Probably unlikely --- at least one using the inductors used for the main project. SDC capsules will tend to have lower capacitive values than LDC, so you're likely to need alternative inductors.This is a pretty long thread so I'm gonna ask, has anyone managed to make a good small diaphragm DIY RF mic?
The terminal inductor here works just the same in resonance with T1 and capacitors. I can't describe the math exactly yet, but the fact is that it can only have one best value. And you can see how the current in it jumps sharply.I don't think you'll get a useful mic without the the high 'Q' noise free gain the second inductor adds to the assembly.
I always encountered way too much noise, when trying to rectify the signal from the bridge directly.
As regards the FET biasing. T2 secondary essentially grounds the FET gate, so it's automatically 'self biased' to the optimum setting for this dual infinite impedance detector / differential audio driver role....
The capsule currently in it is single-sided, and had a capacitance value that I measured at around 80-90 pF - I have two of the same make/model meter, and I'm at least getting consistent readings between the two.What is the measured capacitive value of each side of the capsule?...... and what value of C4 did you choose to match that value?...
I've ordered four of 'em. (I needed some capsules anyway...)That was made using one of these capsules: https://www.aliexpress.com/item/1005003678385151.html
The schematic notes do include a comment that 2 capacitors in parallel may be required (bottom line of the notes - in brackets)......... As a result, I used a single 68 pF cap, taking the small amount of space and the instructions to use the 'closest but smaller' to mean 'single' cap size........
I'm aware it's experimental; I've read (or tried to read?) most of the posts. The first step is replication, of course.The schematic notes do include a comment that 2 capacitors in parallel may be required (bottom line of the notes - in brackets).
I maybe need to make that more obvious? -- it can be quite important!
I should perhaps repeat that this project is very much an experimental one , and not a completed 'hobby kit' style project.
You will probably have noticed that there are some component locations on the PCB that are not used.
Khron had very patiently changed the board layout several times as the project progressed, and different aspects of the project were discussed and modified. The current layout has proved to be very useful -- and without Khron's input the project may well have foundered a couple of hundred posts ago!
Some project details are still subject to discussion and change, so it remains very much an ongoing experiment.
I'm always fascinated to learn what others discover and suggest as it continues....
I'm afraid a capsule with that low a capacitive value is not likely to perform very well in this case......... the new device measures at 49.5-49.9 μF - repeatedly.
That means the 68 pF C4 cap had to go, and be replaced with a 47 µF cap. Unfortunately, after replacing it and re-tuning, performance didn't change much - I wouldn't be surprised if it's somewhat similar to the issue shot saw last year, where it's sensitive to some unknown property in the capsule.............
Then it's time to wait for shipping. That's not a huge loss, as it's time to learn more.I'm afraid a capsule with that low a capacitive value is not likely to perform very well in this case.
The capsule is in series with C4, so for a capsule value of 49pF in series with a 47pF cap fitted for C4 you are going to get an effective value of around 24pF as the load across T1 secondary.
Using a 10MHz crystal, that would ideally need a inductor with a value around 10.5uH. Significantly above the max 8.2uH figure posted for the Spectrum inductors.
So a much lower 'Q' than is ideal. Lower sensitivity - and thus more noise.
T2 and C8 adds to the loading across T1 secondary - although the maths required to calculate exactly how much extra 'loading' have so far proved elusive.
To date, I have had my best results with capsules above 65pF in value.
Edge terminated capsules tend to have a higher sensitivity, which is also useful for this type of project.
Centre terminated 'K.67' style capsules tend to be more effective than 'K.47' types - again because of the higher sensitivity.
It does seem to be a bit of a lottery, when it comes to buying these cheap Chinese capsules though......
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