L'Fake - A Cheap- A** Expedient to Replace the Elusive SIT

[juma]

To all the people reading the thread ?

[wrenchone]

Here's "L'Fake Lite" a brief transitional stage in the evolution of the amp, just to show it can be done if you so desire. I'll post a schematic later, as this beast turned out a trifle different from the first schematic. One of the important differences was using an IRFP260 rather that the original Fairchild output device, as the larger area die in the 260 has more transconductance than the original Fairchild fet I show in my first schematic.

With 2 bulbs per side in parallel as drain loads and a 40V Vcc from a bench supply, each output stage was running just under 2A bias current. The amp sounded ok(but not very loud) when driven by a Discman.

The bulb filaments glow a dull red - the digital camera exaggerates their brightness, as it sees more near-infrared than the human eye.

I'll tell you the reasons why I hate this amp (even though it came up without a lot of back-talk) before I move on to the next stage of the project.

[wrenchone]

Here's a bit more construction detail... I used fender washers (hardware store) to even out the pressure of the screw on the TO-247 output device packages for a little better thermal interface. I also used aluminum oxide washers (Mouser) for good thermal transfer from the output devices to the heat sink. I'll post some details on the heat sink I used when I can get down to the basement again with a tape measure, but you are going to basically be on your own in that respect, anyway. I picked up my heat sinks on Ebay from someone in Ohio that looked to be parting out old motor drives. Use the overall dimensions I post as a rough guide to what might be necessary.

At present, the heat sink is dissipating around 90W. If you bother to adjust the drop across the output devices to the proper level, this should go down somewhat, but expect to dissipate at least 80W for 2 channels. Putting each channel on a separate heat sink will help matters somewhat - that's up to you...

The amp is pretty listenable, even though I detest it from an engineering standpoint (more on that later). Right now, it's being driven by a Panasonic Discman into a pair of Celestion F15 bookshelf speakers, which are what I have in place in the basement for initial listenability tests. Ok, so far... The heat sink gets to about 50-55 degrees C (finger test) over the course of a CD's worth of listening - no doubt it would do better if I propped up the heat sink so that the fins and natural convection could do their thing.

I'm going to post a generalized schematic and some comments so that people who want to build this (or something similar with alternate parts) can do so, but I think I can do better. In fact, I already have - check my "Half Nelson" thread in the Pass Amps forum for one approach. I'm hoping that this thread will reveal an alternate approach, though at the cost of some more parts than what I've shown here so far..

[wrenchone]

Here's a schematic to get people started on the "L'Fake Lite"(if they want to start at all). The circuit is simple enough to build up on a piece of perforated board (a good thing - no PCBs, unless someone with more spare time than I have want to gin something up and offer it separately). The circuit is very tweakable, but you'll have to tweak on your own.

I chose Q2 as a mosfet for high output impedance, but a PNP bipolar transistor would also work. A device in the TO-92L or TO-126 package would be best, as the dissipation will be about 150mW with the specified output voltage - a little high for a TO-92 device.

If you decide not to do any selection for Q3, R11 will be necessary to tune in the drain voltage for Q4, which should be around 20V for a 40V supply voltage. Once the quiescent operating voltage is tweaked in, you can measure the pot and replace it with a fixed resistor. Using a "premium" device like the 2SJ74 for Q3 will likely yield inferior results due to high gate capacitance. NOS devices like the J175 or J271 look to b optimal - if there are any euro-equivalents, let us know here, as I don't do Euro-silicon.

I specify the Feit 300M as a load bulb, as it's available on Amazon. If you want to use something else, you'll have to characterize it with a bench supply in the manner I describe in the first post of this thread.

[wrenchone]

I've perhaps decided to keep the "L'Fake Lite" for the time being. I 'm building up the final incarnation of L'Fake, and If I can cram both sets of modules on the heat sink, I'll bring the ungainly chimera to this year's Burning Amp with some means of switching between the two amps.

[wrenchone]

It happens that I have a heat sink large enough to hold both the "L'Fake Lite" and "L'Fake" boards. It's also one of the filthiest heat sinks I've ever bought - maybe it came from a chain smoker who lived in a chicken coop (that's right, filth plus yellow stuff)... At any rate, the holes are mostly drilled & tapped, so I'm going to see what some Scotchbrite and orange cleaner/Bon Ami will do before I proceed. The construction will be slightly less rough than the original Nelson Pass "DeLite", but not by much. I'll post pictures, but people who want to experience it in all its sleazy glory will have to come to this year's Burning Amp.

[wrenchone]

The filthy heat hink got cleaned up last night, and I installed the L'Fake Lite and L'Fake boards on it. Attached is a picture of the beast on the bench while one channel is burning in. I'll talk about L'Fake when I've finally finished with L'Fake Lite. As a teaser, though, it's a "Half Nelson" amp with several nagging issues resolved.

[wrenchone]

After a couple of frustrating moments, both channels of "L'Fake" are up and running and burning in on the new, larger, (and cleaner) heat sink. It's standing up, so I'll see how simple natural convection copes with ~80W output device dissipation (both channels). I Think it'll do fine, from what I've seen (and felt) so far. The output devices are coupled to the heat sink via alumina washers (Mouser) with fender washers on top to even out the pressure from the mounting screw on the device packages. The fender washers can be seen in the picture embedded in the previous post.

For people looking to do this or similar things, the heat sink I'm using is 10" by 12". The web thickness is 1/4 ", and the fins are 2" long. Similar heat sinks aren't too hard to find on E-pay. Also, a little air flow from a DC fan with the input voltage dialed down can work wonders. I don't think it'll be necessary in this case, but people trying to make do with a smaller heat sink or higher power dissipation may want to consider such an option.

[wrenchone]

Last night, I burned in the L'Fake modules on the new heat sink for over an hour, and the equilibrium heat sink temperature appears to be around 40-45 degrees by finger test. How to tell? 40C feels hot, but you can keep your hand on it, 50C is 10 sec uncomfortable, 55C is ~5 sec uncomfortable, and with 60C it's one second or less. 70C is "immediate ouch". Your fingers are a pretty good non-linear thermometer.
So, the heat sink is a little large for dissipating the 80W required (40W per module), but the heat sink had to be kinda big to fit all four modules. Something between 2/3 and 3/4 the area would probably be ok. As only one pair of modules will be running at a given time, I can forget about the thermals and try to figure out how to package this baby with the power supply.

[wrenchone]

When I'm characterizing an amp, I like to whack its input with a 10kHz square wave adjusted in amplitude such that the amp doesn't clip. This gives me an idea of the amp slew rate and also helps to smoke out snarky oscillations and other antisocial behavior.

Attached is the square wave response of one of the channels of L'Fake (not L'Fake Light - I didn't want to drag the light bulbs in to work). There's no compensation cap. The top trace is the input, and the bottom is the output. Slew rate is decent, though much faster on the rising edge than on the falling. There is a bit of overshoot on the rising edge that will be cured with a small compensation cap. I used 470pF on the other channel, and it's heavily overcompensated. I think 100-150pF should do the trick. After I install these, it's time for listening tests with a bench supply.