Removing U67 Filter Circuit

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tesco_1

Active member
Joined
May 16, 2014
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33
I have a DU67 with a genuine Neumann Capsule w Nos/ valvo/telefunken tubes, and was interested in removing the high frequency filter circuit to open the top end of the mic and increase clairty.  Or even making a switch to be able to change between a brighter more open modded 67 sound and the stock 67 sound.  Can anyone tell me what components would I need to remove to remove the HF feedback circuit?
 
I think that you need short C17.
There's a difference between "clarity" and HF bump.
Like all mxl or rode nt1/nt2 which use flat circuit with k67 style capsule...
You could try to use three position swith with different values of capacitors or two values and a jumper.
 
First off, it's your mic, so it's your call, obviously.
Also, if you're able to undo it, you'll likely be able to redo it.

I would strongly advise against it, though. After the treatment, your U67 will no longer be a U67. It will basically be like the countless modern mics with seriously hyped tops. Well, with better parts than most. But nothing special really.
If that's what you're after, you could also get or build such an animal from scratch.

The U67 circuit is a very sophisticated design. Lots of brainpower went in it. And there's nothing like it.
And of course the K67 capsule was designed with that intricate high frequency filtering in mind.  The highs in the K67 are pre-emphasized, so without de-emphasizing the capslue will be overly bright.
A healthy U67  should not sound dark BTW. The original plots are flat, with a very slight rise even, depending on polar pattern. They have been called dark, but that would be as compared to modern LDCs, many of which are not flat. Also, quite a few, if not most of the originals are tired to some extent and should be sent to an expert. They will sound fresher upon return!

Of course you could install a stepped filter. or you could use an equalizer to tame the highs after the filtering circuit has been removed. And yes, often filtering is better than amplifying.
But there are exceptions to that rule and the U67 is one of them. The U67 takes lifting of the highs like no other microphone. That is, if you have a good equalizer.
And with the circuit intact it acts like a(-n active) de-esser (as if there's a compressor in the side chain). Or should I say a de-eshjzer? There's undistorted sibilance and there's distorted sibilance. I'm talking about the latter. Again, here's a paradox. The U67 has lots of negative feedback and often that would mean a less clean/fast/pure path. But it was put there deliberately and the whole of the circuit (with nothing left out) fights distorted sibilance and similar artifacts that can occur in condenser mics. And consequently you can lift the highs more than you would expect, without getting spikes and filth.

This is my miserable way of explaining, mind. Others, like top recording engineers who have worked with U67 when both the mics and the men were in their prime, have done a better job explaining it. Still, it remains an intricate design, made by true masters of the craft (who were given almost unlimited financial means for designing), so it's not so easy to grasp.

Apart from the filtering system as discussed above, there's also a broadcast filter. You have the DU67 and Dany's version is true to the original, so the broadcast filter is there, too. Another U67 cloner here decided to leave it out from the start, which makes his not true to the original in that regard. But that's a lot less radical.
So IF you decide to go ahead and take filters out, I would start with the broadcast filter.

But again, it's your call.



Henk
 
Im not lucky (rich) enough to have a real U67 sadly (tho i have breadboarded the circuit with a flat capsule & different transformer)

I was just wondering if Tesco 1s U67  isnt performing full spec making him feel it needs a lift.......

All the best
 
gary o said:
Im not lucky (rich) enough to have a real U67 sadly (tho i have breadboarded the circuit with a flat capsule & different transformer)

I was just wondering if Tesco 1s U67  isnt performing full spec making him feel it needs a lift.......

All the best

Yes, that is a good question.

And in that regard I'll repeat my earlier remark,
A healthy U67 schould not sound dark.
 
Some people are just used to the "modern" or just "chinese microphone" sound which was popularized in 90's by rode.
Maybe here's the clue?
I prefer "dark" microphones for instance.
I did once on audix microphone (chinese style u87 - p48/+emitter follower/2:1 OT/CK12 style on sided) additional switch  for two different options of deemphasis eq and one position without eq. It's usefull especially for different instruments.
 
Like the original poster , I am on a journey getting A DU67 closer to the original .
If Dannys design is spot on , the variables become  capsule - tube - transformer - mic body ?
I have a lovely original to compare with , and my first build with neumann k67 and tele tube yielded a Good sounding mic But not as open as the U67.
 
ln76d said:
Some people are just used to the "modern" or just "chinese microphone" sound which was popularized in 90's by rode.
Maybe here's the clue?
I prefer "dark" microphones for instance.
I did once on audix microphone (chinese style u87 - p48/+emitter follower/2:1 OT/CK12 style on sided) additional switch  for two different options of deemphasis eq and one position without eq. It's usefull especially for different instruments.

Yes, many have grown used to overly bright.
Plus nowadays many ears get shot (as in damaged) from a young age on. (And the cochlear hair cells for the trebles are the most vulnerable.)

Simple filters can indeed be implemented in bright mics.
But it will nowhere be like the "dynamic" filtering that's going on in a U67.

Wolcott wrote:

Like the original poster , I am on a journey getting A DU67 closer to the original .

That's not how I read it.

If Dannys design is spot on , the variables become  capsule - tube - transformer - mic body ?
I have a lovely original to compare with , and my first build with neumann k67 and tele tube yielded a Good sounding mic But not as open as the U67.

Question for you and the OP; what tranny was used?
Also, good Telefunken E86 tubes are hard to find IME. I bought half a dozen from a reputable dealer and they were all unusable. I guess most of those re-enter the market after one or more selecting rounds.
And those genuine Neumann capsules. Are they new and if not, were they serviced?
 
I have 2 DU67s
One Max , and one with the T67
Transansformer .  Maxs transformer Is darker , but both don't have the
Type of top end I hear in the original
But great sounding Mics .
I have a new neumann k67 and a pair of " Bowie "  hand picked ef86  that are dead quiet but who knows .
I have the tab historically correct "
Trans , I just need to install it to check
 
If you take the filter out, you will get quite a large increase in sensitivity and an equally large loss of headroom. I would tread carefully. Perhaps start by just changing the value of the capacitor that "leaks" from the transformer primary to feedback winding.
 
wolcott said:
I have 2 DU67s
One Max , and one with the T67
Transansformer .  Maxs transformer Is darker , but both don't have the
Type of top end I hear in the original
But great sounding Mics .
I have a new neumann k67 and a pair of " Bowie "  hand picked ef86  that are dead quiet but who knows .
I have the tab historically correct "
Trans , I just need to install it to check

Thanks for elaborating.

And good to know you have both the IOaudio and the AMI tranny.
BTW, Cinemag seems to have a nice version now, too. And I've heard great results from Marik's trannies (Samar). However, there's a difference between technically best and most vintage sounding. The proof is in the pudding, as always. And exactly matching the old thing is virtually impossible. And do we know how they sounded when new?

Handpicked tubes by Bowie should be fine.
I'll add that my supplier didn't have the right equipment to fully test EF86s.  6072s  etc. no problem.
 
have experimented into removing C17 or tweak the value ,
if you have the AMI transformer , did you use the errata connection scheme on page 1 ?

D. ;)
 
Thank you for all of your responses!


gary o said:
Im not lucky (rich) enough to have a real U67 sadly (tho i have breadboarded the circuit with a flat capsule & different transformer)

I was just wondering if Tesco 1s U67  isnt performing full spec making him feel it needs a lift.......

All the best

The DU67 is performing great, it sounds like I'd expect a 67 to sound.  I have been reading about how some mods that remove a part or all of the feedback circuit result in a brighter and more open sounding 67.  As a vocalist producer and engineer, I'm always obsessed with improving my sound, even if that means customizing or modding my gear to achieve it.




micaddict said:
gary o said:
Im not lucky (rich) enough to have a real U67 sadly (tho i have breadboarded the circuit with a flat capsule & different transformer)

I was just wondering if Tesco 1s U67  isnt performing full spec making him feel it needs a lift.......

All the best

Yes, that is a good question.

And in that regard I'll repeat my earlier remark,
A healthy U67 schould not sound dark.

Yes, I agree.  I wouldn't call the sound of a mint/new 67 dark.  Originally this DU67 had a vintage Neumann capsule that Danny (Poctop) had reskinned, and I would say it was darker sounding, but voiced beautifully, and sounded very sweet overall.  I loved it.  But the mic was damaged due to a short in the mic cable, and I sent it to another member to be repaired as I didn't have the time or skill to trouble shoot it quickly.  Long story short,  after months of waiting the mic was not fixed, and the vintage capsule was damaged during the botched/failed repair, and I paid to have  the capsule reskinned by the same person (who will remain nameless).  The mic was eventually returned to me, still broken and fixed by another member (pH) who lives in my local area,  he had the problem with the mic sorted in record time (thanks again!).  After the repair I then replaced the twice reskinned capsule with a new Neumann K870/K87/K67, attempting to making it closer to a mint 67.  After replacing the vintage capsule with the new Neumann capsule, I noticed that the mic was a tad bit brighter, making it more  neutral sounding, even more brighter when in omni, needing no eq to sound 'modern' with the right amount  of distance (about 12-16").  I do miss Danny's reskinned capsule, the newer Neumann cap doesn't sound as sweet in the higher frequencies as the older darker reskinned capsule did; but the new capsule sounds great as well, just not as sweet.... For all practical uses I'm sure I'm splitting hairs 



micaddict said:
Yeah, I'm with you there, Wolcott.  :)
And I hope the OP will soon chime in to elaborate.

What tranny did you use?

Henk
       

My mic has the IOaudio transformer.  I pair it with a NOS telefunken EF86, Valvo EF86, and a TungSol EF806 (the TungSol being my favorite with the new Neumann capsule)



ln76d said:
Some people are just used to the "modern" or just "chinese microphone" sound which was popularized in 90's by rode.
Maybe here's the clue?
I prefer "dark" microphones for instance.
I did once on audix microphone (chinese style u87 - p48/+emitter follower/2:1 OT/CK12 style on sided) additional switch  for two different options of deemphasis eq and one position without eq. It's usefull especially for different instruments.

In my quest to find the perfect vocal mic I have owned a pretty good variety of mics, from cheaper Chinese mics, to the more expensive bunch, just to name a few off the top of my head:  Neumann 87, vintage and AI, U67, U89, U87 Innertube mod, TLM 103, 102, TLM 67, U47, M49, Rode K2, NTK, Sure SM7B, Nady 1050, Avantone CV12, Sony C800G, SM57, SM58, Peluso 251, Peluso VTB, Peluso 2247, AKG 414, Pearlman TM1, Blue Bluebird, Kiwi, cactus, Sterling Audio ST55, ST77, ST79AT4040, Manley Ref Cardoid, and the list goes on... 

I find no particular or linear correlation with price and quality or usefulness of the mic.  There are many standouts on that list.  Some of my best recordings were done with a Nady 1050 plugged into a $30 behringer mixer (that was my first condenser mic).. I'm still amazed at listening to how good those songs sound.  But now I've stayed with the Sony C800G, and DU67 as my main choices for vocal records, I can cover anything with those with satisfactory results, but If I could get the 67 to be less veiled right  out of the gate I'm 90% closer to my finished sound.  I'm also curious what the 67 will sound like without the minor smearing/softening of the audio image due to the HF feedback circuit.  I've had the innertube mod for the 87 and the sound of the k67 unbridled, was very open and euphoric sounding, I wasn't sold on the overall sound of the circuit (I sent it back because it was too prone to RF interference), but I do remember liking the clarity of the non de-emphasized K67. 



poctop said:
have experimented into removing C17 or tweak the value ,
if you have the AMI transformer , did you use the errata connection scheme on page 1 ?

D. ;)

Hello Danny, this is the original DU67 that you sent me with the IOaudio transformer and NOS Telefunken tube
 
BTW if you will have opportunity, check valvo or philips E80F tube (not related with EF80).
It is damn good tube for microphones and it sounds much more better even  than Telefunken EF806.
It's longer than EF86, but it should fit easily to DU67.
Pin to pin equivalnet.
It can draw more current than EF86, so a little change of anode resistor value can be made to improve the specs of the circuit.
Even NOS E80F can be found for cheap.
Worth to try.
 
tesco_1 wrote:

My mic has the IOaudio transformer

That tranny is darker than some, but that seems a moot point now, because in your more elaborate reply (thanks for that) you wrote:

The DU67 is performing great, it sounds like I'd expect a 67 to sound.

I have been reading about how some mods that remove a part or all of the feedback circuit result in a brighter and more open sounding 67.  As a vocalist producer and engineer, I'm always obsessed with improving my sound, even if that means customizing or modding my gear to achieve it.

Yes, it will be brighter. "Improving" is a whole other matter.  Many would disagree with that. But yes, in the end it's all a matter of taste.

I do miss Danny's reskinned capsule, the newer Neumann cap doesn't sound as sweet in the higher frequencies as the older darker reskinned capsule did; but the new capsule sounds great as well, just not as sweet.... For all practical uses I'm sure I'm splitting hairs 

Interesting that Dany's reskinned capsule sounded sweeter than a Neumann. :) I can almost guarantee that after the filters have been removed, the Neumann capsule will sound even less sweet.
Also, the total difference after the operation will be beyond splitting hairs. ;-)

But now I've stayed with the Sony C800G, and DU67 as my main choices for vocal records

Those are are pretty "yin and yang" since the C800G is very (!) bright on top. That said, the capsule in the Sony resembles a K67, so both microphones have similarities, too. After the filters in the U67 have been lifted, they will be pretty close actually, although I expect the treble peak in the Sony to be centered somewhat higher. In any case, the contrast between the two will basically be gone, so you'll have a less versatile combo.
Is the Sony yours or do you have easy access to it?

In76d wrote about the E80F. If it fits on the IOaudio PCB, too, you could give that a try. From what I've heard it's a little more open sounding, but a little leaner on the low mids. The proof is in the pudding

Talking about pudding, this is a DIY platform, so yeah, why not give the filter removal operation a try? You can always redo it, can't you?
Do keep us posted, please!

BTW, in the old days in the big studio's engineers liked to use a Pultec on the U67 if they needed more treble. The U67 takes (good) EQ like a dream. This is what it sets it apart from others. And it was designed this way. (!)
Genuine Pultecs are very expensive. But you could build your own via this here forum.
And Warm Audio came out with a ready made clone that is actually affordable.
But there are other types, too, of course. Have you tried lifting the highs with a good equalizer?

 
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