Post by Dan Laufman on May 24, 2016 18:41:14 GMT -5
You can't make a meaningful decision about the quality of the Gen 3 amplifiers based on the number of output transistors per channel.
The Gen 3 amplifiers utilize a Class H power supply that greatly reduces the strain on the output devices and allows much higher efficiency than a conventional fixed rail design. This means that you need less power devices for a given output power. You also improve reliability by signifigantly reducing the thermal stress on the power devices. This is a very good thing.
Also, all things being equal, it's easier to drive fewer output devices in parallel than more. It greatly reduces the capacitive loading on the driver stage and allows for higher bandwidth and lower drive currents for a given output level. This allows us to design a "faster" power stage, so to speak.
Look at the single channel power capability of the XPA G3 amplifier module. It can deliver over 800 watts into a 2 ohm load.
The XPA-2 is an XPA-1 (more or less) repurposed to be a stereo amp. Even though it has 45,000 mf capacitance, it has 180,000 mf of capacitance (just wired differently) the same physical number as the XPA-1. I think that has a lot to do with why the XPA-2 sounds so good. The XPA-5 is not the same thing - though it's good at what it does.
The other thing is Lonnie mentioned the XPA-2 has a faster slew rate. It also has a little bit of extra class A. I may be wrong but I think it's a 5 watt bias.
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As someone who has had an XPA-2, XPA-3 and an XPA-5 (all Gen 1) I honestly couldn't hear any consistent difference between them once the SPL's were matched (my ears, my speakers, my room). I have tested them side by side for stereo 2.1 (cross over at 80 hz) music using a mono input signal ie; the FL and FR were reproducing the same music. I have used this method many times over the years and it picks it if there is a difference to be heard. Maybe if I had tested the XPA-2 full range (no sub) then I might have detected a difference, but that's not how I run my system.
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It's possible that the 'preferred' amp does better into a real speaker than the other. Measures of power into a resistor don't mean all that much, especially when you only use so much, no matter how powerful the amp. Unless you are driving low sensitvity speakers fairly aggressivly and/or the speaker in question is very reactive.
Sure, paralleling output devices lowers impedance and may (Depending on the gain structure of the amp AND Feedback) raise damping factor. DF is a Red Herring number once it gets above maybe 50 or so. Real world speakers AND the wires they connect to the amp with and internal wiring act to lower even the most optimisticly high published DF numbers. Don't forget the resistance of a Crossover is part of this as well.
And I've heard LOTS of controversy. Some swear the lower powered amp in a series sounds better. Some vote for the HIGHER powered amp. This is when you basically take a design and 'scale up'. Some PASS DIY amps use this idea to get various power levels. The Parasound A21 / A23 are cousins in this regard. A high bias scheme gets the A21 more class 'A' power, however.
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