07-11-2015, 03:29 PM
(07-11-2015, 02:29 PM)unrealdave Wrote: I would have thought that to increase the energy out you would need to increase the energy in so that can only come from a bigger aerial (or 2 aerials if your using 2 crystals).
I was working on the basis of a 2-crystal solution and wondered if the high impedance presented by the tuned circuit would permit a one aerial solution to work but the easiest answer would be to suck-it-and-see ...
(07-11-2015, 02:29 PM)unrealdave Wrote: I know that with high impedance headphones 2 or up to 4 K Ohms is normal because of the many turns of fine wire needed to have a useful magnetomotive force to move the plates and create sound waves, would a bigger current or a larger voltage be created from a longer aerial? And is it the current or the voltage that creates the magnetomotive force? any idea ?
The magnetomotive force is created by the product of the voltage and current which, of course, equals power.
High impedance headphones are needed with a crystal set so as to not unduly load the high impedance source provided by the tuned circuit.
The only crystal set I ever built was was quite late on in my early experimenting days and was built into a 2-oz tobacco tin with a jack plug on the end which plugged into a 10W amplifier I'd built which offered a 1MΩ load and didn't need headphones!
When I began experimenting, much earlier on, my pocket money didn't run to the high prices commanded by high impedance headphones but I did find a work around for my first one valve TRF. XWD low impedance earphones were available in large quantities at very low prices because their 60Ω impedance made them unattractive to the majority of experimenters.
However, I had an output transformer from a scrapped set which I used to drive my cheap low impedance earphone! It worked very well indeed!
What surprised me was that there seemed to be thousands of designs around for one valvers in those days and every single one demanded a pair of expensive high impedance headphones - the most expensive component in any set!
With such a plethora of cheap low impedance earphones available, why did nobody ever suggest using the solution that I adopted ...?
Even factoring in the cost of a new output transformer, the overall cost would still have proved a winner ...
Of course, if I'd been born a few years later I would have been in time for the first flood of far eastern 'trannies' to hit the UK market and the cheap crystal earpieces that could be bought to go with them.
I would imagine that the impedance of those devices would have been nigh on infinity (and would have certainly have needed an anode load resistor in my one valver to enable the valve to work!
Anybody ever found out what the breakdown voltage of one of those crystal earphones is?
