Dual primary input transformer

[fum]

So I'm trying to wrap my head around how to use a dual primary (150,600: 15K) input transformer.

The reason would be some cm-75101A's I just recieved. I'm trying to figure out if I should expose a selectable 150/600 switch on the front panel, and also, just want to understand the workings of the transformer.

So the datasheet that Fabio posted says connect pins 1&2, 3&4 to get a 150 ohm input. I'm trying to understand how this works.

Is this saying that pins 1&3 form primary one, 2&4 form the second primary, and that the two primaries are drivin in parallel for 150 ohm input? If so, what happens if you only drive one of the primaries and leave one unconnected?


Secondly, the logic in my head says that for 600 ohm, I should be connecting 2&3, with + input on 1, - input on 4, thereby driving the primaries in series.


Am I gettin this straight?

Regards

ju

 

[CJ]

don't just use one primary winding. this unbalances the core and also, the second unused primary will suck up some of the signal.

 

[PRR]

> what happens if you only drive one of the primaries and leave one unconnected?

Resistance losses will double.

In many input applications, this is no big deal. When used "right" the losses from driving just one winding are under 5%, maybe much less if the secondary load is high.

But in some transformers they interleave the several windings to get better coupling, less stray primary flux leaking where it misses the secondary. And by using just one of the several windings, you ruin this trick and will have increased HF droop.

 

[fum]

So, I'll assume that because you didn't rock my world, I've got it straight =)

Will use both primaries, just had to pose the what if =)

Thanks

ju

 

[CJ]

just a side note, you would also lose all your CMR in a humbucking x-former if you only used one primary.

it is kind of confusing, when you series the primaries, you quadruple the impedance of any one pri.
but when you paralell the windings, the impedance stays the same . whats up!
well, with theprimaries in paralell, it acts like a single piece of two strand wire. so the amount of turns is really the same as one primary, even though there are two primaries hooked together. since the two primary wires are connected at each end, the core see'd the same amout of turns as one primary.

 

[Guest]

[quote author="fum"]
So the datasheet that Fabio posted says connect pins 1&2, 3&4 to get a 150 ohm input. I'm trying to understand how this works.

Secondly, the logic in my head says that for 600 ohm, I should be connecting 2&3, with + input on 1, - input on 4, thereby driving the primaries in series.
[/quote]
There are two primaries at the trafo, every one with 300 Ohms.
If you connect them to the paralel, you get 150 Ohm,
if you connect in serie, you get 600 Ohm.
This is noise-optimal, because all the primary copper is used.

xvlk