Apogee Electronics Groove D/A headphone amplifier
The Apogee Groove USB DACheadphone amp has a USB Micro B input at one end and a mini (1/8") headphone jack at the other. Its machined-aluminum case is just under 4" long, a bit over 1" wide, and half an inch thick; it feels very solid, and has a nice, grippy rubber pad on the bottom. You could probably melt down four AudioQuest DragonFlys and pour them inside. A small recess at one corner, bridged by a metal bar, forms a leash loop. There are also 30th Anniversary models in silver and gold ($595), which Apogee describes as offering enhanced performance. I listened to the standard black version.
On the Groove's top surface are two black, rubbery, ½"-diameter pushbuttons, for volume up/down. Between them is a column of three multicolor LEDs. Rather than indicating the incoming signal's sample rate, as with other DACs, these LEDs function together as an output-level meter, responding to the music's dynamic peaks and dips. When you push either volume button, the lights temporarily turn from blue to violet, to indicate level; although there are only three of them, each dims as you lower the volume, so the Groove gives a pretty good idea of the volume level.
The Groove employs asynchronous clocking, and uses an eight-channel ESS Sabre 32-bit DAC chip in a Quad Sum design (four DACs per channel) that can handle PCM streams of resolutions up to 24-bit/192kHz. Powered solely through the USB bus, the Groove got noticeably warm after a few hours: My infrared thermometer measured about 100°F at the center of the top panel. I'm guessing it will drain a laptop's battery quicker than most.
A note about connecting: As with all of the USB-powered DACheadphone amps I have, I was able to easily use the Groove with my Mac laptop, but it didn't work with my iPad Air or iPhone 6 when attached to them with Apple's USB camera adapter, generating the warning "The attached accessory uses too much power." I've seen mentions online of some USB-hub workarounds to solve this problem, but I didn't try any for this review. The Groove can also be connected to powered desktop speakers.
A Current Affair
The circuits of almost all headphone amplifiers99% of themare "voltage drive" designs. What makes the Groove special is that it is a variation of a "current drive" headphone ampor, as Apogee calls its approach, Constant Current Drive.
All headphones have a set impedance (resistance) determined by such things as the thickness and amount of wire used in their voice-coils. They also have what's called a reactive impedance, due to driver resonance and other acoustic factors. Therefore, the total impedance of some headphones, measured against their frequency response, will look like an undulating curve. In other words, some headphones' impedance can bounce around a lot when reproducing music.
The amp's job is to provide power to the headphones in the form of voltage (pressure) and current (flow), which respond to the headphones' impedance and the volume level selected. A good rule of thumb: An amplifier's output impedance should be lower than the impedance of the headphones you're using by a factor of eight or so.
In a voltage-drive amp, the voltage gain is kept constant while the current fluctuates to accommodate the changes in impedance. The opposite is true for current-drive amps, where the current gain is held constant and the voltage fluctuates with impedance. Because current is what moves headphone diaphragms back and forth to move air and thus create soundwaves, having the current modulate with the impedance is not a good thing, and keeping the current steady should be an advantage. Apogee's Constant Current Drive is a variation of this second approach.
But if current drive is a better design, why doesn't everyone use it? I asked Tyll Hertsens, editor of our companion website Inner Fidelity, and he cited two reasons: 1) Headphones are generally already voiced to compensate for the impedance problems with voltage drive. 2) Current-drive amps inherently have very high output impedances and, typically, poor damping factors.
When I asked Tyll if he was aware of any other current-drive headphone amps, he mentioned the ones made in Slovenia by Erzetich Audio, and Bakoon Products' HPA-21 (which Tyll used in the summer 2015 headphone-testing extravaganza he called Big Sound 2015)but not much else. And none of the other portable DAC-headphone amps I had on hand used current drive.
Tyll told me an interesting story about the Bakoon. Because the frequency responses of headphones are voiced for voltage-drive amps, the sound of headphones like the Sennheiser HD800swhich have a wildly varying impedance curvechanges quite a bit when mated with a current-drive amplifier. When his Big Sound 2015 panelists used the HD800s in a blind test that compared the current-drive Bakoon HPA-21 with a voltage-drive amp, they could easily tell the difference: The headphones sounded obviously warmer with the Bakoon. Comparing two voltage-drive amps with the Sennheisers, they could barely hear any differences at all.
Apogee gets around this problem with a unique circuit in the Groove that provides a variable output impedance, depending on the headphones connected to it. Apogee claims that this circuit can compensate for "frequency response induced impedance non-linearity in headphones" and can also "dynamically compensate" for other types of nonlinearities, be they acoustical, mechanical, or electrical. As a result, Apogee says you no longer need to worry about matching the Groove to different types of headphones and their wide range of impedancesand the problem of damping factor also goes away. Voilàthe benefits of current drive without the headaches.
The advantages of Constant Current Drive don't really benefit powered desktop speakers, which typically have a very constant impedance over their entire frequency range.