Hi RL, Are you measuring a UTC 0-15 transformer. I'm guessing not because of the 50R:1Meg, the UTC 0-15 is 15k:1Meg per their spec's.
Do you have part number for the transformer you are measuring?
Mikek
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Hmm... maybe it's elaborate because the impedance is not regularly defined, the matching circuit can be adjusted to match the Q of the tank and the diode used, over a wide range of frequencies, signal strengths and diode characteristics. Mikek
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The capacitor is an integral part of the AM demodulation circuit, as is the resistive bleeder. Ceramic headphones are self-capacitive, but may need assistance in bleeding, if the detector diode isn't leaky.
The less effective the circuit is in peak detection, the lower the audible amplitude will be, ideally by a factor nearing the peak to average value (1.4), or a 2x the power level.
Once the signal is demodulated, it can be applied to an impedance matching transformer at the lower audible frequency.
RL
The low frequency bandwidth/inductance relationship of any audio transformer follows the same ballpark rule; where inductive reactance needs to exceed the characteristic impedance of the winding at it's lowest useful frequency.
The high frequency 50R/1Meg signal transformer I'm using as an example is from the Triad 3499 series, PN SP208-014.
RL
OK, nothing I could use for 300 hz audio. Mikek
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You are correct, it is part of the detector circuit. Obviously helpful, but 10 out of the first 35 crystal radio schematics in a google search show neither the cap or resistor. Then some have the cap only, and some have the resistor only, and some have both. I ask my question and didn't get any help,so I simplified the circuit hoping someone would find that an easier analysis. All the simplification did was open a a can of worms. Mikek
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The 'impedance' marking is simply an indication of turns ratio. There is nothing to stop anyone from hooking up a train of 100:1 impedance ratio windings to get effectively 100:1, 10000:1, 1000000:1. (turns ratios 10:1 giving effectively 10:1, 100:1 and 1000:1)
The limitation on LF performance will still be the inductance of the first high impedance winding attached to the signal source.
A 10:1 turns ratio transformer hooked up with it's windings in series becomes an autotransformer with a total inductance of 1.1^2 x the original 10 turn inductance. This inductance depends on the total numbers of turns used and the effective permeability of the core structure. Turns can be tightly controlled from unit to unit, permeability can't - so two identical parts in series will not guarantee a 50% split of the applied signal.
If however you connect windings from two transformers in parallel, the original relationship between turns, with respect to the source, is restored.
You can re-label most of your drawings with turns ratios and they will end up becoming simple voltage dividers. This is usually easier to understand for a ball-park assessment.
If multiple couplings are made where the voltage division does not agree, accidentally or intentionally, inserting a current limiting resistor in series with one or more of those couplings will allow resolution without abnormal loading effects, as the copper winding resistance is intentionally too low to do this.
If the parasitic inductances and capacitances determining the frequency response of one transformer begins to dominate, then the deliterious effects are shared by the combined structure.
Once you have determined the voltages present on the nodes of a combined transormer structure, an impedance determined by required bandwidth, apparent indunctance and stray terms can be assigned to the input signal terminal. The impedance of all other nodes is
(Vs/Vn)^2 x Zs = Zn
Where Vs = input signal Vn = voltage measured (or calculated from turns ratios) for a node Zs = assigned impedance of signal input terminals Zn = expected node load impedance when Zs is satisfied.
RL
turns ratio of Bogen transformer taps (with respect to 8R winding)
300 - 6.124:1 1200 - 12.25:1 10K - 35.36:1 40K - 70.71:1 300 to 10K branch 35.36-6.124 = 29.24 (Z = 8x29.24^2 = 6K8)UTC 1Meg:15Kohms N= 8.165:1
Z reflected on 1Meg winding is actually 6K8 x 8.165^2 = 453K
If intended input impedance is 1Meg, then LF corner can be expected to be degraded by loading of the Bogen part, if both transformers have the same rated bandwidth at the labeled impedance levels. The magnetizing current of the coupled winding will dominate the characteristics of circuit at a higher frequency than was originally intended.
If the parts were both rated to go to 50Hz on the low end, then your aim of 300Hz might be realistic.
Looking at the websites and projects on them, I've got the feeling that the main advantage of the crystal AM receiver, its simplicity, is lost on these guys. It looks like they're having fun, though.
RL
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