BF862 Preamp

[wrenchone]

The Farnell socket looks like it's a better quality - at times, you do more or less get what you pay for. Instead of removing contacts (a direct solution - I still may do that), I marked the socket with white splotches indicating orientation). I may still amputate contacts... Chop, chop (think first, chop later...)

[jameshillj]

Thanks for the info - that makes it easy now - off to market and get sock(et)ed!

[njepitt]

Hi everyone,

I've spent a little time this afternoon fiddling with one of my versions of this preamp, and I found something I don't understand. (Again....)

Referring to the schematic in post #28, C4 (with C10, C11) and R9 (with the input impedance of whatever it's connected to taken into account, I guess) form a high-pass filter, blocking any DC. (Right?)

But what is R20's role? Is it a "safety resistor", as NP referred to something similar in his B1 article? And if so, how do you decide on its value? I notice that Juma changed the original 33R to 15R in the 2SK170 version in post #283, but why?

Apart from just wanting to understand, I would like to switch R20 for something a bit better quality I have kicking around, and am wondering what replacing the 33R with 56R would change (except for some loss of volume, I presume). I should add that I am not using C10/C11, and have a 10uF polypropylene in place of C4. I could just try it and see, of course, but I like to understand things when possible.

Regards

Nigel

[juma]

Originally Posted by njepitt

...what is R20's role? ....

R20 is there to isolate the input capacitance of the next stage (including interconnects). We don't want to load the preamp with pure capacitance...

[njepitt]

Hi Juma,

Originally Posted by juma

R20 is there to isolate the input capacitance of the next stage (including interconnects). We don't want to load the preamp with pure capacitance...

OK, I can see part of that - R20 would form a low-pass filter with the input capacitance (say Cin), and hence the value of R20 (say R)$ would determine the frequency of the filter - but I don't see how that would determine the value, since the input capacitance is surely pretty low, and hence the frequency (1/2(pi)R*Cin) is certainly high enough not for audio, isn't it?

Is 33R just a sensible balance between "not loading with pure capacitance" and not losing too much gain? If so then 56 R should work just fine, right?

Regards

Nigel

[Zen Mod]

two 56R in parallel are even better



just take it as counter measure for ultrasonic oscillations , and (inter)connecting procedure safety

don't go above 33R - it directly adds to preamp output impedance

[njepitt]

Hi ZM,

Originally Posted by Zen Mod

two 56R in parallel are even better

Unfortunately I only have two of them - they were left over from another project. Not a problem - I was hoping to fire up my soldering iron and tweak the preamp a little this evening, but I can always put a couple of 33R on my next parts order and do it some other time.

just take it as counter measure for ultrasonic oscillations , and (inter)connecting procedure safety

Ah, the point being that the low pass filter deals with the "ultrasonics", and has nothing to do with the audio band, as I (incorrectly) thought above...

don't go above 33R - it directly adds to preamp output impedance

Thanks ZM, and Juma, for the help.

Regards

Nigel

[Zen Mod]

well - put there anything from 10 to 47 Ohms

whatever you have in drawer

[njepitt]

Originally Posted by Zen Mod

well - put there anything from 10 to 47 Ohms

whatever you have in drawer

I'm searching through the drawer as we speak... Trouble is that I usually use cheap no-name resistors I can get locally - for learning and having fun they're fine (and the 33R that are in the preamp now work just fine), but I suspect for critical points like this it would sound a little better to use something of more reliable quality. (Although I'm not a great believer in boutique parts...) The 56R I found are PRP but, as you say, too large a value.

Regards

Nigel

[Zen Mod]

it depends ;

if next stage ( amp ) isn't current hungry ( low input impedance and/or capacitance ) , everything up to 100R is OK

maybe best thing for test is to put shortie instead ; that's best resistor - isn't it ?

but - I'm not expecting that you are going to hear too much difference