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Chord Mojo - volume control question ...

Discussion in 'audio' started by beammeup, Jan 13, 2020.

  1. beammeup

    beammeup pfm Member

    How 'internally' does the Chord Mojo DAC adjust volume?

    Is it based on adjusting the line output voltage only?

    I am thinking of bypassing a pre-amp and plugging it directly into a power amp so I am intrigued.

    Thanks.
     
  2. MotelBlues

    MotelBlues pfm Member

    I can’t answer your question but I can report that I’ve been using my Poly/Mojo directly into a Quad 303 with no issues that I can detect.
     
  3. lordsummit

    lordsummit Moderator

    Likewise, my Mojo plays into a Quad 306 very nicely indeed. I presume the volume control is on the output stage.
     
  4. John Phillips

    John Phillips pfm Member

    I am not sure how it impacts your thinking but according to Rob Watts' presentation at the launch event, the Mojo's volume control is done (digitally, I assume) in the Artix FPGA chip (the programmable logic DAC core). This will, of course, change the output level.
    I have successfully done this myself with other DACs.

    Perhaps this is relevant: according to the big thread on the Mojo at Head-Fi it seems that the Mojo can be set to provide a fixed output level for use via a pre-amp with volume control. But it reverts at power-on to variable output level, using the last volume control setting, for the use case you are contemplating:

    "To set the output level to 3V ( line level ) for connection to a preamplifier press both volume buttons
    together when switching on the unit. Both volume balls will illuminate light blue. This mode is not
    remembered so when you switch off it will reset back to the previous volume stored for safety reasons."
     
  5. Whatsisnaim

    Whatsisnaim pfm Member

    On my Hugo I found it crucial to set it to variable volume and dial it down a bit. This sounded better, and gave the preamp volume control a reasonably wide usable range.
    If you’re using it direct into a power amp, always double check that it’s on variable volume, suitably dialled down, every time you switch on. It’ll only take one accidental switch on at full output to give your speakers/ears/cat a very nasty shock.
     
  6. beammeup

    beammeup pfm Member

    OK thanks. I was under the impression that the highest quality way of attenuating volume with a DAC is to adjust the voltage to the output which then eliminates the need to use software volume control or similar at the risk of chopping off bits or introducing processing overheads which can somewhat impact the sound quality.

    Instead adjusting the voltage output is about as pure as it can get - better than a passive of any sorts (LDR, stepped attenuator, Music First types etc) - so I was interested in how Rob Watts implemented this into the Mojo. I mean a MF passive pre can cost £6k - surely it's better to find a method to reduce the voltage of a line output stage and reap the benefits there instead. I believe some DACs do this (I think Metrum have one or two in their range).
     
  7. Whatsisnaim

    Whatsisnaim pfm Member

    I understand your thinking, but as I said, my experience with both Hugo and Dave is that when used with a preamp, they sound better with the volume set to variable and dialled down. I’ve tried them with active and passive preamps, and felt there was always an improvement. Haven’t heard the Mojo, so I don’t if the same would apply there. All I can say is try different options and see what works best for you.
     
  8. Julf

    Julf Facts are our friends

    Most modern DACs do the digital volume control using 32 bit (or more) for the calculations, so there is no impact on sound quality.
     
    John Phillips likes this.
  9. beammeup

    beammeup pfm Member

    OK thanks - I wasn't sure. I know that Metrum for some reason do think that using digital volume controls are detrimental to SQ hence their design of changing the reference voltage on the output as their way of attenuating the signal (on the higher end models).
     
  10. Julf

    Julf Facts are our friends

    They can be detrimental if badly executed, but the same applies to analog attenuation.
     
    John Phillips likes this.
  11. AndyU

    AndyU pfm Member

    Here are some quotes by Rob Watts, who designed the mojo:

    "So when you have an app that has a volume control, and no bit perfect setting, then set it to full volume on the app on the assumption that this will keep the data closer to the original file. The volume control function on Mojo is much more sophisticated than the PC as I employ noise shaping and I do the function at a very high internal sample rate. Hopefully using the volume set to max on the app will mean the volume coefficient is 1.0000000... so it will return the original data."

    "If I were to use rounding then what you say is correct, there would be small signal non-linearity.

    Mojo categorically does not use anything as crude as rounding to convert bit depths as the volume function is running at 16FS (705.6 kHz or 768 kHz) - I use extensive noise shaping to change bit depths. Mojo's noise shaping from beginning to end through all the intermediate paths (that is digital input to the 4e pulse array outputs) ensures 200 dB performance in band - that's better than 32 bit performance. The benefit of this is small signal non-linearity is much better, and this is essential for depth perception - the tiniest error in small signal amplitude, no matter how small, is audible in terms of truncation of perceived depth of sound-stage.

    Your assertion that analogue does not have these problems is incorrect. Any metal to metal interface contains oxides and other impurities - and copper oxide is diodic, and so attenuates small signals and creates small signal distortion. Moreover, carbon track volume controls are also non linear, as carbon composition has significant voltage dependency of resistance - another source of non-linearity. Analogue electronics additionally suffer from RF noise pick-up, which when added to an active stage will then create more noise floor modulation due to audio signal and random RF noise inter-modulation.

    Mojo, unlike all other non Chord DAC's, has no measurable noise floor modulation, and zero distortion of small signals, with no measurable fundamental signal non-linearity. This is not something that other DAC's can do, nor is it something that an analogue volume control can do too. And the benefit of all this is refinement and transparency - key ingredients for musicality."

    They are from the head-fi forum here:
    https://www.head-fi.org/threads/cho...in-3rd-post-◄★☆.784602/page-648#post-12274209
    https://www.head-fi.org/threads/cho...n-3rd-post-◄★☆.784602/page-1753#post-13030578

    If you have a question about any of his products it is worth posting on the dedicated forums on head-fi. He will usually answer, and as he probably knows more about how his products are designed than most people his answers are usually worthwhile.
     
    nobeone likes this.
  12. beammeup

    beammeup pfm Member

    Great! I used to think that passive pre's were as pure as you could get for attenuating volume - now I'm thinking that a properly implemented digital volume control (when using digital only as a source) is the way to go.
     
    Julf likes this.
  13. lordsummit

    lordsummit Moderator

    There’s no doubt that adding my quite decent passive pre based around a rather nice pot isn’t as good as the Mojo and it’s volume control. I sometimes wonder about trying a Quad 34 pre to match the 306, but I guess I’d be just spending money to prove that in my one source system, I don’t need a preamp.
     
  14. beammeup

    beammeup pfm Member

    Software volume control - even Volumio agree you can't get bit perfect volume control from its software... read below.

    https://volumio.github.io/docs/User_Manual/Volume_Control_and_audio_quality.html

    Volume Controls and Audio Quality
    Introduction
    Volumio is designed to maximise audio quality - ideally, bit-perfect playback. Some say that allowing the listener to control the playback volume gives up that bit-perfect ideal, but that's not correct in all circumstances.

    There are two ways to get volume control:

    • Software mixer: the audio stream is manipulated to get the desired volume change. This makes the stream not bit-perfect and degrades sound quality.
    • Hardware Mixer: some DACs (not all) have an array of internal resistors they can use to change the volume. In this mode, you can change the volume while keeping the audio stream bit-perfect and avoid any audio quality degradation.
    How to get the best sound and Volume Control
    Volumio can detect if your DAC has a Hardware Mixer, and enable it automatically. If your DAC does not support it, Volumio will allow you to enable the software mixer. To change this behaviour:

    • go to Playback Options -> Volume Settings
    • Select the Mixer Type:
      • None = No volume control
      • Software = Volume control but loss of Audio Quality
      • Hardware = Best of both worlds, provides ability to change volume without loss of quality
     
  15. beammeup

    beammeup pfm Member

    I often use software volume control on the Auralic Mini when using older DACs - but as Xuanqian from Auralic says:

    Use volume control equal to break bit perfection, on any device using any software.
     
  16. Julf

    Julf Facts are our friends

    Technically, of course adjusting the volume digitally is not "bit perfect", as it changes the bits, but...

    This is of course patently wrong. Any attenuation, be it analog or digital, will change the signal, and constitute "degradation" based on the definition in the first part.
     
  17. Julf

    Julf Facts are our friends

    See above.
     

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