I have no way of super-imposing one measurement on top of another but if you look carefully at the two graphs you will notice some very interesting effects. Let’s break it down and start with the woofer. This woofer uses a first-order Butterworth network and a Zobel so the only change to this part of the network was the additional suppression of undesired HF content to the driver (one bypass capacitor needed here). The yellow vertical line shows the 125Hz point and the green vertical line at the RH side the 450Hz point.
The midrange crossover consists of a second-order Bessel network on both the LPF and HPF sides also using a Zobel network (three bypass capacitors needed here). Band-pass for the midrange driver is 450Hz-2.7KHz as noted by the two green vertical lines. Midrange attenuation is -5.4dB as implemented through a high-precision non-inductive T-pad.
Measured differences in this range are not as drastic as was the woofer’s, however the audible subjective changes were quite noticeable. First, the midrange also appeared smoother as was the woofer, but the contributing “sound” of the bypass capacitor could also be heard. There is now an edginess to the upper midrange that is slightly unnatural but at the same time there is a mellowness to the lower midrange that is extremely appealing. Hollowness in the woodwinds – the bell-sound of a clarinet and the same of an oboe or bassoon – are now very representative of what these instruments actually sound like. Fullness of a trombone is likewise. BUT the first sign of distress occurs in the vocal region where sibilance is over emphasized. Before there was a smoothness to Norah Jones’ voice (Nightingale’s “sing us a song”) where “S” sounds were natural and well controlled. Now there is a slight exaggeration of this region that – well let’s see how the tweeter is affected since this change ripples into the tweeter’s region.
The tweeter uses a second-order Bessel design crossing just above the mathematically-measured point (actual is about 2,800Hz) and the crossover point is indicated by the green vertical line on the LH side of the images. One bypass capacitor is used here. Attenuation is currently -6.69dB using a mixture of non-inductive and inductive resistors in a high-precision T-pad (I am still tweaking the final attenuation level and I may also shift its crossover point). The yellow vertical line is a reference 8KHz point.
As with the midrange, the measured differences to the tweeter’s response is minor with the greatest impact at the 8KHz point. However, the roll-off is improved just above that point at the expense of a minor over-emphasis in the 4-6KHz (sibilance) region. Subjectively, the sound is duller below 8KHz but slightly brighter above. More very HF content is audible (although still lacking and unbalanced) making it a net positive gain. The sound of this particular capacitor in this region is – well mixed. Like a wishful-sinful, there are some things about this capacitor I really like and some I absolutely hate. Its extension and strain is effortless but its contribution to sibilance makes me want to tear out my hair.
In summary, the subjective effects of a bypass capacitor can be confirmed by objective RTA measurements. The overall sound of my system is greatly improved and I will keep these bypass capacitors in place while waiting for them to burn in. Before swapping them out, I will make another RTA measurement to see if the subjective burn-in effects after 100 hours or so can also be confirmed by objective measurements.
The style of this capacitor (Xicom polyester) is definitely wrong for the midrange and tweeter but satisfactory for the woofer. A good Teflon may do much better but my next venture will be the highly-regarded 0.1uF/160V Vishay MKP1837 (http://www.mouser.com/search/refine.aspx?Msid=61310000&Mkw=MKP1837). Stay tuned for my results in the next round of experiments with bypass capacitors and crossover network tweaks.
I am looking for donations of ultra-high-quality capacitors to complete this experiment. I will need pairs (up to 12) of whatever you wish to send me since I plan to one day change the woofer’s crossover network design from a first-order Butterworth to a second-order Bessel. You can email me at "Philip at OkStateAlumni dot Org" if you wish to help me out in this regard.
The Vishay 1837 Review and Modification
Mundorf Supreme Capacitor Review - Part 1
Mundorf Supreme Capacitor Review - Part 2
Capacitors: All Things are NOT Created Equal - Part 0
Capacitors: All Things are NOT Created Equal - Part 1
Capacitors: All Things are NOT Created Equal - Part 2
Capacitors: All Things are NOT Created Equal - Part 3
Esoteric Shunt Capacitors - Part 1
Esoteric Shunt Capacitors - Part 2
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