Digital, Passive or Amp volume control

[The Messiah]

Have always used integrated amps in the past but have become interested in hearing what a passive preamp or a Dac's digital volume control can sound like in comparison.

Is one way or another generally regarded as superior and do you have a preference for one ?

 

[RayDunzl]

I use either or both, as necessary or as desired, and don't worry about it.

Ultimately, it depends on the implementations, though, so YMMV. How many bits in the digital volume, is it dithered after, does the analog control track well, etc.

 

[The Messiah]

Thanks, that's good to know.

Do you know if the bits drop upon lowered digital volume on the likes of a Topping E30/D50s ?

 

[Doodski]

Digital has the advantage of channel balance and less crosstalk although a passive volume control can be built using high quality resisters and they have good matching.

 

[Weeb Labs]

My preference is actually digital analog volume control, which overcomes most of the shortcomings of potentiometers and purely digital control.

With the exception of my desktop setup, all of my volume levels are controlled using PGA4311UA boards. I find them to be very convenient as their channel balance is essentially perfect at all levels and with four channels, they can handle both subwoofers and full range outputs simultaneously.

They also enable me to automate global volume control with HASS via ESP32, which is very convenient.

 

[RayDunzl]

Do you know if the bits drop upon lowered digital volume on the likes of a Topping E30/D50s ?

Lose 1 MSB for every 6.02dB attenuation is the basic rule.

It's really not as bad as you might think.

In the following image, the top trace is full volume, the middle trace shows what it"looks" like with 60dB attenuation using the same visual scale (it looks like there is nothing left), the bottom trace zooms in on the -60dB (1/1000th) version to show what's left, which isvery close to what you started with.

-60dB takes your playback froom "loud" to "almost inaudible".

The traces represent "voltage" that would be output by the DAC.

 

[The Messiah]

Lose 1 MSB for every 6.02dB attenuation is the basic rule.

How many MSB would it take to be audible drop in quality ?

 

[RayDunzl]

How many MSB would it take to be audible drop in quality ?

I've never noticed a change under normal use.

See the above graphic.

 

[Weeb Labs]

So long as your signal chain is at unity gain (or below at the final stage) and you are not adding makeup gain, it should never present a problem.

 

[restorer-john]

It's really not as bad as you might think.

It depends on the digital attenuator and the source bit depth. A 16 bit attenuator will introduce tons of distortion as you lower the level. A 24 bit less and a 32 bit even less.

Consider 16bit PCM audio at -80dB exhibits ~10% THD.

 

[AnalogSteph]

Audio processing these days is done in 32-bit or 64-bit float.
High-performance DACs have >120 dB of dynamic range (~20 bits), with >110 dB (~18 bits) of good linearity or better.
110 dB (~18 bits) will give very good coverage of human hearing needs.
90 dB (~15 bits) is about the limit for actual (acoustic) recordings.

All this potentially means a lot of latitude for digital attenuation.

If you insist on maximum SNR under all circumstances, you will combine digital attenuation and analog gain settings. Some RME DACs even have an "Auto Ref Level" feature that chooses the most fitting analog output stage gain for each volume setting, bringing total available dynamic range up to 139 dB unweighted (approaching 24 bits). Or for a more pedestrian example, your average CS4272 based midrange USB audio interface will have an analog monitor (output) gain control to complement the DAC's dynamic range (108 dB(A) is the best real device spec I've seen for that). Ditto for the even more pedestrian Behringer UM2/UMC22 with their PCM2902 (clone?) and a ~95 dB(A) 16-bit DAC, or the venerable Sandisk Sansa Clip+ DAP of similar DAC specs with a built-in PGA - it was by far the most power-efficient solution at the time. (High dynamic range analog stages generally need lots of power. The latest DAC IC implementations have been resorting to applying Class G/H techniques for this very reason, yielding a rather dramatic reduction. See e.g. CS43198 and the like.)

 

[brimble]

Audio processing these days is done in 32-bit or 64-bit float.

Excuse my ignorance, but is data sent through the USB interface as floats?

 

[RayDunzl]

How many MSB would it take to be audible drop in quality ?

Audibly?

I can only drop the digital side about 40dB, and still maintain the same volume level, by turning the preamp up all the way.

That would be loosing about 7 of 24 bits, taking the signal to around 17bits maximum.

Sounds the same to me here.

Might measure a little differently on the electrical side. Less audible signal to still inaudible noise.

---

Attenuating any more than 40dB digitally and I can't "turn up" the analog path enough to maintain my normal volume level.

I guess that also infers I have at least 40dB headroom, so I've got that going for me, which is nice. (probably not really but it feels good)

Losing 40dB makes the electrical signal 1/100th of the original, whatever that was.

 

[RayDunzl]

Excuse my ignorance, but is data sent through the USB interface as floats?

Ummm... Don't really know.

In my case, as far as I know, my USB sends 24 bit, the DAC turns that into 32bit for volume control.

I suspect it would be controlled by the USB driver and the capability the DAC and Source advertise when they shake hands.

"The data requirements of a USB Audio system depends on the number of channels , the number of bits to represent each sample, and the sample rate . Typical channel counts are 2 (stereo), 6 (5.1) or much higher for studio and DJ use. Typically sample size is 24 bits, although 16 bits is available for legacy audio, and 32 bits for high quality audio. Typical sample rates are 44.1, 48, 96, and 192 kHz. The latter is used for high quality audio."

https://www.edn.com/fundamentals-of-usb-audio/

My Focusrite 4Pre USB has at least 10 audio channels available, presumably with 24bits, and maybe 8 more if the ADAT function is active, and maybe a channel or two for MIDI, so, ???

 

[brimble]

In my case, as far as I know, my USB sends 24 bit, the DAC turns that into 32bit for volume control.

Thank you. I guess what I really want to know is: does the USB driver ever smush the signal down to 16 bits? Because if it does then losing some bits to digital attenuation prior to the USB driver could be a concern. My guess is that it never does, but I've been too lazy to read actual specs (which I've heard are long-winded), and the summaries I've found online are all about file formats and not about whether there's ever a bottleneck at the USB driver.

So I think you've totally answered my question, but let me check:

> 16 bits is available for legacy audio

I think that means that a modern operating system is always going to use at least 24 bits - have I understood that right?

 

[RayDunzl]

> 16 bits is available for legacy audio

I think that means that a modern operating system is always going to use at least 24 bits - have I understood that right?

Probably means if the DAC says "I can only handle 16 bits" that is what it will get.

Skim this (or look for something simillar - googled "usb audio bit depth negotiation"

https://source.android.com/devices/audio/usb

Devices talk to each other before they decide how to work together.

 

[brimble]

Right. Now I think I understand fully. Thank you! This has been bothering me at the back of my mind for years.

 

[yoggi]

View attachment 95430

My preference is actually digital analog volume control, which overcomes most of the shortcomings of potentiometers and purely digital control.

With the exception of my desktop setup, all of my volume levels are controlled using PGA4311UA boards. I find them to be very convenient as their channel balance is essentially perfect at all levels and with four channels, they can handle both subwoofers and full range outputs simultaneously.

They also enable me to automate global volume control with HASS via ESP32, which is very convenient.

Hi Weeb Labs,

I am interested in finding a PGA4311 based volume control board like the one in your picture. May I ask where you got it from?

yoggi
Sweden

 

[Weeb Labs]

Hi Weeb Labs,

I am interested in finding a PGA4311 based volume control board like the one in your picture. May I ask where you got it from?

yoggi
Sweden

They were designed by Crazy Audio and you can find the Gerbers here. You will need to populate them yourself and a dual power supply is required.

 

[yoggi]

Thanks, I'll have a look at it!
yoggi