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Old 21st September 2012, 06:08 AM   #1
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Default L'Fake - A Cheap- A** Expedient to Replace the Elusive SIT

OK, Michael Rothacher has an excellent thread detailing a series of SE amps made using a Yamaha power JFET/SIT. What if you're too lazy/cussed/cheap to pony up for said part?

There's hope, but not if you're too lazy (the folks that inquired regarding a kit for the Delite ( (one active part. for chrissakes!), should just buzz on, as they'll get no sympathy here. This is a DIY thread in the purest sense of the word.

At any rate, this thread will talk about a simple amp (3 active parts + some passives) that makes a SIT-ish amp using readily available power mosfets. There may be a preferred part for this particular application, but I'm too lazy to try and find it, so I'll make do with what I have on hand, as should the rest of you. You'll need a big N-Channel part of some sort in a TO-247 package. Lord knows there's enough of those out in the world...
I'll start with a really simple Delite-ish amp using the classic light bulb drain load as detailed by Papa Pass in his Zen series and the Delite amp introduced at the Burning Amp festival a few years back. I'm not going to even talk about the amp circuitry just yet, just the light bulb load.

I'm a path-of-least-resistance person, if you get my drift, so the first place I looked for an appropriate light bulb load was the local hardware store - in this case, Orchard Supply, a local (N. California) hardware store chain that's been around long before the other Big-Box stores reared their ugly heads. Right on the shelves there was a 120V/300W clear light bulb with standard Edison base made by Feit Electric (home of cheap (yep)-and-cheerful??? lighting products from China). The part number is 300M - I assume that the M might stand for "mogul" socket. At any rate, they were there, and reasonably cheap, so I nabbed four of them.

At home, I hooked one up to a DC bench supply. At 25V input, the bulb passes a steady-state current of ~1A, at which point the filament is glowing a cheap-and- cheerful orange (this means long, long bulb life). 1A is a reasonable starting bias current for a MOSFET class A SE amp. For a symmetrical swing capability, that means you would need a 50V supply, and you would set the mosfet to drop 25V at 1A. This is 25W dissipation, or 50W dissipation for 2 channels, not too bad for a reasonably big heat sink sans fan.
If you wanted more bias current, you could use more supply voltage, or use two bulbs in parallel and adjust the total supply voltage to taste. For an unknown bulb, you'd do a similar exercise with a bench supply to figure out the necessary supply voltage for a given bias current. For European types, this is likely to work better for 115V and lower bulbs than for the standard 230V bulbs available there - be warned. You may need to use foreign bulbs with a 115/120V rating or higher wattage bulbs - some have tried the linear high-wattage Quartz-Iodine bulbs. Automotive bulbs in series might be another option.

At any rate, you're on your own, but light bulbs are cheap, so potter around until you find a combination that will yield a reasonable bias current for a reasonable voltage. Assume that the bulb/s will drop ~1/2 the supply voltage, and also assume somewhere between 1-2A bias current. And no, I'm not going to find your "sweet spot" for you - that's your problem. Go away and do some homework - I won't be back for a while.

Last edited by wrenchone; 21st September 2012 at 06:54 AM.
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Old 22nd September 2012, 02:34 AM   #2
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Been doing some thinking since the first post and two paralleled light bulbs and a ~40V supply might yield the most power for a simple setup. This would be at the cost of ~40W dissipation in yer mosfet of choice, so you'll need one with reasonably large die area for a good junction-to-case thermal resistance. More on that theme later...

If you do a Google search for "Feit Electric 300M", there are loads of hits - you can even buy the blasted things from Amazon...
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Old 22nd September 2012, 04:57 AM   #3
Pano is offline Pano  United States
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Originally Posted by wrenchone View Post
Automotive bulbs in series might be another option.
I've done that and it works. But they glow very brightly, unlike the undervolted 300W lamps. You may or may not like the bright light.
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Old 22nd September 2012, 11:26 PM   #4
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Not a problem for me, as I already have the 300W bulbs in house. Others may or may not like, but some people do dig the brightish glow from thoriated tungsten filaments. One could find creative uses for the excess light.
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Old 27th September 2012, 07:33 AM   #5
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Here's a prelim schematic that I'll most likely be firing up this weekend. There are 4 active devices rather than 3, as I used a ring-of-two current source to bias the J175 jfet rather than a Supertex DN2530N3 depletion mode mosfet, my usual choice. I've been having a lot of trouble with bad Supertex parts lately - I suspect it's because I've been using an ungrounded cheap-a** soldering iron in my upstairs assembly location. I just purchased a new Hakko soldering station (the venerable "888") with proper grounding, so this may fix the problem - more on that later. The Fairchild HUF***** mosfet I'm using in the output stage was from an old Goldmine Electronics buy and is not critical to the operation of the amp - remember this circuit is based on feedback. Possible substitutions are many - IRFP140, IRFP240, IRFP044A, IRFP150, IRFP250, IRFP350, IRFP360, etc. I suspect that most any mosfet in a TO-247 package with voltage rating of 500V or less and >50V will have a low enough junction to case thermal resistance to properly dissipate the power required in this design.

That's enough for now...
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Old 27th September 2012, 11:01 AM   #6
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Hi wrench,
what's the OLG of that amp?
I mean, with 26dB CLG there might not be any gain margin left for a usable feedback?
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Old 27th September 2012, 05:30 PM   #7
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I'll look at my simulations tonight.
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Old 1st October 2012, 07:34 AM   #8
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A valid point, Juma. A more reasonable gain with some negative feedback is 5-6, or 7-8 dB if you prefer. I'm lucky I have a line stage with gain to drive this thing. In the course of looking at the circuit further, I found other things to hate about it, mostly as a consequence of the resistive drain loading (how do I hate thee, let me count the ways...).

As a result, I'll only have the light bulb loaded incarnation of the L'Fake alive long enough to light up some bulbs with a bench supply and drive some speakers, just to say that I've done it (with a trophy photo, of course). I already have places on the boards for a front gain stage that helps the simulated characteristics quite a bit (more gain/lower distortion, while still yielding an attractive harmonic structure). An outboard current source load is easy to add. I'll post more about this as time permits.

An attractive/interesting variation on SIT emulation is the "Half-Nelson" amp built/discussed back in 2007 (search out the thread - it was a Schade feedback scheme/SIT emulation before I knew about the concept. It got its first public appearance at the first Burning Amp in 2007, driven by a bench supply.

I just finished up with revamping that amp in the basement - the last "improved" incarnation had some residual hum, which I managed to vanquish by moving the filter caps around. The DC fan cooling the heat sink was replaced with a 230V AC fan running at 115V (slow and quiet, with still enough airflow to keep the heat sink real cool). I'll be carting this puppy to Burning Amp this year along with everything else that I manage to finish. The HN sounded pretty fine down in the basement, driving a pair of bitty Celestion bookshelf speakers with some Hamiett Bluiett from a Discman.
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Old 1st October 2012, 08:03 AM   #9
juma is offline juma  Germany
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Originally Posted by wrenchone View Post
....An outboard current source load is easy to add. ...
Yes, the CCS will create higher OLG - it's only logical to follow that path. The Schade feedback will be more effective and the amp will be more linear.
The downside is the rise of the heatsinking demands (light bulbs don't need the heatsink and the CCS does) but if you are fine with that...
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Old 1st October 2012, 05:33 PM   #10
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Transparently obvious.
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