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Common Mode ( long ) from Dennis

Posted by drlowmu (M) on December 3, 2006 at 10:56:56

Subject:
--common mode--
Date:
Sat, 2 Dec 2006 00:11:17 -0700

Hi Jeff---

Here's one for you. I notice discussion concerning a common-mode choke-- a guy tried it and did not have excellent results -- apparently.

I wonder what kind of amplifier he tried it on? This business of trying to separate the amplifier design from the power supply design can cause lotsa trouble.

You should like this: If you're using a S.E. amplifier, trying to apply a common-mode part in it-- either in a bucking mode, or in an accelerating mode would, of course, run into collision with the single-ended forces in the gain section of the S.E. amp. No matter which direction you wired the common-mode choke, part of its action would "buck" and "reduce" some parts of the music signal-- doing this thru the power supply.

Now, if one wished to apply this same thing to a really good push-pull amplifier (the best will have common-mode rejection designed into each stage)-- if you hooked up your common mode choke up in one direction, your amplifier would behave differently from hooking it up in the opposite direction-- or phase. So when one uses a common-mode choke, they should pay careful attention to what is happening.... to see that the wiring direction is agreeing with other places in the amplifier and power supply-- where common-mode boosting or bucking action is occurring. One should never "boost" in one place, and then "buck" in another. This action will cancel a lot of hum, and make a quiet amplifier with a "black" background. Such an amplifier will also cancel a great deal of micro-dynamic musical information-- this will separate it from the truly great amplifiers-- it will be inferior.

You can now understand the reason that I say layout is the most important factor of all in amplifier design. Your amplifier should be layed-out so that hum pickup does not occur. This method will also reduce ripple to some extent, because ripple is a part of the A.C. hum-- it is combined with it. If you try to reduce hum by "bucking it out"-- OR by using large chokes as filtering mediums, or by using large capacitors as hum-reducers, you will in fact, "buck-out", or "short-out" a great deal of musical information.

Now you can see the layout requirement-- all parts and wiring must be positioned to minimize hum pickup and all wiring must "float" in the chassis in a 3 dimensional setting--not touching anything-- certainly not another wire-- the worst possible thing-- and why "lacing"-- or zip-tying together-- of wiring containing differing signals or power-- is totally obsolete-- no matter how neat it may look. You can, of course, do it with the SAME signal-- such as twisting A.C. filament leads together.

Proper layout is your friend, and any and all other hum-reducing methods are necessary evils-- lurking there, eating-up your musical signals. You want to use as little of these things as possible, and you want to keep their action to as little as possible. After all, they are "working over" your music-- butchering it. Layout, therefore, now becomes of paramount importance.

Get layout a bit off, and you may have to double the value of a capacitor or choke to get back to the hum levels you could have had by studying where things must go. Ripple rejection is a part of this complex, and like any other interference signal-- or signals, you want to pick up as little as possible to avoid having to over-do the size of your ripple reducing components.

Ideally, you wouldn't use anything at all to reduce it-- it would just not be there. In the real world it is there, but proper layout allows us to deal with much less of it.

Certain members of the forum and myself have been arguing for months over nothing-- their most accomplished member has been mostly right-- in his world, despite his ragging on me-- and I have been right in my world. The methods most of you are using were designed for push-pull circuits, not for Single-Ended.

I have been remiss in that I did not-- at first, or at anytime thereafter-- adequately explain the differences. The following will explain that: In a balanced circuit arrangement-- in studio wiring, and in push-pull amplifiers, you have a side of the circuit for each side of the musical signal. In a push-pull, you want each side to be as transparent and as wide-band as you need, so you want and need the best possible POWER SUPPLY REJECTION-- by the audio circuits. This allows you to use large-value chokes, and large-value capacitors to reduce both ripple and hum on the Power Supply supplies.

In Single-Ended circuits, and in Single-Ended amplifiers, you have only one circuit to transfer TWO sides of a musical signal. Since the other side of the circuit must be carried also, part of it is carried by ground circuits, and part of it is carried by the Power Supply components/circuits. All of you know what would happen if you inserted a 100uf. or larger capacitor into say--- your RCA interconnect-- your music would be degraded. You also know what would happen if you put in a high-resistance, high-Henry choke into your RCA wire-- you would "choke-out" a LOT of music. In Single-Ended design, this is our major problem: everything we use in the Power Supply-- or in grounding systems and the ground wires used themselves-- has a PROFOUND effect on the musical signal. Where in push-pull, we wanted maximum POWER SUPPLY REJECTION-- now, in Single-Ended, we must have maximum possible POWER SUPPLY INCORPORATION, and MAXIMUM GROUNDING SYSTEM INCORPORATION.. BOTH must now meet the same musical standards as your RCA interconnect, or your "signal path" (as understood in push-pull terms-- push-pull had two signal paths)-- now, in Single-Ended, we have THREE signal paths, (1) The active "signal path" as is commonly understood, (2) The grounding system signal path, and (3) The Power Supply signal path!

So now you know why you cannot use large-Henry or large DCR chokes in a Single-Ender, and now you know why large-value capacitors can't be used therein, either. Further, whatever wire you use for grounds should be equal to, or superior to-- the best interconnect wire money can buy-- and this same wire must also go into all Power Supply rails.

Now you know-- all of this and proper layout are everything in a Single-Ended amplifier. Leave out just one thing, and any good Push-Pull can beat it up in many ways.

Why do we build these terribly demanding things? (When we coulda have had a good push-pull!). It's because of two things: (1) Push-pull, with its common-mode rejection, sees some parts of the music as "common-mode" and it rejects that too! (2) You don't have to maintain balance of the active devices-- you only use one device, instead of two, in each amplifier stage.

To each his own. This is not meant as a put-down of push-pull, or of its design methods, or of its experts..What I am giving you is understanding-- why layout is of supreme importance in Single-Ended, and why small-value parts are necessary, and the best possible wire, wiring, and placement of everything is of paramount importance.

It's a different world, and I apologize for not making this obvious from the start of our discussions-- almost a year ago.

----Dennis----

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