Bass leaves after amp warms up?

I don't understand-after my Musical Fidelity M6i amp warms up for about an hour I notice the deep bass & kick drum aren't the same.
They sound less musical with loss of weight/depth.The notes are there but the moving of air have left.Sound is has much less impact and boreing.
I had the same problem with Bryston amp so there is no defect with amps nor with the rest of my equipment/
PSB Synchrony one speakers,AQ cables,Bryston CD Player.
My question has anyone heard similar & is there a plausable reason?
I guess that transistors don't have enough heat sink and go hotter than supposed to be in normal operational state.
The noiseless fan can probably rectify your situation
It's your ears, not the equipment. If you were to take a measurement, I'd bet you there'd be no change.

I frequently find, within the first 5-10 minutes, that I increase the volume about 3 dB. That's at an average level between 80-85 dB. It's a bit like your eyes; they adjust to brightness—your ears to sound pressure.
Nah,I'm a professional musican and know what I'm saying.
Besides significant other hears the same thing as myself

Could it be that the bass is tightening up as the amp(s) warm up and stabilize? Is it going from bloated to more lean with better definition?

Could it be that you are not hearing less bass just tighter bass as your entire system warms up ?
I prefer to let my system play for about an hour before I do any serious listening. It could be your room or placement of your speakers. Not your amp.
Just a thought.

Most likely your system is on the edge of lean and when it warms up it goes over the edge. Make adjustments in speaker placement and things that make a difference in tonal balance like selecting cables after the system is fully warmed up.
Perhaps,but the bass is taut & punchy while not being bloated before the amp warms up-It sounds right & enjoyable.
The change is dramatic & unmusucal after warming up
I had the same problem with Bryston amp ...
So it would seem that the problem is most likely unrelated to the amp, and that it is related to something else that is occurring during that hour.

Does the same thing happen whether or not the CD player is spinning a CD during that hour?

Does the same thing happen whether you are listening to vinyl or to CDs (I see in your system description that you have a turntable in your rig)?

Does the same thing happen if you are not playing music during that hour, so that the speaker drivers are not being warmed up/loosened up during that time?

-- Al
Okay fish, you're a professional musician. You've dismissed all ideas and suggestions out of hand. I guess you're on your own. Good luck and I hope the bass just mysteriously reappears for you.
Yes this phenomena happens with Vinyl or CD source
I believe wheather it's spinning or not the sound changes after the amp is on about an hour or less.
Also if I change source from CD to vinyl the sound frequently changes for the worse faster.As if I've distrubted the circut
All amps are no perfectly efficient and produce heat as a result. If the heat produced is not dissipated and temperatures rise to much accordingly, I would expect sound quality can be affected.

CAn't say why this might be occurring in the OPs case, but better ventilation around the amp, maybe even use of external fans, might help.

Based on specs the amp should be largely up to the task of driving the PSBs, but driving most speakers to very high volumes might tax most any smaller integrated amp I have seen to some extent.

I had a Musical Fidelity A3CR power amp that I believe was heavily biased to Class A and was of modest size yet quite heavy and tended to run quite hot accordingly. Might be a similar case here. I moved to a very efficient Class D amp that never breaks a sweat regardless of volume with my lesser efficient OHM speakers. Bass and dynamics is never an issue these days at any volume.
What about my third question? If, after the system has been unused for a period of time that is typical of your listening habits, you turn the amp on, play no music for the first hour, and then start to play music, what does it sound like?

-- Al
One red flag I see that might help account for what the OP hears is that his PSB speakers show 4 ohm nominal impedance and the MF amp specs indicate power into 8 ohms only. Together, that might account for what the OP hears. Amp specs or bench measurements in a case involving a 4 ohm nominal impedance load would be more informative.

BTW those PSBs are one of my favorite speakers.

I have heard them run quite nicely in a smaller room and at fulfilling volume off of 80-100 W Rogue tube amp or slightly higher powered Rowland Class D amp.

I did not notice any dynamic compression at the time I heard, however audition was limited.

I would be wary of most any smaller integrated to match this performance though. Tight physical proximity of power amp and pre-amp in the same box creates real limits on what can be achieved compared to separates.

I would want to run those PSBs of the best and beefiest standalone amp I could find if it were me, to help assure OPTIMAL, not just good performance.
Ron, because he said in the original post that the same thing occurred with a different amp.

Best regards,
-- Al
Hmm, why could it not be both amps tried are being overdriven driving the 4 ohm PSBs to the user's satisfaction, especially if listening at very high volumes, and performance deteriorates over time accordingly?

Which Bryston amp was used specifically? Were the limitations over time as pronounced as with the MF integrated? Does the problem mostly occur when listening at higher volume?
The complaint is one I have come to associate with MOSFET output transistors. A small fan can indeed help. The amp may not run all that hot but MOSFETs have a quality where the warmer they get, the more they limit current. So you want to keep them as cool as possible.
Go to your friendly neighborhood Radio Shack and buy their relatively inexpensive sound level meter and get the Stereophile Test CD and run it on your system and in your room to see what is happening. You can get on the Stereophile web site and order the test CD for quick delivery. The Sound level meter is really nice also. This will give you a very good indication as to what your system and your room is doing without spending tons of money. you will see if there are holes in your bass response for your system/room.
Fishing716 mentioned also on another thread that switching sources can bring back the bass. Same happens when he "adjusts" speaker cables. It would imply bad connection somewhere. It is not amp because he tried two, not source since he switched them, not speaker cables because he had this with spades, bananas and direct wiring. I also believe he tried different speakers? It is not line voltage since he stated it is 120V. It is not outlet because he tried few.

Is it related to equipment or room/building? It might be bad connection somewhere in the building (defective breaker, loose screw in the panel etc.) I would set-up the system with small speakers and just CDP to verify that it is still happening and then would get it to my another house.

I considered defective jumper from pre-out to power-in but Bryston has switch.
Good call. Bad connections happen and could easily produce the effects observed.

If the problem occurs at lower volumes as well as high, that would better indicate that temperature inside amp over time when driving speaker load is not a factor as well.
I said "to my another house" - it should say "to friend's house".

I hope it is the house, as hard as it is to change, because the only alternative is Fishing716 himself (common factor) - musician, 60's, LSD. If that's the case he has to be brought up at completely different forum of "experts".

I'm making jokes but I feel your pain Fishing716 and read your other threads on the subject. You'll find it eventually, I'm sure.
I don't know as much as many posting here, but the amplifier/speaker match always comes up sooner or later when we know speakers aren't playing up to their potential especially in the bass. I've always seen in the forums that proper matching of the amp/speaker is crucial to proper performance. Sounds like a bad match between the Synchrony 1's and the Musical Fidelity amp. You may want to try a high current amplifier that "doubles down." (the power of the amplifier doubles as the impedance of the speaker is halved). By the way, Bryston and Musical Fidelity amps do not do this.
Interesting question I think Ralph may have hit on. I have lots of amps and they all exhibit the same behavior potentially. But I don't think it's limited to mosfets. I have both mosfet and bi-polar types and the same thing applies to both. This phenomenon is more prevalent when an amp is listened to "cold" and as it warms up, performance seems to fall off a bit. Not as noticeable with an amp left on 24/7. Mine are on 24/7 and sound better and better as they warm up. Why? Because quiescent current is higher when it's cold or cooler than when it's 'cookin'. So my solution is to set quiescent current to optimum when it's good and hot. The op has good ears. Not many people care about that even if they notice. It has to do with manufacturers' warranty issues. The cooler they run, the longer they last.
I am finding my Icepower Class D amps by far are less subject to dynamic compression at high volumes than any other SS amp I have used in recent years, including 360 w/ch Carver, 180 w/ch Tube Audio Design (I still own and use this and have a/b compared to Class D Bel CAnto), and 120 w/ch Musical FIdelity A3CR.

Only with the BC ref1000m Class D amps (500w/ch) does the volume continue to go up proportionally to raising the volume and the music continue to expand without compressing at some point prior to target lifelike volume levels.

The Class D amps seem to perform more like tube amps in their ability to go louder and louder without compression and fatigue, an attribute often attributed mainly to tube amps otherwise.

The 180w/ch TAD Hibachis (same ballpark power-wise as OPS MF I would sy) are lovely sounding amps even at fairly decent volume, but they do run out of gas sooner and at a point that some who like to listen loud and clear might find deficient.

To answer your 3rd question
If I turn the amp on and everything sounds good I listen as long as it lasts(about one hour).
I have not tried turning the amp on and waiting an hour to play music.
Also after the full sound leaves I tend to either reconnect speaker cables(which works if I am persistent) or come back later.
It's safe to say I never know if things will be the way I like them with full deep punchy bass or white washed and un emotional.
It's really perculular
Generally the amp plays better cold than warm but sometimes The sound dosen't change for a few hours,but inevitably does.
Foster_9, that's true, some amps are better than others but his amp plays fine for couple of hours and then looses bass. Switching source or adjusting speaker cable can bring back the bass performance instantly.
Fishing716, what about amp's power cord? Is it possible that you used the same cord with both amps? Defective cord would be better news than defective house.
The problem is not related to the amp because I had two different Bryston's prior with the same result. Powerful full sound then it vanishes.
Instruments & vocals no longer ring true
While watching DVD Sountrack like James Bond
The growling & bass effects obviously add enjoyment to the experience.After my system no longer reproduces these effects the movie is no longer worth watching.
I never listen at high volumes.I have a downstairs tenant & don't enjoy music loud
Fishing, also I am still wondering if volume of listening is a factor. How loud do/can you listen? Does the problem occur mostly at lower volume or high, or does volume not matter, same results?

If associated with lower volumes as well as high, a connection issue becomes more likely. COuld be a power cord conenction even as Kijanki suggests.

As you describe it, I would expect teh problem to be most noticeable at higher volumes where bass "pressurizes" the room more normally, but if clearly noticeable at lower volumes still, amps ability to drive the speakers optimally may not be the prime issue to address first and then see.
If you had the same problem with the Bryston amp it's very unlikely a power amp issue. Sounds like a power issue. Either line/conditioner or pc. Have you tried switching power cords? Maybe a defective conditioner? Voltage flucuation?
Simply would like to know if anyone has experiencd even a noticable change of music being reproduced from one time to another?
It seems the symptoms I hear have no precident..
Which bothers me the most I think
I've looked on line + spoken to all the manfacutures of my equipment..Most won't even accept my calls any longer.
" I never ever listen to music loud"

Good to know. Its probably not the amp then.
Have you tried different speakers, as Kijanki thought you might have? And if so, did the same phenomenon occur with them?

I'm still suspicious of the possibility that the performance of your speakers is changing after they have been playing music for the hour.

-- Al
I have seen cases where old switches in pre-amps or other gear cause a very significant loss of power over time as they oxidize, etc., and good contacts are lost. Same true with volume controls, balance controls, etc.

If there are any manual switches in play with your gear, next time you hear it, try twidling with controls and switches and listen for any differences. THe twidlling alone might help but if so a good cleaning is probably in order.

This happens with my old NAD 7020 receiver from 30 years ago that I still use. A few twiddles with the input switch on occasion when I notice things have faded out sets things right again until thenext cleaning of switches and controls. Not an uncommon thing...
I had PSB Synchrony twos now I have PSB Synchrony ones which are all around better when everything works.
Auditioned Thiel speakers for about a week but results were inconclusive

Could PSB being 4ohm possibly be the reason?
No one else has these symptoms & my dealer disputes this.
Ironically I keep buying PSB anyway..
I have Kimber PK 10 powerkord for the amp & Audioprism foundation II power conditioner.
I've also went direct into the wall.
None has proved to be the issue
Did the Synchrony Twos exhibit the same symptoms as the Ones?
Could PSB being 4ohm possibly be the reason?
As Mapman asked earlier, what models were the Bryston amps?

-- Al
Yes Synchrony II' exhibited same symptoms as current one's

I had Bryston B60R SST & B135 SST
Both fine sounding amps

Thought I needed more power so got MF M6i 200 watts
One more possibility: dynamic compression from overheated woofer voice coils.
On two consecutive set of speakers?
Others would have found problems if this was prevelant in PSB speakers.My hearing is very good but the drop in dynamics is night & day.
PSB Synchrony one' speakers are rated top 25
Csontos, I did read it but never experienced such big effects from wrong bias. There might be some temperature effect on Mosfets' rds(on) but I cannot imagine why amp would suddenly play fine again after changing source or moving speaker wire. I was asking if the same power cord could be the cause but no answer. I was suggesting trying in different building to isolate the problem, but got no answer to that as well. I'm out.
I used stock,Kimber & audioquest powerkords with no benifit.
I am unable to play my system in a different building but that would certainly be illuminating.
Peter (Csontos), your point is an interesting one.

But in this case, given that we now know that the problem has been observed with two different speakers (having similarly low impedances; here is the plot for the Synchrony One), and given that the specs on all three of the amplifiers are not suggestive of strong performance into low impedances (the M6i has no 4 ohm spec; the B135 has no 4 ohm spec in the manual, although there is one at the website; and none of the three amps are rated to double power into 4 ohms), I'm now thinking that the problem is simply that none of these amps can hack it with these speakers. And that raising the bias, if it is practicable to do so, would risk damage.

Not sure what else to suggest at this point.

-- Al
Kijanki, its not about incorrect bias. Its about current limiting due to temperature.

I always liked the MOSFET and later types of amps over the earlier bipolar designs because they sounded smoother. But one thing I didn't like about them was that the bass would dry up shortly after you started playing them.

Now two amps have been involved but this could still be the issue. However Mapman made a good point earlier- a dirty selector switch (on the preamp) or the like can also dry up the bass, but IME there are usually other symptoms- like noise when you operate the switch. It could also be an intermittent cable, but I would again expect other artifacts- hum or a squeal. Loose speaker cables could do it, but I would expect the signal to also occasionally drop out entirely on one channel.

One telling thing is the problem appears to be in both channels. This strengthens the idea that it could be power related, but likely not due to a power cord.

You might see if the problem shows up at a certain time of day. If yes, power coming into the house is indicated.

Here's something to try: Next time this happens, stop playing the system for about 1/2 hour but *don't* turn it off. After 1/2 hour play it and see what it does. See if you can repeat the process (have it go bad, 'rest' the system and have it be OK).

Plan B: when the bass dries up, shut off the stereo and let it sit for 1/2 hour. Play it until the bass dries up. Make a note of how long it takes for that to happen.

I am still really suspicious that this is current limiting due to heat in the amp; these tests will confirm. Of course, you could put a fan on the heatsinks and see if that fixes it.

Good Luck!
Your conclusion is very on point..I came to the same one but was "convinced" by Dealer that wasn't likely.
You would suggest I get 8ohm speakers?
And no one else on this planet had similar problems?
PSB Said "we sold many thousands of this model" not even one similar complaint.
Ralph, I have 4 Acoustat TNT amps just rebuilt by Roy Esposito. They are TO2 mosfet amps. I assure you they get better as they warm up. So much so that they are 'eerily' life like when in mono mode. I don't know what Roy did, but I have not experienced this kind of sound in my life. However, may not have anything to do with your assertion.