SMPS repair tip

That's OK at low frequencies, but can have a high VSWR t VHF and up.

--
Anyone wanting to run for any political office in the US should have to 
have a DD214, and a honorable discharge.

"Cydrome Leader"

** I have sometimes added a Zobel ( ie 22nF and 56ohms, 0.5W) ) across the output of an amplifier that did not need it.

Why ? As a tell tale in case of supersonic oscillation.

Incorrect installation and bad cabling practices can cause almost any power amp to break into oscillation at an inaudible frequency resulting in rapid overheating and failure of output transistors etc

It's nice for a repairer to have proof when this has happened.

.... Phil

I listen to distant AM radio in the shop. An amp that breaks into oscillation (often caused by "replacement" actives with way too much bandwidth, BTW) will cause interference on the AM (MW) radio. It is as reliable an indicator of oscillation as fat blurry traces on the Oscope. Also, the amp pulls more current than it should when it breaks into oscillating. Now a SMPS without EMI suppression already makes the interference so all bets are off.

"dave"

** Errrr - that is "parasitic" oscillation at radio frequency.

My post was about a quite different matter.

Maybe you have never seen it - it kills output stages in 30 seconds.

..... Phil

"Phil Allison"

Not really.

** Then kindly explain how the PWM signal is being generated.

** There is very little HF at the output to go on, just some harmonics at 300 to 500 kHz.

output of an amplifier that did not need it.

Why ? As a tell tale in case of supersonic oscillation. "

Good thinking if you suspect that. I take it you mean mainly in analog amps.

** Is there another kind ???

( Apologies to Jack Nicholson ..... )

amp to break into oscillation at an inaudible frequency resulting in rapid overheating and failure of output transistors etc "

Think about what would happen with piezo or electrostatic tweeters. Even ribbon tweeters.

** Almost instant destruction.

When it happened to my buddy, it must have been high because if it was only

50 kHz or so, with the power, those tweeters should heve been toast. ** 50kHz or thereabouts oscillation does no harm to typical woofers - the inductance of the VC protects them.

..... Phil

I know exactly what we are talking about. Radio Frequency starts at

10KHz. My traces fattens. The radio hisses. And the dim bulb glows brighter. The dim bulb tells me to cut power way before any destruction in 30 seconds happens. Modern IC opamps have too much bandwidth.

** No you don't.

** No it does not.

** That is RF parasitics - not FULL POWER supersonic oscillation due to positive input / output feedback. ** ROTFL.....

.... Phil

Relaxation oscillator to generate a sawtooth wave, comparator to chop it up into PWM. It's not that hard really. The chips that do it are just easier to use than to design it with discrete components. After tha you got your P WM square waves, then you design the drive circuit for the outputs. What's more, even though it is BTL, you only need one, it is simply inverted for t he other phase.

Looking at the picture, there are plenty enough transistors and whatnot on the board to do it.

Of ocurse the question is why. Why didn't they just use some off the shelf chip for that ? To answer that we need to know things we are not going to f ind out. Amybe some parameter was too hard to get out of the chip, maybe th eir chopping frequency is lower than the chips like to operate at. Maybe ce rtain effects were easier to implement, though I'm having a hard time fatho ming any at the moment.

Or it may be attributable to audiophoolery. Some people like discrete compo nents, myself included. I know damn well thaat what's in the chip is the sa me as what's in the transistor package, but...

Like if you bought a Marantz that uses an STKXXX instead of a Luxman that u ses the same STKXXX, what did you get different ? Maybe us nuts just like t o make the engineers work harder.

But really, among musicians you will find a healthy dose of audiophoolery, so it may be discrete for marketing purposes.

Of course in class D it doesn't mean a thing as far as I can tell. But then if ONEE golden ears can prove he hears the difference, the market will bea t a path, to a certain door.

No, they don't. Some people just don't know how to use them properly.

--
Anyone wanting to run for any political office in the US should have to 
have a DD214, and a honorable discharge.