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.

Headphone101_GrooveCurrentSourceMeasurements_Photo_TestingNext 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.

COMMENTS
kais's picture

From your measurements it's quite obvious that this amp uses a combination of voltage and current feedbacks of some kind:
Voltage negative feedback keeping the output voltage stable, and a current positive feedback via a current sensing circuit (maybe just a resistor), counteracting the voltage feedback to create some "current source" effect.

So - no matter how the output stage is build, it does not react much like a high impedance current source.
Interesting is the the slight increase in output impedance with lower load resistance. This can make a little difference on low impedance headphones, it's a little bit kind of "hyper current source" effect.

Two other measurements would be interesting:

1st comparing to a voltage amp, the frequency response with a headphone having a low impedance with strong variation over frequency, like the "Enigmacoustics Dharma Production 2015", to see if the changes in frequency response follow it's impedance curve.
According to your measurements I would expect a drop of 3.4dB@2.5khz relative to 500Hz.
If you would use the transfer function you could very much improve the accuracy of your measurement.

Here's a describtion how that works:
http://www.studiosixdigital.com/audiotools-modules-2/acoustic-analysis-m...
A bit of smoothing would make the curve clearer readable.

2nd it would be interesting how the maximum unclipped voltage is on different loads, if current or voltage is the limiting factor.
Here's what Apogee claims:
225mW into 30 Ohm (= calculated 2.6Volts, 7.5mA).
40mW into 600 Ohm (= 4.9 Volts, 8.1mA).
Not that much IMO, a standard NE5534 as output stage can do 12V, 35mA.
Combined with your measurement I'd say their output stage is current limiting, say not very strong.
But enough to hurt your ears with most headphones.

Best Kai

kais's picture

Correction:
Voltage negative feedback keeping the output voltage stable, and a current NEGATIVE feedback (higher current -> lower voltage) via a current sensing circuit (maybe just a resistor), counteracting the voltage feedback to create some "current source" effect.

Phoniac's picture

> 225mW into 30 Ohm (= calculated 2.6Volts, 7.5mA)

No, it's 86 mA. Back to the class room...

And a single OP wouldn't do it, especially at the low THD values the Groove has.

kais's picture

right

thune's picture

Unless the measurement technique led to unrealistic results, and ignoring the slight adaptive behavior or the amp, it seems realistic to say the Apogee Groove is a voltage-source amp with 'about' 20 ohms output impedance.

kais's picture

I'm not quite sure what the hype about current source amplifiers is.
According to the measurements the Apogee behaves more like a normal voltage amp anyway, with a little bit of "current source effect" on top.
Apogee might not have been sure if a real current source amp is such a good idea for driving all kinds of headphones, so they compromised.

I'm waiting for comments on the sound, but it will be hard to isolate the "current source effect", as it's high output impedance will change the frequency response with most low impedance headphone.
You can't have one without the other.

Using the Audeeze LCD X could sheed some light on this.
15 Ohms, almost independent of frequency and a high efficiency (= some amount of electrical damping might be possible due to a strong motor) makes it a candidate for a comparison.
Comparing it's impulse response on both amp types could be interesting.

Adding a "real" current source amp to the comparison would be nice too.

For those measurements it would be better to show only one channel, left or right, to clearyfy the graphs.
Best Kai

Bill Brown's picture

Thank you VERY much for your efforts.

mindbomb's picture

This might be an amp with 24 ohm output impedance and an automatic gain switch. So the output impedance isn't decreasing, the gain is increasing as the load increases.

steaxauce's picture

Thanks very much for exploring this, Tyll. It's really hard to tell what's going on here. How did you calculate the output impedance?

Jlle's picture

Made me think of how to insert current feedback behaviour in a voltage feedback design as described on this website:
http://education.lenardaudio.com/en/14_valve_amps_7.html

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simplestatus0's picture
saintkrusher's picture

Hello Tyll. What do you think what amp should be better for Elear headphones: Audioquest Dragonfly Red or Apogee Groove? Or anythink else?

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