McIntosh Power Guard Peak Limiter

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McIntosh Power Guard Peak Limiter

Postby SarnoMusicSolutions » Fri Aug 28, 2009 7:08 am

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I know this isn't the buy and sell section, but figured this is the place to show this piece. It's a mint/new condition McIntosh stand-alone peak limiter device. Likely made in the late '70s. This box is really pretty cool. You run your audio signal thru the input/output jacks on it right before your power amp. Then you also run a second set of small speaker wires from the power amp speaker terminals (in addition to the actual speaker wires) to the speaker inputs on this P438. Then this P438 senses the output power of the amp and depending on where you set the knob, this limiter will control the audio signal level. It's a passive device and is only powered by the power amplifier outputs. This is the patented McIntosh "Power Guard" limiter circuit that's in many of their amplifiers. Very effective at protecting speakers and putting a loudness "cap" on your rig in various situations.

Includes the box and manual.

$260 shipped in the US
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... and it's just like any other day that's ever been...
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Re: McIntosh Power Guard Peak Limiter

Postby RiverRat » Fri Aug 28, 2009 10:27 am

Any chance you'll crack it open and document the circuit?

Reverse engineering this would be awesome!!!
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Re: McIntosh Power Guard Peak Limiter

Postby strumminsix » Fri Aug 28, 2009 10:30 am

RiverRat wrote:Any chance you'll crack it open and document the circuit?

Reverse engineering this would be awesome!!!



A Peak Limiter would be a nice add-on to your preamp!
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Re: McIntosh Power Guard Peak Limiter

Postby tigerstrat » Fri Aug 28, 2009 11:35 am

strumminsix wrote:
RiverRat wrote:Any chance you'll crack it open and document the circuit?

Reverse engineering this would be awesome!!!



A Peak Limiter would be a nice add-on to your preamp!


Yeah, even though it would work more closely with the power amp, I could see something like this actually fitting inside the rear of a Dual Showman Rverb head(?) Just how big is it, anyway? kinda looks to be about 4x6x3deep?
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia
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Re: McIntosh Power Guard Peak Limiter

Postby playingdead » Fri Aug 28, 2009 12:02 pm

Maybe it's the mystery knob on Jerry's rig!

Image

(Kidding.)

How's it sound, Brad?
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Re: McIntosh Power Guard Peak Limiter

Postby strumminsix » Fri Aug 28, 2009 12:54 pm

playingdead wrote:Maybe it's the mystery knob on Jerry's rig!

Image

(Kidding.)

How's it sound, Brad?


n00d. such a n00b. that knob controls how cheezy bobby gets.
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Re: McIntosh Power Guard Peak Limiter

Postby waldo041 » Fri Aug 28, 2009 1:30 pm

:lol: :thewave: :lol:

the power guard is standard in all mcintoshs made AFTER the mc2300. the same is true for the current limiter. i, as well as some others believe that this is what killed that mojo jerry gets used inconjunction with the Sentry Monitor in all later Mc's with it. That's why you can't just buy any mc and call it a day. the power guard "monitors" and alter the levels of the direct input signal by what it sees at the speaker(output) terminals. the Sentry Monitor, monitors the input signal at the power transistors drivers and alters that by what it sees at the output of the power transistors. This threshold needs to be peaked as jerry always did to get it to have that slight hair. but that's not what it's there for. it's there to protect itself from a severe short circuit. the Power Guard prevents that slight "hair" phenom from happening as it will no longer reach that threshold because the Power Guard is preventing the Sentry Monitor(current limiter) from reaching it's threshold. Mc's are made to be powerfully clean Hi Fi stereo systems and this was an improvement jerry proved they needed. That said, it would be very interesting to hear it without a current limiter as it most likely will work just like one only in a different position.

Brad, am i correct when i say that alone, without a current limiter, the Power Guard should allow for pushing the output transistors cleaner, but up to breakup, because the input stage is limited by what the final output stage sees? ie.. your not distorting the input stage, as most amps fall victim of, but rather are able to clip the output stage alone. which is essentially what jerry was doing with the Mc2300 only he was current limited at that clipped threshold?


btw, that's a steal for the price! it should go for a little more on ebay as they are rare and his is mint!
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Re: McIntosh Power Guard Peak Limiter

Postby SarnoMusicSolutions » Sat Aug 29, 2009 5:51 pm

Yes,

this Power Guard limiter would let you push a transistor amp and really lay into it without the risk of clipping or overdriving the output transistors as long as you set the limiter knob to a power level just below the maximum power of the amp. I wouldn't try to use this limiter beyond the power limits of an amp that doesn't have good built in protection (current or VI limiting). I would treat this device as a clean limiter, and operate the amp within that range. Something like this would let you use a power amp without the risk of going over the limits and as a guitar player or bass player it lets you "bang" on it the way music typically makes you do when things get going and get loud.

So this won't exactly let you do what the Mc2300 Sentry type limiter (like in Jerry's 2300) would do because it's a different "action". This really is intended to be an invisible and musical limiter that gently and cleanly handles peak limiting. The Sentry limiter that Jerry used, or more like "abused", was only intended for short circuits on the speaker terminals, or severe attempts at asking the amp to dump more current than it wanted to. The Mac's were just so damn rugged that Jerry was able to abuse them the way he did for so long. Oddly, the result was very musical in that it not only limited and smoothed out the spikes, but also added a bit of distortion that just worked for him. Basically this limiter here is meant to be used under more clean conditions.

Did I understand the question correctly?

Brad
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Re: McIntosh Power Guard Peak Limiter

Postby mijknahs » Mon Dec 07, 2009 1:49 pm

[quote="waldo041 i, as well as some others believe that this is what killed that mojo jerry gets used inconjunction with the Sentry Monitor in all later Mc's with it. That's why you can't just buy any mc and call it a day. ...This threshold needs to be peaked as jerry always did to get it to have that slight hair. but that's not what it's there for. it's there to protect itself from a severe short circuit. the Power Guard prevents that slight "hair" phenom from happening as it will no longer reach that threshold because the Power Guard is preventing the Sentry Monitor(current limiter) from reaching it's threshold. [/quote]

I'd have to disagree with you Waldo on the Macs. Have you ever "pushed" an MC2100? Ever "pushed" an MC2120 with Power Guard? To me, they sound virtually the same. Same great tone of a Mac. The MC2200 I used to have (it was too heavy) had a really authentic tone. Especially on those low E and low A open chords. They growled. Also the 2120 I have has a switch on the back to turn the Power Guard on or off. I've tried both setings and don't notice a tonal difference. The JBLs will start clipping if they get saturated enough even if the power amp is not clipping. I do hear this. Especially when I'm just using 1x12 (through my MC2120).

I also don't agree that Jerry's sound always had that slight "hair". If you want to hear "hair", listen to 5/31/83 JGB at the Roseland Ballroom. Jerry is maxing out. You can really hear it. It's great. But usually he was much cleaner in the GD. I'm thinking GD 1982 when he first started using the Super IIs. Every show is really clean but what great tone! 1980 and 1981 are "dirtier" sounding but that might be due more to the Dual Sounds (maybe even in series?). 1984 is probably my favorite GD Jerry guitar tone and it does have a bit more "hair" than 1982 but I don't think he's playing any louder (stage volume wise) than in 1982.

Just my opinion on the Macs from playing them for the past 13 years. They all get that Jerry tone (compared with most other power amps) even at living room volumes. You don't need to crank them to the point of overdrive to enjoy THAT sound.

Jim
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Re: McIntosh Power Guard Peak Limiter

Postby Pete B. » Tue Dec 08, 2009 9:18 am

Can anyone post a list of the Mac amp models that definitely have the limiter (the ones that should sound just like Jer's)?
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Re: McIntosh Power Guard Peak Limiter

Postby mijknahs » Tue Dec 08, 2009 11:51 am

http://www.roger-russell.com/amplif1.htm
http://www.roger-russell.com/amplif2.htm

It would be any Mac that has the "Sentry Monitor" but no "Power Guard" (or you can at least switch the Power Guard off - like on the MC2120).

Brad's hypothesis is that it is the sound of the Sentry Monitor (limiter) smoothing out the clipping on an overdriven Mac that creates the "Jerry sound". Not sure this hypothesis has been tested or not. I think it's just an educated guess.

At least I haven't noticed any big difference from playing a 100W Mac with Power Guard and one without it.

Jim
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Re: McIntosh Power Guard Peak Limiter

Postby strumminsix » Tue Dec 08, 2009 12:12 pm

mijknahs wrote:http://www.roger-russell.com/amplif1.htm
http://www.roger-russell.com/amplif2.htm

It would be any Mac that has the "Sentry Monitor" but no "Power Guard" (or you can at least switch the Power Guard off - like on the MC2120).

Brad's hypothesis is that it is the sound of the Sentry Monitor (limiter) smoothing out the clipping on an overdriven Mac that creates the "Jerry sound". Not sure this hypothesis has been tested or not. I think it's just an educated guess.

At least I haven't noticed any big difference from playing a 100W Mac with Power Guard and one without it.

Jim

Jim, do you really push your Mac to where you'd hit that ceiling for the monitor? I know you have a great band and it wouldn't surprise me if you did just curious if you have a hot preamp signal from a twin going into a cooking Mac driving 3 12".

I'm very curious what the monitor does. Input padding/limiting? peak limiting? output limiting...
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Re: McIntosh Power Guard Peak Limiter

Postby mijknahs » Tue Dec 08, 2009 12:45 pm

I'm usually using an MC2120 (with Power Guard off) into 1x12" E120. It is often (especially on the higher notes) making the "limit" light flash and I can maybe hear some smooth clipping at times. I've used it with the Power Guard on and it doesn't make much difference sonically (still making the "limit" light up). My Twin is cranked up anywhere from 6 to 8, so its a pretty hot signal.

I don't really think Jerry is getting this kind of clipping all that often (yes, on some recordings more than others - like the 5/31/83 JGB show). Usually though he sounds much cleaner to my ears. Especially in the Grateful Dead.

I think Brad explained the difference bewteen the Sentry Monitor and Power Guard in the above posts. I really can't explain.

Jim
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Re: McIntosh Power Guard Peak Limiter

Postby strumminsix » Tue Dec 08, 2009 1:27 pm

Good info, Jim, thanks. I am always curious about these things.

Since using a limiter/comp in my Bobby rig and it's probably the single best thing I added to my rig for me to properly sit in my band's mix.

I know, I know, going off topic. Just sayin...
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Re: McIntosh Power Guard Peak Limiter

Postby waldo041 » Tue Dec 08, 2009 2:10 pm

strumminsix wrote:I'm very curious what the monitor does. Input padding/limiting? peak limiting? output limiting...



http://www.roger-russell.com/mcintosh1. ... trymonitor


mijknahs wrote:I'm usually using an MC2120 (with Power Guard off) into 1x12" E120. It is often (especially on the higher notes) making the "limit" light flash and I can maybe hear some smooth clipping at times. I've used it with the Power Guard on and it doesn't make much difference sonically (still making the "limit" light up). My Twin is cranked up anywhere from 6 to 8, so its a pretty hot signal.

I don't really think Jerry is getting this kind of clipping all that often (yes, on some recordings more than others - like the 5/31/83 JGB show). Usually though he sounds much cleaner to my ears. Especially in the Grateful Dead.

I think Brad explained the difference bewteen the Sentry Monitor and Power Guard in the above posts. I really can't explain.

Jim


no, jerry definately had some really clean bite. that "smooth clipping" of the sentry monitor is exactly what "hair" brad and i speak of as jerry almost always was in that power range. it's not a Dist+ or Mesa Preamp hairy at all.

The power guard simply monitors the actual output signal at the speaker terminals for a certain threshold then pads the input if that is met. So, if your not blowing out the input of the very robust input stage of the Mc, or pushing the amp to it's limits, then odds are, the power guard is working but not actually doing anything. so on or off Unpushed, wouldn't make a difference. The mc's VI limiter, or sentry monitor, monitors the output(voltage & current) at the output transistors output. if the threshold is met then the circuit switches on and limits the input of the driver transistor. jerry was known to bury the needle of one side of his Mc2300, 300 watts out of a 2 ohm tap is pushing a substantial amount of power and would have the sentry monitor("smooth clipping") always engaged. if jerrys mc2300 had a clip limit lamp it would almost always be on when he was playing. the sentry monitor is activated the majority of the time. a different scenario then running it at lower volumes vs. really pushing the limits of the amp.

the way jerry pushed his mc2300 would, imo, most certainly engage a power guard and limit the sentry monitor from reaching it's threshold and smooth clipping at pushed volumes. that is my theory on why a mcintosh without a power guard, or at least switchable on/off, is more in line with jerry's. also a lower wattage mc'amp would allow you to push the limits of the amp without needing 300 watts. but even a 50 watt mc'amp pushed is way to loud for the average bar gig.

peace,
waldo
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