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SMSL VMV A2 Review (Stereo Amplifier & DAC)

gvl

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I'm trying to make sense out of the highlighted statement, maybe it meant that the digital input bypasses some form of ADC, means there's no analog to digital conversion that is happening inside the process of generating the PWM.

That's my guess as well. One would think they should be resourceful enough to hire a qualified English technical writer to avoid this type of Google-translate nonsense.
 

tjf

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Francis Vaughan

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The digital chain is going to be unbroken right through the system. There is no discrete DAC*. There is a 16 core XMOS processor that does all the USB, S/PDIF and DSP, including sample rate conversion duties, EQ and volume control. The webpage is a bit odd, it implies DSD is directly converted to audio, but this is a trifle difficult. The AX5689 only sucks on PCM over I2S. It is hard to imagine that the DSD conversion is not performed in the XMOS processor. The ADC, one assumes, feeds into the XMOS processor as well.

The overall configuration of both the SA400 and VMW A2 are a bit odd. The SA400 does not present any digital inputs, and uses (we assume) a couple of the spare ADC*s on the AX5689 to provide analog input, and they add specialised analog tone and volume control chips.
The VMW2 could easily have forgone the analog input altogether and satisfied 99% of users. The only sensible use case is for intrinsically analog sources - so tape or phono. The obvious configuration is a digital only device, one that basically packages up the digital inputs of the AX5689 in a useful manner. Somehow it seems that their marketing advice is that they need to produce products that are just a tiny bit weird. The additional BOM for the analog inputs here probably adds $50 to the RRP. Personally I can't see ever needing or wanting them.

The performance differences between the SA400 and the VMW A2 remains a puzzle. All indications are that the power amp is identical. In both cases it seems the analog inputs have been messed up. Given that is all the SA400 has, it seems quite a shame, and makes the SA400 a product without a reason. There could and should be a middle ground product, similar price to the SA400, digital inputs only.

*The subwoofer output is another matter. This could be obtained in a whole range of ways. There could be a DAC and the XMOS processor creates the filtered signal, all the way through to a bit of analog filtering sucking directly on the speaker outputs. I fear it is the latter. Clearly we are on the cusp of the near perfect building block, but somehow nobody is prepared to make the last step. The plate amps from Hypex with internal DSP are achingly close as well, but not available as a turnkey consumer product.

*ETA, I originally wrote DAC, which is clearly wrong, the AX5689 doesn't contain any discrete DACs, but does have spare ADCs.
 
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Francis Vaughan

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Looking for an alternative ?

Also using Axign: https://truebluebox.com/

Interesting. Very much a boutique operation. Amplifiers are custom made to order. Pricey compared to the SMSL, a bit hard to compare as their prices almost certainly include VAT, but even then higher. All amps include phono. They look a bit hobbybox, but the CNC cut wood panels are cute.

What is really interesting is that they sell the amplifier as a module. No price or datasheet, but looks interesting. But difficult for them to compete with Hypex's dedicated Malaysian operation. You are on your own for a power supply.
 
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amirm

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The digital chain is going to be unbroken right through the system. There is no discrete DAC*. There is a 16 core XMOS processor that does all the USB, S/PDIF and DSP, including sample rate conversion duties, EQ and volume control.
I am not seeing SMSL having the signal processing background to write a multi-threaded resampler on Xmos processor. They are much more likely to glue a DAC with analog out, and then analog into the amplification than anything else.
 

tjf

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Are we sure that the Axign AX5689 is at the heart of the SMSL VMV A2?

There's also this family of PCM input to PWM output devices:

https://www.ti.com/product/TAS5558


Is the only evidence for the use of the AX5689 chip this image from the A2 web page??
1625192253477.png
1625192253477.png
 

SylphAudio

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Are we sure that the Axign AX5689 is at the heart of the SMSL VMV A2?

There's also this family of PCM input to PWM output devices:

https://www.ti.com/product/TAS5558


Is the only evidence for the use of the AX5689 chip this image from the A2 web page?? View attachment 138560View attachment 138560

Only AX5689 digitizes the post filter feedback on class-D, the evidence is in the performance. Flat freq. response, THD+N touching -100dB at 10W 4ohms, power vs distortion graph. SMSL folks are hiding this chip for unknown reason and only market it as STM chip which is only equivalent to a mosfet driver + halfbridge mosfets, it's like marketing a class-d with a GAN power stage.

ax3.PNG

ax1.PNG
 

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Francis Vaughan

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I am not seeing SMSL having the signal processing background to write a multi-threaded resampler on Xmos processor. They are much more likely to glue a DAC with analog out, and then analog into the amplification than anything else.
XMOS provide an open source DSP library with all these capabilities for xCore.
https://github.com/xmos/lib_src

Indeed, they provide pretty much a turnkey system, including RTOS, all open source.
 
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Rottmannash

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May you suggest a NCore stereo Amp + an external Dac that would set you back only 600$?
The Buckeye 2 ch 252 with a Topping D10S. Amp $499 + D10S $109 on Apos or for $149 the vaunted E30.
 

SylphAudio

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The overall configuration of both the SA400 and VMW A2 are a bit odd. The SA400 does not present any digital inputs, and uses (we assume) a couple of the spare DACs on the AX5689 to provide analog input, and they add specialised analog tone and volume control chips.
The VMW2 could easily have forgone the analog input altogether and satisfied 99% of users. The only sensible use case is for intrinsically analog sources - so tape or phono. The obvious configuration is a digital only device, one that basically packages up the digital inputs of the AX5689 in a useful manner. Somehow it seems that their marketing advice is that they need to produce products that are just a tiny bit weird. The additional BOM for the analog inputs here probably adds $50 to the RRP. Personally I can't see ever needing or wanting them.

The performance differences between the SA400 and the VMW A2 remains a puzzle. All indications are that the power amp is identical. In both cases it seems the analog inputs have been messed up. Given that is all the SA400 has, it seems quite a shame, and makes the SA400 a product without a reason. There could and should be a middle ground product, similar price to the SA400, digital inputs only.

Looks like SA400 is using the built-in ADC of AX5689 for the analog inputs, hence the better performance measurements.
According to product brief, paralleling the ADC inputs offers better performance, for the A2 they might be using all 8 ADC inputs for PFFB to get the best performance, since they've used a separate burbrown ADC for the analog input.

1625194388177.png
 
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Francis Vaughan

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Looks like SA400 is using the built-in ADC of AX5689 for the analog inputs, hence the better performance measurements.
According to product brief, paralleling the ADC inputs offers better performance, they might be using all 8 ADC inputs for PFFB to get the best performance,
I agree. Paralleling the ADCs seems a useful trick overall. In the SA400 they could have used 6 of the 8, and used the remaining 2 for input, or a 4/4 split. Whatever happened, it hasn't worked out as well as it might.

ETA, ah, I notice I mistyped ADC as DAC in the post you quoted, that probably makes thing a bit confusing.
 
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restorer-john

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How do these post filter, ADC'd feedback amplifiers cope with highly reactive speakers and cable disconnections/shorts at elevated power levels? I'd really like to see some typical accident scenarios being tested on amplifiers.
 

JohnYang1997

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How do these post filter, ADC'd feedback amplifiers cope with highly reactive speakers and cable disconnections/shorts at elevated power levels? I'd really like to see some typical accident scenarios being tested on amplifiers.
Output filter itself is already highly reactivative, essentially shorting out the load at high frequency.
The ST power stage has built in protection, so shouldn't have any issue.
 

SylphAudio

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Processing is purely digital, it might just be interpreted by the system as lost bits. It has the control of the power stage so it can shut it down anytime.
Would like to see the inside of this A2 amplifier to see the chip haha.

I think there will be no low level analog signal in the future. They will eliminate the DAC, it will be all ADC + class-d.
Well, the risk is more to have the modulator going nuts with unpredictable oscillation frequency.
 

boXem

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Processing is purely digital, it might just be interpreted by the system as lost bits. It has the control of the power stage so it can shut it down anytime.
Would like to see the inside of this A2 amplifier to see the chip haha.

I think there will be no low level analog signal in the future. They will eliminate the DAC, it will be all ADC + class-d.
If there is a watchdog system in the chip, yes. But I would not take this as a given, some additional testing as suggested by @restorer-john might be interesting.
It would anyhow be a sign of safety, which is good, but not of stability.
Having ADCs with both high speed and high accuracy is not an easy task. For the moment analog loops still have the lead for high quality class D amplification.
 
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amirm

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XMOS provide an open source DSP library with all these capabilities for xCore.
https://github.com/xmos/lib_src
What is there is not capable of the feature set of VMV A2:

"Conversion between 44.1, 48, 88.2, 96, 176.4 and 192KHz input and output sample rates"

VMV A2 goes to 768 kHz sampling, and DSD. CPU rate scales with sample rate so you need massively more optimization or computation power than the stock library.
 
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restorer-john

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This Class D is digital. The only thing analogue about it is the signal at the speaker terminal...
 
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