Line In

[analag]

LineIn
Need a line in stage? There you go, It will take all you got and give you more than you need. All in class A.

analag

 

[Guest]

Yes, with a theoretical power supply... And what is that PFB loop for?

 

[ChrioN]

[quote author="analag"]It will take all you got and give you more than you need.[/quote]

:green: :green: :green:

 

[Kit]

Yes, with a theoretical power supply... And what is that PFB loop for?

I wonder myself. :?

Feedback should be taken from output emitter and in to FET source.
That will lower output Z and improve THD.

But maybe thats not always what we want.

 

[CJ]

Looks nice and simple.
Put it in a potted package, aka Hardy/Forrsell/Melcor for noise and thermal.
Put 8 on a small pcb, you have a mixer.

 

[clintrubber]

Yes, why is this in the Drawing Board ? :wink:

Apart from the PFB & JFET i.s.o. a BJT we have a 'known circuit', as shown for instance in this incarnation:
http://electronicdave.myhosting.net/miscimages/1588B-3.gif


Please tell more

Regards,

Peter

 

[rotation]

I'm also interested in knowing more about it.
How much gain is there and why is feedback positive?

 

[analag]

[quote author="rotation"]I'm also interested in knowing more about it.
How much gain is there and why is feedback positive?[/quote]

The gain is a little over 2 which is where I think a line input stage should be at. PFB is used to set the output level. Not because it is not used ordinarily doesn't mean it can't or shouldn't be used.

analag

 

[clintrubber]

[quote author="analag"]Not because it is not used ordinarily doesn't mean it can't or shouldn't be used.[/quote]
Sure, fully agreed.
But as in an old Ampeg-add I recall, be different to be better, not just to be different.
Is the PFB for grins or are there benefits you think ?

Annnnd... my words might qualify as negative feedback
but that's not intentional.

Regards,

Peter

 

[rotation]

[/quote]

The gain is a little over 2 which is where I think a line input stage should be at. PFB is used to set the output level. Not because it is not used ordinarily doesn't mean it can't or shouldn't be used.

analag[/quote]

Thanks. Is PFB 10k resistor and 100u capacitor from BC560 to 2N5457

Why did you choose 2N5457 for the input? I'm using it in another application and someone suggested to change it for BFxxx (something like this) because it's easy to get and cheaper.
I'm wondering which sounds better.

 

[analag]

PFB is used for one purpose and one only....gain increase.
BF245 comes in three iDSS flavors A, B and C.

analag

 

[Viitalahde]

Thanks, Rowan. Looks like something that can come handy.

PFB is perfectly ok.

 

[solder_city]

using a really weak ass input stage and then trying to boost it with positive feedback might be 'different' but it sure dont make a lot of sense to me.

PFB is perfectly ok.

if you wanna build an unstable amplifier. or an oscillator!

cmon, tell the truth, is this actually built and tested?

 

[analag]

[quote author="solder_city"]using a really weak ass input stage and then trying to boost it with positive feedback might be 'different' but it sure dont make a lot of sense to me.

PFB is perfectly ok.

if you wanna build an unstable amplifier. or an oscillator!

cmon, tell the truth, is this actually built and tested?[/quote]

I built it, tweaked it and yes it's sounding damn good with PFB applied. In fact the gain goes up quite a bit with feedback and no stability problems.
Absolutely dead quiet too. Listening to the opposite of the most often used NFB is a very revealing experience for those haven't ventured into this territory, more dynamic, more detailed are terms that come to mind. Will post the changes.

analag

 

[analag]

LineIn_Ver2.0

Here you go...built and tested in fact I'm listening right now. Rfb can be as low as 47K with no instability issue, I stopped at 47K I wasn't interested in going further. Those who hate NFB will love PFB if done right.

analag

 

[Viitalahde]

[quote author="solder_city"]if you wanna build an unstable amplifier. or an oscillator![/quote]

Well, Sontecs use PFB in the front end for gain adjustment.. So does my version of the EQ. Never heard of anyone using'em as oscillators.