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Romy the Cat wrote:

Well, I have to admit that I did not listen the whole sound of AC vs. DC filaments yet.  I will bring home today my DC lab supply that I use for my electromagnet coil and will give the try. I still at this point am not concerned about the “whole sound” but I rather a bit pissed about some aspect of “targeted listing” . Thos aspects might be not important at all but it is what I am accustomed and so far with AC those aspects are fucked up. Listening the AC on DHT vs. the IDHT single-stage I do not feel a lot of advantage of DHT or IDHT. They giving or taking are compatible if I mentally extrapolate myself out of the differences that are intentionally implanted into the sound of those amps. However, when I discard the “whole sound” and look at the “little sound” then the IDHT single-stage amps destroys my DHT amp and at this point I might presume that it is because of AC. The “little sound” is my own meshuginah that might not be important at all but they are those “some aspect of targeted listing”.  I developed those techniques in 2001-2002 what I was listening a LOT of compression drivers and was trying to see how the drivers’ cones react to different minimal currants from my Lamms ML2.0. Different diaphragms with different suspensions did differently and the best diaphragms did very interesting. I place me head at the mouth of my MF horn and playing music loud I try filter out in my mind all sound and concentrate my attention only at the sound that are just above auditable level. At this point you can almost hear the curvature of the cone. It hard to explain what to listen but since one get what and how to listen then it is a totally new world – the micro-world , what you deal not with Sound itself but with some kind of molecular sound. Those little artifacts of molecular sound that juts pup up from the diaphragm surface are kind of funny.  You never hear then even at 3feet from horn. You need to dive into the horn, to close your year and spend then foe a while unit you hearing will be able to tolerate the “overload” and to concentrate itself on the “molecular sound”.  Then the “new word” would open up… I have to admit that as many drivers I heard in the molecular mode no one did as “interesting” as Vitavox S2 with a specific version of magnet. The S2 present the molecular sound as some kind of fantastically sophisticated texture cut with super fine scalpel. Most drivers do not have even a reaction of the levels where S2 already bulb up a whole labyrinth of micro sounds… The key in here is to observe who the “molecular sound” relates to the “whole sound”… So, my IDHT single-stage is not juts “clean” but it has stunning “molecular sound” quality. My DHT loosing the “molecular sound” butte with HUGHE margin but it is not the worst thing. The worst thing is that “molecular sound” of my DHT is absolutely independent from the rest of the sound. It feels like the DHT’s molecular sound lives with its own live and I very much do not like it. A friend of my advised that it is not only due to the AC modulations but also the non-literally of the DHT triode anode/grid curves. The curves are not straight and the cathode to anode and cathode to grid voltage of the tube varies for a part of the tube by +/- ½ the peak/peak value of the heater voltage.  So, presumably the DC on heaters would minimize this effect. I think the tonight run of filaments from DC lab supply would show off where I stand on the DC subject. If the DC will be able to make the “molecular sound” of my DHT close to what I have from a single-stage amp then it will be a big plus to the DC idea.

I did a few times AC and DC switch with practically no impact. Then, driving the amp from DC I shut doe the B+ with speaker connected the noise is gone instantly. I was looking at my connect Lab Supply and it keep powering the DHT filament. I was looking at the miliampermeter of the DHT current and it she 35mA – the caps still hold the charge.  So, the output stage was fully operation but there was no noise. Ok, it came from the first stage I concluded…

Looking at the first stage I decided that the only element that might be responsible for noise was not shielded filtering choke. I hanged it from anything and who know might be is suck-in some dirt. I disconnected the choke from driver’s grid, and turned the B+ back (the DC filaments kept running on DHT). This time the amp just did not work at all. I was sitting in floor, trying to figure out what I did wrong and then something made me touch to touch anode of driver with a screwdriver. The speaker responded with a very soft click. Holly, shit, it was operational I figured. I tuned “play” in my CD played and the amp was functioning perfectly fine. The scope showed virtually the same level of noise but now it was with zero harmonics – juts a fat thin line.

OK, the problem was found, how about the AC? I disconnected the DC supply and put the AC back. To my huge surprise it was not bad at all. It has some very minor nose if I literally stick my head into the horn. The texture of this noise is WAY more pleasant, it just a soft FM hiss, in a way similar to FM noise, only a bit less uniformed. It is absolutely not auditable from 1-2 feet. Bingo!!! That is exactly what I need!!! Now, with this result like this I can perfectly stay with AC.

Now, I have two problems.

Problem # 1: the MF channel with no filter at all sounded juts spectacular. I play very softly muss with no LF content and with no filter and it was VERY convincing. It was so convincing that I even thought to get a full-range transformer and a speaker-level filter….

 Problem # 2: it is self-evident that my older filtering coil in my custom made shield was good to sitting behind the 12K resistor but it was not good enough with the coil sitting directly on the grid.  (before above  and the after below)

It looks like the choke sitting on the grid acts like an antenna. The new non-shielded coil acts even worse, as in addition it has no resistor between grid… I did not figure out if it is the choke that sucks in dirt or it is the wire from grid to the choke… Nevertheless, the dealing with the filtering choke will be the last bottle-neck that I can envision in this amp. Any ideas are welcomed.

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