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

[Korablev]

hi I try to send the file to your e-mail and is not working ? no worries , here are the gerber files please check from schematic to layout to see if all is in order and for what I read here the NTE-3 transformer connections are like this:

TXI- red wire
TXI+ black wire
TX0- yellow wire
TX0+ white wire
on the NTE-3 tiny transformer the blue wire is not in use

the PTFE insulator part number is 534-11154 Mouser the hole diameter is 3.20 mm you can change this one they are other ones,
the orientation of the transformer would be with the 3 wire down and the good thing is that they will align just fine, the gerber files can be imported to Sprint Layout 6 software if you need to do some adjustments to fit the microphone frame you have if need it.

Today adding more information on C7 1uF capacitor the pitch distances are 34.5 mm, 22.5 mm, 10 mm, 5 mm, and the last one 2.54 mm, I add and extra R* so in series you can get the value you need after Q1 FET final adjustment. The electrolytic caps C8,10,12 are 5 mm in diameter size and PD 2.54 mm, the top insulators have two options of part number Mouser# 534-11154 3.18 mm diameter on the PCB the hole is 3.20 mm the other one Mouser# 534-11150 1.90 mm diameter and hole diameter on the PCB are 2.00 mm "I'm not sure how tight they need to be" Korablev there it is

Thank you very much Juan Vargas! Today I will order the production of boards from Chinese comrades, as well as those parts that are indicated in the BOM of the project. I will definitely write later about how my assembly of this magnificent ORS 87 project is going. Many thanks to the community for this opportunity. Assembling a microphone with your own hands is wonderful! Juan, if you need help with 3D modeling, be sure to contact me, I will help.

 

[Juan Vargas]

my pleasure! have fun please check layout and schematic make sure all is in order before order good day
by the way C12 is C7 from schematic but I change it the ID to C12 22uF 63V cap

 

[Juan Vargas]

hi guys got a question is a good idea to have one of the PCB support to be connected to microphone GND to chassis? I trace a connection to screw support to P1

 

[rankot]

As an aid to troubleshooting, here's some information about the ORS87-Plus testbed I built.

Schematic​

Here's the schematic with the component values used. The changes from the ORS87+ schematic were mostly to follow the original U87/U87A values more closely.

View attachment 138270

For testing, the U87's 'calibration input' arrangement was rigged up by replacing R15 with 560R + 6K8 in series, and the input wired to the midpoint. (I understand the V1.1 PCB has space for 2 resistors). I didn't implement a pad or bass-cut switch. Feedback capacitor C4 was 2.7pF, made by putting 5pF and 6pF caps in series.

To simulate a single cardioid K87-style capsule, there's a 47pF capacitor (Ccap) connected between the FD and BP inputs (the Neumann test procedure includes a fixture which does exactly this).

The BF256B JFET was chosen after a shootout (here) between various types: it's nearly the best by various measures, and cheap and readily available. Biasing the FET was done by adjusting trimmer R7 for minimum THD at the output, when fed with a 100mV, 1KHz sine wave at the calibration input. Values in blue are DC measurements at the bias point, and in red are the measured AC values.

The input stage looks like this:

View attachment 138271

Results​

Results are pretty close to the reported values for the U87A:

Measurement	Value
Overall gain, CAL to Vout, 1KHz (measured at 100mV in)	-1.6dB (0.83x)
JFET open-loop gain (1KHz)	+19dB (9.0x)
THD, 100mV RMS in, 1KHz	0.013%
Max input voltage. THD < 0.5%, 1KHz (RMS)	530mV
Drain current at optimum bias	0.38mA
Polarization voltage	45.0V

I also did a frequency response and distortion plot using REW. This is with a 100mV RMS input level.

View attachment 138272

We're getting a moderate 3-4dB dropoff at 15KHz, and a stronger rolloff after that. Obviously some of the distortion is coming from the transformer, which is relatively cheap compared to many alternatives.

If I understand this schematic well, you removed high pass and pad switches. What was the purpose of Rh/Ch anyway, since you removed them too?

 

[Juan Vargas]

anyway I add then an extra pad to solder a chassis GND if need it also since I know that measure hole distance with a none digital vernier caliper I change the holes shape so it will have a bit more play

 

[Voyager10]

If I understand this schematic well, you removed high pass and pad switches. What was the purpose of Rh/Ch anyway, since you removed them too?

Do you mean R9 and C5? They are an alternative way of achieving a high-frequency roll-off, instead of the C12/C13/R14/R15 feedback network. Nothing wrong with using them, it's just not how the U87 circuit does it.

 

[Екатерина]

I tried many different transformers from Ali and literally everything is better than T13, only T8 is worse,

the best is bv8, but it is very big

Good day, by a fortuitous coincidence, I acquired the adorable little transformer located on the right side of T13. Have you conducted any measurements on its frequency response?

 

[eso]

I haven't posted on this thread for a bit. This thread sent me off on a serious tangent.

I started off with Homero's board on PCBway, and then picked up khron's boards. I got 3 ORS 87s built and then went off the rails. The picture here is before I installed the Cinemag transformers. Cinemag is a local company that makes great products and I like to support them.

I've since completed a pair of U67 mics and power supplies and am getting started on a pair of U47s with parts on hand for a couple of C12s right behind those. For added fun I'm going point-to-point on these newer projects. I happen to have a friend here that I met 50+ years ago who is pretty well known in the mic world and so I've got some excellent tech back up when I need it.

And the members here are amazing. This is an awesome community and an excellent resouce for those of us who like making things.

I've been looking for a link to order some of the boards Juan Vargas modeled a few pages back that includes the mounting for the transformers. Is there such a link? I'd like to rebuild my U87s with such boards if available.

Thanks all,
eso

 

[Juan Vargas]

hi the mounting of the transformer opening need to be orientated according to the mic frame fastening spots, you just need a PCB to hold the transformer? the one layout I made is to fit my TF22 Temu mic frame if you can measure the screw distance top and bottom is gonna be a taper shape I can try to fit it also with the ORS 87 circuit or just a PCB for the transformer?

 

[Korablev]

At the beginning of the discussion in this forum thread there was a message that the capsule works best if it is raised in the grid. So I decided to do an experiment and solve two problems at once.
1. Raise the capsule by 8 mm higher in the HL-95 case.
2. Make a diffuser on a 3D printer in the form of a set of pyramids.
I have absolutely no idea how this will work, since I am waiting for printed circuit boards and components for them for about 2 months.
But I have already printed a diffuser under the capsule. Obviously, you can experiment with the shape and size of the pyramids. For those who want to experiment with sound, I am posting all the necessary files. I use a simple capsule mikparts k87, but for ORS 87 I have something better in stock for the future.

 

[rankot]

Do you mean R9 and C5? They are an alternative way of achieving a high-frequency roll-off, instead of the C12/C13/R14/R15 feedback network. Nothing wrong with using them, it's just not how the U87 circuit does it.

I was referring to this schematic, sorry for not including it. Then I noticed that they are labeled as R9/C5 on your schematic with mods. There are few different schematics circulating in this thread, so I wasn't sure.