Input transformer topology question

[zigzagflux]

Wondering about the practical pros/cons between the two approaches. Assuming all other devices being equivalent, is there any benefit of a center-tapped secondary over matched resistors?

[SY]

One way, the CMR is dependent strictly on the balance between halves of the secondary. the other way, it's dependent on that balance plus the balance of two resistors.

[artosalo]

The resistors forming the centre tap cause additional load to the signal source,
but if this stage has low output impedance, then this matter has no meaning.
With adjustable resistor(s) optimum balance can be adjusted. With center tap it is fixed.

[dhieber]

I can't add to this discussion of the technical merits of the two approaches, but I've built and listened to both schemes in a power amp with an input transformer. Sound-wise, I think I prefer grounding the ct of the secondary.

[zigzagflux]

So even without a center tap I still need to maintain balance from primary to secondary, if I understand you correctly. I guess that makes sense; any common mode noise able to couple into one end of the secondary more than the other end will be transferred to normal mode noise on the secondary, whereby I will amplify it.

I think I lose you with the resistors: if the secondary load is unbalanced, how does common mode on the primary get through? I'm thinking in superposition terms.

I am trying to pick an input transformer, but some of them do not come with center taps. Jensen JT-11P-1 for example. The Cinemag CMOL-2X600T2 is an option, but the Jensen on paper looks a scoche better. Others are 4:1, which drop my signal a little too much.

What is the degradation in performance with say, 0.1% resistors? Is there a simple quantification?

[artosalo]

So even without a center tap I still need to maintain balance from primary to secondary, if I understand you correctly.

The balance between two secondary halves is essential and determines the CMRR.
When a transformer has CT, then the balance is "factory made". Usually perfect.
When there is no CT, then it must be created with two resistors. It is obvious that good CMRR will be achieved with two similar size resistor, but I would test this by fine-tuning one of the resistors and measuring CMRR at the same time.
Maybe this procedure is not necessary, but then it is ensured.

[dsavitsk]

From Audio Transformers - Bill Whitlock Chapter 11 (3MB PDF) From the "Handbook for Sound Engineers" Third Edition.

In all balanced signal connections, common-mode noise can exist due to ground voltage differences or magnetic or electrostatic fields acting on the inter-connecting cable. It is called common-mode noise because it appears equally on the two signal lines, at least in theory. Perhaps the most important function of a balanced input is to reject (not respond to) this common-mode noise. A figure comparing the ratio of its differential or normal signal response to its common-mode response is called common-mode rejection ratio or CMRR. An input transformer must have two attributes to achieve high CMRR. First, the capacitances of its two inputs (to ground) must be very well matched and as low as possible. Second, it must have minimal capacitance between its primary and secondary windings. This is usually accomplished by precision winding of the primary to evenly distribute capacitances and the incorporation of a Faraday shield between primary and secondary. Because the common-mode input impedances of a transformer consist only of capacitances of about 50 pF, transformer CMRR is maintained in real-world systems where the source impedances of devices driving the balanced line and the capacitances of the cable itself are not matched with great precision.

[zigzagflux]

I got that; what I'm trying to understand now is how secondary load resistors affect CMR.

[tinitus]

Originally Posted by SY

One way, the CMR is dependent strictly on the balance between halves of the secondary. the other way, it's dependent on that balance plus the balance of two resistors.

aha!

will the balance of secondary stay unaffected by whatever you may do to the primary side

[SY]

Yeah, that was a bit confusing. It won't affect primary CMR, but will affect the CMR of the active stage (i.e., noise pickup between secondary and grids) as well as balance.