you may want to check out the Rothwell 10 DB inline attenuators for sale here at A'gon.
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I would suggest modifying an input to the preamp by using a voltage divider rather than the output of the DAC. You will have to experiment to find the proper values. You could start by using two equal size, say 47,000, 1/4 watt. You'll need 4 resistors, 2 for each channel. This will reduce the effective output of the DAC by 1/2. You could use clip leads to easily swap values but be careful not to short anything.
On the inside of the preamp (WITH POWER OFF) unsolder the wire from the center of the RCA input you want to use. If the input jack is soldered directly to a circuit board you will have to cut the trace.
Solder one end of one resistor to the lug where you took the wire off.
Solder one end of the other resistor to the ground lug.
Solder the free end of both resistors together.
Solder the wire you unsoldered to the point where the 2 resistors are soldered together.
If you have to cut the trace from the center of the RCA jack:
Solder one resistor so it bridges the cut.
Solder the other from ground to the end of the other resistor that is furthest away from the input.
Increase the size of the resistor soldered to the ground lug to increase the output, lower it to lower the output.
You could use a multimeter on AC to measure the output of the DAC, but with music instead of a reference tone from a test disc it will be constantly changing. Besides, it really won't tell you anything. You don't need to know the actual value. You know your output is too high for the rest of the system so you just need to turn it down.
Depending on the circuit you might be able to get a correct reading on a resistor in a circuit. Without looking at the schematics it would be impossible to predict.
Send me an email and I can email back a drawing if you think this is something you want to do. Let me know if they are soldered into a board.
The correct value,the how-to instructions and the resistors should have come with the dac as they did with mine.It is a matter of swapping one resistor only.Maybe some owners can share their resistors with you.I had Alpha for 5 years until it burned down.I threw away the manual with the resistors about a year ago.There are a lot of CAL dacs out there and i am sure you can get it all from one of the owners who does not need any adjustments.