Here's a copy of a message I posted to the Ampex and Backroom mailing lists (via recordist.com) back in 2004:
--------------------------------------------------------------------
I discovered this line of VERY inexpensive line level transformers awhile back and ran some tests
which I distributed off-list. Bill Ruck's test of the isobox prompted me to share the info with
everyone else. Additional info from the manufacturer here (and, they are actually made in the
USA!):
http://www.edcorusa.com/transformers/audio/matching.htm
The replacement Edcors (600:600 impedance versions) arrived today, and the DCR was MUCH more
reasonable. Approx. 54.1 Ohms primary, 54.5 secondary. Level drop from open loading on secondary
to 600 Ohms was just a bit under 1.5 dB, much what I would expect.
[ Added note for this posting at Prodigy-Pro...the first set of 600:600 trannies that I received had a VERY high DC resistance, on the order of several hundred Ohms, on both the primary and secondary. The other correspondents on the Recordist mailing lists were measuring around 50 Ohms DCR on the samples they had bought, so I contacted Edcor. They gave me some "cock and bull story" about a production line design change, but they replaced the units I'd purchased at no charge with the "old" design.]
For testing, I breadboarded a "zero Ohm source" driver with a 5534 opamp running as an inverter:
10K input and feedback R's, 22 pF compensation cap (between pins 5 and 8) and 22 pF in parallel
with the feedback R to tame any overshoot and ringing from the opamp itself. -3 dB response with
the opamp circuit calcs to around 723 kHz. I powered the 5534 with +/- 20 VDC from my bench
supply, which allowed just over + 24 dBu/m at clipping.
1 and 10 kHz squarewave response through the iron looked very good, loaded or unloaded, with just
a
tiny bit of overshoot and ringing. Frequency response was beyond anything I could measure with my
Amber (bandlimited at around 500 kHz or so) which came up with -1 dB at approx 6 Hz and 300 kHz.
Bypassing the Amber and just watching the scope, usuable response was extended, and limited by the
rolloff of the opamp driver on the top end. With a 50 Hz square wave, there was very little tilt
which indicates LF response well below 20 Hz.
[Another sidebar for this posting at Prodigy-Pro...I recall very little difference between open circuit and a 600 Ohm termination.]
Next I measured THD, and at some of the lower levels I was fighting noise picked up from the
breadboarding! Here are various "spot measurements".
1 kHz, 0 dBm = 0.0085% +10 dBm = 0.005% +20 dBm = 0.0025% (noise was an obvious factor!)
100 Hz, 0 dBm = 0.03% +10 dBm = 0.015% + 20 dBm = 0.05% **
50 Hz, 0 dBm = 0.055% +10 dBm = 0.035% +20 dBm = 0.18% **
20 Hz, 0 dBm = 0.07% +10 dBm = 0.085% + 20 dBm = 0.7%
Spot checks at LF and + 20 dBm: 40 Hz = 0.3% 30 Hz = 0.45%
(Actual 20 Hz numbers may be a bit lower since my 20+ year old Amber was having a hard time
nulling
at 20 Hz).
** Notice the THD dip at the "middle" levels at those frequencies. I also think I detected a THD
rise at levels below 0 dBm but was fighting noise issues.
I have a bunch of other numbers, but you get the drift. Looking at Jensen choices, this Edcor is
just a bit "rattier" than a JT-11-EMCF which sells for $66.85. These Edcors are a bargain!!