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MichaelJHuman 7th December 2009 09:32 PM

What is the primary limiter of power in AVRs
Having read as much as I could find on the topic, I am still unsure.

In AVRs with a shared power supply, does anyone know what the primary power limiter is?

It's not nominal rail voltage, I am 90% sure of this, because some of these AVRs drop to 1/3 rated power when tested with all channels driven.

It could be the power transformer's voltage dropping under heavy load, but that would seem to require a pretty good load, and that's a lot of voltage drop. Admittedly, I don't understand what happens with the filter caps and a heavy load. Does the voltage destabalize a lot if you are drawing so much load, the filter caps are not keeping up? Understand I am talking about an all channels driven test using a sine wave from an audio analyzer where the peaks are all lining up for each channel (I assume.)

A likely factor, based on some snippets I have read, and looking at power vs. distortion curves is that most receivers have some sort of power limiter in their circuitry. Under heavy loads, I assume this would kick in.

I looked at my own receiver's service manual and noticed that there's a limiter circuit who's output supplies the rail voltage to the first two stages of the amplifier. The standard rails are connected to the output transistors. I am not sure how this works as I only have a basic knowledge of electronics.

DigitalJunkie 8th December 2009 01:58 AM

I've always assumed it's because the power supply isn't holding up it's end of the deal.
I have a power transformer from a Sony STR-DE845 setup here,It's only rated 335VA (~280W,or so?),and the amp is rated at 100W per channel (x5). It's pretty clear that the power transformer couldn't keep up with 500W very well if all channels are heavily driven.
It's likely to only make about (280/5) ~56W RMS per channel,all channels driven. That's about what a well built 'Gainclone' type amp can do.

MichaelJHuman 8th December 2009 02:37 AM

The transformer is likely the limiting factor to be sure. But is the limit from regulation (the transformer unable to hold supply voltage,) or is a limiting circuit usually kicking in to protect the transformer?

MichaelJHuman 9th December 2009 11:32 PM

Bumping in case anyone else has ideas on this, thanks :)

Andrew Eckhardt 10th December 2009 12:48 AM

There is usually a thermal fuse packed into the transformer windings. If you manage to overload/drive the amplifiers for an extended period of time without breaking anything else that fuse will open. I wouldn't call this the primary limiter, as usually in better designs there are other types of protection/ lmiting that might kick in first, but it is certainly the ultimate limiter.

MichaelJHuman 10th December 2009 01:48 AM

I read about such fuses. No ideas on whether the primary power limit is from a limiter circuit or voltage regulation though?

Out of curiosity, I shorted a wall wart, and could see a large drop in voltage.

As power demands from the transistors increase, a fairly low impedance would present itself to the power supply from 5, 6 or 7 8 ohm (or less) speakers drawing power, at least that's my understanding.

A load like that could cause the voltage to drop from the transformer, I would think.

AVRs, especially cheap ones, can have pretty low power when benchmarked with all channels driven. I have seen power drop by over 1/3 what their two channel power measurement would be. I found it a bit hard to believe that was all voltage drop from the transformer.

I was thinking that a limiter would explain that sort of behavior.

I will post an example of a power vs distortion curve. Note that this is with one channel driven so nominal rail voltage could be a factor here. But what's happening at the two "kinks" in the graph?

Home Theater: Onkyo TX-NR807 A/V Receiver

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