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Help! Denon AVC-A10SE to subwoofer via power amp - how to lower fixed gain it's loud!

Discussion in 'Subwoofers' started by Rob.Screene, Jan 6, 2003.

  1. Rob.Screene

    Rob.Screene
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    Help! Denon AVC-A10SE to subwoofer via power amp - how to lower fixed gain it's too loud!
    Q: I think I need to reduce a line-level phono signal by -20db, how can I do this?

    Can I make a simple circuit in-line with the phono level 1.1V subwoofer signal to lower the voltage by 25%?
    i.e.
    Can I use a simple resistor, or will that give the supplier or consumer equipment impedence problems?

    Why?
    -------
    I am trying to use a new power-amp with fixed gain to drive my passive subwoofer.

    I currently get about 88db max with my old power-amp, but can get a cleaner 100db out of the same sub with a new power-amp in place of the "150w" one I currently use.

    Sub level limiting feature of the Denon amp...

    The power amp has a fixed input gain at normal, which is too high for me to use the subwoofer output level limiting function on my Denon AV amp. This feature appears to only work up to -18db, according to the on-screen graphics and interestengly translated manual.

    I really like the sub limiting function as it saves my hearing and listening room being destroyed by overly loud sub-bass, while keeping calibration levels correct.

    I think I need to lower the peak signal output level lower than the max -18db in order to stop the power amp cracking the subwoofer against it's stops (at about 100db!).

    I currently use a lower powered amp which has a variable gain, so I can merely lower the amp's input sensitivity to make use of the Denon's subwoofer limiting, which I really like. The problem is the current amp can only drive the speaker to about 88db before noticeable distorsion sets in.

    I would like to effectively lower the new power amp's input gain.

    Any help appreciated.
    regards,
    Rob.
     
  2. mjn

    mjn
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    one word.....attenuator.....fit between the interconnect, usually a 10db drop.
     
  3. Rob.Screene

    Rob.Screene
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    Wey, hey mjn!

    How do I buy one or two line-level attenuators then?

    I've looked on maplin.co.uk and can only see attenuators for VHF and UHF, not line-level?

    thanks,
    Rob.
     
  4. mjn

    mjn
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    i saw some in the current issue of what hi-fi, if i remember correctly. But try www.hificables.co.uk and places like that.
     
  5. Rob.Screene

    Rob.Screene
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    Thanks again.

    I'd appreciate anyone who really knows what they are doing to check my choice at the end, as I can't really afford to replace my AV amp at the moment if I kill it!

    I found some discussion in rec.audio.highend using google groups
    http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&frame=right&th=5430f241ef54e7b1

    It appears attenuators are very common to the high-end audio crowd, in that the ultimate pre-amp is merely a "stepped attenuator", which a collection of about 24 line-level attenuators to allow volume control beween one's hugely expensive source component and evil power amps! They do this to avoid the degredation caused by solid state or potentiometer volume controls.

    Anyway, I'm sold that I don't need to worry about a big quality loss for my sub-bass signal for this one.

    The good news is that's it's a really simple circuit, using just two resistors, one in series with the signal (R1), then one in parallel (R2) to ground to ensure the circuit usually adds up to 10K ohms. This circuit appears to be commonly called an L-pad.

    In the above thread on usenet, Mark Yun added that:
    The equation to work out the output ratio = R1/ (R2+R1)
    e.g. R1=5Kohms, R2=5Kohms = output ratio 0.5

    To convert the output ratio in to dB
    dB = 20*log(output ration)
    e.g. 0.5 = -6.02dB attenuation.

    It appears "Vishay Bulk Foil Resistors – Unquestionably the world’s finest resistors for audio applications. " Are available for £2-3 each at http://www.audiosynthesis.co.uk/components.htm

    Since I'm looking for about -20dB, it looks like I can use R1=680 R2=10K, giving a result of -23.92dB and an impedence of 10.68Kohms using values they list.

    thanks for any comment or advice,
    Rob.
     

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