FET847 Microphone Project Circuit By Jonathan Burtner PCB Layout By Poctop

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dasnevestheo

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Oct 6, 2017
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Hey there,
I've been using my fet847 for a few years now and was very happy with it !
Today I wanted to replace its RK12 capsule for a Beesneeez K47 because I've always wished this mic had more mid presence.
But now the mic as very low volume so when I adjust gain for enough level I obviously get a lot of noise.
Adjusting the bias isn't helpful here.
I've checked & cleaned my soldering many times.

Any clues ?
 

JMPGuitars

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Apr 30, 2018
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80
Hi,

I'm planning on building one of these FET847s. I have the board, and I'm probably going to use a G2-2B body.

Is there a good source for the Cinemag 5722 transformer? What other transformers are good? I'm guessing the AMI T47, or BV8 would be good alternatives, with their 6.5:1 ratios?

Thanks,
Josh
 

fsalter

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Dec 31, 2011
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Montreal,Quebec
JMPGuitars said:
Hi,

I'm planning on building one of these FET847s. I have the board, and I'm probably going to use a G2-2B body.

Is there a good source for the Cinemag 5722 transformer? What other transformers are good? I'm guessing the AMI T47, or BV8 would be good alternatives, with their 6.5:1 ratios?

Thanks,
Josh

I would highly recommend 3U's GZT-84. I've used it on several builds and have not been disappointed. The price is right too.
 

JMPGuitars

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fsalter said:
I would highly recommend 3U's GZT-84. I've used it on several builds and have not been disappointed. The price is right too.

I'm up for trying them out, but wouldn't the GZT-47 be more appropriate with the 6.5:1 ratio?
 

mhelin

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JMPGuitars said:
I'm up for trying them out, but wouldn't the GZT-47 be more appropriate with the 6.5:1 ratio?

https://groupdiy.com/index.php?topic=33987.0

Anyway, maybe you could try that 47 and use two low input capacitance JFET's or single dual (LSK489) in parallel like on Telefunken TF11 (https://www.telefunken-elektroakustik.com/microphones/tf11).
 

JMPGuitars

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mhelin said:
https://groupdiy.com/index.php?topic=33987.0

Anyway, maybe you could try that 47 and use two low input capacitance JFET's or single dual (LSK489) in parallel like on Telefunken TF11 (https://www.telefunken-elektroakustik.com/microphones/tf11).

That doesn't seem exactly relevant? This project is stated to be used with the CM5722 which is a 7:1 transformer. I would think going from 7:1 to 6.5:1 isn't enough of a variance to give concern. Jumping from 7:1 to 10:1 would drop the output, wouldn't it?

Thanks,
Josh
 

mhelin

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Yes there are also other factors like the transformer inductance and the load of course which depends on which preamp you will use.

It's odd that the CM5722 datasheet first says "Microphone Output Transformer 9.5:1 Step-down BV-107 Style" and then later tells the turns ratio is 6.9:1. Anyway the response is specified for Rs=10k and Rl=1.5k.
 

JMPGuitars

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mhelin said:
Yes there are also other factors like the transformer inductance and the load of course which depends on which preamp you will use.

It's odd that the CM5722 datasheet first says "Microphone Output Transformer 9.5:1 Step-down BV-107 Style" and then later tells the turns ratio is 6.9:1. Anyway the response is specified for Rs=10k and Rl=1.5k.

In the OP it says:
● New Cinemag CM5722
transformer is 7:1 for original KM84 ratio.
● U87 uses the 10:1 with essentially the same FET driver circuit so those can also be used for
lower output and different needs.



I'm fairly certain this will work either way, and most of these transformers claim an impedance of 200 ohms. I certainly don't want lower output, so I'll try something in the 6.5 to 7:1 range.
 

craigmorris74

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Sep 8, 2005
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benton, ky
Ricardus said:
Is this project long gone and is there no more board availability? Anyone got 2 boards they aren't planning to use?
When I go to the order page at https://www.vintagemicrophonepcbkit.com/orders.html, the 847 boards are the last thing on the list.
 

Ricardus

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Upstate NY
craigmorris74 said:
When I go to the order page at https://www.vintagemicrophonepcbkit.com/orders.html, the 847 boards are the last thing on the list.

Ooh. Good call. I didn't see a tab on top so I didn't think he was sellng them.
 

mhelin

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There are also some related SQ-KM84-0.9.0 project, wonder how much that differs from the FET847 actually...
https://groupdiy.com/index.php?topic=66424.0
I've got some boards ordered from oshpark, I guess that the above thread contains the design files inc gerbers.
 

soliloqueen

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Oct 5, 2020
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Did some digging, looks like the body is is actually manufactured originally by Shanghai ShuaiYin as their SYC400. they offer several headbasket options and a few versions with different trim, only a few are on their product page. They also manufacture Alctron's products and were the contract manufacturer for iSK for a time? There exists a SYC5/600 which is the same body but with 2 filter switch cutouts.

http://www.shuaiy.com/product02.asp?plt=40&pone=6

There's also a 2-switch version of the 900:
http://www.shuaiy.com/product02.asp?plt=37&pone=6

Nobody seems to be rebranding/importing these at the moment, but they'd be ideal for future development if we could set up a group buy or something
 

JMPGuitars

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Apr 30, 2018
Messages
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Hey there,
I've been using my fet847 for a few years now and was very happy with it !
Today I wanted to replace its RK12 capsule for a Beesneeez K47 because I've always wished this mic had more mid presence.
But now the mic as very low volume so when I adjust gain for enough level I obviously get a lot of noise.
Adjusting the bias isn't helpful here.
I've checked & cleaned my soldering many times.

Any clues ?
Did you ever resolve this? If not, check that the capsule connections are good on both ends (carefully!).
 

JMPGuitars

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Apr 30, 2018
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I didn't see any great photos of populated boards, so here's one I just finished.

A couple things to note: I always insert the insulated pins backwards, because it gives a much better connection, and for better soldering technique. Now that it's all good, I'll be connecting the capsule on the backside of the board on the other end of the pins.

I'll post some more photos later in the week after my capsule screws arrive and I finish building the mic.

Thanks,
Josh

PS: Note that the jumper pins aren't installed yet, see next post.
 

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JMPGuitars

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I finished the mic today. Biasing was interesting. As pictured, first I used a multimeter and set it to about 10V. Then I used a scope, and it was different than expected. Typically I see AWG voltage set at 200mVpp as a starting point, but in this case, anything over 50mVpp gave an asymmetrical waveform, showing that the bias voltage was too high (top half wider than lower half of sine wave).

Anyway, I decided to try it out with the voltage set to 10V, and it works great. I was actually surprised how good/clean it sounds given the budget capsule and transformer.

The only thing I don't like is that the donor body doesn't lock cables in. I know I can grind the catch out bigger, but I was wondering if anybody else had this issue with the BM-800 bodies? I used a Neewer NW-800 body because it's basically the same, and less than $20 shipped from Amazon...but the catch thing is annoying.

Thanks,
Josh
 

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kato

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Indianapolis, USA
This seems ingenious.

I'm now awake to the idea that fussing over tubes in a mic circuit is probably more trouble than it's worth considering 1.) the trouble of selecting a good tube, 2.) the added expense of the power supply, and 3.) the fact that most people can't necessarily "hear" any tube benefits since these tube mic stages are designed to be generally linear and low-noise / clean.

My goal is to put the best capsule I can afford into a solid-state mic and call it good for vocal duties. This FET847 seems like an ideal choice for a cardioid-only vocal mic with a quality capsule, and not fretting about the elusive tube mojo.

I'm aware we're not getting the full S/N ratio available by not polarizing the capsule at a full 60v. That has me wondering if anyone's tried jacking 60v in at some point beyond the output transformer. (Probably a stupid thing to ponder.)

Would there be any noticeable difference between this FET 847 and the D-47fET? Or are they essentially the same?

Thanks for any thoughts.
 
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