Edcor XLR transformer

[M Fowler]

I know you smart amp-aholics out there can educate me on xlr balancing transformers such as the Edcor WSM 10k/600R I'm using in a tube preamp.

Every where I look I see only primary pins 1 & 4 used no Center tap connection but I had AC hum I could not get rid of no matter what I did.

Had a brain fart to connect the primary CT to ground and presto the hum was gone.

Secondary side pins 5 & 8 are used but what about the CT?

Mark

[dorrisant]

Pin 5 of the secondary to pin 2(+) of the XLR.
Pin 8 of the secondary to pin 3(-) of the XLR.
Let the CT (pin 6) float.
I usually grond Pin 1 close to the jack... Somewhere there should do.

I have never grounded the CT of the Primary...didn't know you could. Most of the Lundahls that I've used don't have CTs on either side.

Hope that helps! Nice layout BTW... Looks pretty sweet. But where's the rest of it?

[M Fowler]

Tony I've never grounded the CT either but I decided to do all kinds of jumpering throughout the preamp yesterday to see if I could get rid of that hum.

When I got to the balancing transformer I focused for some reason on the noted CT on the top label of that transformer. Clipped an alligator clip from CT to ground and presto no more hum?

That got me wondering about the secondary CT which is unused for my xlr and 1/4" jack outputs.

That Alembia F2B pdf above has 5 pages to it!

Mark

[martin manning]

Mark, where and how do you have this matching transformer connected in the circuit?

[M Fowler]

Mark, where and how do you have this matching transformer connected in the circuit?

Martin it's in a custom built 5b6 preamp in an old toaster container.

The signal comes of the anode of the 6SL7 through a .1uf/600v coupling cap to the 1/4" jack which works fine.

This same coupling cap goes to pin 1+ of the edcor balancing transformer 10k/600r to pin 2 of xlr, pin 4- of edcor to pin 3 of xlr. pin 1 of xlr to switch/ground lift.

I found out I cannot ground the CT of the edcor as it makes the signal of the xlr out to power amp distort which I should have known that anyway so the CT is no longer grounded or used so scratch the above talk about that.

[pdf64]

Regarding the grounding of / necessity for, coupling transformer CTs, I just happened across #11 of the Sowter FAQ http://www.sowter.co.uk/faq.php

11) SHOULD THERE BE A CENTER TAP on the secondary winding of a transformer to drive balanced or unbalanced audio line.?

For driving a balanced audio line a center tapped winding is not necessary. In fact a grounded center tap can cause ground loop problems and negate the advantages of the balanced line.

I know this does not quite feel right but what happens is that the signal automatically "floats" and assuming the load is balanced it will automatically balance itself about ground. The point is that "ground" is not really defined and may have noise on it so it is not a good idea to try to force the signal to be referenced to it.

Another advantage of not grounding a center tap is that If the load is unbalanced (i.e. one side is grounded) everything works and the source and load are still isolated from ground noise.

The only situation where you need a center tap is when you are driving a push-pull amplifier stage where the grids of the valves need to be tied to ground.

[dorrisant]

So Mark, to be clear... Are you grounding the primary CT of the Edcor XLR TX? I could see this working or not depending on how you feed it, but just not the the secondary.

[martin manning]

Mark I think having the XLR transformer connected to the output and floating is the source of the hum, and you will need to isolate it. Grounding it cured the hum but as you found out the output was severely loaded down. Furthermore, the transformer primary impedance (10k) is too low for the 6SL7 output stage to drive. What you need is another stage with high input impedance and low output impedance like a cathode follower or cascode in between. You might get that done with a MOSFET or two and a few other parts... See the first circuit drawing in Keen's MOSFET Follies, or do something like this: http://www.jensen-transformers.com/wp-c ... k61-1m.pdf

[Leo_Gnardo]

In keeping with the Alembic type quality, consider nicking a design from Pultec for output transformer drive. Note no DC thru the primary. Might be worth a try:

[img:800:686]http://ekadek.com/wp-content/uploads/20 ... er-amp.jpg[/img]

I think you could do without the drive tube & FB, just bring the output drive tube's cathode R to ground. Might sub parallel 12AU7 triodes for the 6AQ5. Or 6SN7 since you seem to be in an octal mood.

[martin manning]

To keep it stupid-simple (kiss) I'd try a MOSFET follower off of the 6SL7's plate... only [edit] four additional parts: ZVN0545A, gate stopper, source resistor, and a big coupling cap to the Tx. The line-out may need padding down too, otherwise it will be ~1/4 the output voltage at the 6SL7 plate.

[M Fowler]

Thanks guys I'll ponder all this for awhile.

Not using the CT.

[JazzGuitarGimp]

Hi Mark,

One of the main features of using a balanced audio connection is that the ground reference between the sending unit and the receiving unit is broken, thus eliminating any possibility of ground loops between the two units. In your schematic, you have pin 8 of the isolation transformer grounded, and so you are forcing the receiving unit to be referenced to the sending unit's ground. This will, more often than not, cause hum. See the attached schematic for reference, with the following notes:

1. If the transformer primary will always be connected to the 6SL7 (IOW, not switched), you can remove your 220K resistor to GND as it isn't needed - the transformer primary provides the ground reference here.

2. In the old days, it was common practice to add a 604R, 1% resistor at all balanced inputs - many units had these built-in. Today, it is more common to see higher input impedances at XLR input connectors. 47K or higher is common. To get the transformer to operate to its optimum specs, I have added a 604R resistor across the secondary. This may not be needed, and in fact, it will force the transformers input impedance to be 10K, which may load down the 6SL7 too much without a MOSFET of CF driver. This resistor could be switched, and it's value could be increased to increase the load impedance to the 6SL7, or you could put a 25K pot in series with the resistor to give the output of the 6SL7 a variable load.

[M Fowler]

I had already removed that 220k resistor to ground off the .1 coupling cap as well as some other changes.

Eventually had to disconnect pin 3 of xlr to Edcor pin 8- to stop hum.

Installed a switch so Edcor pin 5+ can go to either pin 2 or pin 3 of xlr.

I'm done screwing around with this thing, wasting so much valuable time from other projects I need to complete.

[JazzGuitarGimp]

Did you unground pin 8 of the transformer?

[M Fowler]

Did you unground pin 8 of the transformer?

Yes I did.