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Amp Hardware Volume Control?

John Hagensieker

January 03, 2016 10:12

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I haven't dabbled in a while.  Have a couple of DAC + and a Digi+ and just sprung for an Amp+

Ideally I'd like to take a Pi A+ with Amp+ and mount in an antique radio console.  I'd like to be able to manually control volume.  Is that going to be possible on the Pi A+? Or am I going to need a B or better?

Also considering hooking up an SDR dongle to get FM via GQRX.  That I'd happy leave as a software control as a functioning Pi with FM with manual controls is too much for what I want to do here. 

Somewhere in the archives here I took an A+ and DAC+ and mounted inside a tube headphone amplifier.  I still use that system religiously and still think it was a cool implementation.

I'm looking for a conversation piece. Old wooden console streaming music through 2 speakers but I would like that hardware volume control.  What do I need to do.  I've seen some brief explanations but all lack hard details. 

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7 comments

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  HiFiBerry team January 03, 2016 10:25

  Hi John,

  you want to connect a volume knob to the Amp+? The Amp+ itself does not have a feature like this. However, you can add buttons or a rotary encoder to an unused GPIO and control the volume in software. Check out this section:

  http://support.hifiberry.com/hc/en-us/community/topics/200398351-Your-HiFiBerry-installation

  You will find some projects that added volume control with a rotary encoder.

  Best regards
  Daniel

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  John Hagensieker January 03, 2016 22:16

  Where can you make the physical connections to the rotary encoder if the AMP+ is installed as a typical HAT device?  What are people doing?  Attaching pins to the bottom of the pi?  Or using some other method?  I'm aware that some GPIO's can't be doubled up on. 

  I was specifically looking for someone who had done it and demonstrated how they did it clearly.  I still haven't come across that yet.

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  hifi January 07, 2016 18:49

  The latest version of AMP+ has long GPIO pins which protrude enough to connect 40-pin connector to it. Just make sure that you order really latest model of AMP+ because the earlier models don't have such pins. For example I ordered such board from this site directly.
  
  The DAC+ has a place where you can solder 40-pin connector. That will allow cable connection to GPIO as well.
  
  The following page has enough info for connecting rotary encoders:
  http://support.hifiberry.com/hc/en-us/community/posts/201494692-Woodware?page=2#comments

   

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  hifi January 07, 2016 18:58

  Just a clarification - this topic is about hardware volume control. Rotary encoder is definitely a piece of hardware but it actually doesn't provide hardware volume control. Rotating encoder you can only change a volume through your software. For real hardware volume control you would need on-board support in AMP which would allow to connect potentiometers to it. This support is missing right now.

  Edited by hifi January 07, 2016 18:59
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  John Hagensieker January 07, 2016 19:12

  Understood about the hardware and software control.  Thank you.  I need to pick up a 40 pin header.  I'm a good enough solderer I could easily just add the wires but I find my pi-s change missions frequently.  I do something cool then move on to the next thing.  Best to slap the header in.  I think I'm all ready to roll with this project.  Thanks again.

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  HiFiBerry team January 07, 2016 20:00

  @hifi: Unfortunately different people refer to different things when they talking about "hardware volume control". While I refer to "hardware volume control" as a feature the hardware provides, some people think of hardware volume control as "analog domain volume control". Many digital circuits today use a hardware volume control - that means something that is implemented in the hardware - but still implement this in the digital domain. That means internally hardware volume control is often implemented as a multiplication of the input data with a fractional value.

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  hifi January 07, 2016 21:24

  To me the term 'hardware volume control' always means something in the analog stage of the device usually potentiometer (regular or motorized). Any other way of changing volume I consider as a software one. The usage of anything based on ADC (even high resolution) I consider as a software method because it's still discrete/digital method and usually doesn't provide that smooth (linear or logarithmic) nature which analog potentiometer usually provides. But this is just my opinion. Actually I use only software volume control in my RPi projects ;)

  Probably it makes sense to distinguish 'analog' and 'digital' methods to control volume. Where 'software' method is digital and 'hardware' can be either analog or digital or both.

  Edited by hifi January 07, 2016 22:14