what does the 1n3 cap in JLTi VBIGC do?

[sandro600]

WHat does this cap do in Joe Rasmussens tube buffer design?

This cap has been mentioned in despatches several times as a tweakable component but I cant find a reference to say in which direction the sound moves with a change in value.


So,by altering its value how does it change the sound ?

Reason for asking is the bass and midrange are rich and beautiful in my amp using 1n3 (actually 2x680 nf in parallel) .

But the higher notes are not so clear and its recessed compared to the lower ranges. I was wondering if the 1n3 cap is the one I should change as a first step in trying to find better treble reproduction.?

Any other suggestions will be happily incorporated into the amp.
Thanks.

 

[darkmoebius]

Joe R goes into extreme detail on his website...

It keeps the high frequency feedback path extremely short and filters out high frequency slew rate distortion.

Halfway down the page here

Notice the 1n3* cap? Perhaps you have. It serves a couple of purposes, the first being that at very high frequencies it keeps the feedback path short (a la 47 Labs contention that feedback should be short but kinda difficult in a SERIES feedback situ). The second reason, in conjunction with 4K7 it becomes a low pass filter. You could say this is bandwidth limiting but there is more to it. It does tailor the HF response so that it is minus 1.25dB at 20KHz.

When feedback is not fast enough and in an Opamp IC, then slew rate distortion results. That is why many solid-state amp design have a passive low-pass filter (usually a full band-pass filter) right on the input stage. So in that sense what I’m doing is not new, but what is new is this, I did was adjusting this low-pass by EAR.

In order for the final value to be verified further by ear, I asked a friend to take the amp for a period of time and adjust the value. No other changes were to be made except this cap value. His system has what I would call an even balance and no tendency towards either brightness or dullness. I put a 390pF cap in there first and gave him a handful of others with increasing values, one of which was the previously preferred 1n3. This value was preferred as well by my independent tester..

 

[sandro600]

Yep thanks for that moebius . Im not technically savvy enough to interpret what that means in practice. I think what those quotes say is this: In a wave made up of many different frequencies some frequencies will travel through the system at different speed . The result is a wave at the output with a different shape to the original at the input.
the 1n3 cap is meant to prevent that from happening.


But,if i increase the value will high frequencies be dampened or enhanced

or vice versa

if i decrease the value will high frequencies be enhanced /degraded

Or maybe there is no strightforward relationship and it just HAS to be done by ear ?.