fILTERS AND dac CHIPS CUT IN HALF

​Here are the digital filter profiles of the Topping E70 AKM version.

Unfortunately, there is NO choice for the DSD filter on the AKM version. The DSD path is not the 'direct' path; it is the 'processor' path that includes FIR filter, volume control and then re-modulation. The only available cutoff is at 19khz!!!! (At 64fs). This is extremely low for DSD. Honestly, this kind of filtration is very much like one would expect with a PCM decimating filter. There is no decimation here, however. Sample rate stays the same and as expected, the FC doubles with speed. (DSD 128 has 38khz fc, DSD 256 has 76khz fc. I could not verify if DSD 512 follows pattern. I will not assume it continues the pattern, because in some other recent AK chips, higher speed DSD saw a coefficient change in addition to bitstream speed, meaning it is not predictable. A measurement is required)

The DSD frequency cutoffs mentioned in the manual are incorrect; they are correct for the 'old' AKM4499 chip, however, this is not the AK4499. The VELVET E70 uses the very latest and highest end AKM DAC, which is split into two sections, each with their own clock. The digital processing chip is called the AK4191, and the switched resistor DAC that shares the 4499 designation (as opposed to switched capacitor in previous AK chips like the 4493), is where all analog processing is done. This is a VERY expensive chip, and I assume is where the extra cost comes from as compared to the E70 ESS version.


Here are the filter profiles as measured with ARTA software via the RME ADI-2 ADC. (For the most sensitive measurements, such as SINAD, I use the E1DA COSMOS ADC and APU.)

Filter 5, however, is no filter at all. The manual calls it 'super slow' but it is NOS. (non-oversampling)

ALSO, there are some very low level sidebands alongside each test tone in every filter mode. Other than the NOS filter, the presence of sidebands is purely academic, but interesting nonetheless.

A technical look into each PCM filter 1-6 is below. 

Filter 1- Fast Linear Phase
Filter 2- Slow Linear Phase
Filter 3- Fast Minimum Phase
Filter 4- Slow Minimum Phase
Filter 5- Non-Oversampled (NOS) 
Filter 6- AKM Hybrid Filter

Below is the single DSD filter and its response at various speeds.  Normally a doubling of the bitstream speed into the moving average FIR filter means a doubling of the -3db cutoff frequency as well.  Such SEEMS the case here through DSD256.