Hi Ted,

  Note that mass itself does not have much influence on sensitivity,
and high mass makes proper damping harder to achieve. I can see no
advantage to having balanced outputs.

  A seismometer using a vertical spring is the worst design possible.
For example, a vertical spring must be stretched one meter from its
zero length to obtain a natural period of two seconds. You should take
a look at the AS-1 student seismometer

 http://www.jclahr.com/science/psn/as1/index.html

  Also, see the vertical  sensor described on my web site

  http://bobmcclure90.googlepages.com/sitemap

  While you are there, download the "SpringCalc.zip" file.
Experimenting with this application will help you in designing a
vertical seismometer, using whatever spring you might have available.

  Whatever you do, use magnetic damping or shunt resistive damping,
never oil damping.

  Amateur vertical sensors usually have too short a period for
adequately displaying teleseisms, especially the S and L phases. The
AS-1 uses a "bass boost amplifier to make up for loss of long-period
response. However, my WQFilter program can extend the useful period by
many times without the need for electronic compensation. My sensor has
a period of 4.4 seconds, but I use WQFilter to extend the response to
32 seconds. I use an amplifier built by Larry Cochrane, modified to
pass DC by shorting out the interstage coupling capacitor. The
resulting output bias level is somewhat sensitive to ambient
temperature rate of change, but tolerable. I found the amplifier to be
unstable when hooked up to my sensor. This problem was cured by
placing a one microfarad bipolar capacitor across the input terminals.

Cheers,

Bob
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