K47 capsules roll off

[oscarhuang]

I builded some mics with K47 capsules (RK47, beeznezz k47 and Maiku k47 and lots of others). But I don’t really like k47’s sound. It’s so hash on 4-5k and roll off around 12k. I tried ef800 and Ami bv8, Km84 ,M149, u47 fet so many flat Circuit. I like Thiersch M7 capsules a lot but too expensive. Than I tried Schoeps circuit and add a 22NF cap parallel from source to gourd. Magic happened so detailed bright and airy sound.
my question is schoeps has balanced output. If I add cap on source to ground will change any output balance?
Any other circuit can boost HF at 10k or 12k?
This is my first post and very happy to know your guys.

 

[RuudNL]

In the Schoeps circuit, the FET is used as a phase splitter.
Normally, the amplitude of the audio signal on the drain and the source has the same amplitude, but opposite phase.
If you put a capacitor in parallel with the source resistor, the FET will work as a phase splitter for low frequencies, but at higher frequencies the audio signal on the drain will have a much higher amplitude than the audio signal on the source. The FET is followed by two transistors. But the frequency dependent unbalance will stay the same.
When the microphone is used with a balanced microphone input, the result will be as (you) desired.
The lower the value of the capacitor you put in parallel, the higher the 'boost' frequency will be.
(Personally I find 22 nF a pretty high value; it will also have an influence on the mid-frequencies.)

 

[oscarhuang]

Thank you for your reply. I really like your post and This idea Came from your other post and I did a tube Mic from your “ U67”-ish microphone post. That’s help me a lot and so many ideas from your post.
It’s any other way make a HF boost circuit for tube mics?

 

[oscarhuang]

 

[oscarhuang]

Will it work? C13 will be charge to whatever you wanna boost.

 

[RuudNL]

The schematic you posted will probably work. Now you use Q2 as a phase splitter, so amplitude on the collector and the emitter will (almost) be the same, independent of the frequency.
By the way: I would remove R6, R9, RP1 and C10, because they do the opposite from what you are trying to achieve!
(They attenuate high frequencies!)
Also R6 and R9 will increase the self noise of the microphone.

It will be much more difficult to create a HF boost in a tube microphone.
I suppose the only possibility is to introduce a form of feedback for the 'lower' frequencies and reduce the amount of feedback (=more gain) for higher frequencies.
In my "U67"-ish microphone you can simply reduce the value of the feedback capacitor. (120 pF if I remember correctly). The graph in the attachment shows the effect of the feedback capacitor, referred to the standard value of 120 pF. Of course you can get lower than 82pF, but I wouldn't remove this capacitor completely.

 

[oscarhuang]

Oh yes. This PCB is made for test all of capsules. K87 c12 and k47. PCB will be come next week and I will try it.
For u67-ish schematic I didn’t know will boost hf. I think is a flat circuit when you don’t apply the feedback capacitor. Maybe I am wrong. I will post other tube schematic see if that works.

 

[oscarhuang]

 

[oscarhuang]

This idea from La-2a FB. R7 8 9 and c3 may be able to increase the HF. I don’t know how to calculate the value. Will see some smarter people finish this.

 

[Whoops]

In my "U67"-ish microphone you can simply reduce the value of the feedback capacitor. (120 pF if I remember correctly). The graph in the attachment shows the effect of the feedback capacitor, referred to the standard value of 120 pF. Of course you can get lower than 82pF, but I wouldn't remove this capacitor completely.

It's the C17 capacitor on an U67 schematic.
Neumann schematic states a possible value of 80pf to 160pf for the C17 capacitor, but Klaus Heyne says he never seen an U67 with a value higher than 100pf for C17. He says that U67 either had 80 pf or 100pf

you can see it in this thread:

https://repforums.prosoundweb.com/index.php/topic,36297.msg537896.html#msg537896

 

[raywisby]

in the first revisions of the u60, this capacitor did not exist, a special winding of the output transformer does this, I believe for this reason the capacity is so small

 

[RuudNL]

How can a transformer alone provide a frequency dependent feedback? (Needed to compensate the HF lift of a K67 capsule.)

 

[raywisby]

How can a transformer alone provide a frequency dependent feedback? (Needed to compensate the HF lift of a K67 capsule.)

Similar to the pre-emphasis/de-em- phasis scheme in tape recording technology, the K 67 capsule was designed with a large treble boost – which the microphone circuit then compensated for an overall linear re- sponse. This was implemented via frequency dependent negative feedback across the en- tire microphone. The output transformer re- ceived additional windings whose signal was fed back to the capsule with reverse polarity.

 

[Whoops]

We all know that,
But you still didn’t reply to Ruud’s question
“How can a transformer alone provide a frequency dependent feedback?”

 

[RuudNL]

The transformer will produce the full spectrum on the tertiary winding, so no frequency dependent feedback!
(Just 'feedback')

 

[raywisby]

Okay, please explain where the filter is?

 

[Moby]

I builded some mics with K47 capsules (RK47, beeznezz k47 and Maiku k47 and lots of others). But I don’t really like k47’s sound. It’s so hash on 4-5k and roll off around 12k. I tried ef800 and Ami bv8, Km84 ,M149, u47 fet so many flat Circuit. I like Thiersch M7 capsules a lot but too expensive. Than I tried Schoeps circuit and add a 22NF cap parallel from source to gourd. Magic happened so detailed bright and airy sound.
my question is schoeps has balanced output. If I add cap on source to ground will change any output balance?
Any other circuit can boost HF at 10k or 12k?
This is my first post and very happy to know your guys.

My personal opinion is that K47 does not sounds harsh, but yes, compared to the M7 it has some more "presence" what can mask the "air". I speak about geniune Neumann K47 and M7. I suppose that copies can sound diferent or wrong. But if you don't like the sound of the K47 why you insist on it? Why not M7 or Ck12?
Do the search around, some more afordable and possibly better M7's (than Thierch) are available.

 

[oscarhuang]

My personal opinion is that K47 does not sounds harsh, but yes, compared to the M7 it has some more "presence" what can mask the "air". I speak about geniune Neumann K47 and M7. I suppose that copies can sound diferent or wrong. But if you don't like the sound of the K47 why you insist on it? Why not M7 or Ck12?
Do the search around, some more afordable and possibly better M7's (than Thierch) are available.

Well that is a good question. I like build gears for recording and like to try all the different gears. I understand everyone has different ears and different feelings for sound. I bought those capsules just for my hobbies but it’s lots of money as well. They’re not cheap and I tried to made something new.
I tried other M7 capsules but never tried 3U audio yet. If you have good recommendations for M7 capsules just let me know. I would like to try it.

 

[Moby]

Of course. I accept the fact that you don't prefer the k47, thats why I mentioned M7. And yes, above mentioned manufacturer can probably be the best option for you. )

 

[Whoops]

I agree with Moby, you might not like the K47 capsule, try a different type like CK12 , K67 or M7