OPEN SOURCE DIY Mic Project - ORS 87 - Stripped Down u87

[Wordsushi]

Anyone here ever try the tiny Pikatron Prowave K4000004 transformer? I vaguely recall seeing someone post that it sounded good, but I can't seem to track that one down.

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[Highlander]

Have really enjoyed the build, great thread and can really hear a difference with the mic.
Just checking on how to correctly bias the FET.
Im using, JLI TSC-2 Capsule the LSK189 and have set Bias to 15.9v with meter.
Measuring between Drain (C9,C7 and R12 on ORS CCT with 36V Zener) and GND
What am I listening for and is a meter the best way.
Got a very very cheap scope and even cheaper wave generator, is that a better way to go?
Also feel there is some clipping (could be hearing aids) could that be the bias?
Still sounds great to my deaf old ears.
Too many years with headphones way to loud

 

[Wordsushi]

Have really enjoyed the build, great thread and can really hear a difference with the mic.
Just checking on how to correctly bias the FET.
Im using, JLI TSC-2 Capsule the LSK189 and have set Bias to 15.9v with meter.
Measuring between Drain (C9,C7 and R12 on ORS CCT with 36V Zener) and GND
What am I listening for and is a meter the best way.
Got a very very cheap scope and even cheaper wave generator, is that a better way to go?
Also feel there is some clipping (could be hearing aids) could that be the bias?
Still sounds great to my deaf old ears.
Too many years with headphones way to loud

Bravo! Congrats on the build!
Perhaps your bias is set too hot. Going by OneRoomStudios guide in this post (https://groupdiy.com/threads/open-source-diy-mic-project-ors-87-stripped-down-u87.86814/post-1143520), measure the voltage between R12 and R14 and shoot for a bias voltage that is half that measurement.

 

[micolas]

@Highlander used 36V Zenner and an LSK189 jFET, which has different specs than the 2N3819
According to @OneRoomStudio 's simulation, bias is good.

 

[homero.leal]

May be you could try a JFET with a higher IDSS. According to this tutorial from Chunger:

https://studio939.blogspot.com/2012/12/vintage-u87-circuit-clone-microphone.html

"As I understand it the higher IDSS JFETs will yield higher dynamic range before distortion, but the overall output level will be a bit lower than the lower IDSS ones."

So, if you use a JFET with low IDSS, the mic will have a higher ouput level, but lower dynamic range before distortion.

According to the tutorial, recommended IDSS values for the u87 circuit is from 8-12mA.

Regards!

HL

 

[micolas]

May be you could try a JFET with a higher IDSS. According to this tutorial from Chunger:

https://studio939.blogspot.com/2012/12/vintage-u87-circuit-clone-microphone.html

"As I understand it the higher IDSS JFETs will yield higher dynamic range before distortion, but the overall output level will be a bit lower than the lower IDSS ones."

So, if you use a JFET with low IDSS, the mic will have a higher ouput level, but lower dynamic range before distortion.

According to the tutorial, recommended IDSS values for the u87 circuit is from 8-12mA.

Regards!

HL

Correct.
Depending on different sources, you need different sensitivity and headroom. Lower Idss leads to higher circuit amplification and less headroom. For vocal applications I have successfully used in the U87i circuit jFETs with lower Idss between 5...9mA.
For stronger sources, I used 9...12mA.
In this range of values I managed without problems to make the BIAS correct, by all methods.

 

[elskardio]

Quick question.

If I’m using a k87 capsule (isolated backplates) in cardioid only, do I need to connect both backplates together?

 

[Khron]

Quick question.

If I’m using a k87 capsule (isolated backplates) in cardioid only, do I need to connect both backplates together?

Probably not a bad idea. Could even consider tying the rear diaphragm to the backplates too.

 

[micolas]

Quick question.

If I’m using a k87 capsule (isolated backplates) in cardioid only, do I need to connect both backplates together?

Since so far most ORS 87 builds have used capsules with non-insulated K67/870 style backplates and the results have been very good, logic tells us to bond the backplates together.
On the other hand, I've personally had 50Hz hum issues with switch parts/bodies/buttons not connected to GND in the SP CS1 and when using the B2-Pro as a donor body (also with switch bodies not connected in the circuit)
My opinion is that any metal parts that are free, not connected, in the high impedance area or in close proximity can act as interference collector antennas.
So I always connect to GND the rear diaphragm and the rear backplate if it is isolated K87 style. Let's say that between isolated backplates (K87) we have a capacity of approximately 300pF.
By connecting the back to GND we do an additional high-frequency filtering of the polarization voltage and at the same time a better shielding of the capsule.

 

[elskardio]

Since so far most ORS 87 builds have used capsules with non-insulated K67/870 style backplates and the results have been very good, logic tells us to bond the backplates together.
On the other hand, I've personally had 50Hz hum issues with switch parts/bodies/buttons not connected to GND in the SP CS1 and when using the B2-Pro as a donor body (also with switch bodies not connected in the circuit)
My opinion is that any metal parts that are free, not connected, in the high impedance area or in close proximity can act as interference collector antennas.
So I always connect to GND the rear diaphragm and the rear backplate if it is isolated K87 style. Let's say that between isolated backplates (K87) we have a capacity of approximately 300pF.
By connecting the back to GND we do an additional high-frequency filtering of the polarization voltage and at the same time a better shielding of the capsule.

Great info! Thanks

 

[elskardio]

I just finished working on this little guy!

From now on, I will use the phrase: "don't judge a mic by its size" .

I'm pretty impressed with how it sounds. It's pretty close to its big brother!

I will try to post sound clips tomorrow.

Cheers

 

[Wordsushi]

Great info! Thanks

That is really excellent. Nice PCB's for that one! Very impressed.

 

[Wordsushi]

View attachment Stereo ORS87.wav
Took advantage of my Aliexpress getting delivered on a holiday and finished my first attempt at a dual output STEREO ORS87
View attachment Stereo ORS87 ambience test 1.wav
And this is me just a quick stereo ambience test of me pushing stuff around on my desktop with the mic hanging over top.
View attachment Stereo ORS 87 shaker test.wav
And shaking a plastic box full of small mic screws around the mic.

 

[Barry Hufker]

Congratulations on your stereo mic. I don't know if it's me, but I think the plastic screw box sounded a bit "phasey" as you moved it back and forth. Are the capsules coincident?

 

[Wordsushi]

Congratulations on your stereo mic. I don't know if it's me, but I think the plastic screw box sounded a bit "phasey" as you moved it back and forth. Are the capsules coincident?

It's actually a single dual diaphragm, isolated backplate k87 style capsule.

 

[Barry Hufker]

Then it might be frequency cancellation as the sound travels from one side to the other.

 

[Wordsushi]

Then it might be frequency cancellation as the sound travels from one side to the other.

Thinking that the chiseled headbasket is causing more of a null on the sides than a round headbasket would, so I'm planning to migrate this to a Studio Projects C1 body.

 

[elskardio]

@Wordsushi
Are you building your mics with the 24v or 33v zener?
Noticed any difference?

Also how do you find the overall gain with your usual mic preamp?
the ORS has a lot of gain compared to the vintage 87…
I might try lowering the backplate voltage a little.
Cheers

 

[Wordsushi]

@Wordsushi
Are you building your mics with the 24v or 33v zener?
Noticed any difference?

Also how do you find the overall gain with your usual mic preamp?
the ORS has a lot of gain compared to the vintage 87…
I might try lowering the backplate voltage a little.
Cheers

Haven't really noticed a difference between 24v and 33v, but I'm fairly certain that all my PCB builds have been 33v, so that could be why.
Gain overall is about the same as my u87ai. I have a couple that have lower gain, but I've just been pulling FETs out of a bag of them without testing first.

 

[micolas]

Haven't really noticed a difference between 24v and 33v, but I'm fairly certain that all my PCB builds have been 33v, so that could be why.
Gain overall is about the same as my u87ai. I have a couple that have lower gain, but I've just been pulling FETs out of a bag of them without testing first.

Using a higher voltage, so 33V Zenner, will get a bigger headroom, allows higher SPL before distortion.