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Old 21st December 2012, 07:54 PM   #71
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Originally Posted by wrenchone View Post
That's one way - why don't you try it? I have other uses for the switching regulator (remember, it's also knocking down 64VDC to 45VDC), and I'll use the technology in other projects.
I'll work with a circuit like yours in Post #14. It will be a lower power device than you are using. I'll still use the byposs mode circuit for looks.
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Old 21st December 2012, 10:26 PM   #72
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Not really - that's what I'm using, and it eats 45V at ~2A for each channel. The "L'Fake Lite" is hilariously inefficient, as is any solution of that nature (resistive loading). However, Papa swears that the resistively loaded solutions sound better than active loads.
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Old 21st December 2012, 11:17 PM   #73
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The voltage/current regulator is still a work in progress, but results are getting encouraging. Attached are the rise/fall times of the buck converter output voltage waveform. I may place a little bit of resistance (just a few ohms) in series with the gate of the switching fet to clean up the rising edge of the switching waveform.
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File Type: gif HYS_tr-tf.gif (14.2 KB, 245 views)
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Old 21st December 2012, 11:26 PM   #74
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Attached is a picture of some of the operating waveforms for the regulator. The operating frequency is a little high - I want it to be around 60kHz at max load~4A. Shown here is the waveform at 2A load. The top waveform is the switching waveform at the input of the output inductor, and the bottom waveform is the output inductor, these two waveforms give one insight into operating frequency, duty cycle, programmed current hysteresis and the overall health of the converter. The circuit is designed around an LM393 dual comparator, a cheap readily available single-supply device that has the ability to sense near ground. A faster comparator with more output drive would be a good thing, and the LM311 is a possible candidate. I may need to add a negative rail to its supply voltage to make it behave. Efficiency is around 89%, including the losses in the toroidal transformer driving this regulator.

At any rate, I'll post a schematic when I'm satisfied.
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File Type: gif HYS_fop1.gif (8.9 KB, 242 views)

Last edited by wrenchone; 21st December 2012 at 11:31 PM.
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Old 22nd December 2012, 02:16 AM   #75
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Originally Posted by Antoinel View Post
wrenchoneThe ideal name for this negative feedback needs to be Pass Feedback in favor of Black and/or Schade.
I don't think this name would become popular outside of our little sandbox.

It is true, however, that there are several ways of achieving Schade effect.

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Old 22nd December 2012, 02:51 AM   #76
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... Papa swears that the resistively loaded solutions sound better than active loads.
what about reactive loads - does the use of a choke sound as good as using a resistor ?
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Old 22nd December 2012, 12:52 PM   #77
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Originally Posted by Nelson Pass View Post
I don't think this name would become popular outside of our little sandbox.

It is true, however, that there are several ways of achieving Schade effect.

Fortunately, our little sandbox is a worldwide stage due to the proliferation of diyAudio as a social-scientific medium. I hope that DIYers recognize, appreciate, and use the term Pass Feedback in their communications [I will]; because it has special performance attributes absent in generic NFB.
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Old 22nd December 2012, 06:09 PM   #78
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what about reactive loads - does the use of a choke sound as good as using a resistor ?
Mr. Pass is a Physicist, and may view a lit incandescent lightbulb as a thermionic device. Electrons boil off the Wolfram wire, form a mobile sheath/cloud around it, and contribute to its classical solid state resistance. A hypothesis maybe that hot electrons may impart a "better" sound than cold ones meandering through a luke warm wirewound or Carbon composition power resistor.

wrenchone: Consider replacing the power resistors in the source leg of the MOSFET with lightbulbs too. Go all the way. Hot electrons for cool sound! They maybe even hotter and sounding different by way of a lit Halogen lamp load. Rothacher's first generation SIT amp may have a totally different "thermionic" sound with a Halogen lamp instead of a Tungsten only load. It is a variable for study.
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Old 22nd December 2012, 10:07 PM   #79
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You know me, a sucker for incandescence.

A string of 100 small clear light bulbs is used to decorate X-Mas trees. The bulbs are connected in series and operate directly from the power line. I pulled one off a spent string and energized it with a variable DC voltage. It glowed orange-yellow, drew ~60 mA with ~ 1V or less across it. Maybe, 2 such bulbs can be used in BA3 Front End replacing R10 and R11 = 22 Ohms. The capacitor C3 need to be non-polarized film should one bulb fail in operation. Thus, we will have a Light Burning Amp3 [LBA3] Front End. But; will this transformation make a great sounding amp sound even better?
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Old 22nd December 2012, 10:52 PM   #80
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Attached is a prelim schematic for the hysteretic buck converter I'll be using to power L'Fake Lite. It's based around a commonly available comparator (LM393). The converter sits on top of a ~60-65V rail and is biased with a floating winding that happens to be on the same toroid I'm using for the main power. A separate small transformer could also be used.

Floating the regualtor on the DC rail makes current sensing relatively easy, but makes drive a little problematic. The regulator could also sit at ground with the current sense in series with the return leg, but there are stilll issues. I also like to keep the return uncluttered. Let's just say that there are several ways of accomplishing this, none without issues. Drive transformers are out, as I want to operate pretty much up to 100% duty cycle.

Transistors Q3 and Q4 are used to speed up the drive. Q1 is the main switch, and was what I happened to have hanging around. There are some Infineon parts with much lower on resistance I might try when I get around to ordering some. Diode D1 is a 30A 100V Schottky. There are large bulk caps stationed before and after the regualtor.

Resistor R16 provides the comparator hysteresis that make this thing work. Gimmick C6 provides overdrive at the edges of the switching waveform. This goes a long way toward cleaning up the switching waveform. I suspect a ~2pF NPO disc would work as well, but a gimmick can be twisted together from some hookup wire in a jiffy (I used some wire wrap wire), and works just fine.

As this is still a work in process, expect a few changes. I'll probably do the ultimate start-up test tomorrow to test this circuit's ability as a current regulator, but right now, it functions just fine as a high power voltage regulator.
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File Type: gif HYS_BUCK_SCH.GIF (25.3 KB, 179 views)

Last edited by wrenchone; 22nd December 2012 at 10:54 PM.
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