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
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.
I dont know if we talking of the same noise though, as no tapping would help with this.
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.
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.
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...
... Must mean that my left channel isn't defective...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.
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.
Sorry for the delay on getting back, been pretty busy with family issues! The problem doesn't really bother me I just find it funny that out of all the RB-990BX's out there no one else has inquired about this problem. I have a few TV sets and a little EL-84 project to finish up and I think just for craps and giggles I am going to dive into this a little more. Thanks for all the input guys.
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