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?
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