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To test the latter hypothesis I began fiddling with the location of the cables entering the preamp. The one cable movement that did have a small but noticeable effect on the hum level was to move the phono leads as far as possible from the Tron’s power transformer. I ended up dressing them so that they laid on top of the preamp shelf with none hanging down over the back of the rack into the power transformer’s field. As an aside, placing the phono leads in close proximity to the umbilical power cables coming from the Doshi’s external power supply did not affect the hum level. As a final measure I kept the amp on the shelf below the preamp with the two mu metal boxes over the PT and the phono cables properly dressed and then put two pieces of the remaining scrap mu metal sheet under the preamp. This helped a bit more and I decided to get out the meter again. I measured 54dB of hum, equivalent to having the amp on the lower left shelf with no shielding. It would appear that shielding the Tron’s power transformer—the clear source of the field—was most effective but some additional shielding underneath the preamp helped too.

This seemed a good juncture to ask Graham Tricker and Nick Doshi, designers and builders of the Tron amp and Doshi preamp respectively, a few questions. Nick was concerned about the high level of magnetic emissions from the Tron’s power transformer. He suggested that I email Graham Tricker to discuss it. Graham’s response: “There is nothing amiss with the Tron 211 amplifier. Please bear in mind that you have a very large mains transformer running at about 330VA plus there are at least two chokes inside all radiating 60Hz and 120Hz. The HT rails run at over 1000 volts with 60Hz and 120Hz ripple plus filament windings of 5 amps per 211 again all radiating at 60Hz to create a large field. I always recommend placing the power amp well away from any sensitive sources like a preamp or a phono stage. As you know this is standard audio practice anyway.”

Certainly the high plate voltage of many transmitting tube amps—211, 845, 833, GM70 etc.—presents a special case where more care in placement of sensitive components is required. Typical tube amps running at lower plate voltages should produce less of a magnetic field. However the cautionary comments still apply.

Checking the one-page owner’s manual of the Tron amp revealed no special precautions regarding the emission of a high-level magnetic field. A buyer might logically presume that the power transformer is effectively shielded to prevent such high radiation. Graham Tricker took his turn at bat and inquired about the Doshi preamp as follows: "Does the Doshi preamp have mu-metal screened MC transformers in its phono stage? Also, are the internal wires in the Doshi that take the phono signal from the phono RCA sockets to the first stage screened? Are your phono leads from your tone arm into the back of the phono input screened? All of these need to be screened cables." I gave Nick another call and got an affirmative on all counts. Phono stage output (not input) transformers are mu-metal shielded and all wiring inside the preamp is shielded. The Nottingham Analogue‘s tone arm leads are also shielded. In fact I can’t imagine any tone arm being supplied with unshielded phono cables. Nick also told me that his preamp chassis is made of steel so the chassis itself has its own shielding properties.

The last experiment to conduct was to use both distance and shielding. I moved the amp to the lower left shelf and placed both mu-metal boxes on the Tron’s PT. Now I had reached bliss measuring 50dB of hum or lower (this is the bottom of the range on the meter). I had arrived at vinyl playback with supremely quiet backgrounds! Removing the mu-metal sheets under the preamp now had no effect so those were put aside. I sent one more email to Graham Tricker asking him if there was any concern about heat buildup in the PT from two mu-metal boxes over it. He advised me to cut some venting slits in the top edge.

My audio engineer friend Charlie King and I were talking about my experiments and he raised some interesting points. "One has to wonder what kind of energy fields are around our systems and if they are having any negative effects. Most of us are inundated with RF signals from radio broadcasts and a myriad of wireless communication devices. I have had my share of RF trouble when thanks to deregulation a local radio station years ago was taken over by Clear Channel Communications who decided to boost their signal and/or change its directionality. Their broadcast signal began bleeding into my preamp and I could listen to their programming through my system. Fortunately the FCC went after them and the problem was corrected."

Charlie went as far as to suggest that the amount of negative energy fields around a system may actually mask some of the audiophile tweaks, explaining why a tweak works in some systems and not in others. It could depend on the amount of radiation in the system. Almost every audiophile has suffered system hum at one time or another. Often it is caused by ground loops or improper grounding. When it is not, exploring magnetic field interference may pay off. While it is logical to look at high-voltage power supplies as the culprit, a gauss meter takes the guesswork out to identify the source more strategically.

If you have a magnetic-field hum problem, the best solution is to get any low-level amplification devices and cabling as far away as possible. My experiments proved that the mu-metal product can be very helpful. If you are restricted to move components far enough apart, the mu-metal foil can greatly attenuate the hum. If your hum level starts out lower than mine, the mu-metal foil may take care of it completely. For me the combination of distance, careful phono cable dressing and targeted mu-metal shielding of the offending amp’s power transformer suppressed my phono playback hum by 16dB. Tracking down the source of my system’s hum problem was not about playing a component blame game. After all, variations in magnetic field strength, shielding design and grounding schemes are secondary characteristics of any audio component. If magnetic-field induced hum raises its ugly head in your system, mu-metal shielding foil from appears to be a very useful tool to minimize or perhaps even eliminate it.

Publisher's comment: Perhaps Steve's concrete example will encourage manufacturers of fine hifi components to pre-install more effective shielding around known high-radiation offenders so customers needn't retrofit unsightly solutions. It's one thing to say that one knows one's gear to radiate strongly and hence recommends distance (which obviously does nothing to the radiation per se). It would seem altogether more professional to actually do something about it during the design and implementation phase. - Ed.

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