Apogee Groove Current Source Amp Measurements
In an ongoing effort to investigate current source amps I did some initial measurements of the Apogee Grove, which is supposed to be immune from frequency response changes to headphones with varying impedance. For a little more background on the subject and measurements of a pure current source amp check out my article measuring the Bakoon HPA-21.
The Apogee Groove ($295) is a USB powered headphone amp that claims to have a "Constant Current Drive" design that is able to rid itself of the interactions with complex impedance curves found on many headphones. Unfortunately I can't do a full battery of tests on the Groove, but I am able to do a few measurements to learn a bit about it, and to help determine whether or not it is indeed immune to interaction with the headphones.
The plot at the top of the page shows the raw frequency response of the Sennheiser HD 800 using the FFT frequency response measurement of white noise. (This measurement will not be the same, nor as accurate, as a swept tone from the AP, but it is repeatable and useful for comparative purposes.) The blue traces are measurements from a HeadRoom Desktop amp (a pure voltage source amp with about 1 Ohm output impedance) driving the HD 800. The green traces are the Bakoon HPA-21 current source amp, in which you can see the considerable rise in the bass frequencies due to the large impedance bump at the primary driver resonance. The red traces are the Apogee Groove. You can see there is absolutely no rise in the bass relative to the blue traces of the voltage amplifier.
Next step was to have a look at its output impedance and how it reacted to various resistive loads. To the right you'll see a series of pictures metering the output of the Groove of a 500Hz tone with various resistive loads. In the top picture I've set the output to 0.5mV with no load attached. I need to say at this point that a pure current source needs a load of some type to work properly. (If it's trying to output current into an open circuit the voltage will rise to the rail as there's no place for the current to go.) Obviously there is some sort of feedback in the Groove to sense an open circuit and limit the voltage...in other words, it's not acting like a pure current source amp.
I then attached a variety of loads and calculated the output impedance with each load.
- 16 Ohm load; output impedance calculated at 24 Ohms
- 32 Ohm load; output impedance calculated at 21 Ohms
- 150 Ohm load; output impedance calculated at 20 Ohms
- 600 Ohm load; output impedance calculated at 18.5 Ohms
It appears the output impedance is lowering as the load impedance increases. Interesting.
Another quick check of whether or not the Groove is a pure current source amp is whether there's a directly proportional relationship between output voltage and load resistance. (Which is exactly what happened with the Bakoon.) With a pure current source amp, current will remain constant at any particular input level and volume control knob setting. In the case of the Groove and the images to the right, the input and volume levels remained constant; only the resistor value of the load changed. Doing the calculations, (voltage/resistance=current) you can see that the current does not remain constant with differing loads.
- 0.203Vrms/16 Ohms = 12.6mArms
- 0.309Vrms/32 Ohms = 9.6mArms
- 0.441Vrms/150 Ohms = 2.9mArms
- 0.485Vrms/600 Ohms = 0.81mArms
So, neither the voltage increase, nor the current decrease is directly proportional to the change in load impedance.
I've done some initial listening with the Sennheiser HD 800 and I have not heard any tonal changes compared to a voltage output amplifier. Bass seems nicely tight, which should be a problem with the high output impedance...but it doesn't seem to be the case. In fact, I'd say this is a better than average sounding USB powered DAC/amp.
From where I sit, the Apogee Grove is either a current output amp with some type of voltage feedback circuitry, or a voltage amp with a high output impedance...that also sounds pretty good.
I have tried to get Apogee to spill the beans on what's going on inside the Groove to no avail. Maybe once they've got their patent applications in we'll find out for sure, 'til then we'll just have to guess.
Speaking of which...any good guesses out there? Any ideas on a test that would clarify it's nature one way or the other? I know some will just say it's a high output impedance voltage amp, but I'm not so sure. Fire away in the comments.