COMPARING AUDIO TRANSFORMERS

AUDIO TRANSFORMERS, SIZE MAKES A DIFFERENCE

JIG FOR TRANSFORMER A-B TESTING

A friend asked me to compare a small isolation transformer to a larger model I'm familiar with so I set about making a jig so I could easily switch between two transformer-parts under test or bypass. I chopped some alligator test clip leads in half to make it simple for connecting to the transformer under test. I used the standard Crimson input-output wiring color code for the test leads.

MINIATURE 4PDT SWITCH,  SELECTS BYPASS OR TEST POSITION

MINIATURE 4PDT SWITCH, SELECTS TRANSFORMER A OR B TO TEST

I used two 4PDT switches to construct the switching needed to A-B two transformers with bypass as a third option.

L TO R, CRIMSON ISOLATER, 620X, EDCOR 600:600

I tested three audio transformers, two were manufactured by Crimson Audio Transformers in Cape Girardeau Missouri and one is a 600 ohm to 600 ohm part manufactured by Edcor in New Mexico.

CRIMSON 620X

The first part tested is the Crimson 620X, 600:600 +20 dB isolation transformer made with 80% nickel laminations.

CRIMSON ISOLATER

The second transformer under examination is the Crimson Isolater, this transformer is packaged in a steel can and has flying wire leads for input and output, it's also the smallest of the three parts tested. I don't know what is in the can but I suspect it's the slightly smaller size of lamination stock (down from the 620X size) with more windings on the coils since the impedance of the coils measured almost double that of the 620X.

EDCOR 600:600 AUDIO TRANSFORMER

The third part tested is the Edcor 600:600 transformer made with iron laminations.  The Edcor transformers are physically larger and weigh more than the other two.

For reference my studio is aligned to 0 VU = +4 dBu signal level, all of the listening levels were adjusted to be passing 0 VU as an average level through the transformer under test.

SOUND SOURCES
These are audio transformers so part of the testing consisted of listening to music and part consisted of measuring performance with an audio oscillator and a NTI Minalyzer to measure the signal levels.

For testing purposes each transformer-secondary was loaded with the line input impedance of my Souncraft mixing console, the manual says the input impedance is 10K ohms so we'll assume that is the load across the secondary of the transformer under test.

MUSIC LISTENING
I set the Meyer HD-1 monitor speakers to mono since we would be only listening to a single audio channel at one time. I chose "Talking Blues" by Bob Marley as my test piece of music, this tune has a nice deep bass line and snappy percussive tones from the drummer. I looped it over and over so I could listen very carefully and critically to different parts of the song to hear how each transformer affected the sound.

My test jig allowed me to completely bypass the transformers instantly or switch to either of the two transformers under test.

All three of the transformers tested passed the listening test with virtually no audible difference detected between them. To my old engineer ears, all three sounded excellent, deep full bass response and crystal clear high frequency response.

I was surprised to not hear more of a difference. I expected to hear bass roll off or insertion loss or something but all three transformers sounded excellent.

OSCILLATOR TESTING
I used the Minalyzer to calibrate the output of the oscillator which feeds the transformer test jig, then moved the input of the Minalyzer over to the output of the transformer so I could see the affect of changing the frequency.

I focused my frequency tests in the bass region since low frequencies below 50 hz are very challenging for transformers to reproduce. Each transformer was fed 100 hz at zero-dBu for lineup and then the frequency was dropped to 20 hz and the level was measured. This showed the low frequency loss in level by the drop in frequency.

Here is the results of the oscillator 100 hz bass frequency tests:
620X = (-.18) dBu @ 20 hz
Crimson Isolater = (-.16) dBu @ 20 hz
Edcor 600:600 (iron laminations) = (-.28) dBu @20 hz

The loss in signal level was almost imperceptible in the listening tests and the frequency tests showed why, all three transformers had great low frequency level at 20 hz. Before moving on I dialed the oscillator down to 10 hz and each part quickly showed more roll off. I did not measure the level difference since I'm not interested in frequencies I can't hear.

CONCLUSION
Modern audio transformer production is at a pretty highly evolved state with many options to choose from. The Crimson parts had the best audio performance and are smaller and lighter in weight than the Edcor component. Sonically speaking I'd be happy with any of these in my audio signal path.

Good music to all!

SCHEMATIC FOR TEST JIG

TEST JIG SCHEMATIC FOR REFERENCE