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RE: triode wired KT88

Posted by tube wrangler (M) on June 24, 2017 at 21:17:26

In Reply to: RE: triode wired KT88 posted by cpotl on June 24, 2017 at 12:18:14:

Nothing wrong with your understanding as far as diff-amps go.

We're talking about the same thing. Common-Mode signals (common to both positive and negative going parts of a signal), can be found in many
places in a balanced or push/pull circuit.

When you holler at someone, or when a piano plays, there is only
a single-ended signal. Same as when your dog barks at you.

IF we record this with a single-ended microphone, then the signal is
still intact at that point-- it is still Single-Ended.

Now, let's record with a different microphone-- one that has two generator coils (balanced dynamic mike as an example). Then, let us connect a Center Point equidistant from the two windings-- between them.

Now, we can collect positive going at one coil's output, and at the same time, we can get the negative going sample at the other output.

We have a center-balanced circuit now. Anything that is contained in BOTH
outputs AT THE SAME TIME is cancelled by the Center Connection. We have installed processing-- we have installed CMR or Common-Mode-Reduction.

Why is it called CMR (CM Reduction) instead of CMO? (common mode obliteration!). That's because the generator has both resistance and back-EMF (electro-motive force)-- cancellation is not 100%-- Common Mode is reduced, not eliminated.

Another example would be a twin-generator mike where each coil had it's
own twin output leads, and were not Center-Connected.

Each coil could independently drive a S.E.-input amplifier-- the two amps
could be coupled into a single push/pull amp-- and presto! At that point we have once again installed CMR.

There are many points in a P/P amp where CMR can be installed-- an
example is on a tube's cathode. The cathode resistors for a two tube
gain stage could be tied into each other, after the cathode connections,
and then this common point can be returned to ground. CMR has once again been installed. You could do this with a double-winding transformer also.

Another example is G2 operation in a Tetrode, or in a Pentode P/P amp. Ideally, G2 gets its own separate power supply. One could run the B+2 to each G2 and have a little CMR there between the two amp stages-- that occurs in the common power supply.. OR-- one could give EACH G2 its own series resistor, connect the two resistor end points together in a Common hookup, then connect this to B+2. You have just improved CMR in the amp tremendously. It will sound faster, cleaner,
"hotter" and MUCH more "on-time"-- musical transient pace, rhythm, and timing....

Two power chokes can be installed as a CMR device. Even in my S.E. amps,
I am using a double-wound power choke. The two coils are operated counter to each other, but the choke's iron core is common to both windings.

Even though this choke is in series with a single-ended D.C. power
system, Common-Mode-Reduction has been successfully applied to
quieting the power supply. Pioneer, as an example, likes to use
CMR chokes on the A.C. line input to some of their high-end Blu-Ray
players. CMR power conditioning......

There are many more ways to understand the Common-Mode that I
can't think of at the moment, but rest assured, studying this area is
a big deal.

In GENERAL-- the more places and ways you can find to install
CMR in any push/pull amp or in any balanced circuitry of any kind--
the better it will sound. The results of applying CMR are large.

In GENERAL-- any attempt, whether on purpose, or accidentally--
to install any form of CMR to a Single-Ended signal will seriously
degrade it, unless you are just trying to strip the A.C. line hum & noises from the signal. This is a compromise that you have to
think about for quite a while before you get all that you can get
performance wise-- musically. Overdo filtering of any kind here-- and your S.E. amp has just become "ordinary" sounding...

In GENERAL-- one has to decide whether to honor music as #1, or
whether theory is all there is. Of course, one has to look at BOTH.

BUT-- that isn't the question. What combination of all of these ideas, and more, is going to really "kick-butt" and put the musicians in
your listening room-- fully intact, not just impressively for some kinds of music.

"It's A Long, Long Way to Tucumcari".

-Dennis-

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