Stereomour OPT upgrade (part 2, please bear with me)

[Paul Birkeland]

@PB
Thanks for confirming the high pass filter question.
Would 22k be a good starting value?
That should move the -3dB point from 6.4Hz to 72 Hz if I am not mistaken, which would result in a 20° phase shift at 200Hz.

That should be a decent place to start.  PJ may also have some recommendations on decreasing the size of the parallel feed cap to contribute to the same effect.

You could also try jumpering in a 250K linear pot wired as a variable resistor to get a feel for how this all works.

For better amplifier performance, I would see the 22K resistor as maybe too low of a load for the driver stage, so maybe going to a 0.01uF and keeping the 249K resistor would be more to your liking.

-PB

[Paul Joppa]

Yeah, 22K is WAY too low - it will heavily load the driver, cutting gain nearly in half (-5dB) and - more important - causing the driver to distort pretty badly. Use the 0.01uF.

Note that this will protect the ourput tube and transformer from bass overload, but will not protect the driver at all. It is a poor substitute for a filter before the amp, for that reason.

[Fred_P]

I received an answer from Lundahl regarding the LL1663 PPZ.

The first part of the answer is just confirming that this OPT is the right one for this application.
But the second part is rather ...    hhhmmm ...    you decide for yourself:

> If it is not asking too much, could you please describe the differences of the
> amorphous core to a mu-metal core.
> I have heard the amorphous core has the edge, but it is not clear to me why and
> how....

LL1663 is available with amorphous core or with SILICON IRON core. It is not available with mu metal core.
Difference between amorphous core and silicon iron core is a question of taste. I am afraid I am not an audiophile, so I can not comment on this difference. I am not even sure I hear it. But in spite of the high price we sell a fair number of amorphous core transformers, so I expect there is some truth to the difference...

Sorry I can not be more helpful.

- Per Lundahl 

Of course I can't be sure of a lot of things.
May be he heard them in a PP amp for which this OPT was originally designed for, and which is a different story altogether.

But if I come back to my original question, I find that the situation just got worse.
In the beginning I had two to choose from, and now I have three...
To borrow from PJ: A Lotus, a Corvette and a Königsegg.
But are they really any better than a Ford is?
I actually do drive a Ford, and am very happy with it.
 
Now jokes aside, currently I am kinda drifting from the initial cluelessness towards utter confusion...

[Doc B.]

When you get into this very esoteric stuff you can believe the buzz and buy from the guy with the most evangelistic attitude, make the consideration process the more enjoyable part rather than the actual decision and acquisition and spend pleasurable time doing research, or sort it out for yourself. My view of this is that you are always going to be at a disadvantage unless you buy 'em all and try 'em all yourself. Obviously most of us don't have unlimited financial or temporal resources with which to do so. I think Per's answer is awesome in that it is honest. I have had many experiences with well known people in the industry saying this kind of thing about various products/tweaks/etc., and found myself saying it about a few things too. The lesson to be taken from it is that the differences are small and thus the law of diminishing returns is at play. Any of the choices is probably a fine one.