PSN-L Email List Message

Subject: Re: optimal position of magnetic dampter
From: "Geoffrey" gmvoeth@...........
Date: Tue, 2 Jun 2009 10:19:01 -0700


I am currently using heavy damping to
help flatten my response because
my compensation circuit filters
like a LPF with a curve at like
10 seconds.

It seems if I amplify 1Hz to 80Dbv (x10000) I can
do almost anything I want and still get a
signal. Flatness of response is what I'm
looking for.

I am using a 470 ohm damping resistor
Split 50/50 between ground and +/- differential inputs.
for a single 450 ohm geophone.
This produces a current loop between the
geophone and amp that should override
the weaker noise.

I run the geophone coil straight into the diff amp
with no resistors in series.

I expect this to reduce overall gain by 1/2
plus geophone damping effect of raising
the point of roll off to over 1 Hz.

Is it possible to tell what the new
geophone curve is with such heavy damping ?


----- Original Message ----- 
Sent: Tuesday, June 02, 2009 8:22 AM
Subject: Re: optimal position of magnetic dampter

> In a message dated 02/06/2009, gmvoeth@........... writes:
> Do you  really mean Q of 0.7
> Which is a Damping of 1/0.7 ?
> If I make a  filter with a damping of only 0.7 I get a peak in
> the roll off response  instead of a Butterworth profile ?
> Hi Geoff,
>    You set the damping of a Lehman to 0.7 x  Critical value and also on a 
> geophone.
>    You are providing a force from the frame to the  mass which is LESS 
> than that required to damp it fully.
>    This gives about the best compromise for  the response amplitude / low 
> frequency corner. The rise in the  characteristic is only just visable. 
>    You loose signal amplitude rapidly as you increase  the damping over 1.
>    Regards,
>    Chris Chapman

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