Power Guard off?

Discussion in 'McIntosh Audio' started by Big Mac, Mar 13, 2018.

  1. Big Mac

    Big Mac Well-Known Member

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    The McIntosh MC 2120 and MC 2200 have a switch on the back to turn off the Power Guard on the unit. Is it desirable to have the Power Guard off, especially is running very efficient speakers (94-100db)?

    Are the amps better sounding with the Power Guard off or on??

    Thanks in advance!
     

     

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  2. vaathi

    vaathi baski Subscriber

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  3. Big Mac

    Big Mac Well-Known Member

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    Thank you for the link to the thread.
    Why then would McIntosh put a switch on the amps to turn off the Power Guard system?
     
  4. c_dk

    c_dk Addicted Member

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    So we could demo it to all the skeptics when it was introduced in 1977.....
     
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  5. whoaru99

    whoaru99 Epic Member

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    If you don't run the amp with clipping lights flashing regularly one technically doesn't need Power Guard.
     
  6. kevzep

    kevzep Its all about the Music Subscriber

    Speakers with that sort of efficiency? Won't make any difference if its on or off, you'll never reach the threshold of PG anyway.
     

     

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  7. kevzep

    kevzep Its all about the Music Subscriber

    At the risk of splitting hairs, the red PG lamps are not clip lights, they are PG activation lamps, the amplifier does not clip with the PG.
     
  8. whoaru99

    whoaru99 Epic Member

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    That's true but it is indicative of the basic premise that was the point.

    Regardless whether they are clipping indicators or PG indicators, it's a good sign it's time to cut back a bit. :)
     
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  9. whoaru99

    whoaru99 Epic Member

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    Curious though, if one turns off Power Guard do the PG indicators no longer illuminate at all or do they turn into plain old clipping indicators?
     
  10. kevzep

    kevzep Its all about the Music Subscriber

    Or get a bigger amp!! :D
     
  11. c_dk

    c_dk Addicted Member

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    The switch just breaks the shunt to chassis ground from the LDR so it no longer drops the input signal...so the lights would still indicate reaching the distortion threshold of that design......1% on the early models. I believe today the threshold is .2%, has become progressively less over the last 40 years.

    I do not believe most of the "clip" lights monitor the distortion compared to the input waveform.....just a threshold of the DC component of a distorting wave form at best.
     

     

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  12. whoaru99

    whoaru99 Epic Member

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    That's true too. There are various forms of "clipping" indicators, from monitoring the input signal relative to design gain of the amp, to monitoring peak output voltage relative to rail, to input/output type comparators...and probably others as well.

    The good news is that the meaning is pretty universal; turn it down (or get a bigger amp as previously mentioned by kevzep). ;)
     
  13. 62caddy

    62caddy Trust but verify Subscriber

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    I while back I recall something to the effect that some musicians/studios wanted to be able to defeat the PG circuit in applications involving amplified guitars.

    I have no idea how true that is.
     
  14. c_dk

    c_dk Addicted Member

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    The only people worse to deal with than teenage daughters is "talent", when either get something in their heads you might as well surrender.

    When "talent" would get it in their heads that their sound was happening because of distorting main amps....well you know it was their "sound" right up to the point of no sound as the driver burned out.

    Getting them to understand that their sound was created in the guitar's head end, distortion and all and then allow the main amps to pass that on to the main speakers in a controlled fashion......well it was their sound you know......
     
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  15. twiiii

    twiiii Addicted Member

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    First understand that all of us love certain distortions. Maybe its emphasized bass here or boosted treble there. Well when Power Guard was first introduced potential owners didn't understand the consequences of using it. They thought they could turn the volume up and there would be complete freedom from speaker damage and the sound wouldn't be affected. A lot of listeners had been use to hearing the false brightening of the source when the volume was increased and of course that didn't happen with the Power Guard ON. What did happen was the imaging was compromised and the sound was squeezed similar to listening to POP music on a top 40 FM radio station when the Power Guard was on. I f pushed to hard with greater than 30 % PG activation the bass rhythms could modullate the sound. With the false bright ness gone, the imaging and staging collapsing and tonal balance changing customers were turned off. by PG. They didn't understand how to match the amps with power guard to speaker systems either. Speakers at the time didn't have the dynamic over load capability a lot of speakers have to day over the entire spectrum. So speakers rated at 100 watts might only have tweeters rated at 10 watts. Folks reading speaker advertising would read the speakers could handle 400 watt peaks. That meant buying at least 300 + watt amps . When they clipped the 10watt drivers were instantly destroyed . With MC 2120;s which they should have been using the peaks were compromised by the power guard and the tweeters could still be damaged. With Disco and electronic music coming into popularity higher output capability from tweeters was required. So until Mcintosh and other speaker producers started updating their speakers, Power Guard was an unappreciated product not easily sold to skeptics. To day with amps with higher peak capability, more advanced circuits, and better speaker design integration between amps with power guard to protect drivers and prevent irritating distortions are much more appreciated. With todays amps not being continuous duty like the 2000 series. SS amps with power guard also prevents early on set of over heating along with safe operating area protective circuits.

    Customers today appreciate their systems need dynamic head room and above that plateau some methods of protection should be available. With discipline the system can be chosen to provide desired average listing levels with the under standing that a minimum of 10 to 20 db extra dynamic capability maybe required of the system. So if you listen with loud passages that require 1 to 2 watts as I do with occasional 40 watt peaks with an additional 10 db before protection sets in and the speakers being capable of handling all the power before the PG turns on you will have many years of trouble free distortion free listening. Turning off the PG only increasing the distortion, increasing heating in the voice coils and in the amp output and power supply sections shortening the life of all components involved.

    If your system requires the Power Guard lights to flash all but occasionally then either your speakers aren't efficient, enough, or the amp is to small or both. Under no circumstance shall the amps be allowed to clip generating many times the heating power in HF's that can destroy fragile speaker coils and causing the amp to shut down and if repeated guaranteeing mutual self destruction the voice coils and electronic components of the amplifier. So before buying a 2105 that like its bigger brother the 2300 are known to be fragile high frequency driver dragons, be sure you will never use more than 50 watts under any circumstance.and that your most fragile driver can handle that much power and more. Now the 2205 with its 260 watt capability won't clip, just generating 5 times the heating of the 2105 not clipping, but easily half of the 2105 when it does clip. Clipping is not only destructive to your system but very irritating to the ear.
     
    Last edited: Mar 15, 2018
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  16. damacman

    damacman Blown and Injected Subscriber

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    As usual, a thread like this is easily derailed into discussing the effects of clipping and its destructiveness. twiiii, I know you have a certain disdain for the MC2300 in this regard, but there is really nothing magical inside an MC2300 that would cause it to be more destructive to loudspeakers than other similarly powerful amplifiers. In theory, an amplifier driven to hard clipping can produce 200% of the power it can reproduce unclipped. If we factor efficiency losses, it'll be less but close enough for government work. That's where you run into problems. Many loudspeakers can handle the 400 - 420wpc an MC2300 can put out cleanly. Few can handle double that.

    My buddy Garry Springgay of Cogent Audio Labs was recently telling me a story about a training he conducted years ago. One of the areas he was asked to discuss in detail was the effect amplifier clipping has on tweeters. So, he set up a prop with something like a 5w amplifier. He fed it a 15 or 20kHz test tone and then drove the amplifer into hard clipping. The tweeter it was connected was rated at something like 15 or 20 watts. When he began the training, he showed the attendees this, let them look at the clean waveform on a scope, then let them look at the nasty clipped waveform, and then set the tweeter down on the bench and continued. Everyone had their say on what would happen to the tweeter. At the end of the training, the tweeter had survived, undamaged to everyone's surprise. The point he set out to prove was that clipping does not damage tweeters as long as the clipping does not result in overpowering them.

    twiiii, you obviously have more experience in this forum with live sound and large venue sound than anyone else. Maybe you can elaborate a bit on things you experienced that would point to a different conclusion than what Garry arrived at.
     

     

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  17. twiiii

    twiiii Addicted Member

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    I agree I had a lot of experience with professional systems. But when customers started loosing tweeters and mid domes in ML series with 2105's and 230o's guess who got the call. Klipsch corner horns, Belles La Scalas, and Hersey, MWM and MWM-s had issues, too. Altec Carmels, Capistranos, Coronados, EV and JBL speakers were all suffering from SS amps where tube amps hadn't caused any issues at all. Guess who got the call. Most two ways systems it was because the crossover was set at to low of freq. Altecs changed from 500 to 800 HZ almost doubled the amount of power handling capability. Replacing caps help solved ML series issues. Mac built a little box for the 2300 and 2105 that was an external PG that we used quite often. Part of the problem with Mac speakers was they were so in efficient and required and Eq for bass. It wasn't until the XRT 20 and 18 came along the speakers could handle power. For professionals bi-amping came along for more efficient use of power and to get better efficient control of woofers. Power Guard allowed us to realize 4 db more out put level from JBL, EV, and Altec drivers without audible issues. It was an interesting time and great learning experience. We had no trouble selling 2200, 2205, 2120, 2125, 2155, 754, 502, 2250 2255, 2500, 2600, 7100 and 7200 amps. When the 7300 and 7150 came along QSC had their own version of PG and Mac wasn't making a commercial version of 7000 series amps, so the market dried up and went away for Mac amps.

    I agree with that having to little power can be just as much as issue as to much. I have always had to much, with and without PG. And only lost one mid range driver. My Speakers are rated at 95.5 db sensitivity. Most loud passages are about two watts. Some music on direct to disc, CD and dvd can have 10 db peaks above that and I have another 12 db above that head room when the PG turns on. Woofers are rated at 100 watts RMS so having amps capable of 260+ watts before PG works just fine. The mids are rated 130 watts and the tweeters 80 watts and the super tweeters 100 watts all together. I have never had an issue after going back to MAC amps. I lost some super tweeters with under powered D 150 A crown amp. I upgraded to a DC 300 A to the top and added a PSA2 to the bottom. No issues for the next 25 years. I have had 207's for over 10 years now and all is still fine. My original 275's in mono before Bi and tri amping never gave me any issues either. Friends with the same speakers using 3500's never had issues. Folks with 2300's and 2105's were the only amps that ever caused a Bozak driver failure. Friends with ML-2's and 2205 and 2150 never had a problem. Folks with 2105s were having issues with the domes and tweeters failures. Fortunately most of the time upgrading Mac owners to 2205's, 7200's and 2255's wasn't an issue. We had plenty of other ML 2 and 4 owners with amps with PG to prove its value.
     
  18. whoaru99

    whoaru99 Epic Member

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    There is no such thing as an underpowered amp; only overzealous operators. ;)
     
  19. whoaru99

    whoaru99 Epic Member

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    Agree with the latter...SS amps for sure, anyway. The former isn't inherently the case as damacman detailed.
     
  20. TSmith8605

    TSmith8605 Senile Member Subscriber

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    Not sure if I want to wade into this one, but what the heck. You all may ignore this at will and corrections and / or refinements are welcome.

    My understanding:
    Audio engineering analysis is largely based on sine wave analysis as the builders of synthesizers learned in the '60s that any musical waveform can be approximated or synthesized by applying an appropriate number of sine waves at appropriate relative amplitudes and frequencies. So describing how a sine wave goes through an audio power amplifier is a pretty good way to predict how it will react to music for most (but not all) design parameters.

    Tweeters will blow when overpowered causing excessive heat that melts the insulation on the voice coil wires allowing the coil to short into a piece of wire (short circuit) or maybe come apart causing an open circuit. So after a certain amount of time of excessive power, the coil will overheat and be fried. The amount of power a tweeter can stand depends on the parameters of the tweeter so some are more fragile than others.

    When an amplifier badly clips a sine wave, it converts the sine wave into something more like a square wave with the same peak voltage and frequency as the offended sine wave. I can't draw a picture but imagine a sine wave and a square wave drawn together on the same chart with the same peak value and same zero-crossing times. In one-half cycle, the sine wave smoothly raises and smoothly levels out, touches the peak value for just an instant and then starts a smooth decrease back to the zero crossing. A square wave (or a wave clipped at the power supply rails) goes immediately to the peak value, stays there until the half-cycle is about over and then quickly drops through the zero crossing. Superimposed, a square wave of the same frequency and peak value of a sine wave has more area under the curve - i.e. more energy and more power than the sine wave.

    Further, since the square wave voltage is changing so quickly at the zero-crossing times, this extra energy is at frequencies higher than the offended sine wave's frequency. In fact if you look at the spectrum of a sine wave, you see a single peak at the wave's frequency. If you look at the spectrum of a square wave, you also see that same peak along with several smaller peaks at odd-integer (integer meaning harmonic) multiples of base sine wave's frequency.

    So if your amplifier clips a high frequency sine wave, it turns it into a square wave (approximately depending on how badly it is clipped) by adding those unwanted odd-order harmonics (i.e. harmonic distortion). So again, a clipped amp is generating more power and at higher frequencies than the same amp/signal unclipped.

    In effect, the amplifier is channeling a whole lot more energy directly from the power supply to the tweeter than it should.

    This does not mean the tweeter will blow. The tweeter will blow if it gets too much power and over heats relative to its own design. But It does mean that the tweeter is more likely to be damaged by clipping. So a 5-Watt amp probably won't blow a tweeter regardless of the waveform. But, for example, a 50-Watt amp may be fine for the tweeter normally but then blow the tweeter when driven into clipping and the user is more likely to overdrive a 50 Watt amp than an identical 100 Watt amp in the same system. It just depends on the amp and tweeter.

    So I don't see a controversy here, clipping per se does not blow a tweeter but clipping does redirect a lot more power from the amp's power supply into the tweeter making damage more likely in some cases.

    A least that's my understanding of things.

    Another issue with some of the original transistor amps, (like the 45-Watt Kenwood KA-6000 that I had as a teenager that blew my tweeters on a regular basis), is high frequency oscillation. The amp uses a lot of negative feedback to control distortion and early, poorly designed transistor amps had the problem that with high enough frequency, the feedback delay becomes equal to 1/2 the period of the signal turning negative feedback into positive feedback and now the amp is banging the output between the power supply rails as fast as possible - at supersonic frequencies. This, (depending on the situation) can be instant death for the tweeters. Seems like a clipping amp with those high frequency harmonic components is more likely to trigger the positive feedback, so the clipping and oscillation may go hand-in-hand.

    I first heard about oscillation when I proudly took my KA-6000 to a McIntosh clinic. The man running the clinic said that it, in fact, put out 50 Watts per channel and otherwise in spec. I was even more proud. He also sternly told me (like and adult taking to an idiot teenager) that the amp was oscillating and showed me that on his 'scope. I had no idea what that meant and felt slightly chastised but also proud that my first real Hi Fi amp put out 50 Watts when rated at 45. I went merrily on my way, blowing tweeters every week or so until I got bigger speaker that the amp could not hurt.

    I don't image any McIntosh amp so poorly designed as to be unstable and oscillate.
     
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