Bass leaves after amp warms up?

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.

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!

OK. Based on the testimony so far we can safely assume that hearing has nothing at all to do with it.

I am also doubting that the interface between the amp and speaker is an issue. If it was, the bass would be a problem as soon as you turned the amp on. So that is not it.

The only clue we have is that doing something with the speaker cables sometimes has an effect. Letting the system cool off does not.

If none of *that* has any effect then I become more suspicious that its not the amp/speaker setup at all. But- the fact that moving the speaker cables has an effect is an important clue. Let's start with the recommendations above and see what happens.

At this point I would be looking at the cables. How long are they? How tight are the connections at either end? After listening until the bass dries up, is there any heat on either end of the cable? It might be as simple as a loose connection.

FWIW, speaker cables become very critical when used with lower impedance loads! If your connectors are only finger-tight you are missing a bet. You should get the connections as tight as you can without damaging them. I usually use a small wrench to tighten things down. That can have a big effect on bass, especially with lower impedances.

Its also true that you really should not run speaker cables longer than about 5 feet if you have low impedances in the speaker.

The fact that the system seems to work while running only one channel is a powerful clue.

The amps themselves are ruled out, as 3 amps have had the same behavior. Also ruled out is hearing and the speakers. The preamp and source/s are still under suspicion.

This has something to do with warmup, but I suspect not in the amp or the speaker. My suspicion right now is a defective power cord or wall outlet that heats up with enough current through it such that it eventually starves whatever amp is plugged into it. My suspicon leans this way because if it was a preamp it would not care if only one channel was running.

If the wall outlet, or either end of the power cord for the amp gets warm over time then I would think that this hypothesis is confirmed.

If the power cords and wall outlet seem OK, then the preamp and sources are under suspicion.

Here are the things ruled out:
speakers
amps (3 amps used, from 60 to 200 watts/channel)
hearing
electrical

The only clues are: bass might come back on its own or when messing with speaker cables.

This points to another component. What happens if you listen to an LP, and not only have the digital off but entirely disconnected, even unplugged from the wall. This is sounding more like an RF interference problem, and malfunctioning digital can do that.

Mapman, the reason the amps are ruled out is because the bass can come back without shutting the amps off, in fact can come back without the system being messed with. It comes and goes.

If the amps were the problem, once they had warmed up the bass would be gone for good. But its not.

My suspicion is that a digital product in the system is having an intermittent problem and affects the ground of the system as a result, such that the chassis of everything is coated in digital noise, so to speak. If it were me I would unplug all the digital stuff (CDP and video) from the wall, then run the turntable and see how the system holds up.

I agree with Tls49 about the conditioner. IME its rare to have good experiences with them and amplifiers- they might work OK on the rest of the system but the amp should plug into the wall directly.

If the digital was unplugged, seems like we can rule that out as being a problem.

Go ahead and restore the system.

I realize I made a poor assumption. I would not have turned off the amplifier when messing with speaker cables. Most transistor amps don't care, and our amps don't care, so I always leave them on.

The fact that they are getting shut down is significant. As I mentioned earlier, the fact that messing with the cables gets a response is an important clue; turns out the amps were shut down during this time! They have become suspicious again.

The only bugaboo is that I think we did a test where the system was played until the bass dried up, then it was turned off for a while and restarted. The bass was still gone. But maybe it was not shut off long enough.

Next time the bass dries up, shut the entire stereo off for about an hour, then see how it sounds.

Does anyone know if all the amps in question are MOSFET outputs??

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...

Al, 0.7 db across a spectrum can be quite noticeable to the ear, whereas on a sine wave is undetectable.

You might normalize the two tracks, invert the phase of one and play them simultaneously, then you could hear the 'missing' bass.

Csontos, I used to work at the Allied Radio Shack service department, and put myself through college there and at other consumer electronics service departments. Transistor amps that came in with the bias a mile off was a pet peeve of mine. Funny that nothing has changed in 35 years...

I'd be surprised though if a minor deviation from nominal bias is what is causing this. However I'm not saying that isn't it, just that I would be surprised. IMO/IME it is very telling any way you look at it that turning off the amp for a while does the trick. It really does suggest that heat or 'warmup' has something to do with it, and that whatever it is when it is not warmed is preferable to Fishing. That is what made me think of MOSFETs at the get-go of this thread, as I have experienced that plenty of times with MOSFET amps.

It does sound to me a like a fan is in order!

Fishing, Have you tried putting a fan on the amplifier?

Based on all that we have discussed, it does seem that heat plays a huge role. The fan should be able to cool the amp's heatsinks.

Fishing's girlfriend hears the same differences. So there should be no doubt that its a real phenomena.

Fishing, I'm pretty sure I suggesting removing the power conditioner on page one.

The problem with most power conditioners, whether passive or active, is they force you to use a common power cord. If you get a voltage drop over that cord, it will rob the amplifiers of full output power. This is audible and measurable. If your amps draw more power after warmed up, there could be a tipping point. Maybe that takes about 1/2 hour?

Our amplifiers are quite unhappy with most power conditioners, especially the regenerator kind.

If the lack of a power conditioner continues to fix the problem, ditch the conditioner, or just use it on your source components.

No. But if you have problems again when you install it, you will know what to do.

Fishing, break-in does not affect bass in amplifiers so much. It does in speakers though.

If you have not heard the bass come around in a week we can safely assume (as we did before) that it is not the amplifier at fault.