Kurt Albershardt wrote:
Arny Krueger wrote:

Phil Allison wrote:

"Arny Krueger"

An interesting related article can be found at
http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF

Most interesting is:

"Figure 3 shows the effect of preamp input resistance (and
capacitance) on frequency response of a Shure SM57 with 100 feet of
common cable. The upper
curves, 10 kS and 3 kS, are typical of transformer-less mic preamps
while the lower curve, 1.5 kS, is typical of a transformer input
mic preamp. Note
the ultra-sonic peaks in response caused by insufficient damping".

This chart gives some insight into the output impedance of a SM57.
To summarize, there is negligible change in response at 3 KHz with
load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the
source impedance of a SM57 in the normal audio range (20 KHz) is
quite small. Other sources
give it as being 150 ohms or 310 ohms. It may be even less -
perhaps 75 ohms or less. The same charts show an approximate 5 dB
range of response at
20 KHz but less than 1 dB variation at 10 KHz. None of this is all
that audibly significant.

** Fig 3 in the Jensen article shows an overall variation of less
than 2dB at 20kHz for the three load impedances at the end of 100
feet of cable driven by an SM57. At 15 kHz, or the highest
frequency an SM57 actually reproduces, the variation is less than 1
dB while at 10 kHz that variation is less than 0.5 dB.

The HF response variation between different samples of the SM57 is
a lot greater than that !!

This article seems to throw quite a bit of cold water on the many
claims of dramatic sonic differences due to real-world variations in
the loading of SM57s by various preamps and cables.

It looks to me as though it's comparing the impact of different
loading resistors across the inputs of a particular preamp on the
output frequency response of SM57s.

Right, and that's what I said, isn't it?

Many of the sonic difference
claims (and my own experience) show a marked difference between the
sound of an SM-57 feeding a high impedance transformerless preamp
when compared with that produced by one feeding a transformer-fronted
preamp.

Sure, and the effects of transformer loading are not directly addressed
here. But what is addressed here relates to variations in resistive and
capacitive loading.

We've seen various claims about how preamps with various resistive input
impedances can make SM-57s sound dramatically different, right? It appears
that those claims have now been soaked with cold water by these careful,
real-world measurements. There are substantial measured changes in
performance, but they are all outside the audible range.

The bottom line is that these tests show that a SM-57 presents a relatively
low-impedance source to whatever loads it. They show that SM-57s tend to
perform in an audibly similar fashion with various combinations of
resistance and reactance.

Transformer-input preamps add two dimensions that these tests don't address
directly. First there is the response of the transformer itself. It
obviously acts like a bandpass filter of some kind. Secondly, transformers
can present inductive loads to microphones because the transformers
themselves can have inductance. This is shown as Lt in figure 5 for
example. The article seems to encourage us to ignore both of these, because
it does not mention any effects that are associated with them. It may be
that because of their quality, Jensen transformers have reduced the effects
of these parameters to the point where they can be safely ignored.