Audio impedance transformers

When I started building crystal radio`s, some years ago ar an age of around 10 years, I used a piezo-electric one connected directly to output of the diode and ground. But further into the future around 30-35 years  I began building some DX crystal radio`s that needed some matching between the circuit and the headphone.

Here the few auto transformers I have in my collection to use in my crystal radio receivers.


The first matching transformer I used as the "Bogen T-725".
It`s input impedances are: 150Ω, 300Ω, 600Ω, 1.2KΩ, 2.5KΩ, 5KΩ, 10KΩ,
20KΩ, 40KΩ.  Output impedance is a single 8Ω.
Impedance measured at 300Hz


Next one I used was the "Adastra 952.431".
This one has input impedance`s of 5KΩ, 10KΩ, 20KΩ, 40KΩ, 80KΩ.
Output impedance`s are 8Ω and 16

I used four of these to conect the primairy side in series to get 320KΩ. The secundairy side was coupled in 2 transformers each in series and then the coupled in parallel to get an output impedance of 8Ω again. This output was coupled into the secundairy of a "Bogen T-725", the 150Ω "input" was used as an output to connect my soundpowered decktalker.


Next one in the row is the so called "Uberformer" sold by Dave Schmarder. But
when taking of the sticker on top it`s a "Hammond H301126"
Input impedance 600Ω, 1.2KΩ, 2.5KΩ, 6KΩ, 10KΩ, 200KΩ.
Output impedance how you connect it, same as input. Not really low value input.


Here`s another one, model "KPB-02-200KΩ". With lot more impedance value`s. Value`s are: 64Ω, 150Ω, 300Ω, 500Ω 800Ω 1.5KΩ, 2.5KΩ, 5KΩ
10KΩ, 20KΩ, 40KΩ, 100KΩ, 200KΩ.
And 4
Ω - 6Ω - 15Ω - 32Ω output impedance.


Another one is one designed and made by Gerd Reinhofer(sk) of Reinhofer Electronic. This is the 53.48 version for high-Z headsets.
Input impedance 100KΩ-200KΩ-300KΩ-400KΩ-500KΩ
Output impedance 1000Ω-2000Ω-4000Ω-8000Ω

Also available as 53.48spez.
Input impedance 100KΩ-250KΩ-450KΩ-700KΩ-1000KΩ
Output impedance  1000Ω-2000Ω-4000Ω-8000Ω

Another version as 53.48RA
Input impedance 100KΩ-250KΩ-450KΩ-700KΩ-1000KΩ
Output impedance  200Ω-500Ω-1000Ω-2000Ω

These are not cheap!!
My "super ubermatch" is made using a 53.48RA transformer.
Another one I plan to make is four or six of these 53.48RA in series and two ceramic switches to switch between different impedances at the input side.
Input impedances from 100KΩ to 4MΩ
Output impedance 200Ω-500Ω-1000Ω-2000Ω

If using the 53.48spez for the output you can use a headset with other impedances.

Do I need a such high impedance, the answer is yes. With contra-wound toroid coil with a Q of approx 1000 at 1600Khz and 200ÁH coil, the Rp will be approx 2MΩ.
And 3xRp will then be 6MΩ.

So 4 of these is still a low value to impedance match the whole circuit.

I can make a match to get 6MΩ. But measurements have to prove if the loss of power because of the high resistance is worth to do it.

There are a lot more transformers on the market or for sale on HAM- fests from ripped equipment. But these are the ones I have.

PA2MRX  2020