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Driving headphones straight from DAC's line out

juanvicentefont

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May 29, 2022
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Hello everyone,

This is my first post. I'd like to share my experience driving my Drop HD6XX headphones (nominal impedance 150Ω) straight from the line out of a Schiit Bifrost 4490 DAC (nominal output impedance 75Ω), using a custom built RCA to 3mm headphone jack adapter. The results might save some money to those with DACs with capable output stages.

Before I tried this setup I was using the power amp section of a Meridian 551 integrated as headphone amp with a voltage divider made with discrete high quality resistors at the output. This arrangement provided a huge amount of power to the headphones, with peak levels much beyond those I would (and should) ever be listening at, while the background noise was still inaudible. I reasoned that a good power amplifier delivering only a few milliamps into a high impedance load (compared to speakers) would be working in Class A mode all the time and should be quite transparent.

However, I was not completely satisfied. The highs were not a clean as I expected. So I thought of checking the difference between the amp and bypassing it by connecting the headphone straight to the line out of the DAC. I thought that I would have to listen at limited levels due to limited output of the DAC line out. The result? Yes, the Meridian power amplifier stage did in fact degrade the subjective sound quality. But what surprised me more was that the 2 VRMS output of the DAC are more than enough to drive the HD6XX to the maximum levels I listen at. With this setup I usually set the volume in Pure Music (v. 3.0.9c) to between -16 and -6 dB. Please note that I had to adjust the EQ to compensate the boost due to headphone impedance rise at drive resonant frequency.

The Schiit Bifrost 4490 output stage seems to cope very well with the relatively low impedance load (compared to the line input of a preamp) of a headphone like the HD6XX. The output will not clip or audibly distort before 0 dBFS, and running a two tone test at 18 & 20 kHz I cannot hear any intermodulation products even at 0 dBFS. I wonder how many other DACs are also capable of directly driving sensitive headphones with low enough distortion to avoid the expense of a dedicated headphone amplifier. I know that Amir runs output impedance and distortion vs load impedance on headphone amplifiers, but I've never seen these tests run on DAC line outputs. If the latest super-performing DACs are also capable of directly driving headphones then they would be even more valuable for those like me who mainly listen thru headphones. Perhaps Amir might add these tests to his extensive set of DAC tests.

Best regards.
 
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DVDdoug

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Please note that I had to adjust the EQ to compensate the boost due to headphone impedance rise at drive resonant frequency.
That's one of the problems. As you seem to know, the DAC's output impedance, along with the headphone impedance makes a voltage divider an since the headphones don't have constant impedance over their frequency range that changes the frequency response so every headphone ends-up with a different frequency response, and different from its specification.

Of course, headphones are measured-specified using a low impedance "constant voltage" source and a good headphone amp will have low output impedance, relative to the headphones... Ideally 1/10th of the headphone impedance or less. Line-level signal voltage is "close" to headphone levels but headphone outputs are capable of more current (lower impedance loads) and headphone outputs always have a volume control and some line-outputs don't have a volume control.

Then there's the issue of the "output impedance" spec. Unless you measured it yourself it might not be 75-Ohms. It might actually be lower which would be a good thing. Usually the "output impedance" is really the recommended minimum load. i.e. An "8-Ohm" output on a power amplifier is the rated/recommended speaker impedance,. The effective output-impedance is a lot lower (damping factor).

Most DACs & preamps are designed to drive a line level input between 10K and 100K. Most are actually capable of driving 1K but if you start driving headphones "unpredictable" things can happen. The output voltage might drop, or distortion may increase and there's even a possibility of damaging the device by drawing too much current, but that's unlikely.
 
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jasonhanjk

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Without proper measurements of your headphone, it is hard to compensate using EQ. With headphone amps as cheap as $99, I don't see the reason to resort to using DAC output.
Apple donggle already $10 and it's very good. :)
 
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juanvicentefont

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I actually measured the frequency response using REW and a calibrated microphone with a low impedance source and then with the DAC, and adjusted accordingly. Please note that my original setup wasn't a very low output impedance either. The equivalent output impedance was about 20Ω due to the voltage divider.

The Apple dongle may be good (I use it on my iPhone), but its output is limited to 1 VRMS, that is, 6 dB less than standard DAC line output.
 
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