No need for a second arm. With the JMW arms if you just get another 10.5 arm wand all you need to do is set up your mono cartridge on it. set your vtf, vta, azimuth, etc. You are ready to go. The second arm wand will have it's own counterweight. When you want to go from stereo to mono just unplug the arm from the lemo connector and lift the arm wand off. Set the armwand with the mono cartridge on and connect the lemo. You are ready to go. Once you have the mono cartridge set up on the second arm wand it can't take more than 30 seconds to change them and start playing your mono LP's.
