I have a headphone amp with a DAC. The amp has a USB input.
I want to use the amp/DAC with  an older CD player.
Is this possible? Is there an adapter for this purpose?
Yes im sure there is , check at Fryes or another electronic parts store, Heck Best Buy might sell a cable or adapter.

If the CDP has digital out, YES, it might be possible to send signal to your headphone amp/DAC assuming compatibility (with or without an adapter) between the output of your CDP and the input to your DAC.  IF the CDP only has RCAs outputting an analog signal as would go to a pre-amp or amp, then I don't think you will be able to do it.  Got to be able to access the digital stream in the CDP and send that to the DAC.  Or so I think.    
Just to clarify. 
I want to use the DAC in the headphone preamp.
The headphone preamp has a USB input, a pair of RCA inputs and a pair of RCA outputs. 

david - to me there are 2 questions that need to be answered to address your original inquiry about using an older CDP w/the DAC in your HP amp...1) Can your CDP provide a digital signal that the DAC in your headphone amp can process? 2) If the CDP does have a "Digital out", can you find a way to get that signal into the USB port on your HP amp?  If the answer to both of those questions is "Yes", then you should be able to listen to that CDP though your headphones using the DAC in the HP amp.    If either is "No", you might still be able to listen to the CDP via the HP amp using conventional RCA-type interconnects but I suspect this won't take advantage of the DAC in the HP amp.   Hope that makes sense.

Thanks for the reply.
I haven't purchased the CD player yet.
Of course I will buy one with a digital output.
To keep costs down I'm planning on buying a older carousel player.
The DAC in my headphone amp is modern and reportedly very good.
So, how to use the USB to the digital out in the CD player?
Unless there is such a thing as a coax or optical to USB adapter, you won't be able to.  Have you looked for that?
Mahler's point is Question 2 in my previous post.
D99, Your CD player probably has a SPDIF output.

Sending SPDIF Input to the DAC's USB input cannot be accomplished by a simple cable type of adapter/converter

You will need an electronic converter. 

USB - SPDIF converters are common, but I do not know of any SPDIF TO USB converters

Are you sure your amp does not have any other type of digital input? Such as optical

What amp do you have
Check out the Mutec-3 or even less money the Mutec-1.2
Williewonka 03-20-2016 9:23am edt
Sending SPDIF Input to the DAC’s USB input cannot be accomplished by a simple cable type of adapter/converter

Outputting a USB signal requires either a computer or a device that emulates the USB-related functionality and processing of a computer. A converter having that capability would certainly not be a simple adapter. I am not aware of any such converters, but if some exist I wouldn’t be surprised if you would be better off in terms of sonics as well as cost if you simply connect via analog, and use the player’s internal DAC.

-- Al

Zmanastronomy, thanks for pointing out the Mutec devices, which certainly look interesting if not particularly inexpensive (the 1.2 appears to sell generally in the area of $450).

However, after looking at the manual for the 1.2 ( I’m not at all sure that it would do what the OP requires. If the device is provided with a S/PDIF or AES/EBU input it can convert those signals to a USB output, but while it is not made entirely clear in the manual or the website description I believe a computer or equivalent would be required to receive the signal from that USB output. For example, note this statement on page 16:
The digital audio signal received at the selected input will be converted into an USB audio stream and send through the USB port into your computer, laptop or music server.
A typical audio DAC will certainly not be able to interrogate a USB output in the same manner a computer can.

And note that the USB connector on the device is USB type B, which is ordinarily used on peripherals that interface to a computer, not on the USB ports of computers or computer-like devices that communicate with those peripherals.

-- Al

Thanks all for the information.
It appears I can't use the DAC as intended.
Any recommendations for a vintage but well built and reliable carriage CD player ? 
I have 2 leads on highly modified Art di/o DACs.