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Re: M60's on a raised platform?
BobG #406589 08/15/14 10:43 AM
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4200 sq feet? That's 3x bigger than my house. You sure about those numbers? wink


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Re: M60's on a raised platform?
BobG #406590 08/15/14 01:33 PM
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Cubic feet is ft^2, right ?

D'oh !!


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Re: M60's on a raised platform?
bridgman #406593 08/15/14 05:22 PM
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Since I can no longer edit the post and there's a page break between Ken's bug report and my original post I guess I'll put a fixed copy here.

Originally Posted By: bridgman
BTW this is getting terribly off topic but I do have to say that the bigger amp really made a difference in my enjoyment of the system. The speakers are in a large room (~4200 ft^3 for the immediate listening area) and that space opens directly into at least another 5000 ft^3.

BTW is there a well-understood technical term for the point at which a power amp starts clipping (ie continuous power + headroom) ? There used to be a lot of talk about "peak power" and I *think* that meant "power at clipping" but IIRC there were a few different potential interpretations depending on whether or not you factored in rail voltage droop and how hard the amp had been working before the peak came along.

Reason I'm asking is that I'm wondering how much of the improvement I would have seen with a high quality ~100W power amp. It's probably fair to assume that such an amp would have more relative headroom than the amp in my receiver so the effective power increase would be more than the 65W-to-100W change would suggest, but I'm not sure -- and am trying to talk myself out of picking up a used 100W amp to test with.

I don't really have time to get into amp-swapping at this point, and Schmidt's law still applies ("if you mess with a thing long enough it'll break").

That info seems like something that would be useful for people thinking about upgrading but not sure if it's practical to spec that way or if the dependency on "shape of the signal" is too much to work around. I guess some kind of standard test waveform would be needed and that's 20 years lost in a standards committee right there.

Last edited by bridgman; 08/15/14 05:39 PM.

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Re: M60's on a raised platform?
BobG #406594 08/15/14 06:57 PM
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I'm in love with my Pioneer SC-67 with 140 watts X 9 channels. It was a little on the pricy side, but I've never hit a volume level yet that has hurt my ears. I don't have a need for that high of a volume, but the headroom is very high. Makes for good movie watching.

Re: M60's on a raised platform?
bridgman #406603 08/16/14 02:38 AM
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John, there really isn't a well-accepted term for the beginning of amplifier clipping. To be meaningful it would have to involve an audible level of distortion, not simply the beginning of the flattening of the top of the sine wave when viewed on an oscilloscope. Various distortion levels have been used in published lab tests to represent "clipping". At one extreme the previous Sound & Vision tests used a 0.3% THD level for their clipping power level, although acknowledging that this was well below actual audibility on music. More commonly(e.g. Home Theater Magazine) a 1.0% level has been used to represent the threshold, although this still wouldn't be audible on much music.

So, it isn't really necessary to consider a term such as "headroom" in determining the maximum audibly clean output available. If well-run lab tests on a particular unit are available, an effective clipping level might be considered to be the point at which distortion exceeded 1.0%.


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Re: M60's on a raised platform?
BobG #406609 08/16/14 05:27 PM
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Thanks JohnK. I think you're saying that the only really useful measurement of "what an amp can do for you" is a distortion vs power graph - am I understanding you correctly ?

I guess what I'm wondering about is that even such a graph could deliver different results depending on how long the input signal lasted at each power level. It seems to me that there are sorta-kinda three levels that matter :

1. Continuous power -- no clipping, no overheating, no overloading devices (do I have that right ?)

2. Power at "clipping" (with the caveats you mentioned) where the test signal is long enough to drag down the power supply rails but not long enough to overheat

3. Same as #2 but where the test signal is short enough that the main power supply caps don't discharge much and so rail voltage stays high

It seems to me that "headroom" as people talk about it is probably closer to #3 than to #2, but still somewhere in between since the amp is already being driven at a lower average level which may be enough to start dragging down the rail voltages anyways.

I imagine that on some (most/all ?) powerful amps the power supply is sufficiently beefy that rail voltage drop & power supply ripple don't get too bad even at full rated power, but I'm also trying to figure out what a suitable "headroom or whatever we want to call it" rating for typical amps found in average receivers might look like.

Put differently, in big-ass amps everyone talks about headroom "over and above" the continuous power rating while for typical receivers & integrated amps you don't hear those comments (although that could obviously be a function of target audience), so being me I'm obviously trying to figure out if there really is a difference (I think there is) and if so to quantify that difference.

http://dilbert.com/strips/comic/2007-04-05/

Last edited by bridgman; 08/16/14 06:00 PM.

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Re: M60's on a raised platform?
bridgman #406621 08/17/14 01:57 AM
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John, yes that is pretty much my view of the matter after studying over the years the significance(or non)of more complex factors. Headroom is the power available above the typical 1 watt(not the maximum continuous rating)used at a comfortably loud level. Brief peaks on highly dynamic material such as some classical recordings can be as much as 20dB above the average level, which would therefore require in the neighborhood of 100 watts. A receiver amp which can supply this(at the more meaningful 1% THD level)is entirely sufficient in headroom for this use.

As you indicate, there can be higher power outputs stated for extremely brief periods of time. This was embodied in the previous IHF Dynamic Power rating, which was largely continued in the present EIA/CEA 490-A voluntary standards (see section 5.1 et seq). Yamaha briefly discusses this in a simplified manner here . The amp is tested with a 20 millisecond burst followed by a 480 millisecond period down 20dB at the continuous level and then repeated. 20 milliseconds is viewed generally(not totally accepted)as the time involved in music's transient peaks. The power available for the burst(at 1% THD or clipping observed on an oscilloscope)is the rated dynamic power and typically may be on the order of 20% higher than continuous power. For example, my least powerful receiver has a 75 watt continuous rating and a 95 watt dynamic power rating. Not all manufacturers specify a dynamic rating, but it's a valid measurement which can be given a small amount of consideration and may be the closest thing to the number that you're asking about.


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Re: M60's on a raised platform?
JohnK #406624 08/17/14 03:55 AM
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Originally Posted By: JohnK
Headroom is the power available above the typical 1 watt (not the maximum continuous rating) used at a comfortably loud level.

That one sentence answers a lot of questions wink

Thanks !


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