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Review and Measurements of Matrix Audio Element X DAC/Streamer/Amp

JohnYang1997

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If they're using digital attenuation (say, built into the DAC chip) to drive an analog stage with a fixed gain-feedback loop, wouldn't that make analog noise at the output roughly constant at all volume levels? If so, perhaps digital volume controls aren't the way to go if you want the best possible SNR at the low to moderate levels people actually use when listening. You're making the analog stage pass a small input signal, and it might be cleaner to give it a big signal then squash it with less than unity gain.

For example: assume we want 50 mV of analog output from the DAC chip whose full scale output is 2 Vrms, and offers built-in digital attenuation. What would be cleaner:

A) Use the DAC's built-in digital attenuation, so the analog output of the DAC is 50 mV, to drive an analog stage having unity gain.

B) Use the DAC's full scale analog output, to drive an analog stage having less than unity gain (1:0.025 or -32 dB).

C) Use the DAC's full scale analog output, to drive an analog stage having unity gain, followed by a volume pot attenuating by -32 dB.

D) something else?

PS: my question is in italics above. Grammatically it's a statement, but presented as a question.
Like I said combination of digital and analog attenuation. Preserve snr and doesn't get to complex like hpa4.
 

Julf

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Can you be more specific? What combination of digital & analog attenuation best preserves the SNR at low analog output levels?
Adjust the analog attenuation to give a suitably loud volume with digital set to no attenuation, then do volume adjustment with the digital attenuation.
 

MRC01

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That sounds like option (A) from my prior post, assuming that "suitable attenuation" is unity gain. It's a reasonable assumption since the analog ouputs on a DAC chip are typically around 2 Vrms. In this case, when you digitally attenuate within the DAC to 50 mV, you're passing a small signal (attenuated by -32 dB) through an analog stage at unity gain. If most of the noise & distortion comes from the analog path (not the DAC), then you expect 32 dB worse SNR when you shrink the original signal by 32 dB. Essentially, you shrink the signal while noise level remains constant.

Ideally, when you attenuate the signal you want to attenuate the noise too. Since some of the noise comes from the amount of gain in the analog stage (lower gain = lower noise), why not take the full-scale DAC signal (say 2 Vrms) and pass it to an analog stage with less than unity gain -- that attenuates it by -32 dB (gain of 1/40 or 0.025). Now you have the same 50 mV output, but your analog stage has attenuated a full scale signal, so noise & distortion should be lower.

Would that work?
 

Julf

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That sounds like option (A) from my prior post, assuming that "suitable attenuation" is unity gain.
Technically unity gain is no attenuation.

It's a reasonable assumption since the analog ouputs on a DAC chip are typically around 2 Vrms. In this case, when you digitally attenuate within the DAC to 50 mV, you're passing a small signal (attenuated by -32 dB) through an analog stage at unity gain. If most of the noise & distortion comes from the analog path (not the DAC), then you expect 32 dB worse SNR when you shrink the original signal by 32 dB. Essentially, you shrink the signal while noise level remains constant.

Ideally, when you attenuate the signal you want to attenuate the noise too. Since some of the noise comes from the amount of gain in the analog stage (lower gain = lower noise), why not take the full-scale DAC signal (say 2 Vrms) and pass it to an analog stage with less than unity gain -- that attenuates it by -32 dB (gain of 1/40 or 0.025). Now you have the same 50 mV output, but your analog stage has attenuated a full scale signal, so noise & distortion should be lower.
Let me try again. You want to use the whole digital range when playing at full volume - so adjust analog volume to that. Doesn't matter if it is unity or not. What matters is that at a digital full scale signal, the analog volume is adjusted to give the loudest level you might need. That optimizes the gain structure and signal to noise ratio - and then you can use the digital volume control to adjust to your current situation.
 

JohnYang1997

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That sounds like option (A) from my prior post, assuming that "suitable attenuation" is unity gain. It's a reasonable assumption since the analog ouputs on a DAC chip are typically around 2 Vrms. In this case, when you digitally attenuate within the DAC to 50 mV, you're passing a small signal (attenuated by -32 dB) through an analog stage at unity gain. If most of the noise & distortion comes from the analog path (not the DAC), then you expect 32 dB worse SNR when you shrink the original signal by 32 dB. Essentially, you shrink the signal while noise level remains constant.

Ideally, when you attenuate the signal you want to attenuate the noise too. Since some of the noise comes from the amount of gain in the analog stage (lower gain = lower noise), why not take the full-scale DAC signal (say 2 Vrms) and pass it to an analog stage with less than unity gain -- that attenuates it by -32 dB (gain of 1/40 or 0.025). Now you have the same 50 mV output, but your analog stage has attenuated a full scale signal, so noise & distortion should be lower.

Would that work?
If there is extra output buffer, we need to assume that the output buffer has less noise than the dac. So attenuate the signal and the noise the same time using step attenuator will be able to preserve the SNR. Opa1612 is easily 0.3uV noise. It will be better than input noise of most devices.

If you use opamp as attenuation then the least noise is 1 times the noise of the opamp itself which works similarly but a bit higher noise in every volume setting.

The way to preserve the SNR is to track the digital volume. Lets say we have a step for every 6db of attenuation. So at -6db it switches from digital attenuation to step attenuator and set back the digital volume to 0db. 6 steps will give roughly 36db of extra dynamic range.

However when the dac has noise of lets say 0.6uV, it doesn't really matter in the entire system for the most part as the next component in the signal chain will likely have higher input referred noise. Wantting higher SNR is not really necessary. It can be done to just as stated above.
 

Julf

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However when the dac has noise of lets say 0.6uV, it doesn't really matter in the entire system for the most part as the next component in the signal chain will likely have higher input referred noise. Wantting higher SNR is not really necessary. It can be done to just as stated above.
And again, as a reminder, if you are using a 24 bit DAC (with 20 or 21 effective bits), you have 24-48 dB of digital attenuation before it starts affecting the SNR.
 

JohnYang1997

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And again, as a reminder, if you are using a 24 bit DAC (with 20 or 21 effective bits), you have 24-48 dB of digital attenuation before it starts affecting the SNR.
That's not true. The noise doesn't decrease with the signal. One db of decrease in digital level will result in -1db of snr. The output noise is mostly independent from the digital input level.
 

Julf

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That's not true. The noise doesn't decrease with the signal. One db of decrease in digital level will result in -1db of snr. The output noise is mostly independent from the digital input level.
My point is that the analog noise floor is somewhere between -90 and -126 dBFS. 24 bits gives you 144 dB. Thus you have at least 18 dB, more likely up to 54 dB attenuation range before you get above that noise floor.
 

JohnYang1997

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My point is that the analog noise floor is somewhere between -90 and -126 dBFS. 24 bits gives you 144 dB. Thus you have at least 18 dB, more likely up to 54 dB attenuation range before you get above that noise floor.
Let's say 0dBFS is 2V and noise is 2uV. So you have 120dB of SNR. When decrease the level to -20dBFS to 200mV. Now you only have 100dB of SNR. The noise doesn't decrease with the signal. Only way to preserve the SNR is in analog. Stepped attenuator, Potentiometer, inverting amplifier etc are possible to preserve most of the SNR.
 

MRC01

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...The noise doesn't decrease with the signal. Only way to preserve the SNR is in analog. Stepped attenuator, Potentiometer, inverting amplifier etc are possible to preserve most of the SNR.
That's what I thought. When we see the SNR drop by 32 dB going from 2 Vrms to 50 mV, we're seeing the SNR drop by the same amount the signal dropped. So noise is constant, and we're probably seeing the effect of digital attenuation in the DAC chip upstream of the analog stage.

By "inverted amplifier" are you referring to the fact you can't get less than unity gain with a non-inverting op amp? So inverting amplifier means an analog "gain" stage with less than unity gain. I would guess the following from cleanest to least clean: (1) inverting amplifier (less than unity gain), (2) stepped attenuator (unity gain, then attenuated with metal film resistor voltage divider), (3) potentiometer.
 

JohnYang1997

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That's what I thought. When we see the SNR drop by 32 dB going from 2 Vrms to 50 mV, we're seeing the SNR drop by the same amount the signal dropped. So noise is constant, and we're probably seeing the effect of digital attenuation in the DAC chip upstream of the analog stage.

By "inverted amplifier" are you referring to the fact you can't get less than unity gain with a non-inverting op amp? So inverting amplifier means an analog "gain" stage with less than unity gain. I would guess the following from cleanest to least clean: (1) inverting amplifier (less than unity gain), (2) stepped attenuator (unity gain, then attenuated with metal film resistor voltage divider), (3) potentiometer.
The second one you need an extra buffer AFTER the attenuation. Because you don't want the output impedance to be super high, which can cause frequency response change or distortion or unwanted attenuation when interfacing the cable and the next stage.
The third is equivalent to the second one.
The first one inverts the signal which isn't necessarily desirable. And using the same parts, inverting amplifier always has higher noise than non-inverting for the same amount of gain (extra 1time of noise gain).
 

MRC01

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...
The third is equivalent to the second one.
...
I don't think so, as metal film resistors are quieter than pots.

using the same parts, inverting amplifier always has higher noise than non-inverting for the same amount of gain (extra 1time of noise gain).
Even so, the gain is so much lower (-32 dB) it might be quieter overall.
 

JohnYang1997

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I don't think so, as metal film resistors are quieter than pots.
No they aren't. They are the same if the resistance is the same. You can use 1kohm pot and it will give very good noise performance (equivalent of 250ohm noise at maximum). Less than 0.3uV for 1kohm pot.
And I was comparing to with stepped attenuator and potentiometer with non inverting buffer.
 

MRC01

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To restate the question: suppose you want to build a state of the art cost no object DAC with variable analog output level, optimized for highest possible SNR at the low to moderate levels used to drive headphones (say 50 mV). You're using a typical modern DAC chip so you can use its built-in digital attenuation, or use its full scale output and attenuate it downstream.

How would you design it?
 

JohnYang1997

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To restate the question: suppose you want to build a state of the art cost no object DAC with variable analog output level, optimized for highest possible SNR at the low to moderate levels used to drive headphones (say 50 mV). You're using a typical modern DAC chip so you can use its built-in digital attenuation, or use its full scale output and attenuate it downstream.

How would you design it?
That depends on what I want. Personally I just won't allow digital volume control for simplicity reason. And add high performance preamp in the next stage.

And what I love to see are, if possible like the one Benchmark did, stepped attenuator resistor array. High performance, fine tune attenuation. Low noise.
Also more practically, combination of digital volume control and simple voltage divider is pretty good. This probably has SNR fluctuating within 0db to -6db range. This is also very good.
 

MRC01

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...metal film resistors are quieter than pots. ...
No they aren't. They are the same if the resistance is the same. ...
If we consider the thermal noise of a resistor, sure it depends only on the resistance and temperature. But that is the theoretical minimum noise. In the real world most resistors are noisier than this, depending on their type. Wire wound are closest (lowest noise), followed by metal film, then other types like carbon. Most volume pots use carbon and are noisier than metal film. But your response made me look into this further and I discovered "conductive plastic", a type of resistor that I had not heard of before. Apparently high quality pots are using these because they're quieter than carbon. Are they as quiet as metal film or wire wound?

...
And what I love to see are, if possible like the one Benchmark did, stepped attenuator resistor array. High performance, fine tune attenuation. Low noise.
...
I agree this seems the best approach. But if you think metal film resistors don't have lower noise than a pot, what is the benefit of a stepped attenuator?
 

w1000i

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When I look back and compare these measurement to Chord Qutest, the 3rd,4th, etc.. harmonic distortion is way lower with Qutest.
I'm I reading that correct ?!

Qutest :




Matrix

 

Veri

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When I look back and compare these measurement to Chord Qutest, the 3rd,4th, etc.. harmonic distortion is way lower with Qutest.
I'm I reading that correct ?!

Qutest :




Matrix

As far as harmonics go: yes, but the qutest has no proper balanced output so you end up with comparatively much more noise, even in single ended the matrix still does better in regards to noise.

Those harmonics mean nothing if they get buried in noise in practical real-world situations, the SINAD simply means that distortion+noise wise the matrix does better objectively, harmonic distortion or not.
 
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