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DAC+ Can I break my audio equipment?

martin3

September 07, 2014 08:45

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My Pi B+ with Hifiberry DAC+ is connected to the "AUX" port of a Bowers & Wilkins Z2. I have made two observations:

1. With the hifiberry testimage, the sinus wave is played at full volume. I can't gradually turn down the volume: One click to "-" on the Z2 silences the output completely. One click on "+" and I have full volume again. I find this strange on an analoge input.


2. Still struggling with Volumio, I managed to produce output. However, I hear a crackling sound periodically, even if the Z2 is not switched to "AUX" (e.g. if it plays back from the connected iPod).
   

Specifically this second point worries me: How can an analoge input cause a crackle throughout the device? Is there a voltage spike that is way out of spec on the cinch ports? Can this damage my Z2?

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16 comments

• 0
  
  HiFiBerry team September 07, 2014 09:23

  Hi Martin,
  
  we can't tell why the volume control on the your B&W speaker is not working correctly. The DAC output full 2V rms. As many cheap audio sources and mobile devices output only 1Vrms (or sometimes even less) it is possible that the output signal is too high for your B&W speaker (check the datasheet). However reducing the playback volume on the DAC with alsamixer should fix this.
  
  Best regards
  Daniel
• 0
  
  martin3 September 07, 2014 09:38

  Thanks for your answer! I'll try to reduce the volume.
  
  I'm new to this Hifi Audio Business, so please excuse me if I am asking simplistic questions.
  What exactly is the audio out signal from the HifiBerry? I'd think that it is a PWM signal, so what is the voltage of a "high" pulse? Is this voltage standardized for Class D amplifiers?
  
  I'm still wondering why the Z2 crackles as soon as a signal from the HifiBerry is on the AUX in.
• 0
  
  HiFiBerry team September 07, 2014 09:55

  Hi Martin,
  
  this is not a PWM, but an analogue output the peak voltage is a bit more than 3V, which results in about 2Vrms.
  The DAC does not use PWM or Class-D amplification.
  
  Unfortunetaly there is not really a standard, every vendor has it's own interpretation. The 2Vrms where common for standalone-CD players, but today we see everything between 0.3Vrms and 2Vrms. Therefore I recommand checking the specifications of your speaker.
  
  Best regards
  Daniel
• 0
  
  martin3 September 08, 2014 17:06

  So one could not e.g. connect one of these to the Hifiberry DAC (+): http://www.adafruit.com/products/1752
  Since this is a Class D amplifier, one would need the special form of pulsed "analog" audio-out.
• 0
  
  martin3 September 08, 2014 17:18

  Sorry, please ignore my question. Apparently the pulse generation happens inside of a Class D amplifier, so one can feed it with a continuous analogue signal.
• 0
  
  HiFiBerry team September 09, 2014 06:22

  Have a look at our upcoming HiFiBerry Amp:
  http://www.hifiberry.com/amp
  
  Best regards
  Daniel
• 0
  
  martin3 September 13, 2014 14:35

  Apparently the crackle is indeed because the average voltage is to high, i.e. the volume is too high.
  
  I could validate this with Volumio, where I can lower the volume on the GUI. However, in other respects I have too many issues with Volumio right now, so I want to validate everything with regular raspbian first.
  
  Therefore please excuse my noob question: How can I lower the volume when I use mplayer? I'd like to use the hardware volume control of the Hifiberry DAC+
  
  Thanks! Martin
• 0
  
  martin3 September 13, 2014 15:27

  Sorry, I should say that I tried with "-volume 50".
  The crackle is gone, so this is good.
  
  What I get is this output:
  [Mixer] No hardware mixing, inserting volume filter.
  
  So I think the volume is adapted by software, not hardware.
• 0
  
  martin3 September 13, 2014 16:32

  One last update, because now I stop trying ;)
  
  When I use mplayer with these options:
  -volume 80 -ao alsa -mixer-channel "Playback Digital"
  
  Then I see no error messages. However, there is no audio output either.
• 0
  
  HiFiBerry team September 13, 2014 16:50

  You can just use "amixer" or "alsamixer" on command line. With amixer you have to set everything from command line, alsamixer gives you a simple GUI.
  There is no need to to this with mplayer.
  
  Best regards
  Danniel
• 0
  
  martin3 September 16, 2014 15:53

  Sorry for asking again... When I connect a little oscilloscope (a DSO 112) to line out, it claims to measure a Vpp of 6.2V (when playing the test tone). The iPhone headset connector shows 0.6Vpp, at full volume.
  Unfortunately it can't calculate Vrms.
  
  So I still wonder if something is wrong with my HifiBerry DAC+.
• 0
  
  HiFiBerry team September 16, 2014 16:31

  Vpp of 6.2V is correct. Vrms = 0.3535 * Vpp which is a bit more than 2V.
  
  Unfortunately today's mobile devices have very low output volume. If you're B&W is designed for this it might clip at full volume. Therefore you need to reduce the volume. The datasheet should say what the expected input voltage is. You can reduce the volume with the mixer or use a voltage divider at the output. However no audio device that is designed carefully will be damages with an 2V input level. 2Vrms is the quasi-standard from the era of CD players.
  
  Best regards
  Daniel
• 0
  
  martin3 September 16, 2014 17:46

  Sorry, I am still struggling to understand. The highest number that I can find here: http://en.wikipedia.org/wiki/Line_level is 3.472, which would be the Vpp of a "pro" line level. Vrms is 1.228 in this case.
  The measured 6.2 is almost double this value. What I am understanding wrongly?
  
  Unfortunately the data sheet of my amplifier does not state anything useful. It looks like I have to send their support a mail.
• 0
  
  HiFiBerry team September 16, 2014 18:35

  The levels you are referring to are nominal levels. They are used as a reference for measurements. As db values are relative to a reference, different dbU and dbV values have been defined as reference points. This has nothing to do with output levels of devices.
  If you want to read more about db, dbm, dbV, dbU and these references, I recommend the "Audio Measurement Handbook" by Bob Metzler. You can download it for free after a registration at the Audio Precision website.
  http://opt-in.ap.com/AP-Starter-US/
  
  Best regards
  Daniel
  
• 0
  
  Ben Potter December 30, 2015 22:35

  I have been considering various options for my hi-fi system, including removing my Quad 33 pre-amp altogether and feeding my Quad 405 directly from the DAC + Pro I recently bought.  My simplistic reading suggests that the levels will sit very uneasily, working from the input levels mentioned here:

  http://www.audio-times.com/2014/07/19/quad-405-stereo-monitor-amplifier/

  Would any member of your team care to offer some wisdom, please?

  Many thanks,

  Ben

• 0
  
  HiFiBerry team December 31, 2015 08:22

  The input sensitivity of this amplifier is 0.5V for 100W power output. That means you can't operate the DAC+ at full volume. While you can reduce the volume with alsamixer, it is still a bit risky. Setting the software wrong will not only result in full volume, but also excessive clipping. I would recommend at least a stereo potentiometer between the DAC+ and the power stage.

  Best regards
  Daniel