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Old 27th June 2019, 07:48 PM   #11
Mark 461 is offline Mark 461
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Join Date: May 2019
Thanks a lot. I understand now.
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Old 27th June 2019, 08:27 PM   #12
aurel32 is offline aurel32  France
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Quote:
Originally Posted by phofman View Post
An ideal situation for the lowest signal/noise ratio:

DAC - no attenuation

analog volume control - no attenuation

amplifier - just enough gain to attain the required listening level.

But every recording is different and you need some volume control. So in half-reality you pick just a bit higher amplifier gain and fine-tune with slight analog attenuation, as needed.

But in full reality you have no control over your amp gain and must use its analog volume stage for all the control. Still, keep the DAC output at max, of course below limiting.

For the blocks you have listed above, if they all more have a signal/noise ratio in the same range, I agree it is the thing to do. If yo opt for a more high-end version of some blocks, it's good to estimate the noise floor of each block as it sometimes give some surprises.

Let's take for example the PCM1792/94 DAC. With a good implementation, it can reach a noise floor around 0.9µVrms in the audio band for a 2Vrms output. I have seen on many schematics on the web that it is followed by a volume control based on a PGA2310/11. This is even the case in an Elektor article as "discarding bits in software is not an option". The output noise floor of a PGA2311 is around 2.5µVrms, so it's like 1.5 bits are lost. In that case the analog volume control is not the good choice and a software volume control actually makes more sense to get the best SNR. A potentiometer based volume control would probably be a better choice there, but again it will only improves the SNR if the total noise floor is not dominated by the output amplifier.
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Old 28th June 2019, 11:57 AM   #13
soundcheck is offline soundcheck  Germany
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Quote:
Originally Posted by Mark 461 View Post
Thanks a lot. I understand now.
I'd be surprised if that'd be the case, after phofmans answer


The key issue is the amp gain, which has to match the speaker sensitivity.
If you don't get that right you'll never be able to accomplish a good setup.

With todays 32bit digital VCs quite a huge range 8bit=48dB can easily get attenuated. Within this range digital beats all analog VCs.
And the noise floor logic as outlined by phofman becomes irrelevant.

Within this range it doesn't matter, from a voltage perspective, what VC analog/digital you run.
However. The actual type does matter. Analog VCs perform much worse within this range. Analog VCs do have several issues.

And very important:
Running 2V out - no attenuation (as outlined by phofman) is not always possible. Especially if you run direct DAC -> amp connections. You need to know the maximum input voltage allowed on the amp.
Feeding a full swing 2V - 100% signal into an amp that comes with 1 of 1.5V max input voltage will cause serious issues.


Fill out below list:

DAC = ?
DAC output voltage = ?
AMP max input voltage = ?
AMP gain = ?
speaker impedance = ?
speaker sensitivity = ?
digital volume control = ? (application/DAC)
analog volume control = ?

Know your numbers!

We then can continue discussion on a real world scenario.
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Old 29th June 2019, 03:19 PM   #14
phofman is online now phofman  Czech Republic
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Quote:
Originally Posted by soundcheck View Post
With todays 32bit digital VCs quite a huge range 8bit=48dB can easily get attenuated. Within this range digital beats all analog VCs.
Even the best DACs have 22bit max real resolution. No volume-control headroom for 24bit signal, small headroom for 16bit signal. Every digital attenutation drops the S/N ratio since the noise is constant while the signal drops. If the volume control is at least 22 bits (the DAC real resolution), extra less-significant bits are irrelevant. 32bit volume control is easier to implement and use technically, therefore it is used.


Quote:
And the noise floor logic as outlined by phofman becomes irrelevant.

However. The actual type does matter. Analog VCs perform much worse within this range. Analog VCs do have several issues.

...

Running 2V out - no attenuation (as outlined by phofman) is not always possible. Especially if you run direct DAC -> amp connections.
That is why I talked about passive attenuator in between. Every decent potentiometer drops both the signal and the DAC noise. Just measure it with any reasonable soundcard.

Is the difference between digital and analog volume control audible? Very unlikely. Is it measurable? You bet it.
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Old 29th June 2019, 07:44 PM   #15
aurel32 is offline aurel32  France
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Quote:
Originally Posted by phofman View Post
Even the best DACs have 22bit max real resolution.
I fully agree with that and there are very very few DAC actually able to reach 22 bits.

Quote:
Originally Posted by phofman View Post
No volume-control headroom for 24bit signal, small headroom for 16bit signal. Every digital attenutation drops the S/N ratio since the noise is constant while the signal drops.
I fully agree.

Quote:
Originally Posted by phofman View Post
If the volume control is at least 22 bits (the DAC real resolution), extra less-significant bits are irrelevant. 32bit volume control is easier to implement and use technically, therefore it is used.
Agreed.

Quote:
Originally Posted by phofman View Post
That is why I talked about passive attenuator in between. Every decent potentiometer drops both the signal and the DAC noise. Just measure it with any reasonable soundcard.
I also agree. However the output of the potentiometer is almost never used directly, it is usually followed by an amplifier (e.g. a follower). This amplifier adds noise, and this noise is usually of same the magniture as the DAC noise. In practice this also limits the effective number of bits of the whole system to around 22 bits, even considering a perfect 32-bit DAC. It's possible to get a few tenth of bits by using a much higher output swing out of the DAC, and get a |gain| < 1 at the amplifier level.

Quote:
Originally Posted by phofman View Post
Is the difference between digital and analog volume control audible? Very unlikely. Is it measurable? You bet it.
When using a DAC + potentiometer + op amp, the difference is usually quite small, even at low volumes.

Let's take for example a PCM1792/94A with a 2Vrms output, able to get a typical noise floor of 0.9µVrms in the audio band.

With a digital volume control, the noise floor is kept at 0.9µVrms for 0dB, -20 dB or -100dB.

With a potentiometer, the noise at the output of the potentiometer is reduced by the volume attenuation, so it is 90nVrms at -20dB and 9pVrms at -100dB. Now after a low noise amplifier like the OPA627 configured as a follower, the noise at the output of the whole system is 0.995µVrms at 0dB, 0.424µVRMS at -20dB and also 0.424µVrms at -100dB.

In short there is a small advantage in using an analog volume control, but those are really simple computations assuming a perfect potentiometer and only the input voltage noise. It's therefore doable to get a lower noise with an analog volume control, but it's not something automatic and requires a careful design. Using an op amp with a slightly higher noise, or a PGA2311 like in the example of my previous post will just make the analog volume control worse than a digital one at the DAC level.
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