DAC not working

Hello, this is my first posting hope you guys can help me. Going to try and keep this brief.Have a Sony Cd player model CDP-CX355 will hold 300CDs. This player has toslink optical out. Hooked it up to Audio Alchemy DAC in the box with Zu audio RCA cables out of DAC Also hooked up same with cobalt 307 Dac Ran cables to Carver C1 and Rogue Magnum 66 pre Amps with both DAC showing that they had locked up on digital signals only very faint CD sound with volume on Magnum 66 turned all the way up. If I hook up ZU RCA cables from Sony CD to either pre Amp sound is fine. Why wont DAC work for me and are they better to use? Thanks Jay! jnrogers@hughes.net
I didn't quite catch the manner in which you have this stuff all hooked up, but are you sending a single source to two different preamps at the same time? This isn't necessarily a bad idea per se, but some (mainly older) preamps short out all the sources that aren't selected - this is to reduce crosstalk in their selector switches.

The logic is this: if it isn't selected, then you're not listening to it, right? So who cares if the signal is shorted? This of course doesn't anticipate another component trying to use the source at the same time.
You report that the Sony has a toslink optical output but that you hooked up the Sony to a DAC using a RCA cable.

Either your Sony has a SPDIF/Coaxial digital output (and not just the toslink as you stated) or you have hooked up your analog output of the Sony to a DAC. This won't work. Not sure what the DAC is saying it is locking onto, if in fact it is an analog signal. Check again, does your sony have a coaxial digital output? This is one output that carries both channels.

you then say that you can hook up the CD to either preamp and everything is fine. When you hook up the sony to the preamps, I am assuming you are first changing where the cables are attached to the Sony before hooking them up to the preamp. If you are just unplugging the RCA cable from the back of the DAC and plugging that same end into the preamp - then you are not running a digital signal to the DAC, you are definately running an analog signal.