M700 upper midrange hardness and run-in period help please

Hi PS Audio team and forumers. I have a dilemma with my M700.

The M700 monos is driving a pair of ATC SCM11 and preamp is Primaluna Prologue. Straight out from the box, they sounded flat and lack air. After 2 weeks of use it has slowly become more open sounding but also picks up nasty upper-midrange hardness and forwardness especially with female vocals. The high frequency is smooth though. When I swap back to a Pass Aleph 3, the upper midrange hardness was gone.

I’ve had the M700 since mid September. Playing on average 1-2hrs a day. From Stereophile and Positive-Feedback reviews, the M700 has a smooth midrange. Can my midrange issue be due to lack of running-in? If so, how long do I need to get to that smooth midrange tone? Can I just leave the power amp on without feeding music to run-in the amp?


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What are you using for source? One other thought is cables. What are you using? IC’s and Speakers?

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Those amps do take awhile to burn in. I ran mine continuously at very low volume for a couple weeks. I can’t say at what point they would be considered properly broken in but they definitely improved over time. I also use a tube preamp in that system so I connected the amps directly to my DAC and used software volume control while burning in.

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Considering you’ve only had them for two weeks, and have been running them 1 to 2hrs a day, that’s max 28 hrs, which is not nearly enough time to break in new electronics. From my experience every piece of gear I’ve owned has taken a minimum of 200 hours before things started sounding good. I’d say you have a ways to go with break in.

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I noticed the same with mine. I’ve had them for two moths now and they get better every week. I leave them on but they can still sound brittle in the upper-mids for the first 30 minutes or so.

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My experience with Class D amps from four different manufacturers proved to me that they need to be left powered on all the time and need several hundred hours with music or some other generated noise going through them. Unlike tube, Class A and Class AB amps you can’t just warm them up for an hour and you are good to go. When Hurricane Michael took the power out here for almost 5 days it has taken the IcePower 1200AS based amp in one system days to get back up to snuff unlike the AB amp in the Main system that only took a couple of hours.


The M700 in all frequency spectrum sounds wonderful except the upper mids. I think I will just need to blast with signal for 500hrs. Can I run the CD player output direct to the power amp without connecting to the speakers for this running in period?

No, the amp needs to be connected to a load. One trick to reduce the perceived volume is to place the speakers directly facing each other, reverse the phase of one speaker, and play a mono signal.

You bet.

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Is not a load necessary to completely burn-in the output stage?

There’s continual debate about that. Some suggest yes, others like myself don’t find that to be true.

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Thanks, Paul

I have never compared so I cannot opine. I also tend to just using a new product, rather than concentrating on break-in, so I am a poor observer in this regard. :slight_smile:

Ted has explained how the DS’ output stage benefits when breaking in under load. It makes sense to me this would also be true of amplifiers.

Ahhh, yes indeed. Ted’s DS design has an output transformer so I suppose any product with an output transformer would benefit. Good thinking. Thanks. I’d forgotten that one.

Whew . . . :slight_smile:

I have a sprout100 on my maggies and it doesn’t sound harsh.
Reading that and the fact i wanted to upgrade to s300 or m700, i am a little bit confused. Is finally the sprout100 delivering me smoothness because overdriven by the maggies and i will lose that smoothness going with more powerfull amp underdriven ?

I’ve never heard a Sprout100 although I may get one for my KEFs after my Ohms show up. I have owned the S300 and now own the M700s. Upgrade to the M700s if you can. I heard some harshness in the upper mids early on. The problem is now I listen for it. I realized that when it does show up now it is due to the file I’m playing. I have 27,000 320kbps MP3 rips on my ROCK. It is only a few of those that exhibit the problem. Now I relax and enjoy the music. I broke my amps in the old-fashioned way. I listened to a lot of music. As far as S300 vs M700s are concerned, the M700s are far better in my system and I expect better results from the new speakers as they harder to drive. I believe they would be a great match for maggies.

Rather than play the speakers you can just hook-up these after playing the speakers at just above listening level. Leave the volume control and let the amp “burn in” if you believe that happens. For some that can’t have music playing in their house at all hours this is the ideal way around it.

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So I have left my M700 turned ON continuously for the past one week (without and occasionally with load). I have noticed improvement on the midrange hardness. Let’s see after 300hrs and 500hrs how it goes. I am also going to change my Primaluna Prologue Preamp rectifier tubes to the Mullard GZ34 F31 fat base to see if this further sweetens the midrange. Will update as I past these running in hours milestones.

If you are still running the Prologue with the supplied Primaluna branded Shugang 12AU7’s that may be part of your problem. You need to look into changing out at least the two input tubes with something else. Warmer NOS tubes would be preferable but even current production Gold Lion’s would be more of an improvement than the rectifier tube.

Currently two tubes on Mullard CV4003 NOS.