iSK Little Gem internals

[RuudNL]

Yes, exactly what Khron already wrote.
Something in the range of 6.8 to 10 nF in parallel with the drain resistor will usually work, depending on the capsule used.
Maybe you have to experiment a little to get the amount of HF rolloff that you like.

 

[xeawr]

I always thought that's a no no:

Henry (?) from audioimprov writes:

A popularly suggested cure is to increase the capacitive load on the FET’s drain, that is increase C14, or add an additional cap across R7. That works, but not well. For one thing, it rolls off highs at XLR pin 3, but it also boosts highs at pin 2, and of course it unbalances the output signals. This may or may not cause big problems depending on the common mode response of the following preamp input, but it’s probably bad practice.

So does it now unbalance the signal or not?

 

[RuudNL]

The capacitor attenuates the signal at the drain.
It doesn't boost the signal at the source, because it is a source follower.
The impedance balance after the two PNP transistors doesn't change.
The differential voltage at high frequencies is reduced.
I prefer this way of HF correction over the way that is advised by mr.  Spragens, because the addition of two 10K.ohm resistors on the bases of the PNP transistors will increase the self noise of the microphone.
(He denies this, but I did measurements and the change in self noise was obvious, measurable and audible.)

 

[xeawr]

Thanks RuudNL, I appreciate the input. To sum it up: It DOESN'T unbalance the signal (pin 2 vs pin 3)?

 

[Khron]

Even if it does, the output impedance (which matters most re: CMRR, common-mode rejection ratio) is influenced mainly by the output PNP transistors.

https://sound-au.com/articles/balanced-2.htm

A short quote (all-caps as in the above-linked article):
"SIGNAL SYMMETRY HAS ABSOLUTELY NOTHING TO DO WITH NOISE REJECTION - IMPEDANCE IS WHAT MATTERS!"

 

[xeawr]

awesome! Good to know!!!

Would a SIGNAL imbalance cause any issues for a input transformer coupled mic pre?

 

[RuudNL]

No, because there will be no DC offset.

 

[xeawr]

hey there! I know this is getting slightly off topic but I wanted to let you know that I had very bad results with the drain capacitor (cap across drain resistor). Although it does roll of HF, I had a lot of (LF) distortion, that rendered the mic useless with the cap installed.
I see that the added noise by the 10k resistors is not something one would want, but it's less obvious than the distortion ... any way, just felt like sharing my experiences...

(I used a 8.2nF PET film cap in a t.Bone SC400)

 

[RuudNL]

Did you check the FET bias?
(I never had higher distortion with a capacitor in parallel with the drain resistor. There is even no real explanation for this. At low frequencies the FET acts as a phase splitter; at high frequencies the FET works as a source follower.)

 

[Gus]

The capacitor attenuates the signal at the drain.
It doesn't boost the signal at the source, because it is a source follower.
The impedance balance after the two PNP transistors doesn't change.
The differential voltage at high frequencies is reduced.
I prefer this way of HF correction over the way that is advised by mr.  Spragens, because the addition of two 10K.ohm resistors on the bases of the PNP transistors will increase the self noise of the microphone.
(He denies this, but I did measurements and the change in self noise was obvious, measurable and audible.)

The impedance balance might get a little better at higher frequencies.

 

[xeawr]

Did you check the FET bias?
(I never had higher distortion with a capacitor in parallel with the drain resistor. There is even no real explanation for this. At low frequencies the FET acts as a phase splitter; at high frequencies the FET works as a source follower.)

Hi Ruud,

sorry, it took me forever to get back to this - it somehow slipped my vision. Anyway, interestingly I tried the 8.2nF cap in two different schoeps-style microphones, recording before and after samples and both of them exhibited the same kind of distortion. Then I removed the capacitor and the distortion was gone, it was really obvious. I have used one of those two microphones for recording vocals in a hotel room (before and after modification) and it worked very well. So, what could be the problem? Would the cap throw the fet bias off?

 

[Khron]

"The 8.2nF cap" - the same one tested in both microphones? What're the odds the cap itself is damaged / faulty?

Try a 10nF one (that you know is good), perhaps.

 

[xeawr]

"The 8.2nF cap" - the same one tested in both microphones? What're the odds the cap itself is damaged / faulty?

Try a 10nF one (that you know is good), perhaps.

Sorry, my bad - not very well put. I meant that I installed two different caps but of the same type/brand and value - and that didn't seem to work in both mics. Since then I've actually installed some foam around the base of the capsule holder and I have the feeling that smoothed out the high end as well.

 

[RuudNL]

I would check the drain voltage of the FET before and after installing the extra capacitor.
This all sounds very strange to me, since I never had distortion problems after adding a drain capacitor.
(There is even no explanation for this to happen.)