The Groove uses Apogee’s trademark Constant Current Drive output stage, whose output voltage is dynamically altered by impedance variations of the connected headphones, which has the effect of smoothing-out the frequency response non-linearities that occur with traditional headphone outputs. ‘Apogee’s Constant Current Drive is a truly revolutionary approach to dealing with the wide impedance variations of headphones available on the market today,’ says Betty Bennett, Apogee Co-Founder and CEO. ‘Because of this, the Groove unlocks the highest possible quality of any headphones you have, allowing you to listen to the music you love with incredible life-like clarity.’
Groove's output circuit is not a regular voltage out. It is current out. Which is a difficult concept to explain, but the net result is that a higher impedance will imply a higher output voltage.
Thanks for your email! The Groove’s Constant Current Drive reverses the relationship between voltage and current from how headphone amplifiers are traditionally built.99% of all headphone outputs in the world use a voltage-drive design, where the current varies in order to compensate for load fluctuations and maintain a constant voltage. The disadvantage to this is that it is current that actually moves headphone drivers, and under certain conditions where the headphone itself isn’t perfectly matched to the headphone amp the actual diaphragm produces different audio levels at different frequencies - in effect, the frequency response becomes non-linear.The Groove instead varies voltage to compensate for load fluctuations and maintain a constant current. By doing this, the circuit is built to compensate for frequency response non-linearity caused by impedance fluctuations. The headphone drivers get the consistent current they need to output a linear audio signal. In practice, the Groove’s circuit is compensating for both electromechanical and acoustic non-linearity, offering extremely consistent performance with any type of headphones.The effective output impedance ends up being relatively high, actually, but that’s not a negative because it’s not a factor in how it’s moving the headphone driver like on a standard voltage-drive design. It’s kind of like asking for the MPG on an electric car - it wouldn’t be measured in the traditional sense.Sorry if that was a little confusing, but I hope it helps!
The membranes of magneplanar drivers have a large surface and very low mass. Therefore they are already highly dampened by air. Proper electrical damping is of less importance. ...Changing the effective output impedance only had very little effect on the current. It is the reason that current-amplifiers (instead of voltage amplifiers), like the Bakoon, which by nature have an extremely high effective output impedance, do work well with magneplanar headphones.
Axel Grell, the chief-designer at Sennheiser, once told me that their top-of-the-range headphones are always designed to sound best at a low output impedance.
but since you'd also need really high PSU rails to go with it wouldn't work all that well in a USB powered device.
This is my chief concern. But then consider the bakoons: max power listed is 1W at 50ohm... that means ~2.23V and ~447mA, which is within USB 2.0 spec, right?
