Keith_W DSP system

[Keith_W]

I have resisted making a system thread on ASR for quite some time for many reasons. (1) ASR members seem to be anti-high end audio. (2) I do some things in my system that many in ASR might deem controversial.

I will start off the thread with a few posts describing the architecture and hardware of the system, then I will post measurements. I have considered in detail every aspect of this system.

The system is a mish-mash of different philosophies for a simple reason - I changed my mind. I am open-minded about learning, and I can and do abandon beliefs if I am persuaded. I might seem stubborn to some of you, but that is because my brain is a little bit slow, but eventually I do come around. The evidence that I can and do change beliefs can be seen in the hardware you see before you.

When I first bought these speakers 15 years ago, I was a subjectivist. I loved the way the top end sounded, but I immediately noticed a bass problem with the speaker. I went through the usual recommendations which left me unsatisfied - high end speaker cable (I still have them), then more powerful valve amps. Then I moved on to some real solutions - first, I bi-amp'ed the speaker (SS amp for the woofers, valve amp for the top). This brought improvement, but not enough. So in came a pair of subwoofers. It was better, but not good enough. Then I bought a DEQX and started my DSP journey. I never obtained satisfactory results - in hindsight this was because I could not operate the thing properly, and did not know how to interpret the measurements. I got rid of it and bought a Marchand passive crossover. That worked better than the DEQX, but I was still not satisfied.

Around 2015, I heard about DSP with Acourate. I saw a post on WhatsBestForum here by "Blizzard" (aka @dallasjustice on ASR). To me it sounded incredibly advanced, and the more I read about it, the more excited I became at the possibilities. @dallasjustice and I exchanged a few PM's, and he rang me from the USA to sell me on the idea of using Acourate. This was truly a life-changing moment for me in this hobby. Gradually I started to shed subjectivist beliefs as I learnt more, could hear the improvements being made to my system, and marvelled how they were REAL improvements with measurable results instead of imagined ones where I wasn't sure of what I was hearing.

What you see in these pictures are remnants of a long abandoned approach to audio, a time when I believed in high end DAC's, amplifiers, cables, and the like. Mixed in with this older equipment are more modern stuff. For example, this fully active system requires 8 interconnects. Two pairs of these interconnects are high end audio cables, which I have owned for years. The rest of the cabling is ultra cheap stuff from pro audio shops.

Hardware Architecture

The speakers are Acapella High Violons. These are delivered as a 3-way speaker with a horn loaded plasma tweeter, a midrange horn, and a conventional woofer. The speaker has been dissected and modified. The woofers that came with the speaker were replaced with a pair of prototypes from SGR Audio in Melbourne, manufactured by Lorantz (also in Melbourne). The passive crossovers have been bypassed. Each driver has its own amplifier channel and DAC channel and individual filter made in Acourate.

Not shown in the diagram is some room treatment. Normally they are stacked behind the TV, but I can deploy them around the room if needed.

Also not shown is the measurement setup - I own two Behringer ECM8000's, an Earthworks M30 mic, and a RME Fireface UC.


Software Architecture

The software setup is pretty simple, the above diagram explains it all.

Although the diagram shows two convolvers, I actually have licenses for 3 - JRiver, Acourate Convolver, and HQPlayer. I have uninstalled HQP from the system because it is CPU intensive and inconvenient to use. I use Acourate Convolver daily because I like the filter bank feature, I can switch between different filters and do A-B comparisons. I can also control it from my tablet, so no need to get off the sofa to change filters! Not shown in the diagram is the VST pipeline. I use a Pultec VST, pKane's PKHarmonic, and uBACCH.

 

[JayGilb]

I'm not anti-high end components, just personally not willing to spend that kind of money.

Looks like a wonderful system and I hope you enjoy listening to it, because that is all that matters.

 

[Keith_W]

Plasma Tweeter

For those who have not seen the inside of a plasma tweeter before, the Acapella TW-1S looks like this:

You can see it has 3 chambers. On the left, the control electronics and oscillator. On the right, the power supply. In the middle, the PL519 valve, the coil, and the combustion chamber. Within the combustion chamber is a little nickel electrode surrounded by quartz. Both electrode and valve are service items and need to be replaced periodically.

The native frequency response of the tweeter is in black. In red was an experimental crossover that I made in Acourate.

The plasma tweeter is massless, so the impulse response is pretty impressive.

 

[bluefuzz]

ASR members seem to be anti-high end audio

Not at all. But I think the majority are against needlessly wasting money on dubious undocumented claims coated in irrelevant bling ...

 

[Mr. Widget]

Personally I think the majority of ASR members are all over the map... thanks for sharing!

I love well behaved horns. Also, thanks for the internal shot of the Acapella tweeter, I have seen and listened to them, but that is the first time I was able to see the guts. Interesting.

 

[Keith_W]

Midrange Horns

The horns are 45cm diameter spherical horns. In theory, the lowest frequency supported by the horn is lambda = 2x diameter, so 90cm wavelength = 381Hz. The upper frequency is 3 octaves above this, so 3000Hz. Below this limit, the horn loses efficiency because the wavelength is not "horn coupled". Above the upper limit, the sound starts to beam.

The horns are built into a separate cabinet which sits on top of the woofer cabinet. Access to the driver is obtained by unscrewing several inconveniently located Allen screws. It then comes off like this:

The enclosure is filled with sand. The driver is protected by a rubber boot. Remove the rubber boot, and we see:

A Dynaudio D-52H driver with Acapella branding.

A friend of mine noticed the construction of the cabinet and said it was very German. Because only Germans would construct a cabinet out of thick plywood, line it with lead, and then think to themselves "let's fill it up with sand".

At the bottom of the cabinet, we see the passive crossover. It was bypassed at this stage:

To reassemble the cabinet, the horns are screwed back into place. There is a small access port at the rear of the cabinet, where sand can be slowly filled with a funnel:

This is a very old measurement of the horns with the native crossover in situ:

And this is the measurement of the horns with the crossover bypassed, with no linearization and no crossover applied:

You can see that the lower limit of the horn (440Hz) is fairly close to our calculated theoretical lower limit of 381Hz.

That impulse response looks pretty good to me.

And this is what the horns look like once I have completed driver linearization.

 

[ZolaIII]

Well you objectively have very good system and response. I use JRiver my self and find it as most comprehensive DSP tool chain there is. I do it with more budget components but results are main thing that counts. You didn't say do you use JRiver Loudness or at least tried it. If you didn't it's simple. You do white noise calibration to 83~84 dB mono, 86~88 dB stereo enable it and dial down the volume trough JRiver to your likings.

 

[Keith_W]

Woofers

I mentioned in my first post that I was not happy with the bass of the speaker. When I eventually obtained a measurement setup, I was able to take a look. And it's not pretty:

This is a very old measurement comparing the nearfield native response of the old woofer (in red) with the new woofer (in green). Not shown is the overall step response (it was horrible).

The old woofer was a SEAS CA 25 ACA. I spoke to SGR Audio (a speaker manufacturer in Melbourne), and they recommended a new woofer. Here it is:

You can see it has a much more substantial construction.

Because the new woofer is deeper than the old woofer, the cabinet had to be modified slightly.

Picture of the new woofer mounted in the baffle. (EDIT) silly me, this is a picture of the NEW NEW woofer with the "NEW" woofer. I forgot to mention, that after I replaced the woofer and used it for a while, I noticed that it was linear from 80Hz up to 200Hz, but not linear up to 800Hz. I wanted it linear between 80Hz - 500Hz. So I spoke to SGR again, and they supplied another prototype. The new woofer is mounted in the baffle, with the older replacement woofer on the right. Look at the thickness of the magnet.

Once again I used Acourate to flatten the phase through the passband (80Hz - 500Hz).

 

[Keith_W]

Subwoofers

I wanted the subwoofers to match the design of the entertainment cabinet. I chose Rythmik Audio DS1204 drivers, with two drivers per sub, mounted in a dual opposed configuration for force cancellation. This was because I wanted to use the space above the subs and I did not want them to vibrate too much.

I drew the subs in Sketchup and sent the drawings to Red Spade Audio (another Melbourne speaker designer, best known for his PSE-144 unity horn). The CAD drawings came back:

Once I had the CAD drawings, I sent them off to a machine shop to have the panels cut out on a CNC machine. Then the panels went to a custom furniture company. My request was for veneer matching for my existing entertainment cabinet. After the cabinets were made, they were wired up and the subs installed. The result:

Having four of these drivers in a medium sized room like mine (6m x 7m x 2.8m) is overkill. The subs are way too powerful and can literally rattle the house. This is good, this means I have plenty of headroom to cut with DSP.

This is the native response of the sub, with no corrections. There is some difficulty measuring the subs because they are almost impossible to take outside given how heavy they are, so I am forced to do in situ measurements. Not ideal.

And this is the nearfield driver linearization:

And again, Acourate to flatten the phase:

 

[Keith_W]

Putting it all together

Once I have completed linearization for all the drivers, I now have a full set of crossover filters with amplitude correction, time alignment, and phase linearization built in:

I then position the microphone at the MLP and do sweeps. All the drivers are mismatched, and I have to adjust the gain on each amplifier channel to make sure they are somewhat close. I deliberately left the subwoofers LOUD because my intention was to cut the volume substantially (sacrifice headroom for linearity).

Once the target curve is applied, it looks like this:

The step response looks great:

So I have now finished explaining the basic system. What is interesting about this system is all the DSP that is in it. I will make more posts later with all the experiments I have tried. Some of these experiments are very stupid, but sometimes you can learn from coming up with stupid ideas.

 

[MaxwellsEq]

Thanks, absolutely fascinating.

 

[Keith_W]

I'm not anti-high end components, just personally not willing to spend that kind of money.

I mentioned earlier that most of these components are remnants of an abandoned audio philosophy. I knew nothing about good speaker design when I bought the speaker, all I knew is that I wanted horns, and I liked exotic technology (like that plasma tweeter). In hindsight, I probably would not have made the same decision. You can see that some parts of the speaker are wildly overengineered (like the cabinets), but some parts are surprisingly poorly engineered (look at the bass response). That's high end audio for you.

By sheer stroke of luck (and no special foresight on my part), this speaker happens to be a great candidate for DSP. Many of the flaws are not irreversibly baked into the hardware, and can be managed. Some problem components can be swapped, like the woofer driver.

I have evolved this system close to what it is ultimately possible to do, within limits of the hardware. There is nothing I can do about the directivity, and nothing I can do about the fact the drivers are so spread out so it is hardly a point source. So I have been considering getting another speaker so that I can go through the entire journey all over again

I love well behaved horns. Also, thanks for the internal shot of the Acapella tweeter, I have seen and listened to them, but that is the first time I was able to see the guts. Interesting.

Thank you There are problems with horn speakers like mine, which I will describe by showing more measurements later.

There are some disadvantages to living with a plasma tweeter. As mentioned, the PL519 valve and the electrode are sacrificial parts and will wear out. When the valve wears out, the tweeters start to make noise - a persistent hiss, intermittent crackles, and sometimes a whistling sound. I last changed my valves 8 years ago, and the tweeters were dead silent for a while. Now they are starting to make noise again. I can get rid of the crackles and whistling by "rebooting" the tweeter, but the only cure is to replace the valves. As for the electrode, the symptom of a worn electrode is a different coloured plasma ball and non-stop crackling. From the listening position, the tweeters "look" at you with a pair of purple coloured eyes. When they turn yellow-purple and crackle, it is time for replacement. When new, the electrodes look like a small nail with a pointed tip. When they wear down, they look like cylindrical rods.

Well you objectively have very good system and response. I use JRiver my self and find it as most comprehensive DSP tool chain there is. I do it with more budget components but results are main thing that counts. You didn't say do you use JRiver Loudness or at least tried it. If you didn't it's simple. You do white noise calibration to 83~84 dB mono, 86~88 dB stereo enable it and dial down the volume trough JRiver to your likings.

Thanks Zolalll. There are too many features of this system to mention, but I am using JRiver's ISO226 volume control. IMO it is one of the killer features of JRiver, and it makes a noticeable difference. I typically listen at very low volumes, usually around 60dB. The system preserves correct tonality, even at these low volumes.

 

[3ll3d00d]

This is the native response of the sub, with no corrections.

Nice set of posts and interesting to see the whole system

The sub response is pretty unusual, how did you measure it exactly? Nearfield?

 

[Robocop]

Keith W you indeed are a rare HiFi follower and becoming rarer by the day not unlike steam train enthusiasts. Great to see what you have done. It is a hobby after all whether you enjoy the equipment or the music or both. I'm half technophobe half music lover and always enjoy probing the old stuff. People talk about Dacs being perfect after measuring beyond our hearing. But we are not close to mimicking live sound with all its variables. Glass of red wine helps in these situations. Robert

 

[Keith_W]

Nice set of posts and interesting to see the whole system

The sub response is pretty unusual, how did you measure it exactly? Nearfield?

The mic was positioned about 5cm away from the subwoofer driver. I know this gives me a nonsense measurement, especially since it is a dual opposed sub. I wondered whether it was even worthwhile doing it, since I would be generating a nonsense correction. The right way to do it would be a ground plane measurement. I reasoned that I will be doing a farfield correction anyway, so hopefully any incorrect driver linearization won't matter so much.

Having said that, there is something subtly wrong with the bass subjectively that I can't quite put my finger on. The subs seem to "bloom", as if they are oscillating beyond the impulse. I still haven't decided if it's really there, or if I am imagining it. Yes, I have MLP SPS and MLP MMM's showing good subwoofer correction. Yes, I have a great looking step response. I wondered whether I was making the sub cover too many octaves, so I recently redid the subs to cross over at 80Hz instead of 50Hz. That didn't help. This is on my "to-do" list of things to ponder.

Keith W you indeed are a rare HiFi follower and becoming rarer by the day not unlike steam train enthusiasts.

I don't think so. These days if you throw a rock in ASR you will likely hit someone who is using DSP.

 

[Purité Audio]

Keith when I had a pair of Lansche 5.1s plasma tweeter they told me the tweeters had to be returned for servicing at not inconsiderable cost every I think it was 6000 hours depending upon environment does Acapella recommend regular servicing, ozone etc.
Keith

 

[Keith_W]

Keith it costs $900 to buy a pair of replacement electrodes and valves from Acapella. I have a friend who is an audio engineer, he replaced my electrodes for me and he showed me how. I should be able to do it, it's only a lot of unscrewing and nothing too technically difficult.

 

[Robocop]

These days if you throw a rock in ASR you will likely hit someone who is using DSP.

Well yes, but have to say I use DSP with the minidsp flex and don't regret a minute.

Interesting you dropped HQ player. I think convenience and integration is overtaking these somewhat complex audio programs. I use Xxhighend which also has high PC needs and somewhat clunky. I still feel Xxhighend/HQ player are the best sounding but!!! Tidal streaming is easy to manage for me.

Using Acourate convolver is another level of dsp and J River integrates nicely. I'm certainly looking at this route.

Robert

 

[ernestcarl]

Have you ever thought of selling your main speakers system to start again? Yes, with everything that you now know...

 

[Mikig]

Great system.
And I thought that you, the main contributors of ASR, had put the whole set away in the closet for some time now, to deal with and worry only about measurements!!!
then I see your system, I saw in a JBL review the one about " director” and I am amazed that you also use tubes, double amplifications and 60 kilogram giants….

Jokes aside, it's nice and inspiring to see science mixed with a bit of the past, with use where it's needed of “normal” parts combined with the judicious use of “High-End” components to achieve good listening!!! A right approach in my opinion!!