Dekay, sead: your exchange has inspired me to get back on this forum in a more empirical fashion. The main purpose of this post is to report on a recent OTA experiment I performed.
Purpose: The aim of the experiment was to find out if a 47 Labs OTA phono cable could improve upon previous phono cables in my system (e.g. Fadel Art Reference One Phono, Coincident CST Interconnect, Ortofon AC-5000 Silver, NBS Signature and Statement).
Apparatus: Two 25 cm lengths of OTA were made up. (47 Labs endorses short signal lengths, and I chose to reduce the length of my phono cable by more than 1/4th in agreement.)
Procedure: After 100 hours of break-in, the OTA phono cable was installed between a Simon Yorke S 9 turntable (with Temper Transfiguration Supreme cartridge) and a 47 Labs Phonocube.
Observations: It was very easy to hear immediately that this was the best phono cable I had ever had in my system. More space, instruments were better separated, they sounded larger, more independent, more solid; rhythms flowed better; voices were felt as if the body of the singer were present in the room; low-level detail and dynamic energy was much better than any of the cables listed above: subjectively I had the impression for the first time of being at a live performance. However, I noticed that the installation of this small phono cable had altered the timbre of the whole system: my VAIC monoblocks now had a gutsy, slightly distorted quality more like Jadis. And then, in between songs I recognized that the system was picking up stray radio-frequency interference: its level was low, but annoying. My analogue set-up was now picking up a German FM radio station around 106 MHz, which specialized in kitsch jazz vocalists!
Other signs (intermittent "pong-sounds" from the Phonocube) indicated that the OTA phono cable was picking up sufficient radio-frequency energy that audio rectification (overdrive of the input stage of the PhonoCube by stray radio frequency fields, resulting in the production of interference) was taking place. In my particular case, the RF interference could be eliminated by regrounding the turntable, but returned whenever the turntable or pickup arm was touched again. Not wanting to tamper with the PhonoCube, I rejected the idea of installing a RF choke into the phono stage circuitry. (This can normally be done by connecting a small LC parallel-resonant circuit trap in series with the base lead of the input transistor, or even a small coil tuned to parallel resonance with the RF signal.) Enclosing the turntable set-up in a copper screen structure was not something I wanted to pursue, except in case of emergency. Instead, I chose to re-install OTA, but cut to a length such that it provided trap action with respect to the frequency of the interfering signal. This is ordinarily done by cutting the phono cable to an odd multiple or odd sub-multiple of a quarter wavelength corresponding to the frequency of the interference. For an interference frequency of 106 MHz, a cable length of 70 cm was required. No way! I was not going to triple the length of OTA after what I had gained by shortening it to 25 cm. Returning to a 1.0 m phono cable only confirmed my decision. As soon as a 1.0 meter phono cable was installed, voices took on a reduced quality, seemed covered over, and possessed alot less verve. All instruments sounded flatter, lazier, the three-dimensionality of the recording was lost, everything seemed more boring, dynamics were not as striking, brass instruments no longer had their high-frequency resonance, and the whole sound image lost its live attack. Even though it was not exactly a tuned stub, I quickly re-installed the 25 cm OTA cables with DIY shielding (something like Bill Ying's cable jackets was in my mind as I built my OTA shielding; however, mine are alot more futuristic--silver-- than Shun Mook's Catholic purple). The pickup end of the shielded OTA was left ungrounded and the shielding was only grounded at the phono stage end, so that the RF energy saw a very high impedance at the input end of the cable. It worked! The raucous, gutsy, fuzzy timbre was gone from my VAICs, and there was no more noticeable RFI.
Conclusion: I thought after this experiment that perhaps OTA Kit should perhaps include some simple, grounded cable jackets for shielding OTA in cases where electro-magnetic fields might lead to interference. Perhaps 4 little Shun Mook-like jackets might be included. I cannot help but feel that OTA, WITH the shielding, is slightly better: fundamental tones are slightly more solid, there is a little more relaxation and ease, and the music flows slightly better. The tonal balance even seems to shift subtly when the jacket is moved to different portions of OTA, much like the changes that one can induce by varying the contact area at the ends of the cable. Perhaps these jackets can provide resonance dampening, and current channeling one day in their own way.
