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Post by Boomzilla on Jan 10, 2016 9:43:51 GMT -5
Emotiva's XPA-1L amplifiers have an automatic transition from Class A to Class A/B at a certain output.
Assuming 30 watts of Class A power into an eight-ohm load, what happens to the A - A/B transition into four ohm loads?
If the amp transitions on voltage, then 60 watts of Class A power would be available to four-ohm loads (the voltage @4-ohms is ½ that at 8).
But if the amp transitions on amperage, then only 15 watts of Class A power would be available to four-ohm loads (the amperage @4-ohms is 2x of that at 8).
And if the amp transitions on wattage, then the same 30 watts of Class A power would be available regardless of whether the amp was feeding eight or four ohm speakers (the reduced voltage @4-ohm loads would be offset by the higher amperage draw).
So what triggers an XPA-1L power amp from Class A to Class A/B operation - voltage, amperage, or wattage?
Thanks - Boomzilla
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geebo
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Post by geebo on Jan 10, 2016 10:49:36 GMT -5
Emotiva's XPA-1L amplifiers have an automatic transition from Class A to Class A/B at a certain output. Assuming 30 watts of Class A power into an eight-ohm load, what happens to the A - A/B transition into four ohm loads? If the amp transitions on voltage, then 60 watts of Class A power would be available to four-ohm loads (the voltage @4-ohms is ½ that at 8). But if the amp transitions on amperage, then only 15 watts of Class A power would be available to four-ohm loads (the amperage @4-ohms is 2x of that at 8). And if the amp transitions on wattage, then the same 30 watts of Class A power would be available regardless of whether the amp was feeding eight or four ohm speakers (the reduced voltage @4-ohm loads would be offset by the higher amperage draw). So what triggers an XPA-1L power amp from Class A to Class A/B operation - voltage, amperage, or wattage? Thanks - Boomzilla 35 watts into 8 ohm and 17.5 into 4 ohm loads. emotivalounge.proboards.com/post/721576 |
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Post by Boomzilla on Jan 10, 2016 10:50:04 GMT -5
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Post by garbulky on Jan 10, 2016 10:52:34 GMT -5
Ok according to Lonnie the answer is a bit of both.
The short answer is
at 4 ohm loads
XPA-1 L: 17 watts class A
XPA-1: 30 watts class A
Long answer - which I'm not sure I explained it clearly. Maybe somebody can post Lonnies explanation.
For the XPA-1 L
The class A bias circuit switches whenever the power consumption is past 35 watts of class A as measured in to an eight ohm load. So the amp will always switch (unless temperature prevents it) at that same amount of power output.
However...(also according to Lonnie) due to the way things work
The XPA-1 L will produce HALF The watts in to a four ohm load for class A.
So the XPA-1 L produces 17 watts class A at 4 ohm loads.
(The XPA-1 I assume produces 30 watts class A at 4 ohm loads. )
But that doesn't mean it detects it and cuts the power by half. It still produces the same 35 watts of class A @ 8 ohms at the amp . It just means that's how it works when it goes in to a 4 ohm load. So the speaker sees 17 watts.
Anyway that is unforutnate. But I guess that's electricity. I didn't quite understand though.
As the XPA-1 L will produce 17 watts of class A at 4 ohms. But it will double the power output to I think 500 watts at 4 ohms regular class AB power.
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Post by DavidR on Jan 10, 2016 11:02:25 GMT -5
So W = V x A. Is it that in Class A mode the voltage is higher and amperage is lower than when in A/B mode?
Never mind. I need another cup of coffee.
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Post by DavidR on Jan 10, 2016 12:22:52 GMT -5
Ok according to Lonnie the answer is a bit of both. The short answer is at 4 ohm loads XPA-1 L: 17 watts class A XPA-1: 30 watts class A Long answer - which I'm not sure I explained it clearly. Maybe somebody can post Lonnies explanation. For the XPA-1 L The class A bias circuit switches whenever the power consumption is past 35 watts of class A as measured in to an eight ohm load. So the amp will always switch (unless temperature prevents it) at that same amount of power output. However...(also according to Lonnie) due to the way things work The XPA-1 L will produce HALF The watts in to a four ohm load for class A. So the XPA-1 L produces 17 watts class A at 4 ohm loads. (The XPA-1 I assume produces 30 watts class A at 4 ohm loads. ) But that doesn't mean it detects it and cuts the power by half. It still produces the same 35 watts of class A @ 8 ohms at the amp . It just means that's how it works when it goes in to a 4 ohm load. So the speaker sees 17 watts. Anyway that is unforutnate. But I guess that's electricity. I didn't quite understand though.
As the XPA-1 L will produce 17 watts of class A at 4 ohms. But it will double the power output to I think 500 watts at 4 ohms regular class AB power. I haven't gotten my head around this yet and as a result I don't get it yet either. |
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Post by Gary Cook on Jan 10, 2016 15:55:37 GMT -5
The move from Class A to Class AB is not a switch, it's a change in biasing. Not 17 watts -----> switch ----> 500 watts. It's entirely possible to go from, say, 17 watts in Class A to, say, 18 watts in Class AB if that's what the speakers and the source demands.
Cheers
Gary
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Post by Boomzilla on Jan 10, 2016 19:32:33 GMT -5
Thank you Mr. Cook - exactly correct. The remainder of this is simple volts x amps = watts. Change one, change the other two. The other significant equation is Ohm's law where E/IR applies. E is volts or Electro-motive force, I is amperes or current, and R is resistance (or in the case of a speaker, impedance which takes into account not only the speaker's resistance, but also its inductance and capacitance.
So speaking, specifically, of the XPA-1L, as the resistance drops to half, the current doubles. Therefore, for an 8 ohm load, 35 watts of Class A before the transition occurs. For a 4 ohm load, 17.5 watts of Class A before the transition occurs. For a 2 ohm speaker, you'd get but 8.75 watts. If you had a 16 ohm speaker (old Electro-Voice, maybe) you'd get 70 watts of Class A before the transition.
In a nutshell, this is one of the reasons why Class-A Single-Ended-Triode tube amps DON'T like low impedance loads. They have no "automatic transition to class AB." When their maximum wattage is exceeded, they clip - period.
Clear as mud?
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Post by DavidR on Jan 10, 2016 20:12:34 GMT -5
Thank you Mr. Cook - exactly correct. The remainder of this is simple volts x amps = watts. Change one, change all. The other significant equation is Ohm's law where E/IR applies. E is volts or Electro-motive force, I is amperes or current, and R is resistance (or in the case of a speaker, impedance which takes into account not only the speaker's resistance, but also its inductance and capacitance. So speaking, specifically, of the XPA-1L, as the resistance drops to half, the current doubles. Therefore, for an 8 ohm load, 35 watts of Class A before the transition occurs. For a 4 ohm load, 17.5 watts of Class A before the transition occurs. For a 2 ohm speaker, you'd get but 8.75 watts. If you had a 16 ohm speaker (old Electro-Voice, maybe) you'd get 70 watts of Class A before the transition. In a nutshell, this is one of the reasons why Class-A Single-Ended-Triode tube amps DON'T like low impedance loads. They have no "automatic transition to class AB." When their maximum wattage is exceeded, they clip - period. Clear as mud? Now it makes sense. Thanks. |
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