Monday, March 16, 2015

Bespoke Aria Super Tweeter Review

I have a super tweeter in my system that crosses over at roughly 8.7KHz. This super tweeter is made by an England-based company (I believe that this company is now defunct) called Bespoke and the model is the Aria, a leaf tweeter in a round plastic cabinet with switch-selectable first-order network crossover points (12KHz, 16KHz, and 20KHz) and switch-selectable attenuation (0db, -3dB, -6dB). I initially bought them because of their high sensitivity (claimed 98dB/W/m) to match with the rest of my system and I personally love the sound of a good ribbon anyway.

The Bespoke Aria Ribbon Super Tweeter
 When I received them, I tried attaching them directly to my system with mixed results: I could hear the promise that these drivers had but was sadly disappointed by the built-in network. Great idea; poorly implemented; mediocre sound. I suspected that a compromise was made with the type of crossover capacitors used to keep the cost down and I was right. After all, how can any manufacturer sell super tweeters for $300/pair retail including switchable networks and use high quality crossover components? Remember that your system is like the links in a chain where the strength of that chain is determined by its weakest link, here the capacitors used in the super tweeter network.

So it was off to the workbench I trotted with brand new drivers in hand to undo the switching features and connect the driver directly to the terminals. While I was at it, I also replaced the internal wiring with some hand-made star-quad wiring (silver-plated OFHC copper, Teflon insulation) I usually use when doing such work and added a bit more internal sound damping. The next step was to measure the performance of the "native" driver (without the built-in network).

Ribbon or Leaf drivers always require some sort of minimalist crossover network to eliminate LF content which could irreparably damage the internal transformer. And with the addition of a 8.2uF Clarity SA capacitor in the signal path (Fc=2.5KHz) I was ready to make a near-field RTA measurement. Placing the microphone centered and on-axis as close to the driver as I could without it touching it, I made the following pink-noise measurement with the REW v5.1 RTA software and my calibrated microphone.

Bespoke Aria Pink Noise, 8.2uF Capacitor

The peak at about 4KHz initially puzzled me however with some quick calculations it appears to be a major cabinet resonance perhaps combined with a mechanical diaphragm/suspension resonance (1128 feet per second/4000 Hz=0.282 feet=3.3384 inches or 86mm; roughly the internal diameter of this tube). Given this resonance, the usable bandpass should be at least an octave above this 4KHz cabinet resonance point (8KHz). BTW, note that the performance of the microphone used to make this measurement was woefully inadequate since it is well known that this type of driver delivers excellent and very linear response to way over 20KHz (i.e., this test also revealed the microphone's upper-limit).

Ignoring the band above 15KHz (the estimated upper limit of the microphone) I realized that such an unexpected peak would require EITHER a very high crossover point with a normal second-order network or higher to minimize the interaction of this peak with the tweeter's bandpass. I opted for a fourth-order network ( more complex, more intolerant to tolerances, but a 360-degree phase shift).

Now comes the task of deciding what style of network design (Butterworth, Bessel, etc.) to use. Many online tools are available for free use to mathematically calculate the component values for crossover points and I encourage you to use any one of these tools for your own designs. One of my favorites is here. While there are words that describe the effects of each type of network on the resulting amplitude and phase of the sound at or near the crossover point, one must consider how this design impacts "voicing" of your driver (that is, the effects of that type of crossover network design on your driver's physical and electro-mechanical properties). Assuming that any network design will operate satisfactorily is an incorrect assumption (you know what they say about assuming anything, right?).

A brief virtue/compromise description of the four most popular crossover designs are presented below:
  • Bessel - VIRTUE: optimally constant group delay in the bandpass (i.e., flat phase, fast settling time). COMPROMISE: Slower initial rate of attenuation beyond the bandpass
  • Butterworth - VIRTUE: optimally flat frequency response and low ripple in the bandpass (i.e., flat amplitude). COMPROMISE: overshoot and ringing
  • Chebyshev - VIRTUE: steepest roll-off rates. COMPROMISE: adds amplitude peaks/dips and even more ringing than Butterworth
  • Linkwitz-Riley - VIRTUE: Uniform amplitude at crossover point. COMPROMISE: adds group delay in bandpass
As you can see, design selection is not a simple task since each type of design has its own virtues and its own compromises; such is the drawback of using passive crossover networks as opposed to multi-amplifier configurations with active crossover networks. The other issue complicating a network design decision is this: What may on appear paper or simulation to be the best choice design could measure exactly as predicted, but in reality the listening results may be absolutely the opposite. In other words, what is sound in theory may not be so in practice. Models can only predict from known variables. Much like forecasting the weather, network-driver, filter-phase or filter-amplitude shifts, and who knows what else dynamic physical interactions are unaccounted for in these theoretical mathematical models. You just have to use them to start and then tweak from there.

For example, here is the near-field measurement of this same driver using a fourth-order Bessel network with a crossover frequency of 8.7KHz. One would predict that not only would the 4KHz peak be resolved because of the steep filter slopes but also that the driver would be uniform above the crossover frequency. However, here modeling and reality deviate as shown next.

Bespoke Aria Pink Noise, Bessel 4 Network, Fc=8.7KHz
While it is true that the network functioned somewhat as predicted, the 4KHz peak is still prominent and now only -5dB down from the crossover frequency. So now what? Obviously this is not going to sound well so what can be done? The process is called "voicing" and another filter is added to tame these unwanted characteristics. Voicing is added when a Zobel network is added to a dynamic driver to tame its rising HF impedance curve and also applied to LF impedance peaks. These filters  "nominalize" the impedance of the driver for the crossover network thereby permitting it to operate as predicted. These same filters can be applied to tame unwanted SPL peaks.

For example, adding a LF L-C filter to the Bespoke can change its SPL characteristics. The graph below shows the change of adding a 0.12mH inductor in series with a 2.0 ohm resistor and then placed across the terminals of the super tweeter (shunt across the terminals).
Bespoke Aria Pink Noise, Bessel 4 Network, Fc=8.7KHz, 0.12mH+2ohm shunt
Well that certainly changed things. The 4KHz peak is now gone but as they say you never get anything for nothing. The SPL is very uniform with the expected HF rise of this type of driver design. However, the acoustic contribution of the super-tweeter into the tweeter's bandpass is now an issue. So what can be said? Passive networks have issues when used in loudspeaker designs.

The other drawback to using passive crossover networks is the interaction between other drivers and crossover components. Remember that adding another driver not only permits that driver to more optimally handle its bandpass, but also introduces other dynamic variables to the existing drivers and network. So by adding a super tweeter and its crossover network to your system, it also changes the performance of the other drivers. In effect, adding a super tweeter can change the entire sound of your entire speaker (including woofer performance), and not just add an extension to the top octave.

So how do you decide what network design to use knowing that it also impacts the operation of the existing system? You must listen to the results and listen very carefully. This means that there is both an art and a science to creating proper overall system design (read you never get anything for nothing; there is always a compromise).

For example, I first tried a Butterworth design and while it sounded pretty good, the super tweeter sounded quite off with an unacceptable upper-midrange glare. I then tried a Bessel design and had similar disappointments where the super tweeter just did not have that pristine clarity I hoped to achieve with this driver. So I was at a crossroads: which is the best compromise? Too much glare or to soft an operation? I chose to eliminate the glare since I find this more fatiguing to listen to over prolonged periods (I listen to my rig a lot). What did I compromise? Bandstop issues.

Another factor I needed to resolve at this same time was sibilance. At some crossover frequencies, sibilance became unacceptably pronounced and less so at others. And of course there is another compromise at play: standard values of capacitors. If I could find a crossover point where an off-the-shelf value was available, the cost of the network components would be kept under control. Inductors can be easily wound or unwound but capacitors must be purchased lower-than-desired values and parallel shunts added to achieve the desired capacitance (if a non-standard value is required). If I selected a point where two or three capacitance values were needed (paralleled), it would raise the cost of the network but it is the only way to get the desired value (another compromise).

My philosophy around building crossover networks is to use lower-cost components to find the neighborhood of the solution and then spring for the big-boy toys in the final design. Iteration, repetition, measurements, and subjective comments all help when making any tweaks to maintain your sanity. Change one thing at a time to keep you from becoming overwhelmed and record all of your data.

Once all experimentation is complete, you should sit down and look at your comments about the various configurations. Nothing is more valuable than analyzing your impressions and correlating what you hear with that design. You typically glean insights to what the configuration is trying to tell you by this analysis rather than by staring at the graphs and numbers. And by all means, proceed slowly, preferably over several days or even weeks. Allow yourself enough time to listen to a variety of music and signal sources to accumulate a final impression.

What did I glean?I observed the rising SPL and correlated it to the rising impedance (Z) of a dynamic driver (similar curves, different Q). I tamed this super-tweeter SPL peak with the same type of L-R network used to tame the rising Z of a dynamic driver. I accomplished this through experimentation and measurement rather than calculations. Finding the right combination took a while but in the end the results paid off big time.

Once the final design took shape, I then replaced the cheap signal-path capacitors with quality versions and viola! Not only were all of the issues resolved, the quality capacitors (here Teflon) allowed the driver to perform at its best. Transient response was improved and neutrality achieved. In other words, the network became transparent and permitted the driver to perform at its optimal peak rather than colored by passive components.

Is the Bespoke Aria a good driver? Yes and no. It needs a lot of help to get it to perform at its best. However, once you do, it is very smooth and transparent. I suspect that other driver choices may be more to your liking since they require less tweaking (voicing). However, their cost is double or more of this driver and by taking the time to tweak it you can gain valuable knowledge about driver and network behavior while adding a high-quality top octave.

BTW, the drivers used in this super-tweeter appear to be available from Monacor in the UK for about GBP 46 each (including VAT).

See also THIS LINK on simple modifications to this speaker.

Yours for higher fidelity,

Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate by purchasing one of my eBooks or through a PayPal donation, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style (like that of the Discovery Channel), it targets one star that has never been considered before and builds a solid case for its candidacy.

My other titles include:

Copyright © 2015 by Philip Rastocny. All rights reserved.

Tuesday, March 3, 2015

Music Review - Shaman's Vision Journey

If you have a high-end system, you listen to a lot of music. Most of the software in my collection consists of superbly recorded vinyl and high-quality DSD/24-192/24-96 digital sources. But every now and then, a Red Book format disc (16-44) grabs my attention. While still not among the ranks of a good-quality digital, Red Book CDs can be arranged, recorded, and produced in an amazing manner and this review describes one of them.

I've just rolled into my crossover network some Teflon super-tweeter capacitors and rewired all of the point-to-point connections with Teflon-insulated silver-stranded wire (20AWG,7 strands). Needless to say that after performing this upgrade, I appreciated quite a few sonic benefits, mostly above 2KHz. It also helped reveal more of the nuances buried within a recording like background noises that were once distant being brought more forward.

So with the system tweaked, it's a cloudy Saturday morning, I'm sitting with my feet up, coffee cup on the end table, and I am listening David and Steve Gordon's album entitled "Shaman's Vision Journey." This is a four-song meditation-style CD whose sonic virtues fall in that exclusive category of well-arranged and produced Red Book recordings.

This is a relaxing production featuring bird chirps, flutes, guitars, chanting vocals, Native American drums, and both contemporary and ancient percussion. Most alluring is the cadence seamlessly complimented by the perfect blend of all sounds. None are too loud nor too soft meaning that this timeless music sweeps you away as if cradled by the magic of the performers and the synergistic talent of the recording engineer. It's pretty cool; I think you'll like it!

Each of the four tracks honors one of the four cardinal directions (therein referenced as the four winds) - east, south, west, and north. Each track was inspired by the chants of an ancient culture. The clever cover adds another dimension to this album acknowledging these four ancient cultures from which these four tracks were created. Four crystal "singing" bowls adorn the edges with engraved symbols borrowed from the same cultures whose chants inspired that track. The symbols, left-to-right and top-to-bottom, are:

  • Sanskrit OM
  • Hopi Four Directions
  • Tibetan Double Dorje
  • Buddhist Knot of Eternity
I spent a lot of time in the American Southwest and hiked many of the canyons near Moab, Utah. Randomly adorning the sandstone cliffs I have personally observed white hand prints similar to those shown on this drum face where the spiral on the palm reportedly signifies the spiritual power of the Shaman. I could go into more about the cover design and its integration of sound waves and symbols but I presume you are more interested in the music itself. Suffice it to say that the cover design is well thought out and contains more to it than meets the casual eye.

The attention to detail in the music is similar to that of the cover art. Each song contains a core drumbeat whose rhythm maintains the emotion and foundation of that piece. Enhanced by other instruments, each song takes on a slightly different character although if you did not pay attention you may think that they were all the same song. Such is the indistinguishable nature of well-orchestrated deep meditation music. Not only does the performance lull you into calmness and peace, it also drains away your stress and helps you experience personal insights you may not have previously known.

If I were you, I would follow the directions on the album notes and quietly meditate while listening to this music on your own high-end rig. You nay find yourself fascinated by the accuracy of the instruments or intrigued by the unsuspected appearance of a harmonious instrument, but stay focused on your meditation and let all external influences dissolve. Let your meditation take you deep into your higher consciousness and beyond with the help of these chants. It works as my wife and I can both confirm.

I never thought the integration of playback system and well-recorded sound could make such a difference as to the quality of my meditation. I hope that you find a new use for your rig as I did mine.

I once prayed for the Creator to allow me to hear "perfect sound." I was blessed in a meditation with a single note whose purity drives me to achieve a similar experience with my playback system. While I am much closer today than that day over 20 years ago, I still vividly recall the pureness of that one note not heard with my ears but rather with my inner spirit. Music of this caliber as the Shaman's Vision Journey re-inspires me to continue my journey into revealing that quality of an experience. I hope that this music similarly inspires you to continue on your own journey into audio perfection.

Steve Gordon's Comments
We receive many reviews after releasing an album, but it was particularly gratifying to read Philip’s review.  Since Philip is an expert in audiophile sound, we were very pleased to see that he enjoyed the album and considered the sound quality to be very good for a red book format disc.
When we engineer and master an album like this, we take a long time to get everything to sound the best it can.  In addition to our commitment to composing and recording exceptional music, we are equally committed to excellence in sound.  It’s great to get recognition for this from someone who knows sound.

Yours for higher fidelity,

Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate by purchasing one of my eBooks or through a PayPal donation, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style (like that of the Discovery Channel), it targets one star that has never been considered before and builds a solid case for its candidacy.

My other titles include:

Copyright © 2015 by Philip Rastocny. All rights reserved.