Wednesday, January 26, 2011

Supertweeter integration and buyer's remorse

Listening to a very hot (in many ways) brass band recording called "Hot House" by Arturo Sandeval, it seemed like my Elac 4pi supertweeters were sounding metallic.  Of course, the recording itself is very brassy, and very bright, so perhaps it's not surprising that the supertweeters are adding the brassy sound that the Acoustats are unable to reproduce.  Listening to the supertweeter(s) (or just one) by itself, it was like pure brassiness.  Way over the top.

This has begun to bug me a bit.  My current supertweeter integration, with much higher supertweeter level and lower crossover point than I used last year, still leaves something to be desired.

One sacrifice I have already decided to make.  I might have liked to sit closer to the speakers for a wider stereo image, but can't because the supertweeters begin to become prominently noticeable as a sound source.*  So then I found a new compromise position 10 inches back, about 6 feet from the speakers, which is much better on that score (and six feet back from the speakers is probably a good idea anyway).   But still not perfect.  I still do feel I can hear the supertweeters as a separate source on some music.  Sometimes what I seem to hear is that the acoustats have a bump on the side where the supertweeter is, and it is this whole oddly shaped conglomeration that is the speaker.  I suppose ideally I wouldn't hear any speaker at all, and some recordings can pull off that trick.

(*The correct solution for wider stereo image would not be to try to sit closer than 6 feet from speakers.  That is not good for most speakers anyway.  The correct solution would be to move the speakers more than 5.5 feet apart.  Unfortunately, room geometry and multi-purposing does not permit that.  The speakers are already as wide apart as I can practically get them, except possibly my moving closer to the wall, but then that wouldn't increase the stereo angle either.)

I also heard some brassiness on Computer World by Kraaftwork, though it varied from track to track.  I believe this recording is 100% synthesized, shouldn't sound very brassy.

For what it's worth, I've come up with a new compromise cutoff point for the supertweeter, 15.5kHz.  The old "magic" 14.2 is just way over the top on a brassy recording.  The refined (and accurate by some measurements) 16.5kHz just doesn't have enough supertweeter magic.  I previously argued that 14.8kHz was the top of the magic range, but have decided that 15.5kHz has just enough trickery to still be fun.

Interesting that lots of supertweeter users never even bother to set the supertweeter level.  For many supertweeters, you simply set the cutoff frequency to make the supertweeter unobtrusive.  Supertweeters are generally designed to be more efficient than most loudspeakers, so following the cutoff-only adjustment will lead to excess output,  but the heightened response above hearing range is described as a "feature" making up for hearing loss.

In my case, I'm following the apparent guidance of two ancient uncalibrated Class A SPL meters, and setting the supertweeter level so that the range 18-24kHz is, as best I can measure, essentially the same as the level at 1kHz.  It's possible my meters are so old they are not showing as much high frequency output as they should, so actually I am adjusting the level far higher than it should be (which is what my iPhone RTA app says).

But, for now I'm sticking with the +15dB level setting on the Behringer crossover, attenuated with a 6dB attenuator at the input of the Acurus amplifier.  That seems to be the correct level, and the one that can provide the magic supertweeter effect (including, for example, tighter bass).

I'm thinking perhaps the sound could be improved with a better amplifier on the Elacs, like a Class A transistor or tube amp.  Or perhaps just re-biasing the current Acurus A250 amplifier.  Old Acurus amplifiers are notorious for developing a low bias condition.  The bias was already set pretty low (the amp runs cold at idle) compared with sister upline Aragon amplifiers.  The Aragon amps differed partly by having about 2-3x the amount of bias.  So if the already low bias of the Acurus gets lowered even more through part ageing, you start to get noticeable crossover notch distortion.  Such distortion is especially noticeable around the smallest voltage swings, close to 0 volts output, exactly what is required to drive a supertweeter.  So I mean to put this on my to-do list for real-soon-now: rebias the Acurus amp, possibly to slightly higher than factory bias.  I could also try one of my other spare amplifiers; the Marantz 15 and Parasound HCA-1000A are good possibilities.  Unfortunately, my McIntosh MC225, a 25 watt per channel tube amp with high bias Class AB operation and unity coupled output, needs to be refurbished, but it might be a great amp for this amplification.

One thing I barely think of at all, and that is putting lowpass crossover on the Acoustats above 14kHz or even lower, to help suppress their natural resonance around 13.8kHz.  I just don't want to do it, but it might actually help.  Despite protestation to the contrary, I am still relying on TacT RCS system to do the final frequency balancing, which can't really be done entirely with crossover hacking anyway.

I believe it was Harry Pearson (famous Founder of The Absolute Sound) who said the Acoustats pull off a trick.  They make up for lack of extreme highs (above 14kHz) by having somewhat elevated response below that in the 10-13khz range.  I am trying to undo this compromise with a supertweeter (something Harry, IIRC, suggested that the Acoustat designer James Strickland should look into) giving me fully extended response.  It follows that, at some point I need to remove the 10-14kHz elevation.

A curious thing about the Elacs is that it does not seem to radiate most sound from the center of any face. It seems to radiate from the edges.  This may result from the unique kind of sawtooth aluminum ribbon that Elac uses.  It almost seems that it might operate like a Heil tweeter, but all the information I have suggests that it is a pure aluminum ribbon operating in a circular magnetic field.  The sawtooth construction allows the ribbon to expand in a linear fashion, which is a great idea.  But it may also cause curious radiation patterns, even potential resonances.  Put right next to another speaker, it extends the radiating pattern farther to the side than you might predict.

I might get some benefit putting a damping foam wedge between the Elac tweeters and the Acoustat sides which are only a few inches apart.  Strangely, I do not seem to hear anything like diffraction around that edge.  I can hear the supertweeter perfectly well on the edge across the front of the Acoustat grille...incredible!  But when I put my hand, curved, in between the speakers I though it took away some of the metallic sound.

Finally, one thing which has really begun to spark my curiosity comes from measurements I made a few days ago.  Achieving what looked like a 14kHz acoustic crossover on the Elac supertweeters required
a 16.5kHz crossover setting on the Behringer 2496 DCX crossover.  This was using 48dB Linkwitz-Riley crossover setting (LR48).  Given that the response of the Elacs is quite flat from 8kHz to 35kHz (though I'm having trouble finding a graph right now online), setting the crossover that high should not have been necessary.

That makes me wonder a lot if the built-in crossover of the supertweters, which is something like 12dB/octave at 8kHz, is actually detuning the LR48 crossover somehow, and resulting in less than optimal highpass performance at 14kHz.

I keep thinking about this, and on the one hand it seems impossible and other the other hand quite plausible.  And if, indeed, the steep electronic crossover is being made less steep because of a passive network attached to the speaker, what should be done about it?  (Removing the passive network inside the Elacs would probably not be a good idea for safety protects the driver from damage which is quite possible inside my madhouse).


  1. Dear Charles,

    I don't know any possible way that the effect of a passive 12db crossover and a 48db crossover could possibly not add up to even more than either one by itself. Is it possible that the metallic sound you're hearing is due to inverted polarity of either the tweeter or the CD?

    As you taught me over 28 years ago (and I'll always be very grateful for that as it got me into the mathematics of filters), that every 6db of filter rolloff results in 45 degrees of phase shift. Therefore, the passive 12db crossover has a total of 180 degrees of phase shift between its lowpass and highpass sides and requires the drivers on its lowpass and highpass sided to be connected in opposite electrical polarities, and if it's a Linkwitz-Riley crossover the drivers will be 6db down at the crossover. The 48db Linkwitz-Riley will have a total of 720 degrees of phase shift at its crossover point and will therefore its highpass and lowpass will be in phase and the drivers on the lowpass and highpass side should be connected in phase and 6db attenuated at the crossover point. Therefore if the passive and active crossovers were at the same frequency you still have to opposite relative polarity of the low because they'd have a total of 900 degrees of phase shift. But that's sure results in a huge amount of group delay and could have an adverse impact on transient response. As you also pointed out that the definition of a Linkwitz-Riley crossover is the cascade of two equal Butterworth filters, thus the cascade of the two active and passive Linkwitz-Riley (if the passive crossover is a Linkwitz-Riley) isn't a Linkwitz-Riley crossover and would be attenuated by 12db at the crossover if they were both crossing over at the same frequency but they're not according to you. It sounds like a tremendous mess to me. I'd recommend using either the passive or the active and reversing the phase of the tweeter if you only use the passive and not reversing the phase if you use only the active. But if it was up to me, since I'm partial to having all the drivers connected in the same polarity and all the drivers connected directly to the amplifier, so I'd recommend that you use only the active crossover. However, you told me that your active crossover may have some audible distortion, so perhaps your super tweeter is bringing that out to an unacceptable degree.

    I'm interested on how you finally work it out.

    Best regards,


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  3. For Immediate Release
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    ENIGMAcoustics Inc.
    +1.949.340.7590 office
    +1.626.780.9236 mobile

    September 18, 2013

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