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AK4490EQ Filter adjustment

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I thought I would start a new topic for this, that way it would be quicker to locate. The Klein AK4490EQ DAC has four different filter settings. These can be changed by jumper combinations on the PCB. To remove the PCB from the case use the following procedure. Turn the DAC upside down & remove the two counter sunk screws near the front. Then remove the four counter sunk screws on the rear panel - nothing else. Grasp the RCA sockets & slide the PCB out backwards with the back panel still connected to the PCB. You will see the two pcb jumpers just to the right of the USB board (back facing you) You will need some fine tweezers as the location is a bit tight. You can change these with the DAC running just make sure you don't short anything out with the tweezers. I would suggest let the DAC run in for a few weeks first then you find which filter setting works best for you. The differences are subtle but noticeable on a good system. I have pre-set them to the AKM default which is "short delay sharp roll-off" This measures the best & sounds good to me but you may find a different setting works better with your system. note the super filter AKM talk about is only accessible when software control is used (not on the Klein)

Filter setting for AK4490EQ

IMPORANT Don't over tighten the rear panel screws when screwing back on.

SD pin SLOW pin

open open short delay slow roll-off

closed open slow roll-off

open closed short delay sharp roll-off

closed closed sharp roll-off

Built-in Digital Filters

(images taken from Ayre’s paper

The built-in digital filters consist of 5 selectable filters. They include all the “popular†filters developed so far by different vendors plus one additional filter with undisclosed response (super slow roll-off). The filters are described as follows:

Minimum delay Slow Roll-off (AKM notation: “short delay slow roll-offâ€): this is a “more modern†type of filter also found in the Wolfson WM8741/8742 DACs. In addition to eliminating pre-ringing, this filter also incorporates slow roll-off and this reduces post ringing as well.

The properties of this filter are similar to the “MP filter†found in Ayres latest CD player.

Linear phase Slow Roll-off (AKM notation: “slow roll-offâ€): this is also a “standard†filter found in all DACs. As in the linear phase sharp roll-off filter, it also generates pre-ringing, but trading lower amounts of pre-ringing with letting more aliased image through (theoretically increasing harmonic distortion).

Minimum delay Sharp Roll-off (AKM notation: “short delay sharp roll-offâ€): this is also called the “minimum phase†or “apodizing†filter that was the rage a few years back. Whereas in the past audio engineers have insisted in phase linearity (meaning all frequencies have equal phase or delay), More recent research have shown that a “minimum phase†filter sacrifices some of the phase linearity (adds some phase distortion) for better time response. This filter removes all the “unnatural†pre-ringing but “dumps†all that energy to post-ringing. Implementation of this filter is also found in the Wolfson WM8741/8742 DACs

Linear phase Sharp Roll-off (AKM notation: “Sharp Roll-offâ€): this is the “standard†sharp roll-off filter found is all DACs. It is also known as the “brickwall†filter. It is said that pre-ringing sounds unnatural.

Super Slow Roll-off: this filter is the differentiating feature (in terms of built-in filters) that this DAC provides. The AKM literature says “super slow roll-off filter with emphasized characteristics†(which really means nothing). There is some information in the marketing page as shown below.

Original link with more info

https://hifiduino.wordpress.com/2014/12/07/akm-verita-4490-dac/

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Edited by Gieseler Audio

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Marc   

Thanks for sharing this info @@Gieseler Audio.

Will get a chance on the weekend to do some comparisons between the different settings hopefully.

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 It is said that pre-ringing sounds unnatural.

 

There is a few DACs out now which have programmable filters (upload your own)... some which can take quite a few coefficients.   Have you been following discussion about these?  Soekris DAC one of them.

 

 

Through the intensive testing of these, I have seen a consensus developing in some circles that the filter ringing is actually a red herring....   and that it is simply the transition band shape (amplitude) of the filter which is causing the main differences in sound.

 

I find this particularly interesting, as it is congruent with a lot of other testing which has occurred over the years where people haven't been able to demonstrate the audibility of these type of time domain errors very well  (when they are isolated to purely time domain errors - ie. linear amplitude) ......   but that the audibility of amplitude changes (even quite small ones) can be extremely audible.    Food for thought.

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ljmac   

My Audiolab M-DAC has equivalents of all of these except for "Minimum delay Slow Roll-off" (which sounds interesting to me). My own comparisons of the filters produced surprising and interesting results.

 

The traditional brickwall Sharp Roll-off and "Linear phase Slow Roll-off" filters are built into the SABRE DAC, while the rest are implemented via DSP.

 

I was expecting "Minimum delay Sharp Roll-off" (Minimum Phase on the M-DAC) to sound the best, but instead it was second worst - quite glary and aggressive sounding, although very sharp and detailed. I suspect this is due to the very severe post-ringing of this filter type (far worse than even a brickwall filter).

 

The worst sounding filter to my ears was the "Super Slow Roll-off" (Optimal Transient on the M-DAC), which just sounds too dull - it starts rolling off in the midrange, so although it's very slow the treble is shelved down.

 

Surprisingly, I found I preferred the traditional Sharp Roll-off to either of these, although I liked the Slow Roll-off even better: it trades a very high frequency roll-off for greater phase accuracy, and sounds "just right" to my ears - everything in its right place.

 

The really interesting thing though was that the built-in filters sounded more like eachother overall than to any of the DSP filters, in spite of their opposite phase characteristics. The DSP filters also sounded more alike overall than they did to the built-ins.

 

I found the type of filter slope largely just effects treble presentation; bass and midrange were both far more effected by how the filters were implemented (i.e. DSP vs. built-in in this case).

Edited by ljmac

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ljmac   

I find this particularly interesting, as it is congruent with a lot of other testing which has occurred over the years where people haven't been able to demonstrate the audibility of these type of time domain errors very well  (when they are isolated to purely time domain errors - ie. linear amplitude) ......   but that the audibility of amplitude changes (even quite small ones) can be extremely audible.    Food for thought.

 

Yep. My own experiments with the M-DAC's filters suggest to me that time domain performance is seriously overrated, and frequency domain performance is far more audible. I think many people who like time domain aligned filters actually just like the treble roll-off (making it sound more "analogue-like").

 

Having said that, I can hear a subtle but worthwhile improvement from better time domain performance, as long as it isn't at the expense of obviously compromised frequency domain performance. Hence my preference for the "Linear phase Slow Roll-off" filter, which provides the best overall balance to my ears (only slightly rolled-off treble but excellent phase linearity).

Edited by ljmac

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Thanks for all the feedback guys. Interesting both of the sharp roll-off setting at 20K show the cleanest output on my scope. the short delay slow roll of seems to bring up the mids & presence a bit. Note this was with 44k source, at 96k & above all filter look perfect at 20k.

Edited by Gieseler Audio

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ljmac   

Thanks for all the feedback guys. Interesting both of the sharp roll-off setting at 20K show the cleanest output on my scope. the short delay slow roll of seems to bring up the mids & presence a bit.

Yeah, that was my initial impression of the Minimum Phase filter too - I liked it at first, but in long term listening I found that mid presence emphasis also accentuated glare, which I found fatiguing. I can imagine this filter might be great if your system is on the laid back side though. It would be interesting to hear if the slow rolloff version (which my M-DAC doesn't have an equivalent of) avoids this problem.

Edited by ljmac

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I thought I would add this from the Klein owners thread so it is in the same place as the info on the filters. From Clay's instruction sheet for use of the filter switches which are fitted to later Kleins ...............
Looking from rear

                                                          Left Switch            Right Switch

Short Delay, Slow Roll Off                       Up                          Up

Slow Roll Off                                         Down                        Up

Short Delay, Sharp Roll Off                    Up                        Down

Sharp Roll Off                                       Down                     Down

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Edited by Ancientflatulence

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