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Deaf Cat
2005-09-03, 16:11
Hi

Just wanted to check that it is ok to plug my nice new SB2 directly into my power amp?

At the moment the pre-amp (soon to be replaced with the SB2....?) has a nominal 200mV output level, and a 3V max output level.

The power amp is for use with a 0.8V standard line output level pre-amp. It has an input sensitivity of 24Kohms, and gain of 28.8dB.

I am afraid these figures do not seem to mean much to me and I probably would not have matched the pre/power amps, untill a sales assistant actually demoed them together.

Will the 6.0Vpp of the SB2 RCA left and rights be ok to go into the power amp?

Look forward to hearing any comments.

Wayne1
2005-09-03, 16:28
If you wish to connect a stock SB direct to you amps, you can do this.

Make sure the volume on the SB2 is turned ALL THE WAY DOWN before you turn the amps on.

You will have very liitle range of volume control with a stock unit.

With a modded uni,t the output op-amp is bypassed. The amount of voltage output is quite a bit less.

Deaf Cat
2005-09-04, 02:08
Cheers for that, glad I did not just try it.

Will therefore leave it going through the AV amp and let that control the power amp. Just thought less equipment may be better.

Thanks again.

cbemoore
2005-09-04, 17:13
Deaf Cat,

You need to vote for this bug:

http://bugs.slimdevices.com/show_bug.cgi?id=1601

I REALLY want this functionality, but so far it looks like I'm the only person who's actually voted for it!

Running directly into a power amp (and using the digital volume control) gives a far cleaner signal path than connecting via a pre-amp. But the signal out of the SB2 is far too hot, and its impossible to turn the digital volume down low enough - 1 out of 40 is still really quite loud!

The DAC used in the SB2 already has a built-in attenuation capability, but its not used. If the Slim guys could add an option to make use of it, it would solve all my problems....

Chris

Deaf Cat
2005-09-05, 00:50
Well I never realised we could vote on bugs...cool idea, I have 999 votes left now.

So is it a pure software change for option 2(DAC alteration)?
(Forgive me as electronics is way above my head - unfortunatly)

Cheers

seanadams
2005-09-05, 01:03
Yes, it's a software change. We have a lot on our plates right now - voting on bugs / feature requests, especially small additions like this, helps us to prioritize them.

JJZolx
2005-09-05, 08:43
A secondary volume control setting would be desirable so that SB2 can be limited
to a lower volume range, for use with standalone amplifiers. This could be done
one of two ways:

1) In the server, add an additional attenuation setting which applies to the
main volume control.

2) Add a second "preamp" volume control which only applies to the DAC. The
PCM1748 has built-in volume attenuation capability which we're not currently
using. By taking advantage of this, we could effectively let the DAC be set to a
lower range while still applying the full range to the s/pdif output. This
method may also give better SNR and dynamic range.

#1 is an easy quick fix, but #2 would be better.
Sean,

Why is #2 better?

seanadams
2005-09-05, 11:20
Sean,

Why is #2 better?

Not having detailed specs on the internal architecture of the DAC, I don't know for sure that it is. Either way it's not a big difference... we can still do separate volume controls for DAC vs s/pdif outputs since these are separate data paths in the SB2 - for example the "fixed digital volume" feature does this already.

sbjaerum
2005-09-05, 11:40
Not having detailed specs on the internal architecture of the DAC, I don't know for sure that it is. Either way it's not a big difference... we can still do separate volume controls for DAC vs s/pdif outputs since these are separate data paths in the SB2 - for example the "fixed digital volume" feature does this already.

Please educate me a bit here.
For the s/pdif path, I assume that only a decrease in volume is possible. An increase in volume would result in clipping of the signal leading to severe distortion, or am I missing something here? Assuming 16 bits CD audio, and 24 bits s/pdif transmission, the samples can be left-shifted 8 bits without loosing information (this assumes 24 bits DAC at receiving end of s/pdif). Is this correct?

Then let's move to the analog output of the SB. Is all the volume adjustment done by digital multiplications prior to D/A, meaning that the gain of the analog output stage is constant?

Steinar

mazurek
2005-09-06, 09:37
For the original poster, partsexpress sells line level signal attenuators. These should allow you to use more of the range of the squeezebox output.

Deaf Cat
2005-09-06, 14:23
Cheers,

I'll look up line level signal attenuators, and see what they are and who sells them in the UK.

Had to drop my pre/amp back to the shop today as it has decided it likes humming to its self rather loudly. So a line level signal attenuator may well be handy right now (for the power amp) - untill Slim Guys can sort the software......