PSN-L Email List Message

Subject: PreAmplifier Design for HS-10 1Hz geophone
From: "Geoff" gmvoeth@...........
Date: Sat, 11 Aug 2007 09:12:52 -0700

Hello PSN;

below you will find the ONLY stable circuit design
I can use here. This design gives 40dBv ( X100)
gain to the input and phase shift destroys noise
on the plus input above a certain freq like 60Hz power noise.

I follow this with a buffer amp of also
X100 for X10,000 overall and with
a 12 bit converter ( using the upper 8 bits )
I can see about 16mv/count or
0.016/10,000 = 1.6 microvolts peak
per count.

I have found it best to use a buffer amp
that the preamp feeds directly to the
plus input then you use a resistor with
the same value as the preamps output impedance
( resistance)  so like that means a 50 or 60
ohm from the minus side to ground and possibly
5.1 K to 5.6K feedback resistor.

From the daytime noise here it amounts
to 82 femto watts into 360 ohms.

at night maybe 1/4 less noise.

On the image I give a freq but think
I may be wrong. Possibly the freq is X2
what i say. It will take one of you
experts to set me straight whats really happening.

All i really know is every little thing electrical
and otherwise will screwup my signals
if I do not use this one and only I ever found

I have never seen this design in the books
I accidentally found it by simply searching
for the source of unwanted noise.

Notice it involves the exact matching of all
components. The components are matched within
the resolution of the test equipment ( 3 1/2 digit MultiMeter )
The parts dont have to be the exact values but
for the yin must be the yang so at least try to see
the plus and minus sides are matched.

I had to share this with you because Id hate
to see anyone have the problems I had getting
a stable baseline ( Class " A" for you older folks
which is simply 360 deg waveform reproduction
as opposed to Class "B" 180deg and class "C"
less than 180 usually for oscillators)

Comments welcome as well as improvements.

Anyone give me an easy way to select the proper values
for my sensor to feed into WinQuake ?


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