CanJam at RMAF 2015: Apogee Groove with Constant Current Drive
Current output headphone amplifiers are popping up all over the place these days. It's a technology with potentially significant advantages, but also significant drawbacks. The Apogee Groove ($295) sounds like it may have rid itself of some of the problems.
The magnetic field in a headphone voice coil is directly proportional to the current flowing through it...and not the voltage applied. With a voltage step going into the amplifier, a voltage amplifier will put out a similar step, but because of the reactive impedance the current waveform will lag as it puts energy into expanding the magnetic field around the voice coil. In a current amplifier, the output current will (almost) immediately rise to the proper level, but the voltage will spike significantly to respond to the initially high reactive impedance.
A pure current drive amplifier (transconductance amplifier), however, has a significant drawback when driving headphones and speakers. Because the current is always proportional to the input voltage, its voltage output will change proportionally with the impedance of the load. Many headphones have impedance plots that significantly change with frequency. For example, the Sennheiser HD 800 has a nominal 300 Ohm impedance, but at 90Hz the primary driver resonance increases the impedance to 600 Ohms. With a current drive amp, the voltage will double at those frequencies creating a significant increase in the bass heard at those frequencies. This is why it was so easy for Big Sound participants to identify the Bakoon current amplifier in blind teststhe bass was significantly boosted.
Apogee claims to have overcome this problem with the Groove, which includes some sort of impedance sensing circuit in the design. Unfortunately, this technology is confidential at the moment until patents are in process. I did, however, have the opportunity to hear the HD 800 at the show, and indeed it did not seem to exhibit this problem.
I've long thought that current drive topologies might be a very good thing in audio (Nelson Pass has done some, which include compensating networks to solve some of the problems) as it may dramatically improve articulation in the high frequencies. While the Bakoon current drive amp had problems with tonal balance due to varying headphone impedance, it also seemed to make the treble smoother without loosing articulation.
This is a subject you can expect me to investigate and explain further over the coming months. I do have a Groove in-house, as well as the Bakoon HPA-21 and Erzetich Perfidus, which, as I understand it, is also a current source amp.