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Old 11th October 2012, 08:57 AM   #1771
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by DrDyna View Post
Dan D'augustino would like a word with you Andrew!
What are you referring to?
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Old 11th October 2012, 09:01 AM   #1772
AndrewT is offline AndrewT  Scotland
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Thinking about how to stack multiple speakers can be done from two directions.

Line array for long distance listening:
stack the speaker mouths in a tall column to simulate a line array and the "sound" will travel farther out into the distance due to the different "line array" drop off of level with distance.

Wide bandwidth for closer listening:
where distance is less important there is a small gain in LF bandwidth in grouping all the mouths together so that the corner to corner extremities of the mouth group is as short as possible.
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Old 11th October 2012, 11:34 AM   #1773
DrDyna is offline DrDyna  United States
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Originally Posted by AndrewT View Post
What are you referring to?
I was just pulling your chain a little bit about this:

"Add second speaker to an amplifier and you do not get double the power. Some amplifiers will go into protective limiting, some will give 40% to 60% more power, some will give 60% to 80% more power, none will give 100% more power, none!"

Krell has been making amplifiers that double perfectly into halving impedance for a long while now, unless that's not what you're talking about.
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Old 11th October 2012, 11:42 AM   #1774
AndrewT is offline AndrewT  Scotland
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........Krell has been making amplifiers that double perfectly into halving impedance ..............
I disagree.
As far as I know all Krell amplifiers do not have an infinite damping factor. This indicates that no Krell amplifier has zero output impedance.
In addition no unregulated PSU has zero output impedance.
Combine these two effects and you will find that all amplifiers do not double their output current when the load value is halved. None.
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Old 11th October 2012, 07:35 PM   #1775
djk is offline djk
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It's a paper spec game, under-rate the power at 8Ω and claim it doubles at 4Ω.

I remember seeing these for the first time and thinking 'these are nice, this is how I would build an amplifier if I had lots of $$$ and access to the parts'.
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Old 11th October 2012, 08:07 PM   #1776
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Originally Posted by AndrewT View Post
I disagree.
As far as I know all Krell amplifiers do not have an infinite damping factor. This indicates that no Krell amplifier has zero output impedance.
In addition no unregulated PSU has zero output impedance.
Combine these two effects and you will find that all amplifiers do not double their output current when the load value is halved. None.
I agree with you, but for an amp like the SpeakerPower SP1-4000 which is rated (and delivers) 1300W/ at 8 ohms, 2400W/4, 4000W/at 2 ohms, the difference between a perfect voltage source and that amp is only about 1 dB going from an 8 ohm to a 2 ohm load.

That said, there are many amps on the market that have so much current limiting they actually put out less power at 2 ohms than 4 ohms.

As usual, testing the amp's output SPL with the actual speaker load is always important, specs often don't tell the real story of what will happen with low frequency signals.

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Old 12th October 2012, 09:11 AM   #1777
AndrewT is offline AndrewT  Scotland
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2400W ref 1300W is a 0.7dB reduction when load value is halved. That is a very good figure.
4000W ref 2400W is a 1.6dB reduction when load value is halved. That is a mediocre figure.
The total reduction is 2.3dB when the load is quartered. For a 4ohms rated amplifier that is a very good figure. I would be more than happy with that "stiffness" of supply.

But I would need 1r0 data to assess if this amplifier is rated for 2ohms use.
Yes, I need to see performance into lower resistances than nominal speaker load to complete this part of the assessment.

But for this example where 1.6dB has been lost just going down to 2r0, I would put my experience forward and suggest this amplifier will lose more than 2.5dB when trying to drive 1r0. That for me would be a failed 2ohms capable amplifier specification.
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Old 12th October 2012, 10:33 AM   #1778
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Although the discussion is on doubling of P.A. amplifier power into every halving of load impedance, just wanted to say that Elektor had many years ago published 'A Medium Power A.F. Amplifier' by T.Giffard which doubled power down to 2 ohms. I repeat, it was a 'medium power' amplifier.

So to say, no amplifier can do that is taking the generalization a little too far. It can be done upto a certain power, but perhaps, practically impossible for powers involved in P.A.
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Old 12th October 2012, 11:08 AM   #1779
Mark Kravchenko --- www.kravchenko-audio.com
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So to say, no amplifier can do that is taking the generalization a little too far. It can be done upto a certain power, but perhaps, practically impossible for powers involved in P.A.
It quickly becomes a very expensive enterprise to put together an amplifier that is a true voltage source. Either very large transformers or very robust switching systems, both limited to what can be drawn off of the wall outlets.

It is not an engineering impossibility.

As for the Speaker Power amplifier.

About 3 years ago I did an exhaustive study on available plate amplifiers for a client. At least then Speaker Power was using ICE modules. They were supposed to be changing over to Pascal modules. I don't know that for a fact by the way.

Pascal I have used extensively.

Really good amplifiers, but they do not double power down to 2 ohms.
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Old 12th October 2012, 11:20 AM   #1780
DrDyna is offline DrDyna  United States
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Quote:
Originally Posted by AndrewT View Post
2400W ref 1300W is a 0.7dB reduction when load value is halved. That is a very good figure.
4000W ref 2400W is a 1.6dB reduction when load value is halved. That is a mediocre figure.
The total reduction is 2.3dB when the load is quartered. For a 4ohms rated amplifier that is a very good figure. I would be more than happy with that "stiffness" of supply.

But I would need 1r0 data to assess if this amplifier is rated for 2ohms use.
Yes, I need to see performance into lower resistances than nominal speaker load to complete this part of the assessment.

But for this example where 1.6dB has been lost just going down to 2r0, I would put my experience forward and suggest this amplifier will lose more than 2.5dB when trying to drive 1r0. That for me would be a failed 2ohms capable amplifier specification.
Just to continue being difficult on this issue because I'm a little bored, here's a snip from an old KSA-250 review. Really old, but still fun.

"Looking at the KSA-250's maximum output power into varying impedances (see Sidebar "Class-A?") with one channel driven with a 1kHz probe signal (fig.5, which plots THD+Noise vs output power), we can see it clips (1% THD) at 325W into 8 ohms (25.1dBW), at 635W into 4 ohms (25dBW), and at over 1000W into 2 ohms (actually 1066W at 0.97% THD, or 24.3dBW). When driving 1 ohm, the KSA clipped at 1548W (22.9dBW). This is approaching perfect voltage-source behavior, and could be expected to be closer to the ideal of a doubling of power with each halving of impedance if the AC line voltage were held constant. The AC line voltage sagged during these tests to 116V (8 ohm testing), 114V (4 ohm testing), 112V (2 ohm testing), and 106V (1 ohm testing), from 117V at idle. All maximum power output measurements exceeded Krell's specifications except the 1 ohm measurement, which is a result of the line voltage dropping so low. With a regulated AC voltage, the KSA-250 could probably be expected to put out 2kW into 1 ohm. Although Krell specifies a 4kW output rating in 0.5 ohms, I didn't have a half-ohm resistor that would handle such power."

It would appear that if you're going to do testing on an amplifier like this, you should probably have a 0-0 gauge mains cable and be less than 50 feet from the power substation. :P
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