Bass leaves after amp warms up?

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.

Regards,
-- 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.

Thanks Atmasphere
You're really smart,I will follow your suggestions & report back