Handbook for Sound Engineers

Audio Transformers 303

11.3.1.3 Resistances, Capacitances, and Other Data

Other data which can be very helpful to an equipment or
system designer includes resistances of each winding
and capacitances from winding to winding or winding
to Faraday shield or transformer frame. Do not use an
ohmmeter to check winding resistances unless you are
able to later demagnetize the part. Ordinary ohmmeters,
especially on low-ohm ranges, can weakly magnetize
the core. If an ohmmeter simply must be used, use the
highest range where the current is least.

Capacitances are usually measured on impedance
bridges and, to minimize the effects of winding induc-
tances, with all windings shorted. Total capacitances can
be measured this way, but balance of capacitances across
a winding must be measured indirectly. CMRR tests are
effectively measuring capacitance imbalances.
As shown in Fig. 11-54, sometimes the input imped-
ance of a winding is measured with specified load on
other windings. This test includes the effects of primary
resistance, secondary resistance, and the parallel loss
resistance RC shown in Fig. 11-8 and Fig. 11-13. If spec-
ified over a wide frequency range, it also includes the
effects of primary inductance and winding capacitances.

Breakdown voltages are sometimes listed as
measures of insulation integrity. This is normally done
with special equipment, sometimes called a hi-pot
tester, which applies a non-destructive high voltage
while limiting current to a very low value.

11.3.2 Data Sheets

11.3.2.1 Data to Impress or to Inform?

Data sheets and specifications exist to allow easy com-
parison of one product with others. But, in a world
where marketing seems to supersede all else, honest
data sheets and guaranteed specifications are becoming
increasingly rare. As with many other audio products,
most so-called data sheets and specifications are
designed to impress rather than inform. Specifications

offered with unstated measurement conditions are

essentially meaningless, so a degree of skepticism is

always appropriate before comparisons are made. A few

examples:

• Hum Eliminator and Line Level Shifter products with
  no noise rejection or CMRR specs at all!


• Line Level Shifter products with no gain spec at all!
  Section 11.2.2.4 explains why they won’t tell you!


• Maximum Power or Maximum Level listed with no
  frequency and no source impedance specified!

Other specifications, while technically true, are

likely to mislead those not wise in the ways of trans-

formers. For example, Maximum Level and Distortion

are commonly specified at 50 Hz, 40 Hz, or 30 Hz

instead of the more rigorous 20 Hz. Be careful, specs at

these higher frequencies will always be much more

impressive than those at 20 Hz! There is an approximate

6 dB per octave relationship at work here. A trans-

former specified for level or distortion at 40 Hz, for

example, will handle about 6 dB less level at 20 Hz and

have at least twice the distortion!

Seen in transformer-based hum eliminator adver-

tising copy: “Frequency response 10 Hz to 40 kHz

±1 dB into 10 kȍ load” and “Distortion less than

0.002% at 1 kHz.” What about the source impedance?

Response at 10 Hz and distortion are always much better

when a transformer is driven from a 0ȍ source! What

happens when a real-world source drives the box? For a

full-range audio transformer, measuring distortion at

1 kHz is nearly meaningless. Section 11.1.3.1 explains.

11.3.2.2 Comprehensive Data Sheet Example

For reference, the following is offered as a sample of a

data sheet that has been called truly useful and brutally

honest. Note that minimum or maximum limits are

guaranteed for the most critical specifications!

Figure 11-53. IEC Common-mode test for input types.

10 7p1%Rs/2

Gen

10 7p1%Rs/2

DUT

Buf VMeter

Figure 11-54. Impedance tests.

Z

DUT

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