Rotel RB990-BX oscillates after turning off?

[Vargas]

Is it just me or does any one else that has a RB990-BX have this same problem? This is the second one I have owned now and they both have the same problem. The problem is, just after I turn the amp off, about 10 seconds later I get a high pitch, low volume oscillation noise out of the left speaker, which lasts for about 5 seconds! The funny thing is, when this is happening I can tap on the top or jump on the floor and that will stop it. The first amp I bought new developed the same problem and I returned it back to Rotel for them to investigate. They of course said they couldn't figure it out for sure and assured me that it had no bearing on the performance. Just recently I came across another one used and sure enough it has the same problem. I remember reading somewhere that the oscillation problem might be coming from one of the capacitor's when the power supply caps are discharging. Or maybe a loose solder joint. I havent put it on the bench yet myself as it doesn't really seem to bother the performance but this sure is strange. Anyone else with the same problem? Cheers

[homemodder]

Hi, its not just RB990 model that does this, the nearly all do, especially the higher power models which have large capacitor values. Reason I found is that it has something to do with the capacitors they use, slitfoils, high quality. I cured some by upgrading to fast soft recovery rectifiers or by using a resistor to discharge the caps faster.

I dont know if we talking of the same noise though, as no tapping would help with this.

[alansawyer]

I know nothing of the circuit of this amp, but a possibility is an op amp in the input stage that is unstable at low voltage and oscillates after the supply caps have dropped to a low enough voltage. It could also be because the low voltage is non-equal on + and - sides as it drops.

A cure could be to provide stabilised supply to op amp that cuts to zero after it cannot sustain the required supply any longer.

Production spread of op amp or other parts is a possible reason for one channel only doing this.

As it only happens after turn off where caps are supplying power then the low voltage may well be the issue, meaning normal operation can be completely unaffected.

[jaycee]

The best cure IMO, and it even adds a safety feature, would be to install a DC detect circuit with relays that switch off the speakers when the AC is lost. Quite frankly I'm surprised the Rotel doesn't have this already!

[alansawyer]

Originally posted by jaycee
The best cure IMO, and it even adds a safety feature, would be to install a DC detect circuit with relays that switch off the speakers when the AC is lost. Quite frankly I'm surprised the Rotel doesn't have this already!

A good idea to have DC protection, but if it is AC oscillation and the DC level is zero then it will not cut off the output, and he will still have oscillation. Maybe the amp does have the protect feature already?

[jaycee]

Originally posted by alansawyer


A good idea to have DC protection, but if it is AC oscillation and the DC level is zero then it will not cut off the output, and he will still have oscillation. Maybe the amp does have the protect feature already?

If the problem is only at switch off (and many amp circuits do nasty things at switch off!) then you could detect AC power loss and instantly shut off the speakers, thus curing the problem

[quicknick]

I noticed the same thing on my homemade Elliott Project 101 stereo amp... I use 2x10000uF per rail for each channel so it takes about 30 seconds for the supply to completely discharge after I turn off power. Before the supply collapses I can hear some oscillations on the left channel, very faint, resembling an old radio tuned between stations. I'll monitor the voltages to see if one of the rails discharge faster than the other, but actually I feel relieved to find out it happens with other amps (Rotels even ... Must mean that my left channel isn't defective...

[Corvus corax]

The Adcom 535 used to do this also. Its a fairly common quirk of that amplifier, and as far as I know, they all did it. In the case of the 535, it was considered normal, and no cause for concern. Their explanation mentioned the power supply capacitors, and the lack of output clamping of any sort as the cause of the sound.

Output muting relays of any type sound like garbage, by the way, especially since most high current relays are designed to switch power in the form of AC mains, and the contacts quickly oxidize. Mercury wetted types sidestep this, but they are getting hard to find. Better to put a crowbar circuit in there if the noise concerns you, but it will still be happening, just into a dead short instead of your loudspeakers.

[jaycee]

Originally posted by Corvus corax
Output muting relays of any type sound like garbage

Audiophile pseudo nonsense.

[alansawyer]

Originally posted by Corvus corax
The Adcom 535 used to do this also. Its a fairly common quirk of that amplifier, and as far as I know, they all did it. In the case of the 535, it was considered normal, and no cause for concern. Their explanation mentioned the power supply capacitors, and the lack of output clamping of any sort as the cause of the sound.

Output muting relays of any type sound like garbage, by the way, especially since most high current relays are designed to switch power in the form of AC mains, and the contacts quickly oxidize. Mercury wetted types sidestep this, but they are getting hard to find. Better to put a crowbar circuit in there if the noise concerns you, but it will still be happening, just into a dead short instead of your loudspeakers.

As far as you know they all did it? Wow you have great sources of information.

You also seem to have inclredidle knowledge of relays - "all types" - really? Evidence?

As far as the original problem goes, has the Amp ever been "reaired" or modded?