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

[AdamFrisen]

Which PCB is being used might be good info and some pictures?

Original U87 draws just 0.8 mA current and U87 ORS shouldn't draw excessively more current?

https://www.dpamicrophones.com/mic-university/know-the-basics-about-phantom-power

 

[untamedfrontier]

I'm on the same boat with a very similar BOM, as you can see. I need to do more in-depth analysis with oscilloscope and multimeter, but mine didn't work. One thing that was definitely not good from the outset was that my 1G resistor was 1/4 W, which I later learned may not have been enough. I'll keep digging while keeping an eye on your mic. Good luck!

I used a 1/4w 1G resistor in both of my builds without issue.

Perhaps you have a bad component somewhere (or a backwards cap?) in the build or your JFET is bad. Pictures would probably be best to see if anything jumps out.

 

[Khron]

I'm glad you ask, because that means that I'm wrong. My assumption, as I've seen in BOMs shared around is that it calls for a higher wattage resistor. Homero's BOM in Mouser has a 1.2W resistor, so that was the first thing I was going to check. Thanks for your input!

There's maybe some nA of current flowing through that resistor, so you're looking at some nW (or worst-worst case uW) of power dissipated in that, so... yeah...

 

[Jedias]

Hey guys, I found out that i soldered a resistor on the wrong place and fixed it, now i double checked it. tested it again and only got 1.2 volts on the J-Fet Drain, I can adjust the trim to only get less than that. Got 44v on X2 and X3. Also I got 15v on R12+C7 juction, but 0v on P+, it is right? My C7 is 1uf Poliester, not polarized.
Also got 15v on J-Fet's source, 0v on Gate.

Tried to put 1k sine wave on FD and it worked, I can hear it loud and clean, with only 3 harmonics, but still getting 1v o J-Fet's drain.


Heres the circuit layout:

 

[Voyager10]

If that's the top (component) side of the board in the picture, the 2N3819 is the wrong way round. The source should connect to R10 & C8, and the drain to R12 & C7.

 

[pasarski]

Aren't JFETs symmetrical though? I actually just a minute ago accidentally connected a 2n3819 wrong way around and there's sound. The mic is noisy though.

EDIT: This post reminded me to come and edit this. It turned out the reversed fet was not the cause of my noise problem after all. 2n3818 is practically symmetrical. https://groupdiy.com/threads/comparison-of-jfets-for-mic-applications.86559/post-1166283

 

[Jedias]

If that's the top (component) side of the board in the picture, the 2N3819 is the wrong way round. The source should connect to R10 & C8, and the drain to R12 & C7.

Are you sure? I followed this schematic:

 

[Jedias]

Aren't JFETs symmetrical though? I actually just a minute ago accidentally connected a 2n3819 wrong way around and there's sound. The mic is noisy though.

I made some cool fuzz pedals doing that lol

 

[Dimitrip]

Hey guys, I found out that i soldered a resistor on the wrong place and fixed it, now i double checked it. tested it again and only got 1.2 volts on the J-Fet Drain, I can adjust the trim to only get less than that. Got 44v on X2 and X3. Also I got 15v on R12+C7 juction, but 0v on P+, it is right? My C7 is 1uf Poliester, not polarized.
Also got 15v on J-Fet's source, 0v on Gate.

Tried to put 1k sine wave on FD and it worked, I can hear it loud and clean, with only 3 harmonics, but still getting 1v o J-Fet's drain.


Heres the circuit layout: View attachment 134085

If that's the top (component) side of the board in the picture, the 2N3819 is the wrong way round. The source should connect to R10 & C8, and the drain to R12 & C7.

According to these, the fet looks correct, the drain is on the right.
https://www.mouser.com/catalog/specsheets/2n3819.pdf
https://my.centralsemi.com/datasheets/2N3819.PDF

Btw, I made the circuit on stripboard with centralsemi fet from Mouser, just like the new fixed layout of Jedias. Works fine on both my mics. I'm getting 10.14V on drain on one mic and 9.87V on the other.

 

[homero.leal]

There's 48v at the center tap so you just power the mic from there. Center tap is labeled TXC so if no center tap no TXC and no use for J1-J2.

I already wrote that you can skip the resistors if you have a center tapped transformer but I'm not actually sure.

It seems you have this transformer? No center tap. Yellow wire is for grounding the core.

That's correct, thank you!

Also, please have a look at this schematic:

http://www.sdiy.org/oid/mics/AKG-C414-p48.pdf

The idea is the same. As you can see, the transformer has center tap, thus no need for additional resistors from each XLR line.

Kind regards!

HL

 

[Voyager10]

Aren't JFETs symmetrical though? I actually just a minute ago accidentally connected a 2n3819 wrong way around and there's sound. The mic is noisy though.

I think you just answered the question

Some JFET datasheets (e.g. J201) explicitly say "source and drain are interchangeable", but others don't. In theory the source & drain are just connected to either end of a bit of silicon, but it's possible that in practice there are some construction tricks (e.g. to keep capacitance low) which break the symmetry. So "works, but not very well" is exactly what you'd expect in these circumstances.

Regarding the schematic posted by @Jedias, pin 3 is being used as the source terminal. So it's either a mistake, or intended for a different JFET, or the author intentionally wants it connected the other way round to the datasheet.

 

[Khron]

Some JFET datasheets (e.g. J201) explicitly say "source and drain are interchangeable", but others don't.

They may well be "interchangeable" when using those JFETs as switches. But the Vgs(off) parameter doesn't magically turn into Vgd(off) if you turn the JFET around, so... yeah

Gate-to-source voltage DOES matter when using them as common-source amplifiers. Plus not all 3819's are equal, i've read that some may have reversed pin assignments.

Are you sure? I followed this schematic:

View attachment 134095View attachment 134096

On a side-note, arbitrary "numbers" of pins are meaningless, on discrete components.

 

[Jedias]

Thanks guys! Gonna switch the j-fet the other way around and I'll come back here to tell you if it worked.

 

[AdamFrisen]

That's correct, thank you!

Also, please have a look at this schematic:

http://www.sdiy.org/oid/mics/AKG-C414-p48.pdf

The idea is the same. As you can see, the transformer has center tap, thus no need for additional resistors from each XLR line.

Kind regards!

HL

Thanks for the info.

For anyone using UTM0587, record an impulse and check that the phase is correct.

A friend of mine discovered that the wiring of the secondary can be different from the datasheet.

The violet (secondary +) wire might have been switched with the green (ground -).

If you only build one microphone, you might not notice right away but if you build for stereo and record, it's very apparent.

Simplest is to record a hand clap. If the polarity is correct, then it should start with a "negative" dip and the following and biggest amplitude should be "positive" in waveform display.

 

[AdamFrisen]

Just a note:

If you remember Papa Tango Charley recommending capsules. I tried both Vamisound's (double-sided) and the green single-sided below from Aliexpress.

Vamisound K67:
https://vamisound.com/capsules/cts-67/

Green RK-87:
https://www.aliexpress.com/item/100...st_main.5.21ef1802CeqWX9&gatewayAdapt=glo2fra

I used the smaller U87 style body suggested by micolas.

Two identical bodies and Homero's v1.1 PCB with identical components/values with UTM0587 transformers.

At least listening to my recorded voice in cardioid, I can barely tell any difference (if at all).

And that green single-side capsule costs only 20 Euro in Aliexpress. I am both surprised and pleased with the outcome.

The info (rumors) I have is that Vamisound's capsule is actually a Takstar CTS-2 and that green one is not, but the engineer producing it was working for Takstar before.

It will be interesting to compare these with Arienne K87 and 797 Audio's using the same bodies/PCB.

Maybe it is really the case that at least 90% of the sound difference is in the capsule as Neumann claims?

https://www.neumann.com/fr-fr/transformer-balanced-or-transformerless-which-is-better/

 

[AdamFrisen]

Looking at Kingkorg's measurement for 797 Audio capsule in https://groupdiy.com/threads/soliloqueens-k87-k67-and-k47-capsules.81968:

I wonder how I can darken it to the desired taste?
Purple is 797 Audio which I assumed is the CY002 and black is the desired target:



Like:

0-2000 Hz: ±0.5 dB
3 kHz: -1 dB
4 kHz: -1.5 dB
5 kHz: -2 dB
6 kHz: -3 dB
7 kHz: -3 dB
8 kHz: -2 dB
9 kHz: -2 dB
10 kHz: -2 dB
11 kHz to 14 kHz: -2.5 dB
15 kHz to 22 kHz: -3 dB

In the original U87 schematics from here: https://groupdiy.com/threads/neumann.44805/#post-561144,
there are following values:

R8: 1 MΩ
C6: 220 pF
C5: 33 nF
R9: 6.8 kΩ
R10: 560 Ω

I assume some of them can be modified to better match the target frequency response.

Never used LTSPICE (yet), but I wonder if these values would get me closer?

R8: 1 MΩ
C6: 330 pF
C5: 39 nF
R9: 5.1 kΩ
R10: 470 Ω

 

[Khron]

I wonder how I can darken it to the desired taste?
Purple is 797 Audio which I assumed it is the CY002 and black is the desired target:

Are those the responses of the capsules with a flat-response circuit? If they're from kingkorg, they should be. You can also reference this thread:

https://groupdiy.com/threads/akg-perception-p220-to-neumann-u87-5-min-mod-p200-p100-p400-p420.67473/

 

[AdamFrisen]

Are those the responses of the capsules with a flat-response circuit? If they're from kingkorg, they should be. You can also reference this thread:

https://groupdiy.com/threads/akg-perception-p220-to-neumann-u87-5-min-mod-p200-p100-p400-p420.67473/

Thanks. I have a few questions that I'm trying to clarify.

Are the posts in that thread suggesting that the Takstar CTS-1 and CTS-2 might be inside the AKG Perception microphones, or are they just speculative?

In other threads, it's mentioned that the Takstar CTS-2 capsule is considered one of the most accurate in replicating the frequency response of Neumann's original U87 capsule. Some also suggest that the best alternatives are Arienne's K87 first, followed by the Takstar CTS-2.

It's interesting that the circuit in the AKG Perception is said to be very similar to that of the Neumann U87.

If that's the case, I'm curious why a change to such a large value as 680 pF is necessary (though it appears to be, based on Kingkorg's measurements).

Another point mentioned is that the bias might need adjustment. If the bias is correctly set, it might reduce the need to change the capacitor or use such a high value to manage the upper frequencies. However, I haven't come across anyone who has tried "rebiasing" the AKG Perception, so I'm not sure what the outcome would be.

At one point, I was even considering purchasing an AKG P420 just to obtain the Takstar capsule, or possibly moving everything into a K87-style body, modifying the circuit, and experimenting to see how close I could get to the U87 sound.

Currently, I'm interested in comparing alternative capsules for U87 builds, such as Arienne's K87, the Takstar CTS-2 (as I've been informed), and the 797 Audio CY002 after it has been HF-corrected.

 

[Jedias]

Hey Guys, after some changes mine worked.
Here's the BOM.
R7: 1G
R8: 1M
R9: 7k5
R10: 20K Trimmer
R12: 47k
R13: 330k (5%)
R14: 10k
R15: 300k
R16: 56k (5%)
R18 & R19: 2k2 (closely matched)

C5: 33n
C6: 220p
C7: 1u
C8: 22u 50v
C9: 470n
C10: 4u7 35V
C11: 4u7 63V
C12: 10u 63V
C13: 10n

Q1: 2N3819
GR1: 1N4749 24v

Transformer: T-13
Capsule: A cheap one




Here's a sound test:
View attachment Testei Mic.wav

 

[cello4u]

Think I've got KSA board properly populated so far. Can the jfet be soldered in and clipped?

Ali t-13 : word sushi says (l-r) S- yellowP+ blackS+ yellowP- black
so S1 primary and S2 secondary on KSA board?
what do I do with the jumpers?
R1 has a hole that is filled on purpose?

Khron I hope you have had time to situate yourself.