PSN-L Email List Message

Subject: Re: Help with a Lehman
From: ChrisAtUpw@.......
Date: Mon, 10 Jul 2006 00:07:34 EDT


In a message dated 10/07/06, mike8s2@......... writes:

> I've been thinking of building a Lehman and was wondering what practical 
> considerations I might run in to if I significantly lengthen the boom. 

Hi Mike,

       Lengthen the boom to what? 2ft is about enough unless you want periods 
over 40 sec. The main considerations are the damping, which is far easier if 
you use magnet + plate damping and the bottom bearing. The old points and 
knife blades are not satisfactory and are likely to seriously limit your response. 
The rolling ball and the crossed cylinders are fine. See 
http://pages.prodigy.net/fxc/ and links

>  Also, I've read at least one account in which a Lehman device produced a 
> "spurious" signal due to vibration of the supporting wire.  

       This should not be a problem unless you use a really thick heavy wire. 
Thin music wire or SS fishing trace are fine, preferably in a V to prevent 
the boom rotating / oscillating axially. The other main consideration is to have 
a light but rigid boom. I use 1/2" nom SS water pipe with compression 
fittings, but Al and fairly thin wall brass are readily available. 

       Put your pickup coil on the end of the boom and use a non magnetic 
mass, brass or lead are fine. If you put a magnet on the boom you will see a 
great number of spurious responses, mostly from transients in the utility supply 
house wiring, fridges, changes in the Earth's field, movements of cars and 
bikes....

       Regards,

       Chris Chapman
In a me=
ssage dated 10/07/06, mike8s2@......... writes:

I've been thinking of building=20= a Lehman and was wondering what practical considerations I might run in to i= f I significantly lengthen the boom.


Hi Mike,

       Lengthen the boom to what? 2ft is about= enough unless you want periods over 40 sec. The main considerations are the= damping, which is far easier if you use magnet + plate damping and the bott= om bearing. The old points and knife blades are not satisfactory and are lik= ely to seriously limit your response. The rolling ball and the crossed cylin= ders are fine. See http://pages.prodigy.net/fxc/ and links

Also, I've read at least one a= ccount in which a Lehman device produced a "spurious" signal due to vibratio= n of the supporting wire. 


       This should not be a problem unless you= use a really thick heavy wire. Thin music wire or SS fishing trace are fine= , preferably in a V to prevent the boom rotating / oscillating axially. The=20= other main consideration is to have a light but rigid boom. I use 1/2" nom S= S water pipe with compression fittings, but Al and fairly thin wall brass ar= e readily available.

       Put your pickup coil on the end of the=20= boom and use a non magnetic mass, brass or lead are fine. If you put a magne= t on the boom you will see a great number of spurious responses, mostly from= transients in the utility supply house wiring, fridges, changes in the Eart= h's field, movements of cars and bikes....

       Regards,

       Chris Chapman

[ Top ] [ Back ] [ Home Page ]