Does anyone know of a U67 circuit pcb or similar?

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deuce42

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Joined
Dec 31, 2008
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645
Location
Sydney, Australia
Hi Friends

I want to make a u67 mic circuit  to use with a neumann 87 capsule. I have had a look at the u67 schematic and find it a more elaborate circuit than others and my board etching skills are not very sophisticated. Accordingly, I am wondering if anyone knows of any pcb based circuits available for purchase etc.

I am aware that jacob's G7 revolves around a EF86 tube like the U67 did, but my feeling is that hte G7 circuit is more of a u47 without the rare steel tube.  I am also aware that Neuman 87/67 capsules have a huge high end peak and that a non feedback circuit to curtail the high end is needed. I am bit thrown here.

I have done a search for u67 on this board but not much seems to come up.

Any pointers or suggestions of where to start would be so appreciated.

Maybe this shold be posted in the Drawing Board?
 
http://www.groupdiy.com/index.php?topic=22825

I was wondering if the 500pf cap would work on a FET based circuit, for rounding the high end on a K87...

I once did a layout for the shoeps circuit, but that was FET and transformerless, and for a flat cardioid only capsule...
 
You can load the plate down with a cap on most valve mic designs. Just experiment with the value and measure/listen.

It's not the same as using feedback like on the U67 though.
 
Rodabod - I have done some reading but am still a bit confused. I accept that lowering the plate with a cap will work, but why is negative feedback a better approach
 
deuce42 said:
why is negative feedback a better approach

In general, the same reasons why NFB is usually used; better linearity... less distortion.

When you slap a cap across the plate to ground, you're just driving those high frequencies into to earth which is maybe less elegant depending on what you want. That said, it works well in some cases.
 
Its worth noting that the G7 schematic on the Gyraf site does include negative feedback components - they're the 2k2 resistor and 100uF cap labeled with an asterisk and the note "should be omitted". (Schematic here: http://www.gyraf.dk/gy_pd/g7/gic_s.gif )

Careful selection of resistor and cap values here could potentially implement the low pass filter you're after. I have, however, no idea if the current crop of G7 PCBs have places for these components, and there may be good reason for why these components really should be omitted.

P.S. Rodabod's mic with the cap shunting HF from plate to ground sounds good!

Matthew
 
Just to elaborate on Roddy's answer: If you put a cap between audio and ground, the tube has to drive a pretty low impedance at high frequencies. Which means, there'll be more distortion in the treble range. A feedback approach works fundamentally different: Frequency dependent negative feedback is used to reduce gain at your filter frequency. That means there'll actually be less distortion in that area. So while both approaches tame high frequencies, the result will sound different.

The U67 has a very elaborate feedback scheme, which requires a very complicated transformer with special feedback windings. The transformer, btw. , is the main reason why the U67 is not in production anymore. The G7 design originally tried to mimick the U67's global feedback via the transformer, but Jakob apparently didn't like the sound with the feedback winding involved. I suppose it does take a special transformer to make the U67 approach sound good.

In the U67 the HF-compensation is part of the global negative feedback, i.e. it involves that special transformer. But in the U87 the HF-compensation is realized via frequency dependant negative feedback from the FET drain to the capsule. So, in a way the capsule, is used as a coupling capacitor. I haven't tried it myself, but I assume the U87 approach should work in a tube design, too.
 
Will the cap to ground approach work on a FET circuit the same way as in analag´s tube mic schemo?

I would love to do some listening tests on my modded 87, NFB versus cap to ground...focusing only on how good it sounds.       

 
You can simply wire a cap between audio out (pre transformer) and ground i.e bridging the transformer primary. You can roll of treble that way in pretty much any design, but the value of the cap for a given HF roll-off depends largely on the driving impedance. A U87 ought to behave similar to a tube circuit, so 500p -1000p looks like a sensible range for your experiments.

Don't forget to test with high SPL sources, too. A loud tambourine should give you a good idea of the differences in sound.
 
Rossi said:
You can simply wire a cap between audio out (pre transformer) and ground i.e bridging the transformer primary. You can roll of treble that way in pretty much any design, but the value of the cap for a given HF roll-off depends largely on the driving impedance. A U87 ought to behave similar to a tube circuit, so 500p -1000p looks like a sensible range for your experiments.

Don't forget to test with high SPL sources, too. A loud tambourine should give you a good idea of the differences in sound.

Cool, thanks !
 
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