Then that spike is up near 60kHz. Not audible on its own. VERY possibly a measurement artifact, ringing in 'scope leads etc.
Well, we really want to approximate the 30 Ohms of the cartridge. And we want to work at lower level than many sig-generators offer. The attached shows ~~600r to sig-gen and ~~31r to the transformer.
At 60kHz *capacitance matters*. This test adds the ~~30pFd of the oscilloscope; it is not stated if the target preamp is also loading the transformer. If the preamp offers selectable capacitance, you can adjust the loading for best square wave.
Alternatively, _I_ would use 47k low-C, plus a a series R-C of 47k+100pFd, which only kicks in above 30KHz.
Attachments
The 470R in the test circuit to evaluate the transformer includes the generator Zout.
If the generator has Zout=600R, add a resistor across the primary to reduce the impedance
that the primary sees to 470R. That would be 2.17k.
If the generator has Zout=50R, add a resistor in series with the generator to increase the impedance
that the primary sees to 470R. That would be 420R.
This is how Sowter describes the Zobel network and an additional resistor, to tame and load a SUT of theirs:
Attachments
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Scope lead ringing in my experience up well up in the 10--100MHz range, the stray inductance of the ground clip is around 30nH, coupling to a few pF in x10 mode or 50pF or so in x1 mode.
Perhaps larger scale ground loop could provide such a low frequency resonance? But more probably its due to the circuit.