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Fun at Born2RockU's - The Headroom/Liquefying Experiment!

Jeff Matthews

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"You are also completely leaving out the impedance factor (which

determines how much current flows for a specific voltage...and it is

both the current and voltage that defines the power output of the


Or it is sometimes easier to think of it as simply voltage by load which is current but sometimes easier to understand.


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"So, having to turn the dial more to get the same volume, is an insight into how much amp power remains - not what has been used - but what remains. Right?"

Wrong. There is no correlation between the volume control and the wattage out of the amplifier.

One amp could need 1v of input for 300w of output.

One amp could need 2v of input for 1w of output.

A third amp could need 8v of input for 2000w of output.

Plug the same pre-amp into each of them and you will find their volume controls are at very different positions for the same volume. Same volume means same amount of power being put out. And how far the volume control is turned has no bearing on how much power is left between the amps.

All *any* amplifier does is multiple a voltage it is fed. Different amps have different multipliers to that voltage. (Gains/input sensitivity) The gain/input sensitivity is not directly related to the max power available from the amp. The max power is simply a measure of how high the multiplication of the amp can go.

The volume control on a pre-amp simply adjusts the level (voltage) of the signal fed to the amp.

For example....

Amp A has a multiplication factor of 15v per volt. IOW you feed it 1v it spits out 15. Its max power (in volts) is 30v.

You feed it 2v it spits out 30v and is at max power.

Amp B has a multiplication factor of 5v/v. You feed it 1v it spits out 5. But its max power is 100v. So before it 'maxes' out of power you need to feed it 20v on the input. Compared to amp A you need to turn up the volume on amp B much higher for the same volume (power) out of the amp. Yet amp B has far more max power then amp A.

Get it?


Shawn, you da man! I get it. I get the math, but I want to make sure I get the "real world" application. Is it that the multiplication factor (e.g. 15v per volt) by itself, means nothing? Or is it important because in your "amp B" example, you can run out of dial before you reach the amp's max?

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The question I posed above was to ask in the end whether these voltage in/voltage out ratios have anything to do with sound quality, and one ratio might be preferred over another. Would be good to know this ratio when looking at specs of an amp of unfamiliar brand?

Just wondering. But no, I'm not looking for an amp - this is just so you can impart some knowledge to me for who knows when.... Thanks.

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"Is it that the multiplication factor (e.g. 15v per volt) by itself,

means nothing? Or is it important because in your "amp B"

example, you can run out of dial before you reach the amp's max?"

You can run into the situation where an amp needs more voltage to drive

it to max power then the pre-amp can supply. It isn't terribly common

but it can happen. For example trying to run a pair of Quicksilver Horn

Mono's from a passive pre-amp fed from a common CD player you could

easily turn the volume control all the way up and not get full output

from the amps.

"in the end whether these voltage in/voltage out ratios have anything

to do with sound quality, and one ratio might be preferred over

another. "

Not really for sound quality in an of itself. With sensitive speakers

though a lower gain/lower input sensitivity (more voltage needed) amp

can help to hide the noise from a pre-amp/sources more then a high

gain/high input sensitivity amp would.


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Hi Jeff.

Doesn't look like anyone answered your "Why is the first watt of amp A

any different than the first watt of amp B" question of a couple pages

ago. Shawn's post, above, explains one reason why this is so.

Most preamps have some fixed level of noise that will be present at the

output regardless of the volume setting. That means that the

ratio of the output signal to the fixed background noise will vary -

the greater the output signal, the smaller the portion of the output

that comes from the noise. At some point, of course, the output

will start to distort as you begin to exceed the linear operating range

of the circuit, but let's stop short of that.

Now connect that preamp to two different amps. We have one amp

rated at 200 watts/channel/8 ohms that needs an input of 0.707 volts to

drive it to maximum output. We have another amp that has a max

output of 15 watts/channel/8 ohms, and it needs 2 volts to achieve that

level. We have a magic speaker that has an impedence of exactly 8

ohms that doesn't vary with frequency. The speaker will product

90dB at 3 feet when fed one watt of power. We need 2.83 volts

from our amps to provide one watt. I'm not sure I did the math

exactly right here, but I think that equates to needing to drive amp

one with an input signal of 0.05 volts and amp two with an input signal

of 0.517 volts. The signal from our preamp when outputting .517

volts is going to have a much higher signal to noise ratio that when it

is outputting 0.05 volts, and will likely make the system with amp two

in it sound better than the system with amp one.

Add to this the fact that many circuits are less linear at the extremes

of their operating range, so the 200 watt amp might not be operating

within its most linear range.

Now, this is a gross oversimplification, of course. There a many,

many factors at work at each link along the chain (preamp factors, amp

factors, speaker factors) that might make one set of components sound

better at low power levels than another set sounds. My point was

you can point to at least a couple of things that are real, measurable

and will have an impact.

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Don't forget too that the volume control is not linear (this is where it really gets fun). The volume potentiometer is logrithmic. That is it's resistance decreases as the gain increases output more in the first 1/4 turn than in the second 1/4 etc.

IOW the increase from 9 o'clock to 12 o'clock is greater than the increase from 12 to 3 o'clock.



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I had a feeling this would happen. Things got technical way too quick and you probably got lost along the way. In simple terms:

1. Matching of amp and pre-amp is probably more important to your sound quality than either the ultimate power of your amp (assuming it is sufficient to drive the speakers to an acceptable volume) or the cost/quality of the individual components.

2. The relation of the dial to the listening experience is only important at the extremes - which are not usually that important anyway. Generally the more you turn the dial to get to your required volume the better, both as a result of the fixed noise level of the pre-amp and the degree of flexibility it gives you in choosing your listening level.

Obviously with the latter point you do want the pre-amp to at least get close to the full power of the amp when turned to its extreme point. Note that many implementations actually get to the extreme point too early and you will often find that an amp will start to clip with the volume control only a little over half way open. That this happens in integrated amps is unforgivable IMHO.

Further - the reason that I asked if you listened to the VRD at more moderate volume levels is that it is there that you would expect to find the greatest improvement in sonic quality over what you have now. Your Crown amp is designed to run all day near its maximum power - a good thing if all you want is volume, but when it runs at that magic first watt its distortion SHOULD be higher than that of the VRD by a considerable multiple.

The reverse is also true of course - distortion from the VRD will be at its highest when the output nears its limits. That you still found the VRD to be the better amp at these levels speaks volumes for its quality.

Remember that listening at, say, 90 dB at the listening position in Born2rocku's system is going to be less than 1 watt of power from the amp. In your room - with your nearer listening position it will be considerably less again. Therefore, I would expect the VRD to cream your amp in your house, on your speakers at these levels - and 90 dB aint low level listening in my book.

As a final point when you get the upgraded X-overs in I would not be surprised if you find the result too bright. I think your old nackered networks may well be hiding this to a degree. Again - the VRD would rescue the situation and far more dramatically than you have seen/heard to date.

This could get expensive....

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"Well, there you go! 37.5 watts. My $8.00 voltmeter and Duke's 39-watt calculation were just about dead-on. And everyone ragged us hard. "

That is 37 watts *to the woofer*... if the fuse is on the woofer alone. Some of the total power will be going to the other drivers too. IOW the total power would be higher.

If the fuse is on the input to the crossover (everything went silent when the fuse blew) that wattage number likely isn't correct. You would need to know the overall impedance of the crossover/speaker to get an idea of how much power it would take to blow the fuse. If it was an 8 ohm load it would take 50w... 16ohm would need 100w...etc..etc...


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