2 Newbie Questions: matching MOSFETs, and humming power supplies

[psarin]

hey now,

i'm attempting to build the SOZ amplifier as a test project using parts i have lying around, before actually building the real deal with massive heatsinks. the aim of this project is to get ~2W from a pair of monoblocks into 2 altec 755a's mounted in an open baffle (so efficiency is not a consideration, but NOT blowing up the speakers IS!)

here's the 2 questions, i apologize in advance for their complete stupidity, if applicable

1- i set up the circuit to match my MOSFETS's - IRF540 as described in the PASSDIY.com how-to section. Used a 24VDC power supply and a 50ohm 10W resistor. the problem i have is that after plugging in the MOSFETS's the Vgs starts out as ~3.9V, but falls down to ~3.7V, presumably as the transistor heats up, in approximately 1 minute. what am i doing wrong? or how am i supposed to match these?

2- i plan to use 4 regulated power supplies (lab retirees from the university) that each source 24VDC-8A, (running +/- 24VDC rails for the 2 monoblocks). but unfortunately, found that they hum! i.e. mechanically vibrate (electrically rock stable - measured AC is < 0.1mV). is this just because these have old transformers with shaky laminations? is there something i can do to fix them up, i would hard pressed to build something like these on my own! my backup plan was to use 4 12-volt car batteries and setup a relay to put them on trickle charging when amp is switched off. is this a feasible idea?

[leroy]

Let me give my opinion on your questions:

1.
The change of Vgs is normal. You should wait untill it is stabilized, that means: untill the Mosfet/heatsink reaches its final temperature. Depending on the size of the heatsink it can take up to half an hour (very large heatsink). You'll see Vgs get stabile at this point.

2.
By adding a copper strap surrounding the entire transformer at bobbin height you decrease the radiated magnetic field by 20dB or so. Make sure the copper surrounding is shorted. Mechanical vibrations are more difficult to overcome, I don't have a good solution for that.
A battery powered amp is a good thing, in fact you'll get rid of any noise and ripple. So I recommend the last, but you have to spend some serious money.

leroy

[vpharris]

On the FET matching issue, I think as much of an answer as you would ever expect is already here, but you'll have to use the search function. Search for 'match' or 'matching' in he Pass Labs forum, and you'll get lots of info.

On the other, I think it's the transformer laminations, and I don't know what to suggest. Maybe try pouring epoxy over the thing. I fix everything with epoxy.

[GRollins]

Shellac is the traditional answer for "glueing" transformers back together. Easy to work with, cheap, and dries in about 15 minutes. Epoxy will soften at high temperatures; shellac won't.
We're assuming that the power supplies are in good electrical health, yes? No shorted windings, not overloading them, etc.

Grey

[FEThead]

As a "bandaid" maybe you can put those noisy power supplies in another room and run long wires to them.

[cyclotronguy]

Keep in mind that mains supplies are often very dirty. Transformers will hum if DC is present on the line; and it often is. Try running the powersupplies off an isolation transformer and see if they quiet down. Should this be the case, you will need to do some work on the AC.

Cyclotronguy

[psarin]

thanks to all the experienced folk for their advice. i will do the MOSFET matching with properly installed heatsinks for longish periods of time to ensure that the voltage stabilizes.

also, regarding the PS mechanical hum, i will try all the aforementioned options (dipping laminations in shellac, adding copper sheilds). i did already try an isolation transformer ahead of the power supplies, and the hum did reduce. unfortunately the only isolation transformer i have is only 500VA rated, and itself hums quite a bit!

[explorer]

I too had confusion with the MOSFET matching. When I did the checking of IRFP240s for my Aleph60, I wired it according to the diagram in A40 doc. I used binder clips to attach the MOSFET to a heatsink and placed 3 100R 5Watts resistor wired in parallel. I applied 17V into the circuit. The resistors do heated up so I put them on an additional heatsink. I kept the MOSFETs for 5 to 7 minutes to allow it to heat up and stabilize. I got reading of 3.81V to 4.10V for 30 devices. During that time, the voltage registered to say 3.90, later after 2-3 minutes goes to about 3.88V. It was all same scenario for all MOSFETs, that is, it goes 0.02V lower. But to my surprise, when I touched the heatsink, I felt almost zero increase in temperature. Everyone says it must heat up. Have I done the matching correctly? Is my voltage of 17V not enough? (the doc says 15V only, some says 24V)
Anyone has a recommedations?

[yldouright]

If my power supply has a maximum output of 12V, what resistor value and power rating should I use in a mosfet testing circuit? Do I need to mount the mosfets on a heatsink or can I safely test them without one? How long must I wait for the temperature to be right for measurement? What would be the answers to these questions if the supply was set to 10V?

[yldouright]

One other thing, do any of the values or procedures change if the mosfets are substituted? What if I wanted to measure an IRF044 instead of a IRF240?