Removing U67 Filter Circuit

[ln76d]

Once again thank you all for the swift replies!  Tons of info to look into.  So a few days ago I tried the quick and dirty C17 removal to see what would happen, and I put it right back! 

The untamed high frequency boost was for lack of a better term 'ugly', and unbalanced.  Although the high frequency boost was not incredibly bright (maybe due to the IO transformer?) it was notably different from the stock mic, and had a non linear distortion thing going on up there.  I definitively may experiment with other C17 values via a switch as suggested.

It's becuase you removed a part of feedback - removing capacitor. You can't use jumper here, so only lower value capacitor.
What was the value of C17 in your circuit? If 100pF, you shouldn't notice big change in HF comparing circuit with cap and without it.

Also I am aware of the S2 low cut jumper, and it's relatively irrelevant to me as I mostly record vocals, and it's already switchable.

Are you sure is't switchable? Standard high pass  filter in u67 is additional switch which cut low frequency in higher range.
S2 jumpered, always is set to cut at 30Hz.  My suggestion - check and try

Now to get some things straight:  "RF filters" is the correct term for what some call the "broadcast filters" (implemented in compliance with German radio specs)? If I understand correctly removing this filter should eliminate the high-end roll off (I assume around 14k)? And again what components make up these RF filters?

Ultimately I just want to get a little more top end out of the mic with respect to its general design.

As i stated before!
RF is RF, it shouldn't affect 15kHz, rather much higher since caps are 2nF. 
RF is short from radio frequency - https://en.wikipedia.org/wiki/Radio_frequency
It doesn't affect audible range especially 14k-15kHz. Many microphones doesn't have any RF protection an works great - all depends on environment around. Since there's high frequency damping due to deemphasis eq via C17, here shouldn't be problem to remove it. Sometimes lack of additional caps can give sligthly better results.  Worth to try, always you can put it back.

"Broadcast filters" - i didn't found anything in Neumann papers about it.
The only part in the circuit, which i found, which could affect response in 15kHz range is C17.
Am suspecting that this what people on forum call "Broadcast filters" is only highest value of C17 stated by Neumann - 160pF.
Probably that's why is popular to use 100pF.
Test few values and find your best option

 

[emrr]

Just remember many oldtime users have found that no condenser mic takes a high frequency  EQ lift like a U67. (Post microphone that is.)
So basically, tempering with any part of the de-emphasis/negative feedback circuit is like throwing out the baby with the bath water.
Or worse

Yes, you've said this a dozen times in DIY U67 threads!  Let's be clear you are assuming a properly working U67 who's amplifier meets the test conditions Neumann describes, with at least a capsule the matches a Neumann in response.  Around this place, there's every sort of good and questionable DIY U67 replica/variant/wanna-be in play, and most reading this thread will be coming at it from that perspective.  Some asking this are trying to match the sound of a DIY build to an actual U67 they have on hand.  Tweaks likely needed! 

This shunts point C2/C3/R3/R4/R5 to ground, and is likely very similar to the standard C17/S2 modifications.

Can anyone tell me how would I perform this linear mod with Danny's pcb?

I don't know Danny's PCB, but if you can verify he is using the same part #s then it's a simple matter of finding those positions on the board.  You could change the switch or do a quick solder point jumper to have a listen before committing.  What's different compared to removing C17 is it reorients it towards ground, so it's shunting some treble to ground through C3, while the overall mod is taking the feedback path out entirely, so it shouldn't be as wide open as lifting C17.  I suspect it's similar to the Innertube mod in overall EQ. 

Broadcast filters:  They mean the RF suppression caps on the incoming connections to the mic, which are far outside of hearing range, but have also been reported by Klaus Heyne and others to affect the sound of the mic in some way. 

Many people really have removed C17, if I read historical reports correctly.  The other option besides changing the value of C17 is to insert some series resistance in it's path, to lessen the change without moving the frequency of the change. 

 

[Matt Nolan]

RF is RF, it shouldn't affect 15kHz, rather much higher since caps are 2nF. 
RF is short from radio frequency - https://en.wikipedia.org/wiki/Radio_frequency
It doesn't affect audible range especially 14k-15kHz. Many microphones doesn't have any RF protection an works great - all depends on environment around. Since there's high frequency damping due to deemphasis eq via C17, here shouldn't be problem to remove it. Sometimes lack of additional caps can give sligthly better results.  Worth to try, always you can put it back.

Well, to be fair, they will give you a couple of degrees of phase shift at 15kHz. But I don't think anyone can hear that and I'm sure it is swamped by the phase response of the transformer and everything else!

 

[micaddict]

Yes, you've said this a dozen times in DIY U67 threads!  Let's be clear you are assuming a properly working U67 who's amplifier meets the test conditions Neumann describes, with at least a capsule the matches a Neumann in response.  Around this place, there's every sort of good and questionable DIY U67 replica/variant/wanna-be in play, and most reading this thread will be coming at it from that perspective.  Some asking this are trying to match the sound of a DIY build to an actual U67 they have on hand.  Tweaks likely needed!

LOL, yes Doug, I have mentioned it a couple times. It's just that too many great U67s have been butchered in the past IMO and that of others.
And tired U67s should be freshened up, not altered, by a very competent tech. IMO

As for the many knock offs by other brands as well as DIY versions, the only ones that follow the correct circuit are the ones by Dany and Max.
I would treat those, and those alone, the same in this regard.
And for readers who might have missed it, I'll say it for the thirteenth time; finetuning is fine (excuse pun), but respect the de-emphasis/negative feedback scheme as designed by the original makers.

Also, forget the term "broadcast filter". I borrowed that from Dany who, like me, is not a native English speaker.
RF will do nicely, thank you. 



Henk

 

[Delta Sigma]

Also, forget the term "broadcast filter". I borrowed that from Dany who, like me, is not a native English speaker.
RF will do nicely, thank you. 



Henk

I figured English was your second language since you always have good grammar!

 

[Matt Nolan]

Also, forget the term "broadcast filter". I borrowed that from Dany who, like me, is not a native English speaker.
RF will do nicely, thank you. 

I think the terminology is confusing. It is a filter to enable trouble free use in broadcast studios where there might be a lot of RF about, not a filter to shape the audio band to within some kind of broadcasting spec.

 

[tesco_1]

It's becuase you removed a part of feedback - removing capacitor. You can't use jumper here, so only lower value capacitor.
What was the value of C17 in your circuit? If 100pF, you shouldn't notice big change in HF comparing circuit with cap and without it.

Yes, I'm was aware that I was removing the de-empasis curcuit, that why I called it "quick and dirty".  I just wanted to hear what it sounded like without it, and it confirmed what other have said about removing the C17 without replacing it with another value; which is it doesnt sound good.

Are you sure is't switchable? Standard high pass  filter in u67 is additional switch which cut low frequency in higher range.
S2 jumpered, always is set to cut at 30Hz.  My suggestion - check and try

Yes I was talking about the internal low cut jumper, not the highpass switch (that goes up to around 200).  When I said "switchable" I meant I have the option to open and close the jumper.

As i stated before!
RF is RF, it shouldn't affect 15kHz, rather much higher since caps are 2nF. 
RF is short from radio frequency - https://en.wikipedia.org/wiki/Radio_frequency
It doesn't affect audible range especially 14k-15kHz. Many microphones doesn't have any RF protection an works great - all depends on environment around. Since there's high frequency damping due to deemphasis eq via C17, here shouldn't be problem to remove it. Sometimes lack of additional caps can give sligthly better results.  Worth to try, always you can put it back.

"Broadcast filters" - i didn't found anything in Neumann papers about it.
The only part in the circuit, which i found, which could affect response in 15kHz range is C17.
Am suspecting that this what people on forum call "Broadcast filters" is only highest value of C17 stated by Neumann - 160pF.
Probably that's why is popular to use 100pF.
Test few values and find your best option

Yes I understand RF stand for radio frequency, which some have more casually referred to as to a radio broadcast filters; both make sense to me, I just wanted to get the official term for that part of the circuit so I could be using the correct terminology.  Thank you for all of information.

 

[tesco_1]

Just remember many oldtime users have found that no condenser mic takes a high frequency  EQ lift like a U67. (Post microphone that is.)
So basically, tempering with any part of the de-emphasis/negative feedback circuit is like throwing out the baby with the bath water.
Or worse

Yes, you've said this a dozen times in DIY U67 threads!  Let's be clear you are assuming a properly working U67 who's amplifier meets the test conditions Neumann describes, with at least a capsule the matches a Neumann in response.  Around this place, there's every sort of good and questionable DIY U67 replica/variant/wanna-be in play, and most reading this thread will be coming at it from that perspective.  Some asking this are trying to match the sound of a DIY build to an actual U67 they have on hand.  Tweaks likely needed! 

This shunts point C2/C3/R3/R4/R5 to ground, and is likely very similar to the standard C17/S2 modifications.

Can anyone tell me how would I perform this linear mod with Danny's pcb?

I don't know Danny's PCB, but if you can verify he is using the same part #s then it's a simple matter of finding those positions on the board.  You could change the switch or do a quick solder point jumper to have a listen before committing.  What's different compared to removing C17 is it reorients it towards ground, so it's shunting some treble to ground through C3, while the overall mod is taking the feedback path out entirely, so it shouldn't be as wide open as lifting C17.  I suspect it's similar to the Innertube mod in overall EQ. 

Broadcast filters:  They mean the RF suppression caps on the incoming connections to the mic, which are far outside of hearing range, but have also been reported by Klaus Heyne and others to affect the sound of the mic in some way. 

Many people really have removed C17, if I read historical reports correctly.  The other option besides changing the value of C17 is to insert some series resistance in it's path, to lessen the change without moving the frequency of the change.

Well after looking at the wiring diagrams  and schematics of Maxes and Dannys kit, and I tried the mod.  I suck at reading schematics so, let me know if this sound right:

I basically took the first connection  from the high pass switch and connected to the leg of C2, and then took the second wire from the high pass switch and connected it to the 3/C GND (right below to  C16 on Danny's schematic).  I couldn't find any other references to another grounding point on Danny's board.

Does that look right to anyone? 

 

[emrr]

So long as you are on the correct side of C2, which connects to C3. 

 

[tesco_1]

So long as you are on the correct side of C2, which connects to C3.

Ok, so I connected the the low pass wire to the short leg of C2.  The other leg is long and is in a junction with a resistor, the pad wire, and some other wire.  I'm not sure which side is the side that flows into C3, but I assume its the side moving away from the capsule.

 

[emrr]

That sounds right, but....it's better to be on the wrong side of C3 than the wrong side of C2.  If it's the wrong side of C2 you would ground the capsule polarization voltage and have nothing at all. 

 

[bcwiz]

Can someone explain what is happening with the feedback circuit? Removing C17 only accounts for maybe 2db of top end attenuation there's still a low pass happening whether C17 is there or not.
I have a U67 schematic that does not have C17 which has different values for R19, R4 and R5.

 

[Moby]

Can someone explain what is happening with the feedback circuit? Removing C17 only accounts for maybe 2db of top end attenuation there's still a low pass happening whether C17 is there or not.
I have a U67 schematic that does not have C17 which has different values for R19, R4 and R5.

I did not read all the answers here, so sorry if somebody already mentioned.  Feedback circuit include transformers feedback winding.  It feeds capsule back-plate with the signal portion and does the necessary de-emphasis.

 

[bcwiz]

Yes looking for specifics of the R/C network after the transformer

 

[Moby]

Yes looking for specifics of the R/C network after the transformer

Signal from the feedback winding is just HPF-ed, take a look at the schematics. Nothing fancy. But de-emphasis method is "fancy" if you know what I mean.

 

[bcwiz]

It's the HF attenuation/de-emphasis that I don't understand. There's more to it than C17...