As the reviews of Topping's AKM based E70 Velvet start to flood the internet, I have seen a few errors here and there. One of the more well known boards, which I will not name, gets something that is important very, very wrong. At the very least, I can set the record straight for my readers.
I thought about naming names, but, I decided intrepid readers can figure it out.
(By the way, if you are one of my readers, I express great gratitude to you. I never thought anyone would really read this new audio blog/review site, but the many thousands of you that have in just a few short months, well, you have overwhelmed me with gratitude for your time and attention.)
The incorrect information comes from a reseller product representative, not so much from the author. The salesman claims by using fixed output as opposed to variable output, the Topping E70V will use bypass mode for Native DSD.
This is incorrect. The E70V never uses the DSD bypass mode, set in the logic as DSDD= '1'. The E70V always, regardless of mode or volume, is set as DSDD='0'. See the diagram below for visual clarification.
There is only ONE DSD filter available in the E70V, and it has a 19khz fc at DSD64. This filter feeds the volume control DSP as a multibit 'intermediary' and is converted to lower bit Delta Sigma via the modulator.
The second DSD filter available with the AKM chipset is a 39khz fc filter at DSD64, and is only available with the 'bypass mode'. None of this is available on the Topping E70V, and this is confirmed by Topping representatives at the source. Not from a sales guy.
Here is the AK4191 diagram that helps spell it out...
Again, there is no alternative filter choice. Whether the Topping E70V is measured in Fixed output or Variable output, there is a single DSD filter with no ability to change it. This option IS available to any DAC that uses the AK4191+AK4499, but Topping does not provide this option in the E70V. Measurements confirm the 19khz fc, with full stopband around 30khz at DSD64. FC (frequency cutoff) will double as the DSD rate doubles. (38khz at DSD128, 76KHZ at DSD256)
It seems some people are confused by my wording and I apologize. To put it as simply as possible, AKM offers two possible DSD paths. A 'direct' path, using DSDD bit "1", that bypasses all Digital Signal Processing and Modulation. The DSD signal is filtered at the output stage with a 'analog' FIR filter, built of shift registers for the delay line and bitswitches for the taps, followed by a analog 'accumulator'.
There is a 'DSP' path that allows for Volume Control, using DSDD bit "0". In this path, the 1-bit DSD signal is first greeted by a digital FIR filter, the output of which strips the majority of the high-frequency noise shaping. The output of any such filter is multi-bit, which allows for the next stage, which is volume control. THEN the multi-bit signal, which has a longer wordlength than can be processed by the output stage, is converted to a lower bit depth at the Delta Sigma Modulator (the same one used for PCM, yes).
Running the Topping E70V in fixed output mode, disabling the volume control, does NOT switch the onboard DSDD logic from bit '0' to bit '1". The Topping E70 never makes use of the direct DSD mode. It is always in logic mode '0', regardless. Fixed output mode simply locks the volume control at 100 percent. The output path of Digital FIR and Delta Sigma Modulator is still followed, emulating what DACs such as ESS do. Why? Because this is more than just a technique to apply volume control to DSD. It can provide for a very precise conversion of DSD, with lower noise levels and taking full advantage of the advanced Delta Sigma output stage. Others, like me, still prefer to keep DSD conversion 'simple'. Keep it at 1 bit all the way until the final FIR filter converts it to analog.
Which is why I prefer DACs made by iFi, which is 1 bit native all the way to the analog FIR output stage, RME, which uses AKM like the Topping E70V, but has the user selectable option to use native DSD Bypass mode. Other DACs off the top of my head that are true native DSD... usually anything made with Burr-Brown DACs, T+A uses either Burr-Brown or their own proprietary output filter for true Native DSD, there seem to be some DIY and Chi-Fi options out there, and I know I am forgetting a bunch.
No ESS DACS don't make that cut. There is no 'bypass' option. All DSD is re-modulated in their Hyperstream converters. Indeed, the sound excellent, but they are NOT Native DSD converters in the strict sense.
In a couple weeks I will be a proud owner of a DSD only DAC built around the Signalyst discrete native DAC-less converter. I cannot wait to give it a spin!!