superior npn of the two?

[donovas]

i just started using zetex parts which i find to be sonically better than run of the mil 327s that i've been using.

zetex sounds smoother and cleaner.

what of the transistor's spec promotes such behavior and which of the two zetex npns would you say would sound 'better' in an input duty of 50w amp?

http://www.mouser.com/ds/2/115/ZTX857-93446.pdf

vs

http://www.mouser.com/ds/2/115/ZTX851-90982.pdf

my guess is that 857 would sound cleaner with the lower output capacitance.

 

[davidsrsb]

Which is best depends on the circuit. Operating current and voltage, input and output impedances are just some of the key parameters

 

[JMFahey]

i just started using zetex parts which i find to be sonically better than run of the mil 327s that i've been using.

zetex sounds smoother and cleaner.

Semiconductor brand has no effect on sonic qualities.

what of the transistor's spec promotes such behavior

Which behaviour?

and which of the two zetex npns would you say would sound 'better' in an input duty of 50w amp?

Don´t know about "better" , not sure it even applies here, I´m quite certain both will sound exactly the same, to any human ear.
Not sure any human ear could pick one over the other or both over the transistor which was originally there.
Specially because most probably the "input transistor in a 50W amp" is inside a strong NFB loop which would tend to erase any microscopic difference which *might* have existed.

my guess is that 857 would sound cleaner with the lower output capacitance.

your guess is just that, a guess.
You read some specs, and imagine what effect that might cause on the sound.

I bet any competent amp designer will have made his amp immune to such minor changes in a not-that-important parameter (at Audio frequencies).

I also bet that wiring , grounding or layout will have far more effect than that tiny variation you mentioned.

And again, Zetex is a competent Semiconductor Factory, I do not expect surprises from their plain , consumer grade transistors, they will work as well as all other established competitors.

Brand is not a parameter.

 

[simon7000]

For lower distortion one usually looks at current gain vs Ic. There can be significant variations. If the semiconductor is used without local feedback this may make a difference.

 

[dedegogo]

I guess i would pick the ztx851 because all good sounding transistors that i know have high fT (the 857 is 80M). Then there must be compromise somewhere for having Vce 300v. May be the 60vce of ztx851 has lower noise? But the Cob is unusually high. Hard to predict as i think i have never used Q with such parameters in input stage.