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I bought the Japanese version of the FX Audio, which I feel is constructed better than the Chinese version and for more than twice the price it should be,
$68.49 versus $31.99 for the Chinese version, still a gift from the audio g-ds. The Japanese version adds a very critical switch, which enables you to toggle between zero gain and minus 6 db's of gain, the latter setting reduces the voltage by 50%. Both FX Audio's run best on 6AK5's IMO after rolling nearly two dozen other types of tubes in. I can buy tubes for these at $5 a pair from a surplus electronics store, but I left all other types on their shelves because I never go back to second best after I find tubes that I like.
The Japanese version's minus 6 db's setting was just what I needed to tame my recently acquired MAC 1700. The MAC's built in SS preamp has too much gain to my ear, I would have gotten rid of it until I placed the FX Audio in between my stand alone DAC's outputs and the MAC's Tape Inputs. I am now throughly enjoying my MAC, the two pots are both very good and allow me to come up with the desired amount of gain. The pair of 6AK5's also adds a very pleasant tube sound to the SS receiver with a tubed tuner.
I am enjoying it enough to upgrade the capacitors and any other parts that I can.
I have owned several preamps with the common 12AU7's driving 12AX7's and I find the single pair of 6AK5's to be more pleasing.
Don't be deceived by the price, this is one great preamp / tube buffer!