FET847 Microphone Project Circuit By Jonathan Burtner PCB Layout By Poctop

[G-Sun]

I have a GZT-47 transformer
and the BM800 body

If both sides of the metal in the transformer connects with the body of the mic,
will that affect performance?

edit: I'll add some el.tape there.

 

[Wordsushi]

Just curious... Has anyone tried to build one of these with a flat-response (or flatter response, actually) K67 style capsule like Chungers 35mm K67 or 3u Audio's flat K67?

 

[0dbfs]

The K67 style capsules should have a pre-emphasis or "peaked high frequency" response. The U87 and U67 circuits are designed to "de-emphasize" that response and bring the capsule signal back towards "flat" while at the same time reducing HF noise. This KM84 circuit is flat with no de-emphasis so best suited for so-called "flat capsules" like C12, K47, M7 or even maybe a sony C37 style capsule if you were lucky enough to get any of those when Dale was making them.

If you use a K67 style capsule with the built-in pre-emphasis then it will seem bright and spitty but they all sound different right and you can also always EQ it too

Cheers!
-jb

 

[Wordsushi]

The K67 style capsules should have a pre-emphasis or "peaked high frequency" response. The U87 and U67 circuits are designed to "de-emphasize" that response and bring the capsule signal back towards "flat" while at the same time reducing HF noise. This KM84 circuit is flat with no de-emphasis so best suited for so-called "flat capsules" like C12, K47, M7 or even maybe a sony C37 style capsule if you were lucky enough to get any of those when Dale was making them.

If you use a K67 style capsule with the built-in pre-emphasis then it will seem bright and spitty but they all sound different right and you can also always EQ it too

Cheers!
-jb

Yeah, Chunger said his 35mm works nice in flat circuits and I know Guosheng has said the capsule in his GZ67 is for a flat circuit too so I figured I'd ask if anyone had tried one in here just for laughs. 

 

[RuudNL]

A fact is that a K67 capsule has a certain HF boost. But there are a lot of differences.
The amount of boost varies from manufacturer to manufacturer.
Most Chinese microphone manufacturers use K67-style capsules together with a 'flat' amplifier circuit, resulting in an overbright microphone. But maybe you will even like this...

 

[ln76d]

Didn't tested both "flat" options - but this could be possible. I found at least two chinese k67, which weren't not so hyped in highs.
Sometimes also polarisation voltage apllied to diaphragm (not the backplate) helps a little (depends on many factors, always worth to try).  Try one of these "flat" capsules and you will see - always you can try DC on diaphragm as also cap to ground for hf smoothing.

 

[0dbfs]

Yeah. CHeck out the U87 schematic and look at the signal path from top of FET (output) to C9 -to-  R8 and C6 (in parallel) which then feeds to the backplate... This is the negative feedback de-emphasis part of that circuit.

IOW -
The signal coming out of the FET is fed through an eq / filter (R8 and C6) which then causes modulation on the backplate. This modulation signal is band limited due to the filter and since it's on the backplate it is opposite in polarity and therefore subtracts from the signal produced on the front diaphragm and fed into the FET... Essentially a notch filter. I don't know the exact math but you can probably google that for more specifics on the filter frequencies / etc...

Otherwise the U87 and KM84 circuits are pretty much the same with a slightly different transformer ratio and of course the pattern option differences.

You could put a BV08 in this mic and a K47 or M7 if it would fit and it's basically a "Single FET U47" (not a FET47)... I wonder if we could parallel a couple FET's similar to how the ||6028 implementation works - reducing output Z and increasing drive capability of the active device.. I might experiment with that a little but the mic body in this thread would probably be too small for the BV08

Cheers!
-j

 

[ln76d]

Yeah. CHeck out the U87 schematic and look at the signal path from top of FET (output) to C9 -to-  R8 and C6 (in parallel) which then feeds to the backplate... This is the negative feedback de-emphasis part of that circuit.

IOW -
The signal coming out of the FET is fed through an eq / filter (R8 and C6) which then causes modulation on the backplate. This modulation signal is band limited due to the filter and since it's on the backplate it is opposite in polarity and therefore subtracts from the signal produced on the front diaphragm and fed into the FET... Essentially a notch filter. I don't know the exact math but you can probably google that for more specifics on the filter frequencies / etc...

Otherwise the U87 and KM84 circuits are pretty much the same with a slightly different transformer ratio and of course the pattern option differences.

You could put a BV08 in this mic and a K47 or M7 if it would fit and it's basically a "Single FET U47" (not a FET47)... I wonder if we could parallel a couple FET's similar to how the ||6028 implementation works - reducing output Z and increasing drive capability of the active device.. I might experiment with that a little but the mic body in this thread would probably be too small for the BV08

Cheers!
-j

I think that OP understand deemphasis network option - here we are "talking" about something a little bit different than U87 circuit

 

[benqbasic]

Yeah. CHeck out the U87 schematic and look at the signal path from top of FET (output) to C9 -to-  R8 and C6 (in parallel) which then feeds to the backplate... This is the negative feedback de-emphasis part of that circuit.

IOW -
The signal coming out of the FET is fed through an eq / filter (R8 and C6) which then causes modulation on the backplate. This modulation signal is band limited due to the filter and since it's on the backplate it is opposite in polarity and therefore subtracts from the signal produced on the front diaphragm and fed into the FET... Essentially a notch filter. I don't know the exact math but you can probably google that for more specifics on the filter frequencies / etc...

Otherwise the U87 and KM84 circuits are pretty much the same with a slightly different transformer ratio and of course the pattern option differences.

You could put a BV08 in this mic and a K47 or M7 if it would fit and it's basically a "Single FET U47" (not a FET47)... I wonder if we could parallel a couple FET's similar to how the ||6028 implementation works - reducing output Z and increasing drive capability of the active device.. I might experiment with that a little but the mic body in this thread would probably be too small for the BV08

Cheers!
-j

Did you get a chance to try this?

 

[elskardio]

Hi Guys,

I just wanted to share my version of this nice circuit. Have a great weekend 

 

[Wordsushi]

Hi Guys,

I just wanted to share my version of this nice circuit. Have a great weekend 

Love it! Looks fantastic. Any chance of making your boards available?

The badge looks really cool, too.

 

[benqbasic]

Hi Guys,

I just wanted to share my version of this nice circuit. Have a great weekend 

Thats a good looking mic. What type of body did you use?

 

[Khron]

Let me guess - does your version take forever to "come to life" too?

(Yes, i'm developing a pet-peeve with some bits of this design ;D )

Hi Guys,

I just wanted to share my version of this nice circuit. Have a great weekend 

 

[elskardio]

Love it! Looks fantastic. Any chance of making your boards available?

The badge looks really cool, too.

I made this PCB for my personal use only. Poctop is already selling a nice PCB for this project 

The Badge is from Frank Röllen (frontpanels.de) The quality is excellent.

 

[G-Sun]

Finishing my Fet847 now,
firing up to bias fet.

I have some low-fq-noise, like AC hum,
but I get some signal to hum, just like 12db or some.
Any pointers where to get rid of the noise/hum?

 

[Khron]

If the mic body's still open, and/or the headbasket isn't installed, that's where the hum gets in.

But just for the bias adjust, that's nothing to worry about, the test-signal will be way larger in amplitude anyway.

Finishing my Fet847 now,
firing up to bias fet.

I have some low-fq-noise, like AC hum,
but I get some signal to hum, just like 12db or some.
Any pointers where to get rid of the noise/hum?

 

[G-Sun]

If the mic body's still open, and/or the headbasket isn't installed, that's where the hum gets in.

But just for the bias adjust, that's nothing to worry about, the test-signal will be way larger in amplitude anyway.

Oh, thanks a lot! That's good news

btw: Test signal and bias-adjust. How do I practically do it? Do I set it in front of my speakers?
edit, found this: https://groupdiy.com/index.php?topic=63889.0

 

[Khron]

Bit of a thread "necro", but here's a slight update / simplification https://groupdiy.com/index.php?topic=63889.msg853127#msg853127

 

[G-Sun]

Sound, wow,
and pleasant sounding!
I believe I have a working mic  ;D ;D ;D

Bit of a thread "necro", but here's a slight update / simplification https://groupdiy.com/index.php?topic=63889.msg853127#msg853127

Thanks!
I don't seem to have the right cabling/connectors right now, will order some.
https://www.ebay.com/itm/172679352208

 

[G-Sun]

Does the biasing affect more than max spl?