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PC digital volume control instead of pre amplifier

willem

Well-known member
I search of an ever simpler set up, I noticed that if you use a computer as your only source (as I now do in my desk top system), all you need beyond the computer is a USB DAC and a power amplifier - apart from the volume control.

Of course you can do that in the computer (as with tone control) but I read that this only works satisfactorily with a DAC with sufficient bit depth. Is there anyone here who really knows about these things and can tell me a bit more, and what is sufficient?
 
A study of DACs in home use

A study of DACs in home use

You ask an important question given the increasing use of computers as a source for music. As you say controlling the volume in a computer for playback is generally not a good idea because you will not get the best sound quality. It is not the bit depth of the attached DAC that matters but the limited bit depth used in the computer volume control so that the lowest bits (and the corresponding low-level musical information) are thrown away as the volume setting is lowered beyond a certain point.

I’d not claim to be an expert but I have been through the process of setting up a DAC with a volume control, connected directly to my power amp.

To start with you need to check:
1) what is the maximum output voltage of your DAC? This should be stated in the spec or data in the user manual for the DAC.
2) what is the maximum input voltage of your power amplifier? Again you should be able to find this in the spec or user manual.

If the latter voltage is greater than that the DAC can produce then you can use the full range of the DAC volume control without problem, however if the maximum output from the DAC is greater than the maximum input voltage for the power amp you will need to limit the highest volume setting you use on the DAC otherwise you will clip the signal. To give some more guidance: if the DAC maximum output voltage is twice that which the power amp can handle then you would need to reduce the maximum volume you use by 6dB; if the DAC maximum output voltage were 50% higher than the power amp can handle then a reduction of 4dB would necessary.
What is the range and step size of your DAC volume control? This may again be in the specs / user manual but you may need to do some further research online or talk to your dealer.

Assuming a reading of 100 for the maximum output for the DAC, with units of 1dB and a step size of 0.5dB (typical figures for many DACs) then the maximum volume setting to avoid clipping in the above 2 cases would be 94dB and 96dB respectively.

That’s the easy part. More important is to know whether the DAC will reproduce the full resolution of your digital music at the normal minimum volume level you use. There is no universal answer as the “normal minimum volume level” is dependent on many factors: personal preference and constraints, room furnishings, distance from speakers, amplifier sensitivity etc. etc. The only reliable method of determining this level is for you to listen to a range of music and establish your own normal minimum volume level with the DAC / power amp combination you are considering.

Many DACs have a bit depth of 24 bits. CDs require 16 bits to be converted to analogue if all the musical information is to be preserved. Thus the minimum volume at which this will be possible will be 8 (=24-16) bits below the maximum level. Now 8 bits corresponds to 48dB so the volume level below which musical information will start to be lost will be 52dB (=100-48). So if your normal listening level with your DAC / power amp combination is less than 52dB I’d suggest you look for either a DAC with a lower output voltage or a power amp with lower sensitivity. Note that if you had some 24bit music files (e.g. downloads) there would be a loss of bits for all but the highest volume settings.

There are an increasing number of DACs that have a bit capability greater than 24. This does not mean they can resolve more information from CDs or 24 bit files but it gives far more latitude for controlling volume without losing musical information. Thus a DAC with 32-bit processing would allow 16 bit CD music to remain intact down to a volume 16 bits below the maximum level i.e. 96dB down or a displayed volume level of 4dB. Similarly 24 bit music would remain intact down to a volume 8 bits below the maximum level i.e. 48dB down or a displayed volume level of 52dB.

In auditioning possible DACs you may find your power amplifier is too sensitive so that the minimum volume setting you wish to use on the DAC will be below the 52dB level calculated above for CDs. I experienced this problem with a Bel Canto DAC used in conjunction with a Krell power amp set to a gain of 26dB, however the amp had an alternative lower gain of 20dB which brought the minimum volume level I required on the DAC to above 52dB. If your power amplifier is too sensitive for use with the DAC you choose, and you do not wish to change the amp, you will need to insert an attenuator between the DAC and amp.

As you can see things are not as simple as they might seem but if you sort out the matching carefully the resulting system will be ’simpler’!
Best of luck on your journey.
 

Nessuno

Member
Rounding errors?

Rounding errors?

If you want to use the computer alone to control the volume, then what matters happens inside the computer itself. It is a software issue, the following stages, DAC included, have no role in it.

Every DSP data flow is theoretically a lossy one, but if every processing stage, which in case of volume control is simply a multiplication of every sample by a constant value < 1, operates at a higher bitdepth than the source stream (or in floating point arithmetic), the calculation and rounding errors are practically null and no distortion or noise is added to the signal.

So, what needs to be checked is if the software player and/or the OS operate internally at a higher bitdepth than the source files. Modern players and OSs usually do.
 

willem

Well-known member
Bit depth?

Bit depth?

How do I know this? Is the bit depth number equal to the 32 bit or 64 bit of, say, the Windows OS? Since programs are now either 32 bit or 64 bit, does this mean that we are fine using the digital volume control of the software in the computer?
 

Nessuno

Member
Digital volume controls

Digital volume controls

No, they refer to completely different things.

Labelling an OS or an application as 32 or 64 bit simply means it is designed to make full use of CPU internal registers length (you know: pre-PC era and early home computers used 8 bit CPUs, then 16, 32, 64 etc...). This could have impact on speed performances, not on results and results are what matter in the case of software volume adjustment: how many bits the software actually uses to manage the data stream. So the two things are uncorrelated. An 8 bit CPU can be used at will to make computations on 16, 24 or 32 bit data streams as well as a 64 bit CPU can be used to work on 16 bit data streams only. It's a software design choice.

Now you see, at this point a definitive answer to your question cannot be given, information are needed on how the specific software player you use or the OS it relies upon, handle internally the data (it's good to know both because a player could make volume management by itself or just leave the task to the OS and sometimes the same player could act differently on specific configuration parameters).
It depends also on the sources you use: a CD standard stream (16 bit) is unharmed by a software making volume management at 24 bit, a 24bit source needs a software working with at least 32 bit depth (not CPU registers length).

A hopefully not too complex but correct explication on how digital volume works and its caveats can be found here: http://www.esstech.com/pdf/digital-vs-analog-volume-control.pdf . It actually refers to volume control implemented on a DAC chip, but the concepts apply to fully software implementations as well.
 
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