I wouldn't worry so much about LF response. With 100n, the +3dB point is about 12Hz and response at 20Hz is -1.3dB.• I was wondering if C5 and C10 fitted as 220nF extended the LF response too far? Depends on what impedance you're working into of course, but with the BC557B emitter followers having a typical Hfe of around 280, I thought it might be better to try 100nF.
Even if the good capsule measured ar 60-80pF, the one with 33pF would sound only about 6-8dB below, and have no more noise.so I've tested three of those capsules and they have 33pf, 44pf and 48pf of capacitance. Really low!
Yeah, I'll do that!Even if the good capsule measured ar 60-80pF, the one with 33pF would sound only about 6-8dB below, and have no more noise.
You should probably look elsewhere. Can You check the RF level at T1's primary, against teh one that workd good?
I was always able to find a tuning point. Never had a situation that any of the capsules won't even tune. But noise is mostly constant, in some positions it falls off. But at that position sine wave response also falls of significantly, so in practice SNR at that point is even worse.I have built around 10 of these mics, and all have had acceptable noise floors. The lowest noise and best sensitivity came from a version using a 3 micron edge terminated capsule, which measured at around 90pF.
That same capsule would also collapse if you applied more than around 50V DC from a 'conventional' circuit. So I have concluded it probably has a lower tension membrane - which is ideal for an RF mic!
It also allows the system to be run at 8MHz, which is closer to the 7MHz specified on the inductor spec sheet.... So the Q is probably a bit higher.
I have tried several 'K.67' type Aliexpress capsules and have found them to be quite sensitive. Those have all had a capacitive value of around 65pF.
I have also tried a couple of Chinese'K.47' style mics - again with values around 60/65pF - but these were a lot less sensitive (by around 10dB).
If you use a capsule with a significantly lower capacitive value, I've found the bridge imbalance is often too extreme to allow a 'useful' calibration point. With the other bridge capacitor being significantly larger than the capsule, the resonant inductor tuning point generates a higher level of RF carrier - and with a smaller proportion of capsule capacitive value 'variation'.
This results in a lower modulation amplitude, with a higher RF carrier level, and this will tend to generate more noise.
I've also found that a significantly unbalanced bridge seems to allow several 'peaks and troughs' in the inductor tuning range - with no obvious dominant peak setting.
With a higher value capsule - and a more balanced bridge set up - there tends to be very much a dominant 'peak' setting -- which is often quite dramatically obvious. A much improved signal to noise ratio as a result of course...
Ideally, using a capsule with a capacitive value of - say - 40pF you would probably get better results from an inductor value of 10 or 12 uH. Unfortunately I've not been able to locate any such device off the shelf -- although I imagine some experienced hobbyists might consider 'winding their own'?....
Using a 5.3uH inductor with such a low capacitive value will require a lower value bridge 'balancing' capacitor. That will require a much higher oscillator frequency -- which will probably result in a much reduced system 'Q' -- and higher noise!
The more recent project revisions have concentrated on reducing the system current. One major advantage of RF condenser mics is that the low impedance allows for outdoor use, and low current drain is a useful feature for battery powered recorders.
The lower oscillator voltage does reduce the system signal to noise ratio a little -- but not dramatically.
The project notes still include a link to the original circuit set up -- see HERE .
As I say, I've had my best results from a low tension 3 micron 90pF capsule. And the bridge being almost balanced seems to give the best results. (The self biased infinite impedance detector sits automatically at 'Vp' so it doesn't need much to turn it on and off!)
I can confirm that I too have always been able to find a tuning point.....sometimes several, with a particularly 'unbalanced' bridge.I was always able to find a tuning point........