ZTX651/751 are still available. The only difference is that they have higher voltage rating than the ZTX650/750. Current gain. saturation voltage etc are identical
https://www.mouser.co.uk/datasheet/2/115/ztx750-1165283.pdf
https://www.mouser.co.uk/datasheet/2/115/ztx750-1165283.pdf
It depends on what you mean by low noise. The ZTX650/750 had no specified noise figure. However a very good indicator of rbb' is the saturation voltage. For the 650/750 651/751 this is down in the mV regime.
But for MM rbb' is not such an important parameter. You are more interested in low noise performance at an average of 8k source impedance (because of the rising impedance of the large inductance of a MM cartridge. For best noise performance with MM you are looking at a collector current of 100uA typical or lower.
For the Quad 34 MM, which uses a BC214 and BC413 they run at 55uA.
The ZTX anything is the appropriate choice for MC, where they run at 1.7mA
But for MM rbb' is not such an important parameter. You are more interested in low noise performance at an average of 8k source impedance (because of the rising impedance of the large inductance of a MM cartridge. For best noise performance with MM you are looking at a collector current of 100uA typical or lower.
For the Quad 34 MM, which uses a BC214 and BC413 they run at 55uA.
The ZTX anything is the appropriate choice for MC, where they run at 1.7mA
Alas no, the devil is in the details, these are noise figures and a noise figure means nothing without the source impedance and collector current it relates to. Noise figure at 100µA and 5k source impedance could be completely different to that at 1mA and 1k source, and different manufacturer's use different values which is hopeless.
The modern approach is to quote voltage and current noise densities from which you can determine the noise performance more systematically and doesn't depend on the manufacturer's choices.
There's some great info on this site, measurements for lots of transistors: http://www.dicks-website.eu/low_noise_amp_part3/part3.html
Current noise depends mainly on base current surely, for the shot-noise component? Excess flicker noise (current or voltage) is dependent on many things though, but a graph of noise spectral density is so much more useful I feel than other representations as its very easy to compare devices from them.
Current noise depends mainly on base current surely, for the shot-noise component? Excess flicker noise (current or voltage) is dependent on many things though, but a graph of noise spectral density is so much more useful I feel than other representations as its very easy to compare devices from them.
It focuses entirely on the equivalent input noise voltage and ignores the input noise current (and it's mainly the 1/f input noise current that goes up when the base-emitter junction is damaged due to reverse breakdown, so it is not strange that the author didn't measure any difference). It's useful, but it would be even more useful when you want to build an MC amplifier, or convert an MM amplifier to MC.
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Indeed comparing transistor at 10mA collector current is more for MC, where the base resistance is playing a part in the voltage noise. for MM the total noise must be balanced between voltage and current noise contribution. the latter playing a bigger role at the higher freqs as the source impedance is inductive. generally a minimum can be found determined by the HFE wich should be large as Marcel already indicated. We probably have been attending the same school of dr Nordholt. (he used to have a vacation home 1km from my home.)
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See also "Noise and moving-magnet cartridges", Electronics World October 2003, pages 38...43, https://worldradiohistory.com/UK/Wireless-World/00s/Electronics-World-2003-10-S-OCR.pdf Mind you, one of the sections of the gain switch is drawn in the wrong state in figure 5.