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Szekeres amp works, but doesn't get hot

post #1 of 3
Thread Starter 
I finaly got everything together and did it. This amp's first moments in life were playing the first three min of the Eagles Desperado album before the voltage regulator shut down for thermal reasons. Itt took a while to troubleshoot the dang thing, one channel didn't work and had to be almost compleatly resoldered before it finaly decided to coperate. Sounded like crud at first too, but that had something to do with both the transformer being right next to it and the leads for the input and output being around two feet long.

Now it sounds wonderful. I can hear the tape pops from the master tape the cd was made from. The sound doesn't worry me, what does is that for all the talk of the Szekeres MOSFETs getting very hot, mine get warm, and that's about it. I have left it on with music playing/not playing with no load/load, and the only thing that got hot even after three hours is the voltage regulator (which now has a larger heatsink and does not get very hot). It is getting plenty of current and does not distort, should I just be happy and kind of accept it?

On to another note, I am going to use this amp as a preamp for my turntable too (insted of the hissy preamp of my reciever), and am adding a voltage gain stage and crossover to make listening more plesurable. How much gain would be a good for the turntable?

CaptBubba
post #2 of 3
Hey Bubba,

It doesn't change... it's class A. It's working just as hard sitting idle as it is full blast. And it should be heating those mosfets up nicely... How big are your mosfet source resistors? Or if you went with a CCS, how much?

Here's a spot where a few tens of ohms is the difference between barely above room temp and "hey dude, can I borrow an egg?"

My first Szekeres got so freakin hot (20 ohm source resistors) I thought it was oscillating or something was wrong... but nope, looks calm on any timebase. (didn't have a scope at the time, but later went back and tested all my previous frankenstiens)

Turntable preamps can have any gain you want. Try 24-32dB. Or try upwards of 100dB if you want. Depends on the power amp's gain factor/power/input sensitivity, and then what cartridge type/output power.. for where your usuable volume range will be. If your power amp doesn't have any trimmers for input sensitivity, you might want to build them into your preamp. Basically that's what your volume control is already; but to control from whisper quiet to amp meltdown you'll have to tune it. Amps usually need somewhere between 1-5V to go full blast, but that's a huge range. A 250w power amp expecting a max 1V input signal will probably explode if you fed it 5V. Hey, let's find out!

If you want other inputs as well, just make your selector switch come in behind the output buffer stage... you don't want a 1V CD source getting gained that hard by the preamp. (or the RIAA curve)
post #3 of 3
Thread Starter 
I think I've figured out both of my problems. The MOSFETS are biased a bit lower than is in the plans, and as soon as I get my hands on the right resistors they should be on their happy way to burring my fingers.

Also I really thank you for mentoning that possible problem. I can just see myself hooking up a cd player and destroying my amp. I think I have found a solution though. I'm going to get a 4 pole rotary switch and do the following:
1st & 2nd poles: for switching between the inputs
2nd & 3rd poles: for changing the gain of the amp, high for a turntable and low for anything else.

I've found a three positon switch, and three inputs should be plenty.
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