Please help me adapt a power brick for bench work

[Rick PA Stadel]

Is there any reason why I'm testing these things at such a tiny current ..
when the input board on the amp they'll be used in might run 50v, 60v, 70v, etc..?

Your tester is evaluating the device's Threshold voltage, which is the most important parameter for two devices in a differential pair. The job of that part of the circuit is to maintain matching voltages at the two gates; if the V-thresholds are different, so will be the result.

5mA is a fairly substantial current for an input differential pair, which is as Dr Pass 'prefers' (methinks -- but I don't know the man). An input stage is almost never operated at 50 to 70V -- it would serve no purpose (the signal can't need it), and device heating would reduce its accuracy. Also, more accurate parts generally prefer lower voltages.

Regards

 

[Talon]

Talon, you're clearly a gifted writer and capable problem solver -- I was hooked by your OP. Use one of the 13,51V pairs (sorry about the 13,8 typo), finish the transistor matcher, learn some more great stuff, and let's save the magic of offline convertor design for another time.

Will do.

edit: oops, forgot -- Ray Hudson, IIRC, BeIN Sports.

Thank you! I'll have to watch some of his clips and come up with some suitable mocking.

A "switcher" is slang for "Switchmode power supply" also called SMPS. It does not run at 60Hz. Linear supplies run at 60Hz - those are the classic transformer, rectifier and filter cap things. A SMPS rectifies and filters the mains - your AC from the wall outlets - which it THEN switches off and on at high frequency through a MUCH smaller transformer. In a typical ,linear supply, the filter caps are recharged 120 times a second. They have to provide current to the circuit for 1/120 of a second at a time. A switcher can run at 50kHz, 100kHz, even into the megahertzes. The caps there thus are recharged every 100,000th of a second and so can be much smaller.

The SMPS lacks the huge iron transformer, so it weighs FAR less. It also runs at very high efficiency. The large iron transformer is also relatively expensive. A whole SMPS can cost a lot less than just a transformer. SOmeone had to design the circuit, but they are assembled by machine, not people with soldering irons. SO the complexity doesn't matter.

That is quite a good explanation and I thank you for it.

You keep referring to a "brick", and I have to admit I don't know what a brick is. DO you mean a "power adapter" like comes with so many things? What we call a "wall wart?" Or do you mean a box supply like bolts into a desk top computer? (Those by the say ARE SMPS) Point being, the whole using terms unfamiliar works both ways.

I read you loud and clear. I've heard them referred to as adapters, bricks, power supplies, and ugly. In the computer business a few years back they were always called bricks, though I myself called it a power adapter. I had never heard or read the term "wall wart" until three months ago. I recently had a system at my house fail, and it was due to one of the adapters. When the serviceman came out to do the repair I decided to try using the new term. The guy looked at me and said "wall wart? what?" so I pointed at the adapter and he says "oh, the power supply. I had no idea what a wall wart was."

I think its probably another regionalism like catsup/ketchu, sandwich/sangwich and pop/soda.

A linear supply has a transformer that steps the mains voltage - 120vAC here - down to something lower and then rectifies it. If I plug that into a 240v outlet, all the voltages inside it will be doubled. And if it is a 15v supply, that means 30v will come out. An SMPS can accept either 120v or 240v mains input at the flip of a switch. But more modern ones accept that full range of voltage with NO other compensation. By its nature, a linear supply cannot do that. And that is why jj knew what he knew.

For multi output supplies like in your computer, the +5v output is the main one, so if any output needs a load it would be that one. WHy would it need a load? Because the regulation only needs to be within a small range. With zero load required, then it has to regulate over a wider range. Needing a load isn't a feature, it is a side effect. The SMPS is not used with nothing connected as a load, so needing one is not an issue.

I'll have to re-read that last paragraph a few times and get back to you, but thank you for posting it. In the meantime, allow me to complain: for crying out loud, just as I'm getting the hang of magnet transformers, now I have to deal with this?

 

[Talon]

Your tester is evaluating the device's Threshold voltage, which is the most important parameter for two devices in a differential pair. The job of that part of the circuit is to maintain matching voltages at the two gates; if the V-thresholds are different, so will be the result.

5mA is a fairly substantial current for an input differential pair, which is as Dr Pass 'prefers' (methinks -- but I don't know the man). An input stage is almost never operated at 50 to 70V -- it would serve no purpose (the signal can't need it), and device heating would reduce its accuracy. Also, more accurate parts generally prefer lower voltages.

Yeah I keep thinking about stopping in his sub-forum and asking him about this, but then his answer would service two purposes: waste his time and irritate him, because I'd have no idea what he's talking about.
I just looked at the schematic for the GFA-535 and if I'm reading it right, there's 59v at the differential pair. The GFA-545 has 55v, and the GFA-555 has over 70v. And even though I can read these numbers as plain as the birthmark shaped like three 9s on the scalp of my very large and red-eyed cat, who is always lost somewhere in the house every time someone in my neighborhood goes missing, something tells me you're right and I have no idea what I'm looking at.