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Old 7th October 2013, 09:58 PM   #31
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On Saturday (the day before the 2013 BA), I was in the lab at work running gain-phase plots on the Mutant with our venerable HP 4194A analyzer. The thing (the analyzer) is huge, built like a tank, and good from 20 Hz to 40MHz.
One thing I found is that a Zobel RC output network is a requirement, as without it, the gain passes 0dB, curves around, and comes back through zero again - not nice. A 10 ohm +33 nF series RC network across the output terminals makes the gain stay below zero after the crossover frequency. That's not enough, however.

The attached plot shows the gain-phase with the output RC network in place. Gain crossover frequency is respectable at 2.4 MHz, but there is only 8 degrees of phase margin - oops!

The second plot shows the gain and phase with the output RC and a 100pF compensation capacitor across R15 - much, much better. The gain crossover is a little higher at 2.7 MHz, but the phase margin has increased to 89 degrees - whew!. My only complaint now is that the gain is crossing zero where the phase has started to really dive down. I would feel much more comfortable if the crossover occurred a little earlier where the phase is relatively flat. This may be possible by changing the compensation capacitor to a somewhat higher value, say, 150 pF instead of 100. I may try that and report back. At any rate, when I get some time, I'll post an updated schematic showing the zobel and compensation in place. The amp acquitted itself pretty well yesterday at Burning Amp. The new heat sink keeps the output devices cool enough to allow extended operation.
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File Type: gif Le_Mut_G-P1_100513.GIF (27.9 KB, 557 views)
File Type: gif Le_Mut_G-P2_100513.GIF (27.9 KB, 511 views)

Last edited by wrenchone; 7th October 2013 at 10:03 PM.
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Old 9th October 2013, 07:41 PM   #32
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Le Mutant had its day in the sun this past Burning Amp, kicking Variac's open baffle mid/tweeter and ported bass speakers with authority. The basic circuit could yield more power with more output pairs, and could also be used as the kernel for a class AB amp - more on that later.
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Old 10th October 2013, 07:56 PM   #33
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I tried a simulation last night where I used the Le Mutant circuit to drive some source followers for a classic AB setup (heavy on the A), and was gratified at the 10kHz distortion figures, enough so I'll probably build one using Exicon lateral devices for the outputs. What I like about the setup is that you get symmetrical drive with only a single-flavor input stage.
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Old 13th October 2013, 08:33 PM   #34
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The Mutant is playing in the background as I write this, driving a pair of Event 20/20 monitors. It has no problem with the Events, even though they're 4 ohms. The fan I installed is totally inaudible, and gives just enough airflow through the heat sink for 24/7 operation. I've been going through quite a number of CDs in the past 18 hours or so, and the sound is quite special.

I've finished a layout using the Mutant as input and driver stage, appending some Exicon mosfet source followers. I have high enough hopes for this concept to maybe give it my nice Antek case/heatsink combo (bought before the fire). I hope they carry those cases again - nice for a small to medium size amp. All their current case offerings are sans heatsink.
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Old 21st April 2014, 12:28 AM   #35
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I'm proceeding with a build of the Mutant using doubled output devices and +/-40V rails. This should net me somewhere around 70-80 watts of chunky Class A per channel, as the mutant topology offers good utilization of the available supply voltage.
What remains to be seen is how much dissipation I can get away with for each TO-247 package in the output stage. For doubled devices and 40V rails, I estimate about 40W/package, which is pushing things a lot. Tripled devices will get the dissipation down closer to 30W/package, which is much more reasonable. The heat sinks I have in mind to try first are 8" lengths of Heat Sink USA's big 10" wide extrusion that I scored recently from a fellow Diyer. I may also try something with fan augmentation. At any rate, this amp will blow about 160W per channel, so I'll need some impressive heat sinks, with or without fan. If the heat sinking is too much of a pain for doubled devices, I'll go to 3 devices (6 output fets/channel).

Last edited by wrenchone; 21st April 2014 at 12:41 AM.
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Old 12th May 2014, 08:08 AM   #36
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During the course of scaling up this amp, I encountered a couple of circuit techniques useful*in*boosting open loop gain and reducing distortion, while still being able to independently set the output stage bias ar reasonable levels. Simulations look promising, and. I'm laying out a board to see if the simulated goodness is the real deal.

I won't let the cat out of the bag just yet except to say that judicious application of current sources and mirrors is involved. I may have to chage the name of this amp to one more appropriate to its dependence on mirrors (lots of mirrors, but no smoke).
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Old 12th May 2014, 10:08 PM   #37
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Big question now is where I can find a 28-0-28V toroid good for 600VA or so.
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Old 13th May 2014, 04:59 AM   #38
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dumpster ?

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Old 13th May 2014, 05:46 AM   #39
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I haven't dumpster-dived in a while, but scrap here in Silicon Valley is these days jealously guarded, as there's money involved. I'll find a toroid somewhere, or use two... I may try an inductive input filter, since there will be no problem with minimum current for a Class A design like this one. That will take a lot of the heat off the transformer.
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Old 18th May 2014, 06:44 AM   #40
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Here is some food for thought - feel free to speculate why the extra current sources are there. This one's got a real-world layout that will wait for realization until I have a few other things out of the way.
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