|The Bespoke Aria Ribbon Super Tweeter|
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|
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
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|
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|
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..
Yours for higher fidelity,
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