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I made today a bunch of experiments with shielding of the MF filtering choke and I have very good progress. I was talking with different people and had a number of good advises (Come one, people the ward “advise” must have a plural form!)  The interesting impression I had after a conversation with Dave Slagle. David dived into, in my view irrelevant for my case, advocating the “evil of shielding for phonostage step-up transformers” and his “disapproval of shielding”. Right he or wrong is irrelevant in practical life, and  while I consider the whole subject is “beside the point” for my case (as my filter is “subtraction” non-series devise) I asked myself: what is the difference between the concept of phonostage step-up transformers and my RL filter? My driver stage has 17 times gain and the direct-coupled output stage has 3. I have 20 times gain (before transformer) or the similar gain to some step-up do, and my filter sits right on the driver’ grid. So, if my Expressive Technologies step-up transformer has 5 separate shields (isolated with air) then why I shall not treat my filtering choke with the same respect, not to mention that my filtering choke sits inside of 6 channels amp with a LOT of sources of all imaginable dirt?

Well, it was what I did last night and today. I wrapped 3 layers of .002’ Conetic sheet around the David’s choke. Then I conversed with a shielding engineer that explained to me that all MuMetal sheets loose permeability at the bend locations. So, I bought the Supper Shield Nickel spray and very richly sprayed the whole Conetic sarcophagus with 3 coating of that Nickel spray substance.

http://cableorganizer.com/mg-chemicals/super-shield-coating.html#features

 Then I have an isolation layer and 2 sheets of .008’ Netic. Then an isolation layer and one layer of think .01’ aluminum sheet. Then another isolation layer and one sheet of .01’ cold-roll steal. David sent me emails suggesting using copper foil as well. I do not know what it might be for but my Expressive transformer does use a thick layer of copper shield as well, so why shell I not? I have ordered the copper foil from MacMaster and will have it tomorrow.

So, for a time being I connected today the choke without the last 2 final layers: it will be a layer of double-wrap copper and the last externals layer of double-wrap of A276 foil)

(Cat. # A276-10 at http://www.lessemf.com/mag-shld.html )

The effect of issue the choke as it is today was insultingly wonderful. There are no problems anymore with noises auditable through the MF driver. Even I stick my head into the horn I need to listen it for a few seconds to recognize that there is “something” in there. The residual noise is so low that it has become a subject of ego and obsession instand of the subject of practicality. I still will put the last two layers tomorrow, juts to be on the save side and it will be it. The problem with the choke noise is gone…

A bit more about noise. I was playing with different grounding schemas, not listenining it but just playing with noise views on the scope. Grounding of the secondary of the Slagle’s Nickel OP transformer looks like drive noise even lowers, interesting that grounding of one or another side of secondary produces absolutely different effect. In terms of number the best I was able to do was 9mV, it is not RMS but peak-to-peak.  The best that I was able to get with single stage Milq was 0.9mV peak-to-peak; the DH channel has a few dB more gain however.  As you understand that 9mV peak-to-peak is way beyond where I need to worry. I have to admit that those 9mV peak-to-peak do have some harmonics (on scope). I put DC on filaments and the nose drooped to 3mV peak-to-peak with virtually no harmonics. Since the noise is not problem anymore with ether PS  on heaters it would might be interesting to evaluate how the types of heater voltage might affect Sound. I however did not do those experiments this time and I would prefer to stay with AC, thankfully now I can. I understand that all problems of modulation that I was named above still there but who knows if the cure more deadly than the illness, so I will stick with George Westinghouse and Nikola Tesla. In my own version of the “War of Currents”

 I think that when I put the last shields and close the amps I might lose another 2-3mV. In additions the LPAD on secondary will kill ~3dB, so I will be way into deep silence…

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