Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Designing impedance-matched audio transformers  (Read 930 times)
KE6WNH
Member

Posts: 133




Ignore
« on: July 07, 2009, 10:28:05 PM »

One of the things that has stymied me in trying to build old-fashioned crystal diode detector radios is the absence of the high-Z earphones and audio transformers needed to make the circuits work. You'd think there'd be enough kids and other people around the world who still build crystal sets for there to be a market for these kind of parts, but I searched online and found none.

I have loads of transformers which I salvaged from junked TVs and stereos as well as spools of magnet wire and a winding machine for rebuilding them myself... but there don't seem to be any simple equations  along the lines of "X turns of #Y wire will give you an impedance of Z" (except possibly from engineering texts which cost hundreds of dollars). The trial-and-error method is out too, because a test instrument with an impedance range would cost as much as my car.

Anyone else run into this same barrier with crystal sets?
Logged
G3RZP
Member

Posts: 8070




Ignore
« Reply #1 on: July 08, 2009, 12:42:24 AM »

The turns ratio you need is the square root of the impedance ratio. So if you need 100 to 1 impedance ratio, you want a 10 to 1  turns ratio.

How many turns? It all depends.......Try figuring that you want the high impedance side inductive reactance to be  5 or 10 times the impedance, ideally at the lowest frequency, but for crystal sets, figure at 1kHz. That will give you a ball park inductance. Then your problem is how many turns for that inductance. There I can only suggest trying it.

Reference to Harmsworth's Wireless  Encyclopedia of 1924 suggests a stack of laminations 3/8 inch thick and 2 inches high by 2-1/2 inches wide. The bobbin is 1-3/4 diameter and 1-1/4 high ouside dimensions. It doesn't say how many turns: it suggests 44swg wire and about 1/5th of the bobbin for the primary and 4/5 for the secondary. This design also has you cutting the laminations out of soft sheet iron.....

Hope this helps

73

Peter G3RZP
Logged
AA4PB
Member

Posts: 14298




Ignore
« Reply #2 on: July 08, 2009, 05:44:46 AM »

http://www.midnightscience.com/catalog5.html has crystal ear plugs and matching transformers. That's just the first one I found on a quick Google search.
Logged
K4DPK
Member

Posts: 1120


WWW

Ignore
« Reply #3 on: July 25, 2009, 03:13:27 PM »

I've used filament transformers for output transformers on a number of occasions.

Why not try a 12 volt filament transformer?  Sounds silly, but a 120/12 volt is a turns ratio of ten to one, and impedance ratio of 100 to one.  

The transformer is designed for 60 Hz, and the total inductance would still be adequate at 1k Hz, because higher frequency transformers need fewer turns, not more.

Phil C. Sr.
k4dpk
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!