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Here's what happens... You buy a shiny new transistor microphone preamplifier. It cost you a fortune, but it gives you great sound. Every time you plug in a microphone, you congratulate yourself for having bought it.
But, as time goes by, it doesn't seem to give you all the quality you need. You tell yourself that you're probably getting accustomed to the sound and it's not such a novelty anymore. Or perhaps your aural discrimination is better now, and you're not so easily wowed.
Eventually, you start to doubt whether the preamp was actually any good in the first place. Each time you plug in a microphone, you get less and less satisfaction.
And each time you plug in a phantom powered microphone, something else happens.
Naturally you will instinctively pull down the fader before you plug in a mic, because you know you will get a ear-splitting spike coming through the speakers. That's the phantom power kicking in.
But even with the fader down, it gives the input transistors of the preamp a good kick too.
Over time, this causes an effect known as 'zenering' where the base-emitter junction of the transistor gradually breaks down.
This has the effect of reducing the 'beta' of the transistor, which is its intrinsic gain.
All amplifiers are designed with excess gain, which is then reduced to the desired value through negative feedback.
Negative feedback also reduces distortion and flattens the frequency response. It is essential for good results.
But, negative feedback works best when the intrinsic gain of the preamp is high.
As it becomes lower through zenering, negative feedback has less excess gain to work on, hence distortion increases and the frequency response becomes less flat.
Zenering is a real phenomenon, and its effects are real. Ask NASA - they know all about zenering in spacecraft circuitry, and take steps to prevent it.
So what's to be done?
Firstly, the designer of the circuit should implement protection against input spikes, and such protection should not of course affect the signal quality.
Secondly, zenering has been observed to 'heal' over periods of non-use.
Thirdly, it has been said that transistors affected by zenering can be restored through heating. They can also be restored by replacement, which seems a better solution (although not for NASA!).
The real problem with zenering though is that it causes uncertainty. That vintage transistor preamp you have just bought might have been slowly degrading over the last twenty or so years of use. You think you have bought a classic, but you have really bought a dud.
So is there any good news to report? Yes, zenering only affects transistor preamps. Use tubes and you will be totally immune. And hey... you wanted your vintage transistor preamp to have character, didn't you?