Rotel RB-1072 is a RF transmitter!! - diyAudio
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Old 27th July 2009, 01:51 PM   #1
Ograv is offline Ograv  United States
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Angry Rotel RB-1072 is a RF transmitter!!

I have a Rotel RB-1072 (ICE) amp. Soon after getting it, I noticed that any time it is on it emits a lot of RF. It destroys AM radios & my Shortwave radio with interference. I have a Sony HD tuner that is completely un-usable on AM. I know some of you are going to want to comment on the AM part of this. That is not the issue. Also, I have done enough investigation to know that it is the RB-1072 that is the problem.

I'd like to know if anyone else has this problem with their ICE amp or if you've been able to resolve it. - Thanx
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Old 27th July 2009, 05:51 PM   #2
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Do you have long speaker cables?

I'd always suspected Class D amps might cause this kind of thing if not carefully built. I noticed that the Rotel only has a 2-prong IEC power connector. An amp using class D should be grounded in my opinion!
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Old 27th July 2009, 08:20 PM   #3
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I have this same problem with my infinity subwoofer. It really sucks since im a ham radio operator and have shortwave transcivers. Never found a fix for it except to pull the plug when ready to transmit or receive...it even does a number on 2 meter fm radio's as well!!!
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Old 27th July 2009, 11:32 PM   #4
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If your amp is transmitting RF then it must be oscilating somewhere at RF.

I would be looking at the input filter to make sure it filters out RF.

I would also look at the capacitor on the VAS to ensure your not allowing RF through just AF.

The Zobel network should be in your design too.

In my experience of amps oscilating then tend to over heat and destroy themselves in time.
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Old 28th July 2009, 01:13 PM   #5
infinia is offline infinia  United States
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Default Poor design and sign of times to come

One thing easy to try is get a surplus toroid and run your speaker cable about a 6-12 turns close to the amp. You may have to do the same thing with all the inputs as well incl power (3 cores) . I pretty sure this will relieve your problems (RF design guy). Try using a big powered iron or a smaller ferrite core.
I'm surprised this is a problem on a major brand. No safety ground is not only a bad sign for safety ( means no shields) as well as noise conduction and re radiated noise. I would take it back to the dealer and complain, let them fix it!
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Old 28th July 2009, 02:39 PM   #6
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Quote:
Originally posted by nigelwright7557
If your amp is transmitting RF then it must be oscilating somewhere at RF.

I would be looking at the input filter to make sure it filters out RF.

I would also look at the capacitor on the VAS to ensure your not allowing RF through just AF.

The Zobel network should be in your design too.

In my experience of amps oscilating then tend to over heat and destroy themselves in time.
It's a Class D amp. It oscillates by design (at the PWM frequency) using a square-ish waveform that has lots of harmonics. Although, these days, most modern Class D amp designs vastly reduce RFI by spreading the noise spectrum out so there's relatively little power at any one frequency. I'm not sure, however, if Rotel is using the "spread spectrum" technique.

I second the ferrite idea and applying general RFI supression techniques. Commercial gear like the Rotel should be designed for and certified (tested) for FCC and other RFI compliance. But RFI testing is not exactly black and white. I've moved a cable a few inches and made a failing piece of equipment pass at the certification lab. And the Class B FCC level does allow some "possible interference" for home devices.

You might also contact Rotel and see if they have any suggestions? If it's as bad as you say, I suspect they've been down the road before with other customers. They may even send you some free ferrites, etc.

The Apostate brings up an interesting point about subwoofers. I've seen Class D amps used in subs that are not fully enclosed (shielded) internally. As the subwoofer itself is usually made out of MDF (wood) they can be quite nasty for RFI with essentially zero shielding. And some of those are not certified at all.
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Old 28th July 2009, 04:25 PM   #7
Ograv is offline Ograv  United States
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Default Follow up

Thanx for the replies.
I've tried some ferrite beads on the in&out points of the cable. They aren't big enough to affect anything unfortunately.

I have called Rotel & am waiting a call back.

The 1072 runs cool as a cucumber. No heat anywhere during normal operation & just a bit warm when it's really working hard. So I don't think it's gonna blow up due to this spurious RF noise.

I like the amp, but not sure if I will keep it unless I can fix this. Some EE friends I've talked with say all class d's will do this to some degree.
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Old 28th July 2009, 05:02 PM   #8
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Default Re: Follow up

Quote:
Originally posted by Ograv
Thanx for the replies.

The 1072 runs cool as a cucumber. No heat anywhere during normal operation & just a bit warm when it's really working hard. So I don't think it's gonna blow up due to this spurious RF noise.

I like the amp, but not sure if I will keep it unless I can fix this. Some EE friends I've talked with say all class d's will do this to some degree.
I think nigelwright7557 missed that it's a Class D amp so you can safely ignore his comments about it overheating, etc. If it was a class A/B amp and causing radio interference then you'd have something to worry about.

And yes, Class D amps tend to create some level of RFI regardless. It's especially difficult to keep it out of the speaker cables.

One other thought if nothing else works is to try braided style, or possibly even shielded, speaker cables. And make sure you're using good well shielded line level interconnects to the amp and keep all cables as short as possible. Tightly coil up any excess cabling including the power cord and keep the excess cable close to the amp chassis if possible while as far from whatever is experiencing the interference. You want the cables to be as lousy of an antenna as possible.
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Old 28th July 2009, 05:13 PM   #9
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Class D can be designed for low EMI/RFI with not much more effort just some forthought and knowledge. But who knows if it will get better or worse since lowest cost is king in consumer audio these days. Less and less attention is paid to analog transmission reception in general as most of these are being migrated over to digital. Untill everything is digital expect more problems and cross compatibity issues with older technologies. Also beware if you going to mix use of class D seperates with analog preamps and SE analog connections too. We need to let manufactures know that spewing more RFI into the public spectrum is unacceptable and is a form of pollution more or less.

FIY ferrite beads are not equivalent to common mode toroid chokes that I described above.
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Old 28th July 2009, 05:30 PM   #10
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Default Re: Re: Follow up

Quote:
Originally posted by RocketScientist

One other thought if nothing else works is to try braided style, or possibly even shielded, speaker cables. And make sure you're using good well shielded line level interconnects to the amp and keep all cables as short as possible. Tightly coil up any excess cabling including the power cord and keep the excess cable close to the amp chassis if possible while as far from whatever is experiencing the interference. You want the cables to be as lousy of an antenna as possible.
Shielding speaker cables with no quiet ground will be futile and mostobably makes a better antenna in this instance. Also the noise can be conducted down the input grounds to be reradiated by other gear. This is a common mode noise problem.
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