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Old 13th November 2012, 11:58 PM   #61
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Originally Posted by wrenchone View Post
I did a little messing around in PSpice this afternoon, starting out with a current source loaded Schade cell using Darlingtons. If there is no buffer, the distortion into a 10k load is about 0.3% overwhelmingly 2nd & 3rd harmonic . For grins, I tried tacking on my usual p-chanel jfet buffer in front, with the results shown here. I was surpised by the drastic reduction in distortion with the buffer in place.

Edit - in practice, I would use parts in the TO-92L package for a little extra dissipation, as I'm running about 25-26ma of current in the output Darlingtons. The ones I have hanging around are the MPSW45 and a Fairchild TO-92L version of the MPSA63. This might make a nifty little lineamp if you don't mind the signal inversion.
wrenchone: In my schematics above, I assumed that the secondary winding of the transformer behaved like and/or mimicked the P-channel buffer in your schematics. Your simulations are useful and have already generated valuable data. Perhaps a headphone amp too.




Please examine the schematic of F5. I am seeing in it a Schade-style feedback. Here are my tentative observations of its elements:
  • The F5 output stage is comprised of complementary MOSFETs with joined opposed Drains as the output node. This node is intrinsically a current source as compared with the joined opposed Sources node which is a voltage source [by convention]. The current source amp is is an important requirement for Schade.
  • Unlike a classical Schade schematic, the front end of F5 is an inverting amp instead of a buffer; like the case of your schematic.
  • Loop feedback is applied to the summing junction which is the node of the opposed Sources of the front end complementary JFETs. This bullet point and bullet #2 are an interesting modification to classic Schade feedback. It is a different approach ;but I hope with the same beneficial outcome.
  • In the absence of loop feedback, this theoretically modified F5 [with its output at 0 VDC offset] defaults to a non-inverting current source amp.
Brgds.

Last edited by Antoinel; 13th November 2012 at 11:59 PM. Reason: tweak
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Old 15th November 2012, 05:09 PM   #62
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wrenchone. A stereo amplifier of the preceeding schematic sounded great driving the MCM full range loudspeakers. Deep controlled bass and detailed highs from these brute drivers.

Please consider the attached schematic in your pursuit of the Elusive SIT. It has the elements needed to implement the methodology of Schade feedback. The output stage is like that of F5 [opposed drains of complementary MOSFETs], and it has the simple topology of diy F6. Rf and Ri will give a tunable handle on the optimal tilt of its characteristic curves.

I used half-shaded sine waves for positive-going signals, and half shaded triangle waves for negative-going signals; so as to track the correct signal phase through the circuit.
Brgds
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Old 20th November 2012, 10:39 PM   #63
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wrenchone: Mr. Pass called his "open loop" F6 of gain = 38 dB [x 79] a current source amp in the 6moons audio review. Loop feedback was applied to a novel summing junction which is the bottom primary winding of the Jensen transformer. The resultant closed loop voltage gain of F6 is 15 dB [or x 5.6]. The ideal name for this negative feedback needs to be Pass Feedback in favor of Black and/or Schade. Mr. Pass practices this style of feedback in "FET" circuits like Schade did in vacuum tube circuits. For example in ACA and F5 too.

There is high probability that the intrinsic pentode characteristic curves of the output JFETs in the open loop current source F6 amp were reorganized to triode-like characteristics after injecting Pass Feedback in voltage source F6 amp.
Brgds.
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Old 27th November 2012, 08:32 PM   #64
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I've been working on a discrete buck switching regulator to act both as a voltage regulator and current limiter for the L'Fake Lite power supply. This will sit between two filter capacitors and take the rectified and filtered output of a 44VAC toroid (~62VDC) and turn it into 45VDC to run the L'Fake Lite, the object being to allow the AC to be snapped on without blowing the AC line fuses due to the current draw from the cold light bulbs. I'm using a discrete solution to be contrary (as usual), but also to avoid having to baby an IC. I'm about 70% of the way there. The attached picture shows my breadboard and a switching waveform. The remaining items on the plate are to increase the operating frequency (simple), and to reduce the dead time between switching cycles (the squiggly part of the attached waveform - we'll see). The converter was delivering 45V/2A with the heat sink barely warm, most of that dissipation probably due to the catch diode.

One could also do a pretty simple linear solution for this problem using a power darlington or mosfet, but that would blow a bunch of power, and the L'Fake Lite is hilariously inefficient as it is...
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Old 19th December 2012, 06:47 AM   #65
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I started off with doing a discrete borderline discontinuous buck converter (shown in the previous picture) for voltage/current regulation duty for L'Fake Lite to counter the nasty inrush current from the light bulb load seen at startup (it blows mains fuses - not nice). Nelson suggested unscrewing a couple of bulbs before hitting the "on" switch, but I want a more smoothly integrated solution than that, and after all, I am supposed to be an SMPS engineer in the glaring daylight hours, no matter what I do when I climb back into the coffin...

At any rate, the self-oscillating RCC solution has issues with repeatability, as well as relatively high levels of output ripple current and peak switch current, so I devised an alternate solution utilizing a discrete hysteretic buck regulator that has lower peak currents and ripple. I will post when ready, but my proto in its initial stages show 89% efficiency and lower ripple current. When I achieve more consistent operation, I'll let the cat out of the bag...
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Old 19th December 2012, 07:32 AM   #66
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Old 21st December 2012, 05:07 AM   #67
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Self oscillating hysteretic buck gets tested again tomorrow. Hopefully new layout exorcises some lingering ghosties, now that I have other important issues sorted out. Once I am satisfied with overall performance, ultimate test will be to power up regulator using amp with all pernicious light bulbs in place to see if ultimate current limit chacacteristic of regulator works and keeps line fuse from blowing. I'll have to capture a power-up current profile to make sure things are really behaving.
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Old 21st December 2012, 11:13 AM   #68
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Self oscillating hysteretic buck gets tested again tomorrow. Hopefully new layout exorcises some lingering ghosties, now that I have other important issues sorted out. Once I am satisfied with overall performance, ultimate test will be to power up regulator using amp with all pernicious light bulbs in place to see if ultimate current limit chacacteristic of regulator works and keeps line fuse from blowing. I'll have to capture a power-up current profile to make sure things are really behaving.
I hope it works. You know and are skilled in this fascinating science.
Best regards.
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Old 21st December 2012, 02:41 PM   #69
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Hello wrenchone. A possible solution maybe to preheat the lightbulbs by lighting them at low intensity in a standby mode as shown in the attached diagram. The mechanical switches are ganged such that L'Fake is [Off] in the [On] Standby mode, and reverse. The CCS is intrinsically a hi Z load to L'Fake, and can be enabled continuously by deleting its switch.
Kind regards
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Old 21st December 2012, 05:30 PM   #70
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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.
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