O2 Measurements & Review

Discussion in 'Headphone Amplifier Measurements' started by tomchr, Jan 26, 2017.

  1. Poleepkwa

    Poleepkwa Friend

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    I find the Butte much more engaging than the O2, although I am sure the measurements will favour the O2.
    The DIY instructions are excellent and Pete has been very friendly and helpful.
     
    Last edited: Dec 2, 2018
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  2. ultrabike

    ultrabike Measurbator - Admin

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    OK guys, here are some THD sweeps with low gain (2.5X) using different loads (I skipped the 6.5x gain because I don't really use it, and it can indeed clip. Use that one only if your DAC is weak sauce):

    Note: All of these measurements were done with a 14VAC/500mA wall-wart. I don't use this amp from batteries. It really works better as a small desktop amp.

    300 ohms

    THD vs Vin
    Sweep_THDvsVin_lowgain_300ohm.JPG

    Vout vs Vin
    Sweep_VoutvsVin_lowgain_300ohm.JPG

    With a resistive 300 ohm load we easily get 0.0005% THD or so over most of the operating range. The output voltage can reach about 7.5 Vrms w/o issues and 2.5x amplification is achieved. Maximum input voltage range seems to be around 3 Vrms or so. Not 8 Vrms, but should handle most DACs I know IMO. I'm calling this 7.5^2/300 = 187 mW max output power.

    100 ohms

    THD vs Vin
    Sweep_THDvsVin_lowgain_100ohm.JPG
    Vout vs Vin
    Sweep_VoutvsVin_lowgain_100ohm.JPG

    Things are starting to strain a bit. Now things are more close to 0.001% and above 1 Vrms input (2.5 Vrms output) things start to degrade harder. That said 0.001% is not bad at all. I'm going to call this 6.8^2/100 = 462 mW max output power.

    33 ohms

    THD vs Vin
    Sweep_THDvsVin_lowgain_33ohm.JPG

    Vout vs Vin
    Sweep_VoutvsVin_lowgain_33ohm.JPG

    A more difficult load. The system is now proly closer to 0.05% THD or so and we no longer touch 6.6 Vrms output. A 5 Vrms output is as far as we go before shit hits the fan. Therefore I'm calling this 5^2/33 = 757 mW max output power.

    HD600 load

    THD vs Vin
    Sweep_THDvsVin_lowgain_HD600.JPG

    Vout vs Vin
    Sweep_VoutvsVin_lowgain_HD600.JPG

    Now we are talking business. These sweeps are with the HD600 acting as load. Not as clean as 300 ohms, but not bad IMO. This is a bit of a less linear load perhaps. But still things look decent. Max output power seems to be about 187 mW still.

    Making measurements w the HD600 as load is annoying as hell though. The HD6x0 cans are very sensitive, and 187 mW is a lot of power into them. We can hear the 1 kHz tone loud and clear through the house.

    This sweeps are logarithmic and distortion is shown in % format so it can be compared to @tomchr own measurements.
     
    Last edited: Dec 3, 2018
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  3. tomchr

    tomchr MOT - Neurochrome

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    Looks similar to my measurements. Maybe slightly higher distortion. Do you know what the measurement bandwidth of the SRS is? I use 20 kHz (AES 17) for 1 kHz measurements.

    Heh... Yeah... When I developed my DG300B, I performed many sine sweeps both vs output power and vs frequency. The vs frequency measurements were fun. The 300B is a quite microphonic tube and the output transformers will give off some hum at the signal frequency as well, so when testing at 10 W output power, the amp is very audible - even with resistive load. Every time the sine sweep hit 8-10 kHz my neighbour's dog would go bonkers. It was pretty funny. The dog was a very annoying and aggressive pit bull so I didn't really mind getting it in trouble. :)

    Tom
     
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  4. ultrabike

    ultrabike Measurbator - Admin

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    Yup. I think I used a BW of 32 kHz instead of AES17. I'm still working on a better presentation for the THD sweeps and might redo with the AES17 filter as default. If not using it for some reason I'll try to remember to mention it.
     
  5. tomchr

    tomchr MOT - Neurochrome

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    I've made it a habit to list the test conditions in the chart title. That way there's no ambiguity or assumed defaults.

    Tom
     

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