Hi Chris,

All the numbers I quoted were for the geophone *before* any circuit 
was attached. They came directly from the manufacturer's spec. sheet 
and represent the analog equivalent of its electromechanical 
characteristics.  What I wrote had nothing to do with the damping 
circuit, but is simply the circuit I am using in the Spice model to 
represent the geophone.

We are still playing with damping circuit component values.  Dave's 
configuration differs somewhat from the Lippmann design, though it is 
similar in principle.  A severely overdamped geophone behaves like an 
accelerometer, whose signal must be integrated to get back to 
velocity.  In the process, the low-frequency velocity response corner 
can be made somewhat lower than the geophone's original resonant 
frequency.  In the Spice model it will work with complete resistance 
cancelling, though the geophone still has about 32 mH of 
inductance.  In practice trying to get the acceleration response flat 
to DC sometimes results in oscillation, the nature of which I would 
like to study.  Indeed, things like the variation of geophone 
resistance with temperature are important considerations.

All these designs are compromises at best, as geophones have rather 
poor signal to noise performance at lower frequencies which no 
external circuits can improve.  There's only so much you can do with 
electronics.  If you start with a really good, expensive, geophone, 
you could probably do better, but the whole idea of the design was to 
see what could be done with the cheaper 4.5Hz, 375 Ohm parts.

Regards,
Brett

At 10:31 AM 9/24/2012, you wrote:


>Hi Brett,
>
>
>
>     What are the values of RL and Rd in your load circuit, please?
>
>     This seems to be a voltage generator circuit representation. The whole
>
>point of the Lippmann Patent is that you use the geophone as a CURRENT
>
>generator and you follow it up with a current to voltage converter and a
>
>top cut 'f' filter. The damping is then NOT 0.6, but it is massive and this
>
>effectively holds the armature ~still in ~the same position. You connect
>
>the geophone between earth and the -ve input of the opamp. The spring
>
>constant takes no part in the response.
>
>     One practical problem is that the resistance of the copper geophone coil
>
>changes with temperature and you need to keep the compensated resistance
>
>value just +ve, to keep the overall circuit stable. You could guarantee this
>
>by winding a copper compensation coil on the outside of the geophone, or by
>
>using a Pt foil thermometer element with a metal film resistor in series.
>
>
>
>     Regards,
>
>
>
>     Chris


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