Some thoughts related to CS3.7 ’brightness’.
Brightness, or excessive upper midrange energy is a common complaint about Thiel speakers. Knocking those frequencies down will reduce the perceived problem, but I’d like to add some perspective to that solution.
I know intimately how Jim developed and voiced his speakers. He compared and correlated ear, impulses, sweeps and pink noise using floor-standing, aerially suspended, and ground-plane techniques. He achieved his goal of less than 1/10th dB / octave-averaged power of differences. I believe Thiel speakers are flat. That judgement runs contrary to many user’s experience. Following are some thoughts about that discrepancy.
Jim’s design listening position is off-axis via speakers pointing straight ahead. Lots of irregularities are reduced or eliminated at the resultant 25-30° off axis listening position. Note that most listeners, nearly universally, toe the speakers in perhaps half that amount. They then get more high frequency amplitude and more discrepancy between drivers than a properly (as designed) positioned listener.
So why do most listeners prefer listening closer to on-axis than designed? One reason might be that Thiel speakers have unusually wide dispersion patterns. That is by design - approximating how a real sound source would radiate into the listening environment. Look at the off-axis plots of Thiel speakers and you’ll see only a few dB of high frequency droop with very similar response curves to the on axis response. That wide dispersion provides better in-room power response, better stereo imaging and better phase coherence. But it comes at the cost of more side-wall reflected energy than most speakers. I suggest trying straight-ahead orientation while adding sonic absorbancy at the side wall first reflection point. My experience is that the reflection is causing the perceived problem and that absorbing the reflection presents a better solution than reducing the amplitude of the offending frequencies.
I have only heard 3.7s once, in 2012 in Thiel’s listening room, powered by the Krell FPB-600. That room measures 14’ high x 22’ wide x 36’ long, with 1.5" thick wall panels covering perhaps 1/4 - 1/3 of the wall surfaces and wall to wall wool carpet on hair pad. Room is of medium vibrancy, not dead, not live. The 3.6s sounded magnificent without a trace of brightness.
Not everybody has such a room. My point is that rooms make tons of difference which are best addressed via room / acoustic solutions rather than loudspeaker modifications.
I have been wrestling with other causes of the persistent complaints of high-frequency less than best-ness. I have identified (and previously mentioned) wavefront propagation anomalies that when mitigated, control a sonic instability perceived mostly in the high frequencies. The propagation turbulence is actually full-spectrum and my newly emerging acoustic wavefront propagation solution produces a full bandwidth uniquely settled and natural sound.
Another situation is that Jim chose aluminum driver diaphragms for lots of good reasons. However the tweeters breakup harshly around 25kHz. Even though that is above (generally accepted) hearing limits, that breakup mode couples with some wire anomalies to produce less than best transparency in the highs. Thiel’s solid18-2 in teflon from the 1978 model 03 was reconfirmed for the 1988 CS5 and again for the 2006 CS3.7. I like it a lot compared with other contenders. But there is a problem with skin-depth saturation beginning at 17kHz which interacts with the instability of that tweeter breakup mode. My new cable uses two smaller gauge conductors per leg to raise the saturation frequency to 42kHz. The chronic Thiel high frequency blockiness dissolves into grace and air. My assessment is that any perceived high frequency ’brightness’ will vanish without reducing amplitude via a series resistor.
I realize that this new cable solution has been a long time in the pipeline. It's now in its fourth generation prototype, and the design is settled. My 3rd generation hand loom is being built which allows small scale prototypes for beta co-developers and critical feedback. I believe it will be worth the substantial wait.
