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

[Khron]

Don't see what you did there. Went with Primary +/- to S1+/- and Secondary to S2?

and finally? FD to plate and P1+ to the center capsule?

Not at all...

Primary winding has two wires, right? That's what the "P+" and "P-" pads are for... "P" standing for "Primary" (positive and negative, as it were)

FD = Front Diaphragm; BP = BackPlate

If you have a single secondary winding, that goes between S1+ and S2- (by the same logic as the primary).
If you have a center-tapped secondary, center tap goes to S1-/S2+ (they're connected together), and you can skip the two 2k2 resistors. If you have two separate secondaries, same story.

Please, please, PLEASE try to read what i write...

 

[cello4u]

I read it. I don't know if I have a single secondary winding. I'm guessing you might know what the t13 is. It's difficult to follow logic when you don't have the knowledge. I'm guessing you are trying, in your own way, to educate me, which is why I'm trying to complete this project. I am frustrated, but I appreciate the help. FD. = aha.

 

[Khron]

I'm guessing you are trying in your own way to educate me, which is why I'm trying to complete this project.

https://en.wikipedia.org/wiki/Transformer

 

[AdamFrisen]

My personal quest to get a modified circuit for 797 Audio's K87 circuit.

The target was to attenuate compared to Kingkorg's measurement:
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

I didn't reach that with simply changing few values but I think I can live with this two changes and try out as simulation shows what I think could be important:
0-2000 Hz: ±0.5 dB
3 kHz: -1 dB
4 kHz: -1.5 dB
5KHz - 22 KHz ok with -2 to -3 dB

These two changes seem to give a response closer to what I want using K87 capsule from 797 Audio:

C1 - 120pF
R9 - 9K



20 Hz - 4KHz:



2KHz to 22KHz:


Sadly I don't have any 120 pF and is reluctant to order once again from Mouser .
I just have to try with something else. Like 150pF and 8K. Lower attenuation but attenuating at least -1.5 - 2dB over the range 5KHz - 22KHz.

Time to put the results to the test.

 

[cello4u]

https://en.wikipedia.org/wiki/Transformer

Thank you. I need to look at that. If I could ask a simple question pertaining to your pcb? S1 +/- is Primary and S2 +/- is Secondary. If what word sushi said, and the previous sentence is true, then I think I have connected it correctly. That's my logic.

P+ or P- connects to the back plate on your board?

 

[Khron]

If I could ask a simple question pertaining to your pcb? S1 +/- is Primary and S2 +/- is Secondary.

No it is not. Please read my previous reply to this - i'm not sure how much more clearer than that i can make it, without using THE biggest font size available here.

 

[cello4u]

No it is not. Please read my previous reply to this - i'm not sure how much more clearer than that i can make it, without using THE biggest font size available here.

I will re-read and read the transformer segment on wiki. Hopefully I will then know what
S- yellow
P+ black
S+ yellow
P- black

correspond to on your board.

 

[Khron]

S- yellow
P+ black
S+ yellow
P- black

Which is which? Or, what do you expect to achieve, if you mix up the two black wires between each other, and/or the two yellows?

 

[OneRoomStudio]

That would be true, if we were using the exact copy of the entire circuit, multipattern & isolated-backplate and all. That's not the case, since this project is the cardioid-only "reduction" of this circuit.

As both tests and simulations have shown, that capacitor does indeed work the same way, regardless of the exact connection nodes not being completely identical.

https://audioimprov.com/AudioImprov/Mics/Entries/2015/12/20_Modding_a_BM-800_Mic.html

The circuit you linked to is entirely different that the U87 (or "ORS 87").

@anvl is correct that you cannot just add the 10pF cap in the way that @AdamFrisen suggested. The issue comes from the fact that in cardioid, the original U87 circuit puts the rear diaphragm and rear backplate at the same ~47V potential as the front backplate, effectively making them act "as one" from an AC perspective. As a result, the signal feeding off the rear diaphragm does play a part in the feedback network, even in cardiod. There's a somewhat convoluted network:

Which can be simplified as:


The red traces are everything that works as part of the "backplate network" in cardioid, and the blue part is the front diaphragm. As you can see, the rear diaphragm is connected directly to C1...so C3 is not the only path for "backplate signal." There's a reason I left C3 out of the "ORS 87" schematic. Adding it in without the presence of a rear diaphragm doesn't accurately represent the response of a U87 in cardioid.

 

[AdamFrisen]

10-11V on the drain should be about right using a 24V zener (actual voltage before the drain resistor is ~22V):
View attachment 126506

With a 33V zener, the actual voltage is just below 29V, and you should have closer to 14.5V on the drain:
View attachment 126507

I based the simulations on these schematics you posted earlier in this thread.

https://groupdiy.com/threads/open-source-diy-mic-project-ors-87-stripped-down-u87.86814/page-7

The one with 33V zener:



Maybe I should simulate the U87 classic schematics to find out. That could be fun!

 

[OneRoomStudio]

I based the simulations on these schematics you posted earlier in this thread.

https://groupdiy.com/threads/open-source-diy-mic-project-ors-87-stripped-down-u87.86814/page-7

The one with 33V zener:

View attachment 134744

Maybe I should simulate the U87 classic schematics to find out. That could be fun!

Ah, that was my mistake. I didn't remove the 10pF cap after simulating the frequency response of having it in. The point of that screenshot was to show the DC voltages for bias purposes....not to be the "official" schematic.

I agree with the conclusion that @kingkorg reached when we discussed this back on page 5/6:

Ok, get it now. I personally don't like the idea of using this cap. I agree with what @OneRoomStudio said, but this thing throws off the whole de-emphasis network, which he did point to. I feel like it's extremely easy to end up chasing your tail. It would be extremely difficult to tell if the changes are due to FR, or THD, or level changes. Also if @Wordsushi decides to take this route, make sure you choose one hell of a stable cap for this role. I don't see the point to linearize the circuit, in Neumann's schematic the ''linearization'' of the circuit by introducing C3 is countered with increased value of C6 to 820p.

It just seems way more intuitive and simpler to tune the caps in the negative feedback for frequency response changes, and reduce the polarization voltage if THD is an issue.

 

[AdamFrisen]

Ah, that was my mistake. I didn't remove the 10pF cap after simulating the frequency response of having it in. The point of that screenshot was to show the DC voltages for bias purposes....not to be the "official" schematic.

I agree with the conclusion that @kingkorg reached when we discussed this back on page 5/6:

No worries I was just experimenting with new tool I found (QSPICE).

I see I should have continued to follow the thread. I was stopping here and thought it was worth a shot:



Regardless, I saw similar for KM84 circuit for LDC, FET847 and I was primarily testing it for the flat circuit where a I saw a little cap, 4pF being connected between the gate and drain directly.

 

[anvl]

I would also add that we are trying to follow this U87 circuit in cardiod-only pattern but using a single-side capsule (most of the times). That means all red traces are not present because there is no rear diafragm to connect / no rear backplate to connect.

I wonder if this green feedback path would be the main difference between ORS87 and complete U87 design.

 

[mrchapp]

Why the HUMONGOUS capacitors, though? Ok, "bigger is better" in some instances, but I'll never understand the obsession with 400V capacitors in phantom-powered mics...

It's simple in my case: I didn't know any better! That must have been the first hit in my Ali search.

PS. Flipping the transformer "connector" by 180deg would net you much simpler routing (for the next revision) And you might want to consider connecting the mounting holes to the ground plane(s).

You're right, thanks for the tip!

But was the ORS87-Plus not good enough(?) for fitting into a BM800?

The ORS87-Plus wasn't available when I started this design. Then it was only available on PCBWay (a little more expensive than JLCPCB) and then the BOM started to drift from what I already collected. I really hope to give that a try in the near future!

 

[kingkorg]

I wonder if this green feedback path would be the main difference between ORS87 and complete U87 design.

Nope, c5 and c6 are the de-emphasis feedback path.

 

[OneRoomStudio]

Right, another way to think of it is that C3 is there BECAUSE there is a rear diaphragm connected directly to C1. Without a second diaphragm/backplate, C3 isn't needed. If you add it into the ORS 87 circuit, you're overcompensating and ruining the curve created by C5/C6, as @kingkorg points out.

 

[mrchapp]

I read it. I don't know if I have a single secondary winding. I'm guessing you might know what the t13 is. It's difficult to follow logic when you don't have the knowledge. I'm guessing you are trying, in your own way, to educate me, which is why I'm trying to complete this project. I am frustrated, but I appreciate the help. FD. = aha.

The transformer has two sides: Primary (left in the schematic) and Secondary (right). Each side has a start (+) and an end (-).

In order to determine which side is which in your T-13, measure the resistance between the black pair and then the resistance between the yellow pair. The pair with 450 Ohms (approximately) would be Primary. Connect that pair to P+ and P-.

In @Khron's board, the Secondary wires go to S1+ and S2-.

(As for + and -, don't worry unless your mic's output is 180 degrees out of phase -- in that case connect the Secondary wires the other way around [S2- and S1+]. If you don't care about phase at this point, then don't worry and just connect those wires whichever way your prefer.)

 

[AdamFrisen]

I tried to simulate U87 original in cardioid equivalent. Did I model the double-sided capsule correctly I wonder?

 

[AdamFrisen]

I simulated 3 circuits, U87 original, U87 ORS without C1 and with C1 = 10pF.



And I got this result:



If (big if) I drew the schematics properly, it looks as the U87 ORS with C1 = 10pF looks more similar to U87 in the response than without?

I didn't know it was so easy (and fun) to simulate analog circuits (even if I might have done it wrong).

This is like when I discovered the benefits of writing unit tests in software for the first time.

 

[AdamFrisen]

This was interesting. When I simulate with 4-5 pF as C1, the shape gets even closer to U87 original and also the gain.
C1 = 5pF gives this result:



Darn yet another capacitor I don't have...