Len Polucci lenpolucci@.........
Subject: Robert's circuit




 =20
****Hi Chris,


    You say that the Roberts circuit will lower the frequency response x 10=
 but I thought I read it was x 4 or 5 for a geophone? =20

Hi Len,=20

    I don't know where you read it, but Roberts' article - if you read it o=
n John's web site - quotes a 1 Hz geophone extended to 0.1 Hz. And I use x1=
0 on 4.5 Hz !=20
With the gain increasing as 1/f^2, long period 1/f amplifier noise is the m=
ain limitation.

****Do you know of ANY other 'bass boost' analog seismo amp?=20
  =20
    Yes. The Roberts circuit uses parallel amplification. You can do exactl=
y the same job with a serial circuit.

****I do have Mariotti's EQ PCB unassembled, but I am wondering what it wil=
l do for an SP instrument with a natural period of 5 sec. Will it give me 2=
5 sec. or 50 possibly?


    You will have to recalculate the circuit values. Getting 25 seconds sho=
uld be easy.=20
Trying to go to 50 seconds may run into 1/f noise in my experience, but you=
 could add a 2 pole high pass filter to limit it. The problem does not usua=
lly arise with geophones.
I extended the response of a 1.5 to 4.5 seconds AS-1 out to 20 seconds with=
out any problems.=20
You can also do the same job digitally, but you may tend to run out of sign=
al resolution / have noise problems. 100 x 0 =3D 0 !=20
Isn't there digital compensation provided on Winquake these days ?? There i=
s on AmaSeis, but you have to extract the trace first.=20

    I hope that this helps,

    Chris Chapman


=20











=
Len Polucci lenpolucci@.........

Subject: Robert's circuit













 =20

****Hi Chris,






    You say that the Roberts circuit will lower th=
e frequency response x 10 but I thought I read it was x 4 or 5 for a geopho=
ne?  


 


Hi Len, 


 


    I don't know where you read it, but Roberts' articl=
e - if you read it on John's web site - quotes a 1 Hz geophone extended to =
0.1 Hz. And I use x10 on 4.5 Hz ! 


With the gain increasing as 1/f^2, long period 1/f amplifier noise is =
the main limitation.


 


****Do you know of ANY other 'bass boost' analog seismo amp? 


   


    Yes. The Roberts circuit uses parallel amplificatio=
n. You can do exactly the same job with a serial circuit.


 


****I do have Mariotti's EQ PCB unassembled, but I am =
wondering what it will do for an SP instrument with a natural period o=
f 5 sec. Will it give me 25 sec. or 50 possibly?



 


    You will have to recalculate =
the circuit values. Getting 25 seconds should be easy. 


Trying to go to 50 seconds may run into 1/f nois=
e in my experience, but you could add a 2 pole high pass filter to lim=
it it. The problem does not usually arise with geophones.


I extended the response of a 1.5 to 4.5 seconds =
AS-1 out to 20 seconds without any problems. 


You can also do the same job digitally, but you =
may tend to run out of signal resolution / have noise problems. 100 x 0 =3D=
 0 ! 


Isn't there digital compensation provided o=
n Winquake these days ?? There is on AmaSeis, but you have to extract the t=
race first. 


 


    I hope that this helps,


 


    Chris Chapman