Is this how power supplies work?

[Clmrt]

I ran across a Stewart PA-100B class A/B power amp in a pawn shop. Seems to have a build date of around 2000/2001. In searching for information, I found a PDF describing the amp, which is equipped with a SMPS supplying Darlington circuits (I think).

They say this:

"More than anything else, it's their power supplies that distinguish Stewart amplifiers from components with otherwise similar performance characteristics. A conventional power supply recharges 120 times per second and consists of a large transformer and large storage capacitors. The Stewart power supply recharges 1,000 times faster - 120,000 times per second. As a result, it requires less capacitance for filtering and storage, drastically reducing size and weight."

Does that make sense? Linear supplies are dependent on 60Hz AC "refresh rate", using capacitors to make up the difference, while SMPS run at "processor speeds" to supply steady-state DC up to it's limit?

 

[DVDdoug]

Yes, it's true. But it's not necessarily "better" if they both put-out "pure" DC. Good power supplies are regulated and the regulator adds additional filtering, since it's job is to hold a constant voltage.

A full-wave rectifier is 120 half-sine waves per second (in North America. )

A switching supply also has a rectifier and filter capacitors directly on the incoming AC. (With a "linear" power supply the rectifiers are on the low-voltage side of the transformer.) That DC feeds a high-frequency oscillator and the transformer (usually part of the oscillator circuit) runs at high frequency. At higher frequencies, a smaller-lighter transformer can be used.

The higher frequency has two other advantages - It's easier to filter. DC is "zero Hz" and a low-pass filter is more effective (an/or can use smaller capacitors) at higher frequencies. And any power supply noise is well-above the audio range so any noise that leaks-into the audio circuitry is inaudible. (Sometimes you get power supply "whine" and I'm not sure why something is resonating at audible frequencies.)

...There's a lot of similarity between switching power supplies and class-D amplifiers.

P.S.
Both have transformers. A transformer is analogous to a lever or gear mechanism. It trades-off between voltage and current. With a 120-to-12V step-down transformer you can "pull" 1 Amp out of the 12V side while pulling about 0.1A from the wall socket (there is some inefficiency). It also provides electrical isolation. There is no direct connection between the high & low voltage sides and that's required for safety.
.

 

[solderdude]

Most current amplifier designs use SMPS.
The advantage of replenishing reservoir caps at a higher speed is that (depending on the actual PS feedback design) is that the reservoir caps can be many, many times lower in value and thus size.
A 100uF reservoir cap will be more than sufficient for such power supplies, large 4700uF+ capacitors are not needed in this case.
Of course it all depends on the actual circuit and how well that circuits suppresses the rectified mains voltage that feeds the switching part is.

 

[Clmrt]

Thanks - Obviously I'm not an EE but I like to at least have an understanding.

 

[alex-z]

The reduced size and weight are true, but the marketing department ruined the technical details.

Yes, the higher switching frequency may allow lower bulk capacitance in the power supply but the output section will typically need additional filtering caps to prevent oscillation and RF leakage.

The actual size/weight benefit comes from the higher efficiency, the output transistors spend almost 100% of their time in a full on or off state. Spending less time in a high dissipation state means less wasted current, so you can have a smaller input transformer and smaller heatsink. Those are the biggest and heaviest parts of any amplifier circuit.

 

[Clmrt]

Curiosity is getting the better of me. Since I'm always shopping for that next amplifier, I'm torn between a Fosi or just grabbing this. Here's the blurb. Turns out the quote with which I started this thread was from a review of their much higher-powered 2.1 amp, but the power supply methodology is shared.

Yes, I'm stuck in the 20th century.

 

[solderdude]

Both amps use SMPS.

The Steward is a 'normal' class AB amplifier.
The Topping is a class-D amplifier.

 

[Clmrt]

Yep. Just asking about the blurb. Dawn of new tech from way back.

These amps were pretty expensive - $700ish for 50w of class AB with SMPS from 2001. I get all tingly just thinking about watching it go up in smoke when I play with it.