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  1. #11
    Founder, Slim Devices seanadams's Avatar
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    Qustion: _Why_ is the SB2 output level so high?
    Squeezebox1 was 3vpp and people complained it was too low.

    Now it's 6vpp (~2v rms) which is in line with nearly all high-end equipment. Higher voltage == better SNR along the whole path. This isn't an "oops we made it too loud" - SB2 was deliberately designed (at significant added component cost) to have 2vrms levels.

    This $20,000 cd player, for example, is 2vrms:

    http://www.stereotimes.com/trans120699.shtm

    I don't get what the problem is. If the output were hot enough to clip anyone's receiver that would be an issue, but it's not - the only thing this would potentially clip is really cheap amplified speakers - even a $100 receiver will handle 2vrms. If you're going directly into a standalone amp... well, the only reason you cat do that at all is because we have a volume control. Of course at full volume you're going to drive it to the max - the same would happen with a CD player.

    The ideal solution is to

    a) add a preamp
    b) open up the amp and tune its input pots (asuuming it has them)
    c) add in-line attenuators as suggested above.

    The internal volume control is not ideal for this, but sure we can tweak the software to adjust the range and make it work.
    Last edited by seanadams; 2005-09-08 at 16:30.

  2. #12
    Senior Member pfarrell's Avatar
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    Re: SB2: reduce analog out voltageby 10x?

    On Thu, 2005-09-08 at 16:06 -0700, seanadams wrote:
    > > Qustion: _Why_ is the SB2 output level so high?

    >
    > Squeezebox1 was 3vpp and people complained it was too low.


    You can never make everyone happy.
    I'm happy.

    > I don't get what the problem is.


    Me neither.
    A voltage divider circuit has three resistors.
    You'd need two, so it would be six resistors. Or
    two pots. Clutch coupled, concentric, audio taper
    pots aren't that big a deal for the hand full of folks
    that have a problem. Less than $10 worth of parts.

    --
    Pat
    http://www.pfarrell.com/music/slimse...msoftware.html



  3. #13
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    Quote Originally Posted by seanadams
    If you're going directly into a standalone DAC... well, the only reason you cat do that at all is because we have a volume control.
    Since we're referring to the analog outputs, I assume you mean a standalone power amp, not a DAC?

    As you say, the only reason you can do this at all is because of the internal volume control. But this is exactly why I want to do it!

    If I connect the SB2 to a preamp and on to my power amp, the *only* thing the preamp does is control the volume. Since volume is already controlled in the SB2, it makes sense to remove the preamp completely, and connect the SB2 direct to the power amp. This makes the signal path much cleaner, and to my ears at least, it sounds much better. It also cuts down on the number of boxes, cables and power sockets in my system, which is always nice....

    Chris

  4. #14
    Robin Bowes
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    SB2: reduce analog out voltage by10x?

    seanadams said the following on 09/09/2005 00:06:
    >>Qustion: _Why_ is the SB2 output level so high?

    >
    > Squeezebox1 was 3vpp and people complained it was too low.


    Are you sure? 3vpp is 1.06v rms and I'm pretty sure my SB1 has an output
    that is less that 0dBu (which would be 0.7746v rms).

    > Now it's 6vpp (~2v rms) which is in line with nearly all high-end
    > equipment. Higher voltage == better SNR along the whole path. This
    > isn't an "oops we made it too loud" - SB2 was deliberately designed (at
    > significant added component cost) to have 2vrms levels.
    >
    > This $20,000 cd player, for example, is 2vrms:
    >
    > http://www.stereotimes.com/trans120699.shtm
    >
    > I don't get what the problem is. If the output were hot enough to clip
    > anyone's receiver that would be an issue, but it's not - the only thing
    > this would potentially clip is really cheap amplified speakers - even a
    > $100 receiver will handle 2vrms. If you're going directly into a
    > standalone DAC... well, the only reason you cat do that at all is
    > because we have a volume control. Of course at full volume you're going
    > to drive it to the max - the same would happen with a CD player.


    The problem is that most domestic equipment is designed to operate at
    0dBu, or 0.7746v rms for max level. Sure, the preamp can often handle
    more, but the gain stages are setup so the volume control operates
    sensibly with this level of input. When the input is "hotter" the useful
    range of the volume knob is reduced.

    > The ideal solution is to
    >
    > a) add a preamp


    ....designed to operate with 2.12v input

    > b) open up the amp and tune its input pots (asuuming it has them)
    > c) add in-line attenuators as suggested above.


    ....with attenuation of approx. 2.58 dBu

    d) open up the Squeezebox and replace R37 & R39 with 12.7k or 12.9k
    resistors.

    Here's my calculations:

    Standard Configuration
    R37 4700
    R38 8250
    Gain 1.755319149
    Standard Voltages
    Vout(p-p) 6
    Vout(p) 3
    Vout(rms) 2.121320344
    Vin(rms) 1.208509771
    Desired Voltages
    Vout(rms) 0.7746
    Gain 0.640954685
    Required R37 12871.42475
    Attenuation
    For 0dBu -2.585548412


    > The internal volume control is not ideal for this, but sure we can
    > tweak the software to adjust the range and make it work.


    What effect would this have on audio quality?

    R.
    --
    http://robinbowes.com

    If a man speaks in a forest,
    and his wife's not there,
    is he still wrong?


  5. #15
    Founder, Slim Devices seanadams's Avatar
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    Quote Originally Posted by cbemoore
    Since we're referring to the analog outputs, I assume you mean a standalone power amp, not a DAC?
    oops - edited

    If I connect the SB2 to a preamp and on to my power amp, the *only* thing the preamp does is control the volume. Since volume is already controlled in the SB2, it makes sense to remove the preamp completely, and connect the SB2 direct to the power amp. This makes the signal path much cleaner, and to my ears at least, it sounds much better.
    The thing is... it actually doesn't! The SB2 has a very low noise floor down around -120db. However when you reduce the signal level, the noise floor doesn't change so your SNR becomes effectively worse. Less signal, same noise == less SNR. That's why a fixed analog attenuator set your maximum listening range is the cleanest way to go - it attenuates the signal and the noise together, meaning that the noise floor in your amplifier is the limiting factor (which it might be anyway, in which case you're right and all this is moot).

    It also cuts down on the number of boxes, cables and power sockets in my system, which is always nice....
    I agree. It is also easier to set up and doesn't require buying or soldering together an attenuator.

  6. #16
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    Thanks for the explanation, Sean. That makes it much clearer.

    So do I need to go out and buy some attenuators, or is there a teensy weensy chance you might be implementing something in software???

    Cheers
    Chris

  7. #17
    Founder, Slim Devices seanadams's Avatar
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    Quote Originally Posted by Robin Bowes
    seanadams said the following on 09/09/2005 00:06:
    >>Qustion: _Why_ is the SB2 output level so high?

    >
    > Squeezebox1 was 3vpp and people complained it was too low.


    Are you sure? 3vpp is 1.06v rms and I'm pretty sure my SB1 has an output
    that is less that 0dBu (which would be 0.7746v rms).
    I'm rounding here... but yes I'm pretty sure I measured Squeezbox1 is in the 2.9-3v range. That's taking the outputs straight off the Micronas' built-in DAC. It can actually go almost to 3.3 (Vcc) but it starts to clip at some point and that's where I set the max. Sorry I don't have the exact figures handy.

  8. #18
    Founder, Slim Devices seanadams's Avatar
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    Quote Originally Posted by cbemoore
    Thanks for the explanation, Sean. That makes it much clearer.

    So do I need to go out and buy some attenuators, or is there a teensy weensy chance you might be implementing something in software???

    Cheers
    Chris
    It's on the list. The list is long. I will move it up the list.

  9. #19
    Founder, Slim Devices seanadams's Avatar
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    OK I added this. The setting goes from 0 to 63 db of attenuation in 0.5db steps. Firmware will be updated soon - need to sync up with Vidur on that first.

  10. #20
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    Great! In further reading of my amp manual it says (re the gain pots) “Your amplifier sounds best with these level controls set to maximum…” So – it looks like I can turn them all the way up and set attenuation on the SB2 to match the “correct” level for my amp. Earlier in this thread you stated: “However when you reduce the signal level, the noise floor doesn't change so your SNR becomes effectively worse…” So – from a “best sound” standpoint – would I be better off using the new built in attenuation – or using an external attenuator?

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