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Volume scale...?

Discussion in 'AV Receivers & Amplifiers' started by bones`, Feb 4, 2004.

  1. bones`

    bones`
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    This has been bugging me for a while. I have just purchased a Marantz SR5400 and the volume adjusts from something like -50 up to +30db. As someone who's only other experiance of amps has been pure hi-fi and a couple of mid price Yammy AV's all of which had traditional volume controls i.e. a dial that starts and stops, and I always subscribed to the 12 o'clock school of thought ( 12 being roughly the maximun useable volume to leave headroom and avoid any clipping). Now this Marantz pot goes round forever and I get the db reading in the window. Does this figure bear any resemblance to my amps output. i.e. if it was at say -10 that would equate to 12 o'clock or 50% of the scale. Anyone shed any light on this because I like to know the useable limits and don't want to overdrive my speakers.
    Cheers :confused:
     
  2. Nobber22

    Nobber22
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    Can't your ears tell you the difference between "too loud!" and "just right" in your room? I'm sure your hearing will quit before the speakers do.

    P.S. It seems to be a common thing on AV amps to have a volume knob which spins way beyond 360 degrees these days. Don't know why..... :confused:
     
  3. clayts450

    clayts450
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    I'm experiencing the same thing ith my new Sony 790 (after a Yammy A5) and confess it is a little bewildering.

    At the end of the day, it doesn't really matter what the dB rating is - you'll probably find a suitable setting and then stick to it. I seem to be around the -40dB (loud) to -50dB range (quietish) on my 790.
     
  4. bones`

    bones`
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    Yes my ears can tell the difference between quiet and loud funnily enough :eek: the thing is I like it loud and at least with my older amps I was fairly sure that if I stayed the right side of 12 o'clock on the dial I was safe. With this I want to push a little further but I would like to know how hard the amp is working. The onset of clipping isn't necessarily audible. I just want to know how hard the amp is having to work thats all :smashin:
     
  5. Reiner

    Reiner
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    The volume scale does not directly relate to the volume (SPL) produced since that would also depend on the input signal level, internal gain of the amp and of course it depends on the speakers (impedance, sensitivity), too.
    Thus different amps or different combinations produce a different SPL with the same dB reading of the volume knob.

    If the scale is in dB, as it mostly is nowadays, than this equals to attenuation (negative reading) or amplification (positive reading).
    So if you choose e.g. the -40dB position you are attenuating the input signal by 40dB before it's being fed from the preamp into the power amp stage. In other words volume is increased by reducing the attenuation.
    Note if you turn into the positive side of things (e.g. +10dB) you are overdriving the amp - which is to be avoided.

    So at 0dB the power amps would get a signal with neither attenuation nor amplification and thus operate at it's best level from a technical point of view.

    (BTW: +3dB equals to doubling the power.)

    As as rule of thumb don't go beyond the 12 o'clock position which equals to 0dB on most amps.
    If you hear any distortion you should lower the volume immediately, amp's are usually protected but speakers might get damaged from clipping.
    (Clipping means the amp is over-driven and outputs a near square-wave instead of a sine-wave. A square-wave is DC and usually deadly to speakers)


    That's because the volume on AV amps is nowadays digitally controlled, not via an analog potentiometer anymore.
    Everytime you turn the "digital" dial it will just issue a short signal to the control logic which in turn adjusts the volume. Obviously the knob can tell if you turn left or right and thus provide a different signal to the control logic to reduce or raise the volume level respectively.
     
  6. MikeBz

    MikeBz
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    And given that you have volume control on the remote it doesn't sense for the absolute position of the volume knob to make any sense, if that makes sense...

    Mike
     
  7. calscot

    calscot
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    Yes the main reason for 360 degree knob is the remote control. They used to get round it by motor driving the volume dial, but this is an expensive luxury. They also sometimes use buttons on the unit but people usually prefer a dial and so the compromise is a forever moving dial with no level feedback - that's now the dB level on the display.
     
  8. calscot

    calscot
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    A decibel is a logarithmic unit used to describe a ratio, for example the ratio of to power outputs, P1 and P2, the difference between the two in decibels is defined to be:

    10 log (P2/P1) dB where the log is to base 10.

    So a dB is a unit which changes a logarithmic relationship into a linear one. The ear can only distinguish differences in magnitude of the sound - so it can tell the difference between twice the amplitude but not 10% more amplitude.

    If we used a normal scale we would be looking at the numbers:
    1, 10, 100, 1000, 10000
    instead of:
    0, 10, 20, 30, 40
    and you can see the latter is easier to assimilate.


    You can see that each 10 decibels is a factor of 10 magnitude in the ratio. 3dB is a factor of 2 therefor 9dB is a factor of 8.

    If you understand preamps and power amps, you will know that the power amp has a fixed gain. Therefore to control the volume you have to change its input signal. As described by someone above the signal is attenuated - a simple process electronically using a cascade of resistors.

    This leads to a fractional ratio of the input signal eg:
    1/10 1/100 1/1000 (this is the gain of the attenuator)
    giving the dB equivalent of:
    -10, -20, -30 (this is the gain in dB)

    So when you crank up the volume by 10dB you are increasing the power by 10 times but only increasing the percieved level of sound you hear by a few orders of magnitude.
     
  9. bones`

    bones`
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    Thanks for that info very helpful. So basically the safety threshhold would be 0db on my display as opposed to 12 o'clock on a more traditional dial?
     
  10. Reiner

    Reiner
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    Yes, try not to exceed 0dB, in particular if the 12 o'clock position doesn't happen to be 0dB.
    If there is no dB scale usually the 12 o'clock position is the most you should go.
     
  11. calscot

    calscot
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    It begs the question though, why do they allow you to go above the safety threshold?
     
  12. bones`

    bones`
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    I think to allow the amp dynamic headroom?
     
  13. Reiner

    Reiner
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    If the input signal is very weak. It's not really a safety threshold, the amp actually doesn't care much if overdriven and most are protected against shortcuts and overtemperature anyway.
    It's rather the speakers which will suffer when the amp starts distorting or clipping, the amp itself isn't that prone to damage.

    That headroom would be there anyway (as defined by the max. power capabilities); it would be possible to limit the volume control to 0dB though.
    As I explained earlier the dB scale is about attenuating the input signal in the pre-amp stage, not about changing the power handling of the power amp stage.
     

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