Help finding out what this puzzling addition to an Vintage Mcintosh Amp

[Nordlys]

Hi everyone

I’m the happy owner of a vintage Mcintosh MC2500, lovingly dubbed “the black beast”(as its black & is powerfull enough to possibly use as a welder)
I’ve just had it completely refurbished, and the technician at the repair shop commented on the switch on the front of the amp
There is a “sticker” + the switch, that says: “Power guard - model P613”
He’s one off the official Mcintosh repair guy’s here in Denmark and he’s never come across this, according to him “professional installation” before
That got me searching the interweb + a mail to the danish Mcintosh dealer - all came up “empty”
So I’m hoping that someone in here knows what this is and possibly why this was installed!?

As always I really appreciate your insight

[Nordlys]

Addition:
I’m pretty sure it’s an extra speaker protection!
- But doesn’t this old beast already have some level of speaker protection?
There’s certainly clipping warning lights…
+ this amp drives most speakers to the MAX without sweating

It does have all the dings and dents of an amplifier that’s seen some action, so I’m pretty sure it’s been “on tour”

[Macca]

POWER GUARD®
McIntosh designed and patented circuit that prevents speaker clipping. Photo-optics react in 1/1000th of a second to regulate power levels to speaker, protecting the speakers and the amplifier. A waveform comparison circuit continuously monitors both input and output signals. Power Guard dynamically adjusts the input level to avoid clipping while preventing harsh sounding distortion.

https://www.mcintoshlabs.com/help

Essentially it's speaker protection as you surmised. Typical protection circuit will just be a relay that activates when DC is detected, this one is more advanced in that it won't just cut the sound on overload.

[Barry]

Typing in Mcintosh MC2500 + "P613" into Google comes up with the information that the designation "P613" refers to the specific circuit board responsible for the 'Power Guard' facility, which prevents the amp running into clipping.

The label was likely to have been affixed after the amplifier had been serviced, to indicate the change of circuit board P613, sometimes denoted as P691.

[Pigmy Pony]

Is this anything like the "soft clipping" that NAD used to have in their amps?

[Macca]

IIRC the NAD soft clip circuit just flattens its distortion curve so it doesn't clip as hard but the trade off is more distortion overall.

Maybe this does similar I don't know.

[Barry]

The McIntosh system works by continually comparing the input and output voltages. If the output voltage approaches the clipping level, the input level is attenuated so that the amplifier never clips.

[Macca]

With 500 wpc this thing has you'd need to be going some to clip it.

[Barry]

I don't think the MC2500 was intended for domestic use - more likely it was ntended for professional use.