Update: I think I found the culprit, a busted-off piece of wire stuck in the breadboard on the row where the collector was. I rebuilt the circuit elsewhere on the breadboard and it's behaving properly now (at least in terms of getting me the Q point I was looking for). Next step is to try to find a signal to amplify; I'll have to root around in the basement for a microphone or something.

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Microphones lurk everywhere.

That old cellphone has one, but probably small and too fiddly. Regular desk phones changed from the old carbon granule type to electrets decades ago. Loads of toys have sound activated on/off, also those key ring locators that bleep when you click your fingers.

 

If you want a high gain mic amp, the 330Ω load resistor is much too low. Redesign the amp with ~10KΩ to 47KΩ as the load resistor. It will probably take at least two stages to get an electret to headphone or speaker level.

 

If you want a high gain mic amp, the 330Ω load resistor is much too low. Redesign the amp with ~10KΩ to 47KΩ as the load resistor. It will probably take at least two stages to get an electret to headphone or speaker level.

A few years back I had a design published for an electret booster that was based on a TL431 programmable zener.

With the electret strung from Vin to GND and an 80R speaker as the cathode load, I rigged a 47k pot from cathode to Vin to set the operating point to a suitable value. There was enough gain for howlround, but that was about it - then I realised the 47k pot was feeding AC nfb as well as the DC nfb that set the operating point, the solution was to put a 1k2 between cathode and the pot and AC shunt the tap between the 2 resistors with a 100uF cap.

The booster proved very versatile and experiments with it went on for a while - at one point I stripped the special JFET from a scrap electret capsule and played around with various piezo disks etc - just out of curiosity I also hooked up various types of capacitor, the booster was sensitive enough to reveal most of them were microphonic to some degree.

 

Hello Ian,

Was the booster something like the amplifier in this schematic?

It comes from this techlib page:
http://www.techlib.com/electronics/crystal.html

Bertus

 

Hello Ian,

Was the booster something like the amplifier in this schematic?

It comes from this techlib page:
http://www.techlib.com/electronics/crystal.html

Bertus

No - and that circuit has a coupling capacitor actually *INCREASING* AC nfb!

My first prototype had the 80R speaker as the cathode load - not capacitor coupled.

None of my circuits had a volume control before the 431, the electret capsule was where the 1M is, the 470k was 47k in mine and there was a 1k2 between the 47k pot and the cathode. The 1k2 was to isolate the cathode from the 100uF electrolytic that shunted out the AC nfb so it couldn't cancel the signal at Vin.

One of the early builds had a 2k2 pot as the cathode load with the wiper capacitively coupled to the gate of an IRF540, that had an audio power transformer as the drain load. Not exactly Hi-Fi - but for 2 active devices, the amplifier was immense.

In the experiments with the salvaged JFET from an electret capsule, I tried amplifying various thing - one was a really ancient STC germanium transistor which produced pretty solid noise at high amplitude. A power amplifier was added and a very large piezo disk from a car alarm - the idea was maybe make a small ultrasonic cleaner, but the piezo just shattered into loads of tiny granules.

The circuit has been around for years - typically offered as a phono amplifier, but always with rather disappointing gain.

As far as I'm aware - I'm the first in print to suggest a way to eliminate excessive AC nfb.