Rotel RB-985 buzzing

[jochie00]

I'm a proud owner of a Rotel RB-985 (5 channel amp and RSP-976 since a long time. I haven't used my equipment for a while and after moving I hooked everything up again. (220V - 50 Hz)

With all the speakers hooked up without the interlinks connected, the main amp is dead quite (except the slight hiss which has always been there)
When connecting to the pre-amp to either LF/LS amp or CC amp or RF/RS amp (the 3 are seperate inside) the speakers are still quite.

As soon as I hook-up a combination of left side with right side or centre, a buzzing noise comes out of the speakers.

I also tried different pre-amps, interlinks and wall sockets.

Would there be the remedy?

 

[TLS Guy]

You have a ground loop. First of all, do you have a cable or satellite box connected to the system? If so disconnect it. Now just ground the Rotel power amp. Lift all the other grounds. Now I bet the buzz will be gone. Now connect the cable or satellite box. If the hum returns, put a ground isolator on the connection from the cable system.

 

[jochie00]

Non of the other equipment is connected.
With only the RB-985 connected and for example only one speaker, if I then wire the input of one amp section to the input of another amp section, it starts buzzing.
I checked all cables inside and can't find any loose connections even not for the ground wiring inside. By the way everything is hardwired in the amp.

Yesterday, I also took out all fuses for the power supply except the one on which I have connected one speaker to the amp. wiring the input to another input (from which the fuses were taken out) makes it buzzing.

 

[TLS Guy]

Non of the other equipment is connected.
With only the RB-985 connected and for example only one speaker, if I then wire the input of one amp section to the input of another amp section, it starts buzzing.
I checked all cables inside and can't find any loose connections even not for the ground wiring inside. By the way everything is hardwired in the amp.

Yesterday, I also took out all fuses for the power supply except the one on which I have connected one speaker to the amp. wiring the input to another input (from which the fuses were taken out) makes it buzzing.

What do mean by connecting one amp section to the input of another? That does not make sense. Somehow you have a ground loop. You said if the power amp is plugged in and just connected to speakers, there is no buzz, is that correct? If it is connect pre amp to amp one by one. Report back in the exact labeling of each out to each in and the cable used. Be specific as to which connection excites the buzz. This has to be a ground loop or a wrong connection, causing feedback. I bet its a ground loop though, it has all the hallmarks.

 

[jochie00]

The RB-985 contains three completely separate amps and power supplies but share the thorodial transformer. The left PCB contains the left front and surround, PCB in the middle has the amp for centre channel and the right PCB the right front and surround channel.

When I hook-up only 1 or several speakers, the amp is quite as there is no connection made to the input.
When I hook-up only one channel to one input, the amp is still quite.

As soon as I hook-up another channel going to an amp located on a different PCB a buzz comes out of the speaker(s). For info, nothing else has been hooked up to the pre-amp and it doesn’t matter if its mains are in or out.

Exactly the same occurs when I wire the grounds of each input to the inputs of the other channels.
I also disconnected the fuses located on the power supply of the centre amp and the right amp leaving only the left amp connected. Making ground connection of the inputs between left and right or centre also causes buzzing.

Looking inside the amp, all PCB’s share the same ground and they are all soldered. I couldn’t find any faulty soldered connections inside the amp.

 

[TLS Guy]

The RB-985 contains three completely separate amps and power supplies but share the thorodial transformer. The left PCB contains the left front and surround, PCB in the middle has the amp for centre channel and the right PCB the right front and surround channel.

When I hook-up only 1 or several speakers, the amp is quite as there is no connection made to the input.
When I hook-up only one channel to one input, the amp is still quite.

As soon as I hook-up another channel going to an amp located on a different PCB a buzz comes out of the speaker(s). For info, nothing else has been hooked up to the pre-amp and it doesn’t matter if its mains are in or out.

Exactly the same occurs when I wire the grounds of each input to the inputs of the other channels.
I also disconnected the fuses located on the power supply of the centre amp and the right amp leaving only the left amp connected. Making ground connection of the inputs between left and right or centre also causes buzzing.

Looking inside the amp, all PCB’s share the same ground and they are all soldered. I couldn’t find any faulty soldered connections inside the amp.

The fact that connecting input grounds together proves this is a grounding problem. What sort of mains plugs do you have on your amp and preamp? Have you checked that you house wiring is correct? Are you certain the live and neutrals are correct at the mains outlet and the wiring of the plugs on the units, if the plugs are not molded?

If the answers to all the above are correct, then connect the power amp to the speakers, ALL of them. Then plug in the amp with a correctly grounded plug if it has one. Make sure nothing is connected to any preamp inputs. Is there a buzz? If not proceed. Now plug in the preamp to the mains supply, with the ground lifted. Plug in each channel one by one, Yes connect unit to unit, do not leave an open circuit. If you here a buzz remove that connection, and proceed. Then connect any preamp out that buzzed to an input that did not buzz, and see if it buzzes now. If it does change the cable and see if it buzzes. Report back. Be very specific and do not leave out even the most minute detail. Carry this plan out exactly as I describe.

Trouble shooting at long range is difficult. I have a feeling that if I had your rig in front of me and a multimeter, this problem would crack in minutes.

 

[jochie00]

The mains plug is moulded one which I can't remove. I can plug the plug the other way around into the wall socket but this doesn't cure the problem. I also tried different socket throughout the house even on different phases. (I've got three phases coming into the house)

The first thing I did was with all the speakers hooked up, connecting the pre-amp which had nothing else attached. in both tried it with the preamp attached to the mains and not connected. The buzzing only started as soon as I connected the second set of amps. I tried it from left to centre to right and from right to centre to left.

Also hooked up my Ipod directly to the main amp with the same negative result.
Hooking the ipod up to the LF and LS channel = quite
Ipod to centre only =quite
Ipod to RF and RS = quite
but buzzing with RF and CC or RF and LF or RF and LF
buzzing with LS and CC or LS and RS

I also tried the above with one speaker only connected but unfortunately no improvement.
Th

So I believe I tried all possibilities to concluded that the ground loop is a problem inside the RB-985.
Last night I soldered all grounds of the inputs together and wired them to the central ground point but this only gave permanent buzzing so I removed it.

Which points would you measure with the multimeter?

 

[TLS Guy]

The mains plug is moulded one which I can't remove. I can plug the plug the other way around into the wall socket but this doesn't cure the problem. I also tried different socket throughout the house even on different phases. (I've got three phases coming into the house)

The first thing I did was with all the speakers hooked up, connecting the pre-amp which had nothing else attached. in both tried it with the preamp attached to the mains and not connected. The buzzing only started as soon as I connected the second set of amps. I tried it from left to centre to right and from right to centre to left.

Also hooked up my Ipod directly to the main amp with the same negative result.
Hooking the ipod up to the LF and LS channel = quite
Ipod to centre only =quite
Ipod to RF and RS = quite
but buzzing with RF and CC or RF and LF or RF and LF
buzzing with LS and CC or LS and RS

I also tried the above with one speaker only connected but unfortunately no improvement.
Th

So I believe I tried all possibilities to concluded that the ground loop is a problem inside the RB-985.
Last night I soldered all grounds of the inputs together and wired them to the central ground point but this only gave permanent buzzing so I removed it.

Which points would you measure with the multimeter?

It seems strange that the Rotel does not have a polarized plug. What are using for a ground? What sort of mains plug does the preamp have? Is there a chassis ground on one of the units? If none of the plugs are polarized then it probably does not matter which is ground and which is neutral. However it would be a good idea to know which outlet socket is live and which is neautral.

Connect your meter across live and ground and then neutral and ground, and see what you get. There should only be about 0 to 0.5 volts between neutral and ground. If it is higher than that then you home has neutral gauging and you will need professional help.

 

[TLS Guy]

I have downloaded both of your manuals. There does not seem to be provision for any grounding.

The key to this is probably that you have moved. You state that your house voltage is 220 volt 50 Hz. Where are you living now, and where have you moved from? Are you by any chance somewhere in Asia? If so we need to know you have a true neutral.

If your power seems correct, one side 220 volts to ground and your other 0 to at the most 1 Volt to ground, then do the following.

With you ohm meter carefully check the resistances between the grounds of all the pc amp boards. There should be no measurable resistance.

I'm getting the sense that there is something strange about your power, and you equipment was not designed for what you have, or there is something amiss with your power.

If you power seems correct, and there is no resistance between the grounds of the pc board, then make a true ground connection to your amp to your house ground. If it still buzzes you will have to make a true ground. That means a resistance to the Earth of no more than 5 ohms. That usually takes five good spikes driven deep into the ground.

 

[jochie00]

hmm, just measured between ground and neutral at the main switch board of the house. more that 10 Volts between neutral and ground!

By the way I now live in France but I had the amp for a while in Singapore which should have had the same spec amps overthere.

the plug on the rotel is like:

http://www.attent.com/content/img/gifts/abox/stekker100.jpg

neutral should be absolute zero. down here.

I'll come back on the issue once I know more.

 

[yettitheman]

hmm, just measured between ground and neutral at the main switch board of the house. more that 10 Volts between neutral and ground!

By the way I now live in France but I had the amp for a while in Singapore which should have had the same spec amps overthere.

the plug on the rotel is like:

http://www.attent.com/content/img/gifts/abox/stekker100.jpg

neutral should be absolute zero. down here.

I'll come back on the issue once I know more.

10VAC? Wow.

 

[jochie00]

Yes, but I now have some doubts about my multimeter, so I'll buy a new one right now

 

[TLS Guy]

I think we are getting to the bottom of your trouble. Were those two Rotel units purchased in Asia?

This is important as many Asian countries use a split phase system. In Europe the line voltage is nominally 230 Volts, although is is usually 240 volts. It is single phase with a neutral. In Asia, including Singapore, the voltage is two phase no neutral. That is why the plugs are not polarized. It is a 2, 4 phase system. That is very different to France.

You should not have 10 volts on your neutral. If you really have 10 volts on your new meter then you need to have this verified by an electrician and the power company called.

Now I would make a good chassis ground and see if that cures it. Unfortunately if those units were built for Asia, you will likely have to run them from an isolating transformer. That will provide them with the power supply you had in Singapore.

 

[jochie00]

After getting the new meter, I measure 7 VAC between neutral and earth.
neutral and phase give me 240 VAC and phase and earth 233, which I believe means that some equipt. in my home puts 7 VAC back on the earthing.

Anyway, I moved my amp to the otherside of the house to dismantle it again and cleaned the screwed connection between pcb and chasis.

Doing the checks with a small speaker gave me possitive results.

But now the trouble start. Bringing back the amp to the cupboard, hooking all speakers up, checken and all seems OK.
After hooking up the interlinks, there the buzz came back again.

Took amp out and put it on the floor. hooked up mains from other side of the room, hooked up one speaker and all silent. After plugging in one interlinked (well shielded) and not plugged in on the other side, buzzing starts.
moving the loose and around, up and down and over the floor had influence on the buzzing.
Then the following came into my mind, there is on that side of the house a high voltage power line hanging above the ground less than 120 feet away.

It really looks like the interlink functions like an antenna which picks up the interference, which could explain the cause. I also tried different interlinks all reacting the same.

Meanwhile I also tried the setup outside all same same, but as soon as I move the loose end of the interlink behind the amp, so the amp is in between high power lines and interlink, buzz stops.

Maybe put a metal plate against the wall!?

all other equipment againt same wall like sat receiver, TV or integrated amp are working fine.

 

[TLS Guy]

After getting the new meter, I measure 7 VAC between neutral and earth.
neutral and phase give me 240 VAC and phase and earth 233, which I believe means that some equipt. in my home puts 7 VAC back on the earthing.

Anyway, I moved my amp to the otherside of the house to dismantle it again and cleaned the screwed connection between pcb and chasis.

Doing the checks with a small speaker gave me possitive results.

But now the trouble start. Bringing back the amp to the cupboard, hooking all speakers up, checken and all seems OK.
After hooking up the interlinks, there the buzz came back again.

Took amp out and put it on the floor. hooked up mains from other side of the room, hooked up one speaker and all silent. After plugging in one interlinked (well shielded) and not plugged in on the other side, buzzing starts.
moving the loose and around, up and down and over the floor had influence on the buzzing.
Then the following came into my mind, there is on that side of the house a high voltage power line hanging above the ground less than 120 feet away.

It really looks like the interlink functions like an antenna which picks up the interference, which could explain the cause. I also tried different interlinks all reacting the same.

Meanwhile I also tried the setup outside all same same, but as soon as I move the loose end of the interlink behind the amp, so the amp is in between high power lines and interlink, buzz stops.

Maybe put a metal plate against the wall!?

all other equipment againt same wall like sat receiver, TV or integrated amp are working fine.

Unterminated cables connected to the input of an amp will buzz. Short the unused ends together and see if you have a buzz then. I think you will need to properly ground that amp.