RB-985 MkII Overheating

[Otrembu]

Hello everyone, this is my first post, which sadly will have to be about my overheating RB-985 MkII amplifier. Let me detail this:

I bought this amplifier used on eBay last December. I had been using it perfectly with generic car speakers without crossover, just connected with bare wire, then for about a week with a pair of borrowed Yamaha NS-10M studios, then back to the car speakers. Just recently I obtained a pair of Pioneer CS-G9900, the first "real" speakers to use on my system... as you can see I definitely not have the money for better speakers but these worked and sounded quite nice. A few days later problems began. But when was that? Probably the day the amp was accidentally left on all night (pre and everything else turned off), maybe the night before that when the power went out while the system was on then went back about 30 seconds later. What happens exactly? When played with about 50% volume, for about 15 minutes, the amp will be extremely hot from the left side, before the protection kicks in and turns off the left channel. This never happened before, and I have cranked the volume higher and played the system for hours. The time this happened, I turned everything off immediately, so I don't know if it would have eventually turned off the right channel too. I tested for possible solutions the following days but the amp continued getting way too hot from the left side. I've turned off the amp before the protection circuit could kick in, but it definitely could if I left it enough time, taking in count that the problem is still there, because I used to get an even heating from both sides before.

This is what I have tested:
1. Wiring on the speakers (CS-G9900) themselves
2. Wiring from the speakers towards the amp
3. Speaker impedance (got a stable 7.4 ohms on each one, measuring directly from the speaker terminals)
4. Switching left and right speakers (the left side still gets warmer)
5. Turning on the amp with nothing plugged in (the left side keeps getting warmer)

Is it the speakers? They didn't give any problem the days before, switching them channels doesn't make any difference, even running the amp with nothing but the AC power plugged in causes uneven warming... but I did found the super-tweeter on both speakers is dead and only gives out noise.

Is it some other component on my setup? Again, the amp heats unevenly with nothing plugged in.

So I think it is the amp itself. But what could it be? This is where I come to you looking for help. Thanks in advance.

[madmac]

Oh MY !!. You kinda' lost me about the 'car speaker' thingy!. Am I to assume you are using or used 'car speakers' in a home audio environment??. One thing that I have heard is that Rotel amps DO NOT like 4ohm loads. That would explain you're heat and shutdown issues. There might be damage at this point.

[wkhanna]

...One thing that I have heard is that Rotel amps DO NOT like 4ohm loads.

Dan, you may want to vet your sources a little better.
This amp (like most all home audio amps) is rated at a nominal 4 Ohm load.
Link to owners' manual

Any speaker will simply present a resistive load to the amp.
Leaving the amp powered ‘on’ for an extended time (days or weeks) should have no effect other than normal wear & tear. The amp should be able to survive most common power interruptions, too.

The fact that one specific area on the amp gets extremely warm regardless of whether or not there is a load present would lead me to suspect an internal short of some sort. A short in one of the speakers could possibly cause an overload condition & create some problems in the amp.

The symptoms in your amp could have been caused by any number of reasons, & not necessarily by anything you may have done.

[srb]

Any speaker will simply present a resistive load to the amp.

Although not of any real importance to this topic and situation, just to be accurate, most speakers that have drivers with voice coil "motors" present a reactive load to the amp. That means that the impedance varies relative to frequency, and in many cases, the variation is quite large.

Planar drivers, like the Magneplanar speakers, are primarily resistive loads (but not entirely, usually having a larger impedance variation at the crossover frequency).

But I do agree that both from Rotel specifications and my own personal experience with Rotel amps (RB-1080) and receivers (RSX-1056) that they operate just fine with 4 ohm nominal speakers. In my case, they have driven several different 4 Ohm speakers that I own for many years with no heat issues at all, though it should be noted that none of them go below 3.2 Ohms at any frequency.

It appears, whether by coincidence or from speaker swapping, that some internal component or connection is out of spec and/or failing. You have done enough testing and cable swapping to determine that it is the amplifier (and always the same channel), and the fact that it will overheat with nothing connected to it eliminates the speakers from the equation and the amplifier might be headed for eventual failure.

It is a possibility that perhaps that channel's bias can be adjusted back to specification, but either way it sounds like a job for either Rotel or competent independent technician repair.

Steve

[wkhanna]

Any speaker will simply present a resistive load to the amp.

Although not of any real importance to this topic and situation, just to be accurate, most speakers that have drivers with voice coil "motors" present a reactive load to the amp. That means that the impedance varies relative to frequency, and in many cases, the variation is quite large.

I knew when I wrote that 'generalization' someone with far more expertise in electronics would be compelled to correct me.

I am glad it was Steve, his explanation is far clearer than what I could have come up with.

But this does bring up an interesting point regarding speaker specifications.
As Steve points out, a speaker listed as possessing a nominal (average) impedance of 4 ohms does not produce a continuous load. The load will vary depending on the frequency with low frequency typically requiring more energy. Therefore a speaker 'rated' at 4 ohm will present a varying load over time based on the frequency it is producing. Again, nothing to worry about typically, as this is quite normal & the amp should be able to handle these 'peak' loads for short periods of time.

[Otrembu]

Thanks for the replies... Now, I have heard from some people that this is normal with used amps, and that you can solve it by adding fans to blow on the amp and cool it down. Does this work, and will I be able to give it new life by doing this? Or will it get even more damaged from it?

[wkhanna]

..... I have heard from some people that this is normal with used amps, and that you can solve it by adding fans to blow on the amp and cool it down......

There is nothing normal about an amp that continually goes into thermal protection (overheats) under normal operating conditions. Whether it is used or brand new is of absolutely no relevance.

Continuing to use this amp, fan or no fan, will do it no good & may end up causing far more damage.

JMHO

[PewterTA]

One question, have you opened the amp up (just taken the cover off) and used some compressed air to clean it out?!!? Could be dust is causing an issues as much as anything is going bad in the unit itself. It's worth a shot for the ~8 to 12 screws to get the cover off.

[Otrembu]

I didn't use compressed air but tried sucking dust out of it with a vacuum (at a safe distance) without any results. I have never used compressed air, though. Could it be more effective?

[srb]

Yes, it's usually a lot more effective, unless the dust is anchored in cigarette smoke or other oily residue. If not, take it outside because it will probably blow away a sizable cloud of dust.

Steve

[Glen B]

Thanks for the replies... Now, I have heard from some people that this is normal with used amps, and that you can solve it by adding fans to blow on the amp and cool it down. Does this work, and will I be able to give it new life by doing this? Or will it get even more damaged from it?

After almost two months, I am surprised you have not had the amp checked out. The solution may be as simple as adjusting the bias, which may have drifted off spec. The procedure is a walk in the park for any competent tech. Ths service manual is available from B&W Group NA tech support.

RB-985II Service Manual direct download link: http://bwgroupsupport.com/downloads/...985MKII-TM.pdf

[Aussie Geoff]

Hi, I also recommend a quick trip to an experienced amp service facility. I have experienced both RB-1080 and RB-1090 with heating issues. Generally as a non digital amplifier ages two common faults relate to deterioration in some of the capacitors (particularly the large power supply ones or any smaller oil based ones) and / or drift in the bias. Old Rotels can be magnificent, but often benefit from a health check,bias adjustment and capacitor replacement if any have deteriorated. You should then get another 10+ years of service from it.

Cheers... Geoff

[Otrembu]

Breaking news... After blowing at the internals with compressed air and removing quite a few dust bunnies from inside my amp, hooking it back on and playing some music for about 20 minutes, the right side of the amplifier is warmer than the left side!
You might recall it always heated more from the rear left side... now it has switched to the right side.
I had it unplugged for about 2 months. Could this be the drifting bias?

Thanks once again for your replies.

Edit: I had the amp working at about 55% volume for 1 hour, and it never kicked into protection mode. The left side was just warm, and the right side was actually quite warmer, but not burning hot like before. I turned it off afterwards. My next test will be switching the speakers as I did switch them sides during this period of time.
Second Edit: Well, now the right side of the amp is quite warm while the left side is almost cold. No overheating whatsoever, though. What could it be?

[Otrembu]

Hello everyone, sorry for double-posting:

I just got my amp back from repair. The 8 big capacitors were changed (big audio ones, same capacitance and voltage) as well as 2 transistors. The overheating has been fixed, but I noticed the sound of the RB-985 has changed completely. I would have described it as having a smile-shaped frequency response before, with scooped mids and slightly raised lows and highs. Now the bass has recessed and the mids have been brought up. I had to re-equalize completely to remove the mids.

How could have this happened? Is the current sound the real one, or was it the previous one?

Thanks!

[wkhanna]

I am glad the hear you amp has been repaired.

AFA the change in signature - this is to be expected.
As components age, so too does their performance.
It is a long process & somewhat difficult transformation to realize.
Think of how the shocks on a car wear over time.
For every mile traveled they become ever so slightly less capable of absorbing & controlling the undulations of the road until one day you perceive your vehicle no longer having the control & ride it once had.

Rotel's signature is one of neutrality, with maybe ever so slight an emphasis on the mids.
I would expect its pre-repair sound to be slightly harsh at the extremes of the frequency range with degradation of transient response.

Does the 'new' sound more reflect its natural original one?
Most likely yes, though replacing components with those of equal value but different brands can have some effect on the character of the unit.

Most of us seek out a fairly neutral character, personal preferences varying slightly.
I suggest giving yourself some time with the newly refurbished amp, & see if you do not come to appreciate what I expect is a more natural sound.

[PewterTA]

I also wouldn't worry/judge the sound until you get about 100 - 200 hours on the amp... that should give it and you enough time to break in and get used to the sound. Then see if things are off or if they sound better. Definitely things change with age, so the parts might've been going bad for a while and you weren't getting the real sound from it.

[wkhanna]

Dan (PewterTA) make a V good point.

Allow for ~ 100 or more hr break-in.
You may find the amp to 'Open Up' a bit.
In my experience, a typical occurrence.

[Otrembu]

Thanks for your replies, everyone. I'll definitely keep listening to them for a while to see how the sound "evolves".

I have one last question for now, though:
I'm looking towards getting a pair of real loudspeakers that can be bi-amped. Could the 5-channel surround amp RB-985 be able to achieve this? (Inputting the same signal to two of the amp channels, then wiring those two channels to one bi-ampable loudspeaker?). The manual doesn't say anything about bi-amping, and it's THX certified (does that process the audio in the rear channels some way?), so could it work or not?

Thanks once again...

[srb]

The manual doesn't say anything about bi-amping, and it's THX certified (does that process the audio in the rear channels some way?), so could it work or not?

A THX certified amplifier is a regular amplifier without any processing. THX certification simply means that the amplifier can drive "medium sensitivity" speakers to a 85dB theater reference level with 20dB headroom at a distance of 10 feet or less, and is targeted for smaller rooms.

Higher level THX certifications provide higher SPL levels in larger rooms (THX Select and THX Select2 are intended for medium 2000 cu. ft. rooms at 10-12 feet and THX Ultra2 is intended for larger 3000 cu. ft. rooms at 12 foot or greater distances).

It's not purely a marketing ploy as it gives a reasonable indication of potential performance levels, but there are many very good amplifiers that have not paid the substantial fees for THX certification. THX certification allows you to put the THX logo on your equipment as well as be listed as recommended components on the THX website.

As far as passive bi-amping (using the passive crossover in the speaker as opposed to active bi-amping with an active crossover), it may only give you a small increase in sound quality. My own experience is that a larger more powerful amplifier single wired gives better performance than bi-amping with two lesser amplifier channels.

As an example, I preferred the sound of a single 200W/ch Rotel RB-1080 over bi-amping with two 125W channels of an RB-1075. This is generally more true with two-way speakers where the the power consumed by the LF woofer/midbass can be 10 times that of the HF tweeter. The larger amp can deliver more reserve power where it's needed.

As with anything, your mileage will vary. I last used my Audio Refinement Multi-5 amplifier (125W x 5) to biamp two-way speakers, as the extra channels were not being used and were available. I thought it provided a small increase in clarity and drive, but when I swapped in a 250W x 2 amplifier, it was more of a game changer.

Steve

[Kevin D]

^^^^^^^^what he said...

Kevin D.