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  1. #1
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    High Frequency tone in the noise floor on SB3 R channel

    I just finished recommissioning my old Rotel system as the world's largest distributed alarm clock in the bedroom. 180Wpc ought to wake me up!

    My alarm clock configuration is SB3 -> Rotel RB-890 -> B&W Matrix 805 bookshelf speakers. Note that the SB3 is about 6.5 dB hot on line levels for the RB890 (1V input sensivity). I *do not* have proper analog attenuators installed yet (still looking for an easily orderable Canadian source), so the noise floor is definately double the volume it should be. I have the Preamp Volume Control set to -6.5 dB to keep me from being blown out of bed in the morning by accident.

    Since the RB890 has no trigger I leave the amp on even when the SB3 is in powered off mode (not really off...). The SB3 is set not to turn off audio when "powered off".

    I have noticed that on the right channel only, with no music playing, there is a perceptible high frequency tone. It is very low volume, right in the noise floor. Because the room is so small and quiet, I do hear it even a few feet away. My hearing has always been very sensitive to very high audio frequencies (although this is definately falling off as I age).

    I have tried the following steps to debug this:
    1. I swapped the R/L channels at the SB3. The tone follows the channel (i.e. when I swapped L for R the tone moved to the L speaker).
    2. I flipped the SB3->RB890 RCA cable R/L on both sides. Problem remained on the R channel.
    3. I tried a second SB3 I have on hand, same results.
    4. I moved the wall wart switcher PSU from right beside the amp to around a corner, far away from the amp.
    5. I have changed the brightness levels.

    (1) implies the problem is either the SB3->RB890 interconnect cable or the SB3 itself (problem moved channels, so it is not the amp or interference on the speaker cables)
    (2) eliminates one side of the cable as the problem (if it was the cable problem should have moved to L channel).

    (4) requires more explaination because it is the only change that impacts the tone I have made so far.

    The SB3 has 5 brightness levels (0 - off, 1-4 from dimmest to brightest).

    On display brightness 0, 4, and 1 there is no tone that I can pick out. Brightness 3 has the loudest tone. 2 is much less noticable.

    The pitch of the tone is related to how much text is displayed on the SB3 screen (the tone changes between lots of text and a little text are not large).

    If I had to guess i would say the R channel in the SB3 is picking up some RFI from the computing cycles.

    This rates as a "nit" not a big problem, because I do not hear this tone when playing music (usually at volume 40-50 right now). Its just an issue when not playing music because I leave the amp powered up and the tone is somewhat evident. Also I did *not* notice this in the previous room that I had the SB3/RB890 in (now replaced by Transporter/Bryston) - it was a much larger room and I was much further away from the speakers.

    Has anyone else experienced this?

    Will this go away with proper attenuation?

    Are there any other suggestions to resolve this?
    Last edited by Eric Carroll; 2007-01-24 at 15:47.

  2. #2
    Senior Member empty99's Avatar
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    Have you tried...

    if your SB3 is a wired version, wrapping the ethernet cable around a ferrite rod at the plug before it goes into SB3 body?

  3. #3
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    Quote Originally Posted by empty99 View Post
    if your SB3 is a wired version, wrapping the ethernet cable around a ferrite rod at the plug before it goes into SB3 body?
    Its wireless.

  4. #4
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    I've noticed this noise too

    I just measured the noise floor of my SB; it has a sharp peak at 7.9kHz and at 17.8kHz. This seem to be related to the display, as the relative level of the peaks change when the display brightness is changed, also the peaks disappear when the brightness is turned down until the display is off.
    seems like a pretty significant and avoidable design weakness, I hope R&D fix it in the next revision.

  5. #5
    Senior Member Phil Leigh's Avatar
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    Quote Originally Posted by *Isaac* View Post
    I just measured the noise floor of my SB; it has a sharp peak at 7.9kHz and at 17.8kHz. This seem to be related to the display, as the relative level of the peaks change when the display brightness is changed, also the peaks disappear when the brightness is turned down until the display is off.
    seems like a pretty significant and avoidable design weakness, I hope R&D fix it in the next revision.
    What do you mean by sharp? how far down is this peak.

    I'd hardly call this a significant weakness, since:

    a - you will never hear this in normal usage (see the OP for example of not normal!)
    b - you can turn the display to "off" - just like many expensive audio design - for the same reasons
    You want to see the signal path BEFORE it gets onto a CD/vinyl...it ain't what you'd call minimal...
    Touch(wired/W7)+Teddy Pardo PSU - Audiolense 3.3/2.0+INGUZ DRC - MF M1 DAC - Linn 5103 - full Aktiv 5.1 system (6x LK140's, ESPEK/TRIKAN/KATAN/SEIZMIK 10.5), Pekin Tuner, Townsend Supertweeters,VdH Toslink,Kimber 8TC Speaker & Chord Signature Plus Interconnect cables
    Stax4070+SRM7/II phones
    Kitchen Boom, Outdoors: SB Radio, Harmony One remote for everything.

  6. #6
    Senior Member opaqueice's Avatar
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    Well, from what you say the tone is almost certainly coming from the SB R analogue out and down the interconnect. If you're never turning the SB volume above 50 with -6.5 preamp set, you can put 31.5 dB worth of attentuation in between SB and amp and then turn the SB up to 100 (and turn off the preamp attentuation), which will reduce the tone volume by 31.5dB (almost certainly enough to make it inaudible) while leaving the music volume untouched.

  7. #7
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    Thanks, that is a good point on the attenuation. I was planning on -10 dB to match line levels, but I will never want to use full scale on this power amp - 180wpc is just way overpowered for the room it is in. Its just that "free" power amp beats "new correctly sized" power amp! Also finding a good, inexpensive and small 75-100wpc power amp seems hard. I will probably go for -20 dB of attenuation just in case I feel like turning it up. 45-50 is just the "wake up" volume...
    Last edited by Eric Carroll; 2007-01-24 at 16:03.

  8. #8
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    Quote Originally Posted by Eric Carroll View Post
    I just finished recommissioning my old Rotel system as the world's largest distributed alarm clock in the bedroom. 180Wpc ought to wake me up!

    My alarm clock configuration is SB3 -> Rotel RB-890 -> B&W Matrix 805 bookshelf speakers. Note that the SB3 is about 6.5 dB hot on line levels for the RB890 (1V input sensivity). I *do not* have proper analog attenuators installed yet (still looking for an easily orderable Canadian source), so the noise floor is definately double the volume it should be. I have the Preamp Volume Control set to -6.5 dB to keep me from being blown out of bed in the morning by accident.


    Since the RB890 has no trigger I leave the amp on even when the SB3 is in powered off mode (not really off...). The SB3 is set not to turn off audio when "powered off".

    I have noticed that on the right channel only, with no music playing, there is a perceptible high frequency tone. It is very low volume, right in the noise floor. Because the room is so small and quiet, I do hear it even a few feet away. My hearing has always been very sensitive to very high audio frequencies (although this is definately falling off as I age).

    I have tried the following steps to debug this:
    1. I swapped the R/L channels at the SB3. The tone follows the channel (i.e. when I swapped L for R the tone moved to the L speaker).
    2. I flipped the SB3->RB890 RCA cable R/L on both sides. Problem remained on the R channel.
    3. I tried a second SB3 I have on hand, same results.
    4. I moved the wall wart switcher PSU from right beside the amp to around a corner, far away from the amp.
    5. I have changed the brightness levels.

    (1) implies the problem is either the SB3->RB890 interconnect cable or the SB3 itself (problem moved channels, so it is not the amp or interference on the speaker cables)
    (2) eliminates one side of the cable as the problem (if it was the cable problem should have moved to L channel).

    (4) requires more explaination because it is the only change that impacts the tone I have made so far.

    The SB3 has 5 brightness levels (0 - off, 1-4 from dimmest to brightest).

    On display brightness 0, 4, and 1 there is no tone that I can pick out. Brightness 3 has the loudest tone. 2 is much less noticable.

    The pitch of the tone is related to how much text is displayed on the SB3 screen (the tone changes between lots of text and a little text are not large).

    If I had to guess i would say the R channel in the SB3 is picking up some RFI from the computing cycles.

    This rates as a "nit" not a big problem, because I do not hear this tone when playing music (usually at volume 40-50 right now). Its just an issue when not playing music because I leave the amp powered up and the tone is somewhat evident. Also I did *not* notice this in the previous room that I had the SB3/RB890 in (now replaced by Transporter/Bryston) - it was a much larger room and I was much further away from the speakers.

    Has anyone else experienced this?

    Will this go away with proper attenuation?

    Are there any other suggestions to resolve this?

    Suggestion for attenuation. Have noe expirence with it my self, but i have read some pro audio/home studio use it or an other m - patch version. http://www.smproaudio.com/index.php?...d=24&Itemid=68

  9. #9
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    measurements

    hi,
    here are some measurements taken with an maudio transit. The transit baseline measurement is taken with the input unterminated so the noise floor below ~400Hz is about 3dB higher than it when it is connected to a low impedance output, but is sufficient to allow good measurements as it is still 3 - 9 dB lower than the device under test. You can clearly see (and hear in a quiet room) the junk getting coupled to the right analog output, even with the display off. I don't know if this is specific to to my unit, or is a genral property of the SB.

  10. #10
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    more measurements

    heres with the display off. you can see there is still some junk there but mostly out of band. Sorry the images are kind of big.

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