Wich Preamp to use with an Chipamp? and my SoundCard.

[samsagaz]

Hi guys, im trying to find some good pre desing to use with my onkyo PC Card. can someone point me some good pre designs? Mainly i need to control the volume.

Specs of the card
115dB (2Ch Analog Out)
0.3Hz - 88kHz (2Ch Analog Out Out +0/-3dB)

Another question is, how can i use the digital output of that card? i need an D/A conversor?

its the card
Geek Stuff 4 U - Your best Japanese Gadget Store - ONKYO SE-200PCI


Thanks in advance.

[highfieldrebel]

if you use a DAC, you can just include a volume control in the DAC box. If you're concerned about the output impedance because of a long cable run, you could also buffer after the volume control.
Looking at the price though, I would expect the sound quality from the analogue ports to be excellent already.

[samsagaz]

i dont have any DAC i just want to use that card i purchased 3 years ago and never used the analog or Dig Out. analog will be OK, but need to build some PRE because cant atenuate the vol without it.

[riktw]

that looks like a toslink output.
most amplifiers have a toslink input.
you can also convert it to SDPIF easily if your amplifier has SDPIF and not a toslink input

volume control:
easy mode, use PC software.
still easy mode: make a passive volume controller with a potmeter.
less easy mode: make a pre amp with an opamp with volume control.
hard mode: make a pre amp with a digital volume control IC.

[samsagaz]

yes, have an toslink but i dont have any amp that support it just an chimpamp that build some time ago.

[highfieldrebel]

There is quite a bit of interest in the light dependent resistor volume control, there may be a PCB available. Could be a nice project.

Attenuation in the digital domain will reduce the resolution of the sound (remove MSB). You may be able to perceive this with your setup, if so you should do the attenuation in analogue.

[mt490]

The resolution decrease of using digital attenuation with 24bit generally isn't too bad unless you're going down to micro levels of volume.

But to get the full benefits of this you really have to make sure that the programs and driver setup on your PC is up to it. There's more than one circumstance that you can degrade quality significantly without even knowing it due to software.

Lastly the Onkyo card has a very high level output from the RCA jacks, to the tune of 6Vp-p almost. Many pre-amplifiers don't expect this level of output, so you might find some oddities in control (like only using the first 30 degrees of the knob). All the other outputs from the adapter loom are more usual levels.

[AndrewT]

Originally Posted by mt490

he Onkyo card has a very high level output from the RCA jacks, to the tune of 6Vp-p almost.

most CDP and similar optical disc players have a 2Vac to 2.2Vac analogue output.
2Vac = 5.66Vpp
2.2Vac = 6.22Vpp

There is nothing unusual with a 6Vpp analogue output.

[AndrewT]

Originally Posted by mt490

The resolution decrease of using digital attenuation with 24bit generally isn't too bad unless you're going down to micro levels of volume.

when a 16bit CD or MP3 signal is imported into a 24bit DAC, where do the 16bits fit within the 24bit range?

Do they occupy the LSB upto 16th bit or do they occupy MSB down to 8th bit.

When a 24bit DAC is "volume" adjusted, what happens to the 16bit signal?

[mt490]

Hmm, didn't know that about the line levels - it's a bit higher than your usual computer output, but no doubt they're also trying to get a signal out of 5V or lower on some devices these days.

When the file is played back the player usually decodes the data such that 16bitFS = 24bitFS, and AFAIK sound cards will also maintain this when switched between 16bit and 24bit data modes.

This causes an interesting issue with MP3's and other similar codecs. In these codecs there are peaks which can add to more than the expected FS, which if not treated turns into digital clipping. This is due to the way they get compressed (don't know the specifics myself, it's something to do with how the signal is rebuilt from the psychoacoustically modified information). ReplayGain for example is a method of counteracting this, the files can be pre-analysed for these peaks (and other volume information) and pre-normalised such that clipping does not occur at any point.

But other weird stuff can happen in the interim - for example Windows XP has a notorious 'kmixer' that downconverts everything back to 16bit before applying volume and mixing in various system sounds into the output. Vista and 7's mixers are much better in this regard, as they process in 32bit floating point and also provides an 'exclusive mode' where a program can take exclusive control over the audio chain for ideal quality.