L'Fake - A Cheap- A** Expedient to Replace the Elusive SIT - Page 11 - diyAudio
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Old 18th February 2013, 12:26 AM   #101
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At the operating temperature of the lamp in this application, the iodine will sit around bound to some tungsten, doing nothing. Its purpose is to scavenge deposited tungsten off the bulb envelope and return it to the filament via decomposition, and that only happens at much, much higher temperatures.

I may grab one of the higher wattage lamps tomorrow night.
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Old 19th February 2013, 02:34 AM   #102
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Ok, I grabbed a couple of pairs of the Feit Electric 500W normal duty linear quartz-halogen lamps. I saw no need to go for the extra duty "rough service" lamps (one lamp for the price of two), as this application isn't exercising the lamp anywhere near its design limit.

A bias voltage of 25V gets you 1.8A current through the lamp, and a cheerful orange glow, as compared to the dull red glow of the 300W bulb-type lamps with no iodine or other enhancements. Quartz-iodine lamps run hotter than normal incandescent lamps, as they're designed such that the iodine incorporated inside has a chance to do its thing and scavenge evaporated tungsten from the bulb inner surface. An added benefit is that the higher operating temperature shifts the output a little more towards the visible spectrum. Not a consideration for our application, but interesting nonetheless. If you want to eke out that extra bit of bias current, go for 2 X 300W lamps. If you want one lamp, 500W will work more or less, with a little bit less output power.
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Old 19th February 2013, 03:17 AM   #103
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I get about 12W output power, scandalous when you consider what's blown in the resistive load plus device heat sink, but then again, it's DIY after all. You can leave the kitchen if you so choose....
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Old 19th February 2013, 05:06 PM   #104
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Default Another Feedback Loop

Quote:
Originally Posted by wrenchone View Post
I get about 12W output power, scandalous when you consider what's blown in the resistive load plus device heat sink, but then again, it's DIY after all. You can leave the kitchen if you so choose....
Hello wrenchone. I have no doubt that you have a great [SIT-like] sounding 12W of output power.


Allow me to bring your attention to the following. It emanated from my fooling around with the circuit of the last post in the thread which is entitled "Other Applications of Vacuum Tubes" in the Everything Else Forum. Please consult the attached schematic.
  • The left part of its view is the output stage which you currently use in L'Fake.
  • The right part of its view is an approach you may wish to consider. It shows the output stage of L'Fake using local negative feedback via the loudspeaker. It emanates from the current source port [drain of power output MOSFET], passes through the loudspeaker and terminates at the Source summing junction. Some of us may call it Schade feedback, but I prefer to call it Pass Feedback. Since neither Mr. Schade nor Mr. Pass used/use this style of local feedback around one single power output transistor, I best prefer to refer to it as ANT Feedback; clearly in the abscence of precedence which voids this nomenclature.
I owe my thread [Other Apps..] a follow-up post to report my findings on ANT Feedback. Those DIYers using solo power output devices may wish to consider it too. For example in Mr. Rothacher's " L'Amp SIT amp version 1" which uses a lamp load and a 1 Ohm source degen source. I did not see a source degen resistor in the DIY De-Lite amp of Mr. Pass. But; one may wish to add one and unravel its sonic attributes and new performance.

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File Type: pdf ANTFeedback.pdf (25.1 KB, 57 views)
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Old 22nd February 2013, 06:24 AM   #105
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Here's what a 500W, 120V, quartz-halogen lamp looks like with 25V across it - the lamp is drawing 1.8A, and looks pretty bright. The lamp holder you see may be available in a well-stocked hardware store. I got mine off Ebay, as I've been busy/lazy lately(take your pick).
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Old 23rd February 2013, 06:18 PM   #106
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Quote:
Originally Posted by wrenchone View Post
Here's what a 500W, 120V, quartz-halogen lamp looks like with 25V across it - the lamp is drawing 1.8A, and looks pretty bright. The lamp holder you see may be available in a well-stocked hardware store. I got mine off Ebay, as I've been busy/lazy lately(take your pick).
wrenchone, the yellow- orange glow is eye-pleasing. That is a hefty idle amperage flowing through it.
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Old 7th July 2013, 07:25 AM   #107
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Here is the next incarnation of L'Fake, slightly rearranged and with fewer parts. It made encouraging noises when I fired it up in the basement and fed it with a CD player. I'm hauling the carcass in to work tomorrow to tweak the compensation and do a gain-phase plot. The compensation cap will go across C15 - there's none at present.
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File Type: gif QN_SCH.gif (18.4 KB, 309 views)
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Old 8th July 2013, 01:25 AM   #108
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The new Fake was very well behaved, with only a slight amount of overshoot in the square wave response. A compensation capacitor of 470 pF gave the response shown here.
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Old 8th July 2013, 01:26 AM   #109
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Here's the Gain-Phase plot as taken by an HP4194A analyzer. The top trace is the gain, the bottom, the phase.
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Old 8th July 2013, 02:13 AM   #110
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wrenchone,
I have a few questions, if you dont mind.
Is that a mosfet current mirror you are using as a load for the output? I am not sure this is the case due to the different devices, but do not understand the influence of Q5 on Q3.
THe ZVP33 is just a driver for the mosfet output(q2), correct?
It also looks like the output(q2) is schaded. Is bringing feedback from the output a way of schading input or is it just standard feedback. I hesitate to think it is CF, even though it is brought back to low Z source leg of input Jfet, but I am still a little fuzzy on that whole topic. Thanks. I enjoy your circuits because they make me think, as they are not often obvious for someone like me.
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