OK. After a few days of self-education and harassment of various technical people I got what I need – the truth and the reasons why both of my Sansuis TU-X1 sounded so honorable from 75R FM input and so much better from FA-7 input. Below I will provide the detail expansion. I have to admit that the reason was partially my ignorance but the result of the journey to figure out the mechanics of my improvement was very educational.
Oh, but the way, the readers of my site like to laugh– the indomitable abovementioned Baba Fitzgerald issured to me Cease and Desist ultimatum:
“... If you don't remove my name and private email content from your site forum I will do something about it .... I will contact your ISP, and complain. You may hear from my lawyer for libel, and threatening obscenities. I will also complain to the City of Cambridge, where I have friends on the Police force and in government. Your address and location are known to me. “
Great! He can stop by in my parking lot and fart into all 4 tires on my ugly BMW! What a Moron!
Anyhow, I had a conversation with my technician Ken Bernacky from Stereo Surgeon who calibrated and aligned both of my Sansuis. He for whatever reasons was defensible and felt that I was complaining. Complaining was never my intention, I just wanted to get the rational of this thing. I had no doubts that Ken pushed out on my TU-X1 whatever best was necessary and possible. However, what I heard was also self-evident that FA-7 input with 300R coax connect to front end was incontestably better from point of noise and sensitivity
Ken Bernacky commented on it:
“As you received the item back from Stereo Surgeons FA-7 was not in the circuit, the front end was wired directly to the 75 ohm FM f connector input. Any signal that you were receiving from an FA-7 input was due to stray capacitance and would be greatly attenuated rendering the tuner useless for anything but extremely strong rf levels.
You indicated to me over the telephone that you have opened the item and reconnected the FA-7 coax plug to the front end of the tuner. Reconnecting this provides you an attenuated signal with reference to the sensitivity achieved by my bypassing FA-7. The attenuated signal is due to the increased losses of the crude FA-7 switch which is actuated by a side push in rod found on the side of the FA-7 f connector and additional components. You making this change would only mildly affect the input tuning of the very first tuned circuit in the front end and does not affect any tuning adjustments past the very first tuned circuit in the front end. What you have accomplished is to lower the sensitivity of the tuner. This is not desirable, the desirable way to accomplish the same result is by feeding a signal of lessor amplitude from your antenna thru the 75 ohm f connector.”
I very much not in position to argue with Ken but I accustom to consider that what I hear is what it is. I looked at scope the output of my antenna coax with 60% signal strength on Sansui’s meter: it was .200mV. The output of my FA-7 input was order of magnitude lower, much much lover. It looks like Ken was correct and my signal flood the Sansui input and therefore crate excessive noise. Then why both of my Sansuis have much better sensitivite driver from FA-7 at very weak signals? There was no explanation.
Well, after consulting with a few guys who did not give me a satisfaction I desired to put the heavy artillery into the game – I called Dima. UHF is Dima’s specialty and he looking at the Sansui TU-X1 circuit gave the prefect explanation of what happens with my TU-X1. Dima explain to me that the problem with my tuner was.... my bad technician who juts made to me too sensitive tuner. With the strength of my antenna (the one that Ken incentivized me to install and use), with the sensitively of this tuner and with amount and of the FM stations in Boston Dima proposed that Ken should release this tuner accompany with 40dB coax attenuator as overlay strong signal toss the Sansui’s front end into distortions that produces extra noise. I went and bought the 12dB, 23db, 27db, 33dB, 45dB attenuators and they worked absolutely perfect with regular a high sensitivity FM direct=pass input. In fact playing with attenuators, I concluded that my aligned Sansui has it's best relationship between sound and noise when antenna supplys only 40-45% of signal strength on my TU-X1’s meter. Dima explained that in the TU-X1 the metering is done “properly” and the signal chain and meter chains are completely separated and not even connected. He proposed that the meter chain might be recalibrated for the new sensitivity, but I do not think I need it. I just learns that in both of my TU-X1s,, after the Ken Bernacky’s alignment the 50% of the signal strength become my new max limit over which I should not go, or I pay the price of noise.
Now, if it is so then why with signal attenuation after the FA-7 I had better reception for far-located stations? Dima explained that it has to do with the TU-X1 design. The TU-X1 has (I am paraphrasing) first relatively wide bandwidth first filter and then a second stage/filter that runs automated gain control. (I do not use the TU-X1’s “narrow” setting – bad for sound). So if in the bandwidth of the first filter there is a week signal and a part of strong signal then the second narrower filter's automated gain control adjusts of volume for the week signal, which is overloaded by for the strong signal. He explained in details why lowering the signal strength and this particular tuner should help in this satiation but it is beyond my ability to repeat. What is important that it is works no mater how not initiative it is – with sticking 12dB-23db attenuator on my antenna I “see” more stations in noise and get them much cleaner. I think Ken should release his tuner with some kind of “sensitively warning labels”…
I know, it is kind of contra-intuitive to kill output of antenna to get better sound. But I learned that I need to do it. How about the Sound difference between the FA-7 input (that do attenuation) and the FM input behind a right coax attenuator that would set the signal level of 40-45% on the TU-X1’s meter? The FM input unquestionably better: it has better bass, way better “space” and much, much, much more sophisticated imaging… and the SAME noise level.
So, my “FA-7 alert” was false but it was very enlightening for me to get to the bottom of those thighs. Thanks Ken, thank Dima and ….thanks the TU-X1.Rgs, Romy the caT
PS: BTW, if anyone knows where the source a 75Ohm 0-40dB (not 20dB) LPAD with coax input and output cables going in and out from the BACK of the unit then please let me know.
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche