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Low xmax (Fostex) - vs "healthy" (TB)/heat & real world dynamic capibility-?
Low xmax (Fostex) - vs "healthy" (TB)/heat & real world dynamic capibility-?
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Old 1st May 2010, 03:01 PM   #1
freddi is offline freddi  United States
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Low xmax (Fostex) - vs "healthy" (TB)/heat & real world dynamic capibility-?
Default Low xmax (Fostex) - vs "healthy" (TB)/heat & real world dynamic capibility-?

How does Fostex rate the xmax of their drivers? - is it the difference from voice-coil winding height from the magnet circuit's top plate? There's a rule for 70% BL where Qts has effectively doubled. Are there estimates for other Qts changes?

how hot are the voice coils getting with certain music and listening levels and how much compression there may be occurring? What size amplifiers are most FR'ers using and how often do they clip?

I presently don't have an AB switch but memory says FE206EN subjectively does about as well in an early Karlson 12 as W8-1772 with regards to bass drum. 1772's suspension and motor allow it to travel further on certain passages but that may not be doing much good in this circumstance.

just rambling....
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Old 1st May 2010, 04:00 PM   #2
Andrew Eckhardt is offline Andrew Eckhardt  United States
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As you caution yourself I could easily be talking out my can here but looking at their high Re for some popular drivers and the low Xmax I think you can expect to find real short coils wound of fine wire. That might make sense considering the reports of short term overload burnout. To really know what's going on I'd like to have a couple of these fried drivers to tear down. You'd get all the answers that way, published or home brew electro-acoustic measurments aside. These drivers seem to be prized for their rise time and low end performance in acoustic arrangements that load down the cone (using the usually low Qts) real hard, to limit excursion for satisfying bass, but you probably know all that. They're not going to handle copious amounts of power. Definitely designs pushed far in a certain direction.

Last edited by Andrew Eckhardt; 1st May 2010 at 04:04 PM.
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Old 1st May 2010, 05:27 PM   #3
gripracer is offline gripracer  United States
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Here is a non-scientific test I use to compare the dynamic compression limits of two full-range drivers -- Run as loud of a 20-30hz tone into enclosed driver #1 as you can without audible distortion, and then play a human voice on top. Repeat for enclosed driver #2. You will be able to hear the effect that pushing a driver out of it's best magnetic gap has, and the dynamic ability of each driver will be obvious.

This is easiest when using a PC as a digital transport.
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Old 3rd May 2010, 04:00 AM   #4
freddi is offline freddi  United States
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Low xmax (Fostex) - vs "healthy" (TB)/heat & real world dynamic capibility-?
Default Amplifer power for fullrange speakers -?

is there a sweet spot for amplifier power? with Odenkoza's "Devils On Drums" there's percussion with a lot of midrange-high frequency energy - I almost bet it needs a fair amount of power to play without undue clipping with speaker such as 1772. 1772 sounds good on these transients. I have about 25 watts.
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Old 3rd May 2010, 06:26 AM   #5
gripracer is offline gripracer  United States
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Quote:
Originally Posted by freddi View Post
is there a sweet spot for amplifier power?
Generally, solid-state amplifiers will sound their best near their rated RMS power. SS THD curves generally gradually slope downwards to a certain point, and then rapidly rise beyond that point. With tubes though, the best sound is achieved down low in the watts. People say it is best to listen to a 300B down in the milliwatt range.

If you are referring to a sweet spot in amplifier power, with a relationship to a driver, it depends on the driver.
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Old 3rd May 2010, 09:44 AM   #6
CLS is offline CLS  Taiwan
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So, that means sound quality of the system is closely related to the THD curves of the amp?

I don't have any hard evidence or mearsurement records against that, but I doubt that's the case.

How can we keep an SS amp operating close to its rated power and 'enjoy' its best THD at the same time? With PA speakers and 50m away outdoors?
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Old 3rd May 2010, 10:58 AM   #7
T101 is offline T101  Bulgaria
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Default jnd

Here are some measurements taken from Ymaha MX-1000 (2KW power suply - 2X1 KW transformers - non US model stock!)

All THD is +30 db in order to appear on the scale... The measurement conditions are not laboratory and the tested model is used.

Probably here there are some people capable of understanding the data better than me so I'm not going to make conclusions...

The only thing that I know is that I was told that only the best power amps have such curve of THD - up and down throughout the power band.

Edit: all power on the horizontal axis must be multiplied by 4 as stated in the graphs, so 100W is actually 400W and so on...
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Last edited by T101; 3rd May 2010 at 11:10 AM.
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Old 3rd May 2010, 11:07 AM   #8
T101 is offline T101  Bulgaria
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Here is the other part of the measurements.

All I can say is that there is bass even when the digit indicator states 0.0 watts and the bar graph indicator says 0.03W or even 0.003W as a matter of fact that amp and I guess every amp with such characteristics supplies all frequencies evenly from the lowest possible power to the highest!

I compare with a modern smaller Yamaha RX-497, and system consisting of big bass driver, light membrane full range and ribbon tweeter.

With the small amp at really barely audible levels first you start to hear the ribbon tweeter - lightest membrane I suppose, then you hear the full range join and last joins the heaviest driver in the system - all this happens under 0.1 watt... With the big Yamaha there is no such phenomenon...

From my experience the MX-1000's first "sweet point" is between 0.2 and 0.7 watts mean level not counting the peaks, but that might be my ears as well as system features.

Best Regards to all!
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File Type: jpg Y40R4.jpg (47.1 KB, 6 views)
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Old 3rd May 2010, 02:22 PM   #9
gripracer is offline gripracer  United States
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Quote:
Originally Posted by CLS View Post
How can we keep an SS amp operating close to its rated power and 'enjoy' its best THD at the same time? With PA speakers and 50m away outdoors?
You are assuming that all SS amps produce a lot of power -- of course this is not the case.
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Old 3rd May 2010, 02:55 PM   #10
T101 is offline T101  Bulgaria
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Quote:
Originally Posted by gripracer View Post
You are assuming that all SS amps produce a lot of power -- of course this is not the case.
It is well known that with speaker sensitivity under 98 db/w or so every amp under 100W is inadequate for reproducing the actual dynamics of the record.

Actually most amps have dynamic headroom that is too small, and every thing under 2 db is much less than needed.

And if the case is that one has on his disposal a 100W amplifier, then the maximal mean level when the whole dynamics of symphonic orchestra is available is 5 to 10W. All 90 watts above 10 are headroom reserve for peaks, otherwise for instance bass output will be compromised...

A 50W amplifier is good only up to 1-2 watts of mean level. And will compromise the THD every time the signal pushes it where it can't go. There will be lack of dynamics and THD...

A 400W amp is good to 17-30 watt mean power level... a 1000 watt amp is good up to 30-50-70 watt mean power level.

I read an article on amp power reserves and there were stated the maximal mean power levels available for different powered amps with retaining the maximal usable dynamic range of the amplifier.
The author noted that after you go past the limit for maximal mean level, your power ends very quickly with end result only a very few decibels and very much distorted and monotonous reproduction.

Best Regards!
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