Initial experiences with a common mode choke

[Dr.H]

Hi All,

Thought I owuld post my initial experiences with a common mode choke in the power supply of a tube (6SN7 mu stage) pre-amp. The power supply is a dual mono, fed by a 225VA transformer, with 4 secondaries (2 heater, 2 H.T.).

H.T. is fed via solid state rectifiers through a capacitance mutiplier. Heaters are regulated DC, rectified by silicon, fed via LM317.

I installed the choke (7.6mH, rated for 3Amps)into the primary of the pre-amp transformer (i.e. on mains). The impact on sound:

Soundstage much smaller, Less separation between instruments, Bass not as full, some bass notes seemed lumpy.

I guess that the problem must lie with the choke impeding current flow in some way.

I next plan to try
CMC on secondary of H.T. prior to rectification (one channel)
CMC on H.T. post rectification (i.e. on DC) (one channel)
CMC on heater AC (prior to rectification) (one channel)
CMC on heater DC (one channel)-treid this and could not really discern any change in sound.

Any comments?

[Circlotron]

Did you put a cap across the input side of the choke from active to neutral, and another across the output too? e.g 1uF X cap? And also a 1nF Y cap from each output to ground?

GP.

[Dr.H]

I put a 220nF cap (x-rated) on either side

[SHiFTY]

I installed the choke (7.6mH, rated for 3Amps)into the primary of the pre-amp transformer (i.e. on mains).

I'm no choke expert, but doesnt a choke resist AC and pass DC? therefore it should be in your B+ after the rectifier? My amp has a big sodding choke in series with the filter caps... It has no hum or noise at all even with NFB turned off (it is pp and you need a little!).

What is the point of putting a choke in the AC line? Apologies for a dum question

[Circlotron]

A common-mode choke has 2 windings that are wound parallel but opposite direction to each other and are therefore out of phase. Anything going in along line only goes out along the other unimpeded as per normal electric supply, and the magnetic fields of each winding push opposite directions so the choke has no effect for these. But if BOTH input lines go up at the same time the choke filters out this kind of noise. The noise spike is common to both lines hence the name.

The other kind of double wound noise filtering choke has the two windings wound the same direction and is called a differential choke because it works on any noise that is on one line only.

Dr. H, the Y caps are quite important for the CM filtering to work, in fact if they aren't there it wont really work at all. They are dominantly the "C" part of the LC filter bizzo.

GP.

[Dr.H]

Hi,

This is a common mode choke (CMC), so that the indcutor is in series with BOTH the live and neutral lines (or in your case, HT and ground). The CMC passes differential signals (i.e. signals on one line only) and filters common mode signals (i.e. signals that are simultaneously on both lines, say HT and ground).

Apparently, under the right applications, the CMC is an effective way to filter noise, especially in switched mode power supplies.

I also am using a choke in my powers supply (10H, 250mA) and agree on its benefits.

Ryan

[Dr.H]

I will try the Y caps tonight and see if there is any improvement. Y caps generally connect to earth from live or neutral, so that makes sense, but what about where you have no earth? This happens in a CD player for example that uses a 2 pin plug, with no earth wire.

[Circlotron]

Originally posted by Dr.H
I put a 220nF cap (x-rated) on either side

Hey! Didn't you read the new posting rules? No x-rated stuff allowed here!

GP.