[RuudNL]

Just for fun, I recently put together a "U67"-ish microphone.
It is a kind of mix between a U67 and a U87.
The microphone works as expected, sounds good and I don't hear any difference between this microphone and a U87ai.
Measured self noise is 2 dB higher than a U87ai. Not too bad, but maybe a better EF86/EF806 could improve this.
I used an original Neumann capsule that I had as a spare.
The microphone is powered from an existing tube power supply, delivering +120 V for HT.
De-emphasis is through feedback from the output. (15 KHz is 5.5 dB down referred to 1 KHz.)

For those who are interested: the schematic is in the attachment.

[OneRoomStudios]

Very nice. Which output transformer did you use?

[RuudNL]

This one (3U Audio):

https://www.ebay.nl/itm/3U-Audio-GZT-87-Output-Transformer-for-Condenser-Microphone/111683319855?hash=item1a00d8382f:g:f6kAAOSwiCRUg89Z

Although it has a small core, the frequency response is excellent.
(Measured from 'test input' to output, 20 Hz is only 1 dB down.)

[kingkorg]

This is phenomenal. Thank you for sharing!

[RuudNL]

Picture of the prototype.

[RuudNL]

If anyone wants to try my design: I have a few extra printed circuit boards available.
(The PCBs fit into Chunger's 'budget' SYT-5 body.)

Edit June 07: All PCBs found a new owner...

[Sredna]

Great, sent you a PM! 

[DjaiTeam]

Hi Ruud,

Looking very nice indeed, must sound incredible!
I have a spare K87 capsule doing nothing so why not try this one

Sent you a pm for the pcb.

Best Regards,

Iwan

[Icantthinkofaname]

Nice. I've thought of putting a 1nF capacitor for the high end de-emphasis but in a cheaper FET mic like the BM-600 (or in a DIY U87 clone).

[RuudNL]

In this design the de-emphasis is done by HF feedback. (120 pF)

[Wordsushi]

Just finished building this mic this week and I am super, super impressed at how it sounds. I had a vintage Neumann capsule (circa 1979) that I put in there with the GZT transformer and  brand new JJ EF806s. Really love the sonics of what I'm hearing. Very quiet. I think I'm going to hit up Bowie for a Telefunken or Amperex and see where it goes. Here's a short clip of it through the UAD Avalon 737.

Bravo, Ruud! Bravo! And thank you for designing and sharing this awesomeness!

https://www.dropbox.com/s/532aon0qnxkvtgh/Ruud67Circuit-1.wav?dl=0

[RuudNL]

Glad to read that you have got it working and that you are happy with the result.
I compared mine with a 'real' U67, but I didn't hear any difference!
In fact, the LF response is better than the U67. (Because there is no fixed low-cut, we want the full audio range!)
In the meantime I have experimented with some more tubes.
The JJ Electronics EF806S are very good and a NOS Mullard EF86 was even a fraction quieter.
One thing you can experiment with is the de-emphasis capacitor.
I started with 120 pF, but with a Neumann capsule the microphone was a little brighter than I wanted.
So I changed the capacitor to 150 pF.
But yesterday I have built a second version with a non-Neumann capsule and with that one I ended with 100 pF.
So that means you have to 'tune' the capacitor for the capsule you are using.
To be honest, I never was a real fan of tube microphones (usually they were too noisy for me), but this one changed my mind...

[rock soderstrom]

If anyone wants to try my design: I have a few extra printed circuit boards available.
(The PCBs fit into Chunger's 'budget' SYT-5 body.)

Edit June 07: All PCBs found a new owner...

hello ruud, will you publish the PCB layout for self-etching?  Cheers

[RuudNL]

If you can wait a little: there is a company thinking about selling this design as a complete kit.
(That would save you also the time and effort ordering all the separate parts.)

[rock soderstrom]

If you can wait a little: there is a company thinking about selling this design as a complete kit.
(That would save you also the time and effort ordering all the separate parts.)

Very interesting, thanks again 👍

[Ricardus]

If you can wait a little: there is a company thinking about selling this design as a complete kit.
(That would save you also the time and effort ordering all the separate parts.)

TAKE MY MONEY!!!   

[Winston O'Boogie]

Just for fun, I recently put together a "U67"-ish microphone.

Hey, very nice 

Do you have a personal opinion on your implementation of the high frequency network vs the original?

I have a new U87Ai here that I'm looking to U67-ize and am looking through the various schemes that used the K67,  the valve SM69 has the simplest of the lot but perhaps not enough?

[Gus]

I have installed U67 like HPF in phantom powered microphones(basic cut and paste) some with an added low pass as well.

Waveborne posted a circuit  years ago adding a 67 like network to a MXL IIRC this was before people chased him away.

I don't get removing the HPF.
 
Yes SOLO male voice often sounds better but how does it sit in the mix?

If the room is not treated there is often a lot of low frequency junk that you don't want. If you turn the filter on you will see a drop on the metering depending on how well the room is designed. If it is in a house I would keep the filter on.

[Winston O'Boogie]

I have installed U67 like HPF in phantom powered microphones(basic cut and paste) some with an added low pass as well.

Waveborne posted a circuit  years ago adding a 67 like network to a MXL IIRC this was before people chased him away.

I don't get removing the HPF.
 
Yes SOLO male voice often sounds better but how does it sit in the mix?

If the room is not treated there is often a lot of low frequency junk that you don't want. If you turn the filter on you will see a drop on the metering depending on how well the room is designed. If it is in a house I would keep the filter on.

 I agree on the HPF, the Neumann U67 version is actually pretty clever but I'll look for Wavebourne's circuit, thanks
I was just curious about Ruud's implantation of the hi frequency attenuation vs the u67 scheme.

I think the idea behind Neumann omitting that particular stuff - HPF- on the SM69 might be to do with it being a stereo microphone designed for far/mid field full range recording but, dunno?

Cheers.

[RuudNL]

The Neumann SM69 didn't use de-emphasis or a high-pass filter and used a K67-style capsule.
Because this microphone was mainly intended for 'distant miking', the theory was dat high frequencies would undergo a certain amount of attenuation as a result of friction with the air molecules.
As a result of this, the high frequencies would be more attenuated than the lower frequencies, that have more energy.
The air damping for high frequencies would act as a 'natural de-emphasis'.
(If you listen to a musical event on a distance, you will notice that you mainly hear low frequencies, this is the same effect.)

The U67 has a fixed high-pass filter, that can be removed by cutting a jumper wire.
It seems that everybody disables the high-pass filter, in order to obtain a better LF response.
Also, this has an influence on the phase linearity at low frequencies.
But even without a high-pass filter, subsonic frequencies are attenuated by the transformer.

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