a load than an antenna system. We use power meters to measure output power of transmitters all the time. For the sensor to do it, they become the load.
Don't argue, show us. Please post a link to a power meter that can read the power output of an audio amplifier into a loudspeaker. If such a thing exists then many of us would be interested, especially the OP.
BTW, here's one product that claims to be an audio power meter.
Audio Power Meter KIT (K4307)
It does not measure power. It's a voltmeter with a stepped LED display.
#16
02-29-2012, 12:58 PM
 Registered User Join Date: Oct 2007 Location: Ottawa, Ont
Quote:
 Originally Posted by billfitzmaurice Don't argue, show us. Please post a link to a power meter that can read the power output of an audio amplifier into a loudspeaker. If such a thing exists then many of us would be interested, especially the OP. BTW, here's one product that claims to be an audio power meter. Audio Power Meter KIT (K4307) It does not measure power. It's a voltmeter with a stepped LED display.
Thats a toy.

Here is what we use, along with different power sensors you can cover from frequencies of DC up to 67 GHz, and multiple power ranges as well.

R&SŪNRP2 Power Meter (Rohde & Schwarz Canada - Products - Test & Measurement - Power Meters & Voltmeters)

To use it properly you would need to use a directional coupler that would match impedances for you, allowing the speaker to stay in line. Or you could use a calibrated matching feedthrough, of high enough power handling capability, to match the head directly up to the output.

I use this all the time to measure significantly higher power than any audio amplifier can generate on much more complex loads than a speaker.
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Last edited by Jonyak : 02-29-2012 at 01:00 PM.
#17
02-29-2012, 01:24 PM
 In case you missed it, I work for QSC Audio! Applications Engineer, QSC Audio Join Date: Jul 2001 Location: Costa Mesa, Calif.
Why are you insisting that RF power measurement methodology is applicable to measuring audio power into a loudspeaker load?

I agree with Bill. There is no practical way to measure in real time the power put into a loudspeaker by a normal audio signal--i.e., music, voice, etc.
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#18
02-29-2012, 01:32 PM
 Registered User Join Date: Apr 2010
Let me try something simpler. Your amp will put out its max rating if fed a signal of sufficient strength. Preamps cover that. They are built into the head and take the output from the bass and bring it up to where the amp can max out. If your preamp (gain) is cut back, max amp effort will be less than 100% of potential. So gain limits what your amp has to work with. That is one way of controlling output.

The volume control regulates how much effort your amp puts out. But none of these are linear. so gain (or volume) at one half just means less (but exactly not one half) of full amp effort. Three quarter way up means more amp effort than one half, but not exactly 50% more.

And there is a relation between amp output and percieved loudness, but it is complicated by the charactistics of the speakers among other things.
#19
02-29-2012, 01:48 PM
 Registered User Exar went out of business, so... Join Date: Oct 2005 Location: PDX, OR
Quote:
 Originally Posted by DLESSING ...If your preamp (gain) is cut back, max amp effort will be less than 100% of potential. So gain limits what your amp has to work with. That is one way of controlling output. The volume control regulates how much effort your amp puts out. But none of these are linear. so gain (or volume) at one half just means less (but exactly not one half) of full amp effort. Three quarter way up means more amp effort than one half, but not exactly 50% more.
Unfortunately these descriptions are false. The reality is that an amp's potential is always at 100% when operating in its clean range, or more when operating outside its clean range. The gain knob adjusts the level of the input signal, but it does not generate the input signal! Think about what the amp does when you barely pluck a string, vs what it does when you slap/pop like a maniac. The same problem goes with the master volume knob. Yes it adjusts the level by a fixed amount, but what are you feeding into it? If you turn down a whisper by 50%, or if you turn down a scream by 50%, the amp puts out drastically different amounts of wattage--so it doesn't work to say the knob reduces the amp's "effort" by some amount corresponding to the knob.
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#20
02-29-2012, 01:49 PM
 Registered User Join Date: Mar 2009 Location: West Hartford, CT
I think that's really the issue here: Practicality. But while I greatly respect Bill's knowledge here, his original reply that "watts cannot be measured" is not true.

The real problem is the trouble someone would have to go through to accurately measure ampflier output in real time between proper test setup and investment in the correct measurement equipment. Better to just use your ears and listen for stress on the cones.

Of course this is quite a bit easier for someone who has a relatively clean tone goal versus someone who likes a lot of dirt. That distortion (signal generated) unfortunately can mask overpowering problems (most often mechanically generated).

Also, measuring voltage swing is all well and good, but giving that information to the average consumer and making good use of it is a whole other story, especially when it doesn't have any obvious correlation to ratings on speaker cabinets, which most of the time aren't accurate anyways.

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