O2 Measurements & Review

It's been a while, but here are some further measurements and commentary.

The following measurements were done at low gain, which is what I usually use when driving my HD600s.



Distortion numbers in general are very decent. I chose a 40 kHz BW to accommodate for the 20 kHz THD+N measurement. In terms of voltage available to 300 ohm headphones, consider that 175 mW is equivalent to a fairly respectable 7.24 Vrms.

Frequency Response is fairly flat through out the audio range (+/- 0.1 dB). Below is a plot of the 10 Hz to 20 kHz frequency response while using a 33 ohm load on both channels. Volume knob is at 12 o'clock.



One of the main issues I have with the SR1 is that it does not seem to allow visualization of THD+N vs Vout. However, below is a THD+N vs Vin sweep using 33 ohm and 300 ohm with both channels running.



The cuttoff (1% distortion) for the 300 ohm loads corresponds to 7.24 Vrms, and for the 33 ohm loads corresponds to 4.24 Vrms. One should be very careful when interpreting the distortion numbers. THD+N is referenced to fundamental. For the 300 ohm load, 0.0005% is achieved around an output of 361 mVrms.

To observe the distortion behavior across frequency we can plot the distortion vs frequency at some nominal operating value. This is useful because THD+N is typically done @ 1 kHz only. A device might have no issues @ 1 kHz, but it could strain at other frequencies, particularly in the high frequency audio range.

For 33 ohms THD+N vs Frequency is characterized @ 200 mW output.



While distortion does raise @ higher frequencies, it does seem well controlled. Next we characterize with a 300 ohm load @ 50 mW output, with similar conclusions.



Next we see distortion numbers when the device is driven by 40 linearly spaced tones in the audio range (about 1 Vrms output using the more demanding 33 ohm loads). As can be shown, THD+N is -70 dB, THD is -82 dB and IMD is -75 dB. This is a HiBW measurement (200 kHz).

 

Next we plot the THD+N PSD for 33 ohms / 200 mW and 300 ohm / 50 mW to get a feel for the distribution of harmonics.

Note: The measurements below might not exactly match the ones in the measurements summary, because they were taken at different times, and noise environment conditions might change slightly. However, they should be reasonably close (+/- 1 dB or so).

Below is the 33 ohm case.



And the 300 ohm case.



With the exception of the 2nd harmonic, most odd harmonics are higher in power than even harmonics.

To get a feel for IMD we take a look at the PSD of the CCIF and SMPTE standard measurements also taken with the 33 ohm load:



(NOTE: The plot below is IMD SMPTE, but it is mislabeled as CCIF)



Another device characteristic of interest is linearity. It is perhaps more important in precision DACs. However, it can be applied to amplifiers as well. I expect it to degrade at low signal range (noise dominated), and large signal range (distortion dominated). In both measured cases, a 1 kHz tone was used.

Here is the case of the 33 ohm load:



Here is the case of the 300 ohm load:



Because the 33 ohm demands more current from the amplifier, the 33 ohm case will fall out of the linear region sooner than the 300 ohm case.

Next we take a look at crossover distortion using a -100 dBVrms input into 33 ohms. We use a low level signal to accentuate any crossover distortion issue. This maybe of some interest because more than likely the amplifier is class A/B with the need of feedback for error correction in the audio BW. The plot below shows no obvious issues.

 

Next we look at the HiBW response of the amplifier. This maybe of more interest for Delta-Sigma DACS, or things that have a switching power supply.



The amplifier appears fairly flat all the way out to about 200 kHz.

If there is something of interest besides these somewhat standard measurements, let me know.

Regarding subjectives, I don't dislike the O2. Though I have heard other amps that I like better. Some with more impact, some with more resolution, some a little more engaging. But for $70 or so, it serves me well. It is also an amplifier that has been reviewed and characterized to dead. Therefore it holds some value as a sanity check for a measurement setup.

I enjoy audio and the awesome variety of equipment with all their different characteristics. I also enjoy measuring and understanding how stuff works.