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    EEVblog #1056 – Digilent Open Scope MZ Review

    Dave looks at the Digilent Open Scope MZ Review, an $89 open source oscilloscope, logic ...

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    • Trevor Wilson


      The valves (I refuse to use the American term: ‘tubes’) perform the following function in almost every low level application for audio (including your headphone amp):

      * They introduce a low level of distortion (predominantly 2nd harmonic in single ended amps), which some listeners find pleasing.
      * They introduce a low level of microphonics, which, with a headphone amp could perform a useful (for some listeners) function, by adding an extra sense of depth, caused by the minute delays in the reverberant signals.

      BTW: A triode connected pentode has the following advantages over a pentode connection:

      * Significantly lower distortion.
      * Lower gain.
      * Lower noise.
      * Lower output impedance.

      • Synthetase

        By definition, then, this is not a high fidelity amplifier.

        • Trevor Wilson

          Of course it isn’t.

    • Jim Bean

      Having a Valve in there reminds end user not to bash it around ,
      (so any build quality issues will not be revealed ?)
      Thanks for some nostalgia ,as the number of thermionic devices drops at home !

    • Ken Galer

      If you’re interested, I can test it with my Audio Precision analyzer. I’m sure it doesn’t come close to meeting their specs.

      • Vasily Axenov

        Would be really great to see this test.

    • LinkZ

      Ugly and useless DAVE, as always. You better go and search for a job like EVERYONE, and STOP to do and say bullshit like this. It’s a shame that there are people watching you opening crap that the others sent to you. It’s a shame that a jobless moron earn money without practically doing anything, and a bigger shame that there are people watching you. GO, KILL YOURSELF!

      • Ken Galer

        That seems uncalled for….

    • tlhIngan

      I can tell you the output impedance makes it crap.

      Two op-amps running through 47 ohm resistors makes for an output impedance of around 23.5 ohms. WHICH IS COMPLETELY CRAP FOR A HEADPHONE AMP! If you have nominal 32 ohm headphones (remember, headphone impedance is a function of frequency), the high output impedance will cause distortions – if your headphone impedance decreases, the headphones will get softer, and if it increases, it gets louder, so now you have a frequency-dependence on gain.

      You roughly need between 8-10x headphone impedance before output impedance of the amp doesn’t matter (8 times will get you roughly 0.5db variance due to impedance changes from 0 to infinity). So the headphone amp requires at least 235 ohm headphones (300/600 ohms), which are extremely hard to find. Which may explain why they use the opamp in follower mode – driving 600 ohm headphones is extremely hard.

      Modern solid state amps can easily get half an ohm or less output impedance, driving even 8 ohm headphones adequately.

      More details here – http://nwavguy.blogspot.ca/2011/02/headphone-impedance-explained.html
      And here – http://nwavguy.blogspot.ca/2011/02/headphone-amp-impedance.html

      23 ohms is terrible.

      And tubes have a problem with output impedance – that’s why they have matching transformers in tube amps – tubes have a high output impedance which needs to be transformed into a lower impedance to properly drive speakers. I guess the designer here attemped to use opamps to do the impedance transformation.

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