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"Volume control is by a digitally controlled dual stepped attenuator circuit. I've used it in prior products but the Rx amp is the first portable amp to benefit. The Rx volume control starts at a reasonable level which does not leave the user wondering if the amp is not really on or faulty. For the output stage, I combined what I learned from designing FDA-listed medical imaging products with my numerous analog audio and mixed-signal PCB designs. With an internal gain setting***, the Rx can be set for a gain of 1 or 5, allowing it to optimally drive any available headphone


*** As per Ken Ball, the gain setting ended up being mostly redundant as the delivered amp set up for low gain works just as well with hyper efficient ear-canal phones as it does with standard open-backed designs. But the feature remains accessible by opening the Rx. "The hi/lo gain switch is there though rarely used. We have found it's really not needed.

"I should have sent you the mini-to-speaker cables because this little amp will push speakers. Not terribly loud mind you but amazingly enough it's got enough giddy-up to drive 8-ohm speakers. Grados or any other headphones for that matter are no sweat. The AKG 701/702s, Sennheiser HD800s and to a lesser extent the Ultrasone Edition 8s require a lot of authority in a headphone amp to really shine. We designed the Rx to do exactly that."


The portable concept implies small size, battery power, quick recharge (four hours for complete drain) and long play time. Check all that. With real estate rarer than Hong Kong, there's space only for 3.5mm stereo i/o ports, a teensy power toggle with equally teensy power LED and a nippled volume thumb wheel which doesn't fully turn but only fowards and backs over the distance the nipple has before it hits its stops. Individual short clicks increase or decrease the volume in discrete steps, holding them longer rushes the volume up or down until released. Turn-off resets the volume to low so you don't inadvertently blast your brain if the last user was a cranker. It's a very clever, effective and precise solution. Given the dearth of cubic volume to stuff many or big parts into it, the Rx was bound to use a chip output stage either in class D or by op amp. For further details he and Matt were comfortable parting with, I asked Ken Ball: "The Rx output stage is an opamp circuit optimized for sound quality first, followed by low DC offset, then battery life. The schematic and layout are very minimalist with a less-is-more approach".