Why do crystal radios need high impedance headphones? (10-Analog Electronics)
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- Опубліковано 24 лют 2023
- Example problem: What benefit is offered when the earpiece is high impedance rather than low impedance? Modern radios use low impedance ones - let's see why they used to use high impedance ones instead.
Aaron Danner is a professor in the Department of Electrical and Computer Engineering at the National University of Singapore.
danner.group
Video filmed and edited by Cheryl Lim.
@randomcheryl
I'm a physics major, I once built a crystal radio when I was young, ur video makes total sense now
Very cool!
Thank-you! I’ve been trying to teach myself electronics and have been struggling to get past some erroneous intuition. In particular, I’ve struggled with why a lower resistance isn’t always better (after all, lower resistance must be easier). This example really helps to drive home why lower isn’t always better. Thank-you!
Thank you...a simple straightforward answer...now I understand...
I built a Crystal radio when I was a preteen from memory. I remembered the simple circuit from a Radio Shack 101 Electronic Projects Experimenters kit. I got a 1N34a germanium diode because of the .35 forward bias on voltage, AM slug tuned coil, a 365 pF, 365 mmf back in 1975, air variable capacitor, 2000 ohm resistor and a crystal high impedance earphone, all parts available from Radio Shack back then, and wired it together on a wood board with fahnestock clips. At first I was picking up shortwave radio with it but after I realized I needed to use the entire length of the loop stick coil and not the tap it received am signals.
Except in the case of low impedance rocking armature earphones from old rotary dial phones! I've proven this many times by demonstrating that these very sensitive earphones are an exception to the rule. The value of the earphone coil is 150 ohms Z, but it works flawlessly in the circuit you described.
The one I'm using is made by STC and is second sourced by Plessy, both English manufacturers
Thank you so much for taking the time make this video!!!
Beautifully explained. Thank you.
Thank you for explaining the answer to this problem so clearly. I don't think I've ever seen any videos actually explain why you want High Impedance Headphones for a Crystal Radio. It always seemed counter intuitive to me to say you needed High Impedance Headphones because that's higher resistance to current flow but I only recently began to grasp Ohm's law.
You also need to understand when you need a greater voltage (more push) then you do greater current (Amps - Flow). Don't the higher voltage fluctuations (Amplitude) above the Diode Turn On Point also drive the headphones more forcefully therefore pushing more air into you ear (Volume)? Thank you for sharing.
A higher voltage reaching headphones of resistance R would ordinarily drive more current through them (by V=I*R) and give more volume; however, the antenna here is not a good voltage source but rather a bad one. The voltage just drops when you try to draw more current. This effect can be modeled as source impedance, and the result will be a volume limitation.
@@adanner Thank you for your follow correction. I'm obviously still grappling with this. But, I won't give up.
@@adanner So is this, the antennas inability to supply stable current when demanded by radio (Headphones) another reason in addition to the Diode Bias Needed that High Impedance Headphones are needed. Is the receiving Antenna in this a case a High Impedance Source and the Rule of Matching Impedance maximizes Power Transfer from the Antenna to the Headphones (complete receiver circuit)? Thank you. If you haven't done so already I'm sure we could all benefit from a lesson on you about Impedance, especially regarding RF/AC currents and how major radio components react?
It's a really good idea for an explanation. But the germanium diode is not a switch at 0.3 V, proper demodulation of RF-AM signals also occurs well below this voltage. Nevertheless, the highest possible impedance of the headphones and a corresponding time constant with a parallel capacitor are important. In order to hear reasonably well, only a few ten picowatts of AF are necessary. For crystal earphones, the impedance is extremely high, but the self-capacitance is about 1.5 nF, so a parallel resistor of 100 kOhm is a good choice here.
I think it is better to have headphones or a load with the highest resistance possible to minimize the damping of the tuning coils and thus have better selectivity and amplitude.
Is there a way to communicate with you