dufus wrote:

can i just put a 1k resistor across pins 2 and 3?

Absolutely. This is not that different from what happens when you add many
mic attenuators. The mic attenuators I've examined provide load impedances
in the 1 to 2.5 Kohm range. Attenuators that are connected across pins 2 &
3 are generally recommended because of their minimal effect on CMRR

to my understanding
this degrades CMRR, since the differential signal is knocked down, but
not the common mode.

The real question is not CMRR, its resistance to externally-generated noise.
Do you have reason to believe that common mode noise is currently a serious
problem with your system? Or, are just worrying about possibilities, as
opposed to realities?

how much would CMRR degrade in dB with a preamp
with a ~5k input Z?

Don't know, because this calculation would be based on the source impedance
of the SM-57, not the input impedance of the preamp.

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.

aside The high frequency variation relates to a resonance between the
source inductance of the SM57 and a cable capacitance on the order of 0.035
uF. The resonance centers at about 45 KHz, from which the equivalent source
impedance of the SM57 can be calculated.

I would expect less than 1 dB variation due to adding a 1K ohm resistor.
This means that even in your pessimistic estimate, there would be a 1 dB
loss of CMRR which is almost always negligible, even in a high noise
environment.