Bass leaves after amp warms up?

Csontos, you may have missed the post where Al found out that all three amps have bipolar output devices.

My understanding of bipolars is that the hotter they get, the more current they pass- which leads to more heat. This is why they have to have large heatsinks and some sort of bias feedback circuit to control temperature, else they go into thermal runaway.

I think it might be interesting nonetheless to try a set of ZEROs on the system and see how it behaves. http://www.zeroimpedance.com

Then we could put the whole 'system overheating' idea to bed once and for all.

Right now I am thinking that Fishing does not like the sound of the system warmed up. Right now if it were me I would run the system for at least one hour and see how it sounds, and then see how it sounds 3 hours later, after it has warmed up for all that time. I've seen some amps that you don't want to turn off because they need 24 hours to sound right...

I listened to the two recordings on my main system, via both speakers and headphones, after transferring them to CD-R. I also performed various analyses of their waveforms using Sony Sound Forge 9, a professional caliber audio editing program.

By far the most notable difference was in the volume levels. Would I be correct in assuming that you created both recordings with the same setting of the volume control on the amp, and the same setting of the recorder's level control? And would I be correct in assuming that if the recorder provides AGC (automatic gain control) and/or peak limiter functions, that you disabled them?

Before doing any analysis, I cut out dead times at the beginning and end of each recording, because those dead times were slightly different between the two recordings, and that difference might have introduced a small bias in the waveform analyses, in one direction or the other.

With 0 db representing digital full scale (the maximum possible instantaneous volume), I found that the maximum instantaneous volume that was present on the "good" recording was 0.0 db, and the rms average value across the entire recording was about -18.6 db (the two channels being slightly different, but both very close to that number).

On the "bad" recording, the maximum instantaneous volume was about -1.9 db (slightly different between the two channels), with the rms average value across the entire recording being about -21.7 db (again, the two channels being slightly different).

I then produced a CD-R containing (1)the "good" recording, (2)the "bad" recording, and (3)the "good" recording with a 3 db level reduction. The volume difference between (1) and the others was, of course, completely obvious. My wife commented on it immediately from another room in the house. (2) and (3) had some subtle differences, but nothing that I would remotely characterize as missing bass, or even a fundamental change in the character of the bass. And the differences between (2) and (3) were small enough that I suspect they might have disappeared if I had done enough trials to more precisely optimize the 3 db level reduction.

I also used the software to do waveform analyses after applying brickwall (essentially instantaneous cutoff) 300 Hz high pass and low pass filters to the "good -3db" and the "bad" recordings. The high passed content of those signals showed almost no difference in instantaneous peak level (0.2 db, with the "bad" recording being lower), and zero difference in rms level. The low passed content of those signals showed slightly greater but still small differences, about 0.7 and 0.4 db lower on the bad recording in peak and rms levels respectively.

I would agree, btw, with DRubin's characterization of the sound quality of the recording. Parts of the treble region are boosted excessively.

The bottom line: All of the differences, if any, were slight, except for the difference in volume, which on an rms basis across the entire track was about 3 db.

Regards,
-- Al

Ralph, I need to state that the bi-polar amps I have do in fact reduce current when hot, however not as profoundly as the mosfets do. But I don't think it's an 'overheating' issue at all. Simply an incidental one in that because the outputs are operating under an optimal (factory) setting, the difference in performance from cold to hot is more pronounced.
No proof this is the case but I would be willing to wager bias is set to a conservative sub-optimal level in any brand new ss amp. Whether this is what the op is experiencing is imo unclear.

Csontos and Ralph,

So, if I'm understanding what you're saying, some amps sound better cold than when they're warmed up? If that is the case, anyway it goes, it seems that a person needs to leave the amp on and accept the way it sounds warm.

That is, unless you want to listen for an hour, turn it off, let it get cold again and then turn it back on and listen for another hour. That just doesn't seem to be an ideal way to listen to your system.

If a warm amp is at least consistent even if it doesn't sound as good, at least you have a known sound that you can count on.

Am I thinking wrong?

Chuck

You got it! Unless of course you decide to either set up the amp optimally yourself or have it done if you're too squeamish. If you do, you won't be complaining about the issue I brought up and you can go ahead and leave it on 24/7.