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

[peri valencio]

Hello everyone, my contribution here is to read everything very attentively and countless times. I already have my v1.1 working and I’m very happy, but I’m still testing different components and capsules. Thank you all. I see a lot of discussion about which types of capacitors to use for coupling; there are so many types of capacitors but few insights on the final results. I found this paper from TI, and it seems to align with what has been written here by the more experienced members.

https://www.ti.com/lit/an/slyt796a/slyt796a.pdf?ts=1729861324029

 

[joulupukki]

Yes, you will have more headroom, more signal output and the transformer will saturate a little faster.
Just put a 33v Zenner and reduce the resistance that feeds it (for example in ORS87 R16 - 22...33kohm , you have to experiment, the Zenner will stabilize effectively if you feed it with a few extra volts, also take into account the possible phantom voltage deviation +/- 4v). For Dani B. U87 get inspired by U87ai.

What effect would a lower-value resistor have? Right now I converted it from 56kohm to 47kohm now with the 33V Zener. With the 33V Zener and 47K resistor I'm not noticing that much difference compared with the 24V Zener & 56K resistor.

 

[jp8]

I'm still trying to understand some peculiarities of all the U87 variants and understand the design choices that the Neumann engineers made. I hope you guys don't mind that I will (ab)use this thread to educate myself. But of course, I hope I can contribute as well.

Simulating a circuit in LTspice always helps me a lot in understanding a circuit. So that's what I did. I applied some simplifications to Voayger10's circuit and dropped it in LTspice. Picture attached. I was hoping, of course, to get results that would match the measured data on the real circuit. With the components displayed, I obtained a 0.38mA bias current, resulting in a Drain voltage of 11.4V. So far, so good. Ran a sim and checked the gain and high-end roll-off. As you can see from the resulting AC Analysis plot, mid-band gain equals 19 dB and at 15 kHz, the drop-off reached 3.5 dB. Spot on. Or not...?

@Voyager10 : In your table, you mention an Open Loop gain of 19dB. Which is very close to my simulation, but in my view that would be the Closed Loop gain. My definition of Open Loop gain of a circuit would be the gain with all feedback removed. Feedback is through C4 and the feedback circuit from C8 back to the capsule. When I change C8 and C4 to 0.01pF, effectively removing feedback, I obtain an Open Loop gain of 27.6 dB. See last AC Analysis plot attached. So my question is: do we have a different understanding about what the concept of Open Loop entails? Or is my simulation completely off?

Now, getting to @joulupukki 's question: assuming my simulations are right, then changing the zener to 24V, R2 to 47k and rea-adjusting the bias resistor R7 to 10k to obtain 11.2V again at the Drain, the sim shows a circuit gain gain of 18.5dB. So it does very little to the gain, as you already observed. That is easily understood when you realize that only the Open Loop gain is affected by these changes. Open Loop gain is particularly high in this circuit, so it will have some effect on Closed Loop gain, but not a lot. Open Loop gain with 56k/24V is 26.7 dB, so a drop-off of 0.9 dB, resulting in a 0.5 dB Closed Loop gain.

If one wants more Closed Loop gain, remove C4. But I don't see this capacitor in every U87xx incarnation, so maybe yours doesn't have it. Without C4, Closed Loop gain is very close to Open Loop gain, the odds that you will get more gain from this circuit are negligible. I think the best way to obtain a higher sensitivity is by raising the capsule voltage by using a polarization voltage generator circuit. With 60V, you'd have 2.5dB more gain and at 80V, +5dB. But the question is whether your capsule (Arienne's K87, AFAIK) can handle such voltages. Does anyone know?

Jan

 

[Voyager10]

@Voyager10 : In your table, you mention an Open Loop gain of 19dB. Which is very close to my simulation, but in my view that would be the Closed Loop gain. My definition of Open Loop gain of a circuit would be the gain with all feedback removed. Feedback is through C4 and the feedback circuit from C8 back to the capsule. When I change C8 and C4 to 0.01pF, effectively removing feedback, I obtain an Open Loop gain of 27.6 dB. See last AC Analysis plot attached. So my question is: do we have a different understanding about what the concept of Open Loop entails? Or is my simulation completely off?

My "19dB" figure was with C4 in circuit, i.e. is the gain if the 'outer' feedback loop (C8, C13, R14 etc.) is removed. It will be very close to the closed-loop figure at 1KHz (because the outer feedback loop is largely ineffective at this frequency), so I think this totally agrees with your simulation.

Without C4 I measured a gain of 18x, or +25dB, for the BF256B - that's close enough to the simulation's +27.6dB (25x), especially as my build will have more that 0.01pF of parasitic capacitance...


The theory is that the shape of the closed-loop frequency response curve is affected by the "FET + C4" gain - if you change the FET but adjust C4 to give the same gain, you should get the same frequency response. Higher gain gives more pronounced HF and LF dropoff - see simulation. This effect may be a more likely explanation for "I changed the FET and it sounds different" than mystery un-measurable FET properties.

 

[jp8]

My "19dB" figure was with C4 in circuit, i.e. is the gain if the 'outer' feedback loop (C8, C13, R14 etc.) is removed. It will be very close to the closed-loop figure at 1KHz (because the outer feedback loop is largely ineffective at this frequency), so I think this totally agrees with your simulation.

Etc...

Thanks for the confirmation and the backgrounds of C4! This really helped.

Jan

 

[Papa Tango Charly]

Hello I would like to know if i can use 16V on the C8 condenser 22uF
on the ORS 87 if not what is the lowest voltage i can use
I know that the other should be a list 50V
Thank you best

 

[Voyager10]

Should be no problem, at most there's 3-4V across C8.

 

[Jobair Ahmed]

Can anyone tell me the Primary and secondary voltage of this kind of transformer? it's a 9:5:1 U87 Microphone Transformer
i want to build it by myself but i need to know the primary and secondary voltage.
the main reason is i live in Bangladesh, i can't find these kinds of transformers in local market and if i bring it from Ali express it will cost me more than $60-65 USD for a single transformer.

 

[Khron]

Can anyone tell me the Primary and secondary voltage of this kind of transformer?

Take a multimeter and measure resistances between wires?

Higher-resistance winding goes towards the circuit, lower-resistance winding goes towards the XLR. Compare with a retail mic to check for polarity; if it happens to be reversed, swap the two wires going to the XLR.

 

[Juan Vargas]

Hi I'm new here question, are this test points I'm cloning the PCB and I notice this small rectangles, on the original PCB layout I can see those rectangles on the last image

 

[Khron]

Hi I'm new here question, are this test points I'm cloning the PCB and I notice this small rectangles, on the original PCB layout I can see those rectangles on the last image

@homero.leal seems to think there's some benefit to connecting the various ground points like that I fail to see how or why, but there you go...

 

[Juan Vargas]

sort like star GND? anyway this the first time that I want to build my own mic I usually work designing audio amplifier boards but also I want to learn microphone design PCB, I thought that those were test points okay

 

[Khron]

sort like star GND?

Sort of, but like the complete opposite. But I'd rather let the designer himself explain.

 

[homero.leal]

Direct quote from LNPrimer.doc, by @ricardo The doc can be found on the MicBuilders forum.

"PCB layout is critical for low noise. The first rule is to be very clear what your earths are doing. I always define 3 earths; Clean, Dirty and Chassis. I won’t talk about evil Digital earths but see the above link on www.planetanalog.com Consider carefully what you are doing when you connect anything to an earth. If you connect supply decoupling capacitors to an earth line, you make it Dirty. In our simple circuit, Clean earth is used ONLY for input / output earths and feedback components. The 10k input resistor is taken to BP so this line must be treated like Clean earth. BP is joined to Clean earth via a single 47u right at the input sockets. Dirty earth carries our supply decoupling currents. The 3 earths are joined ONLY at one point, at the output in our simple circuit. Output current does not upset the Clean input and feedback paths; a common source of instability. Some of you will recognise this as similar to "Star Point" earthing in power amplifiers."

 

[Khron]

That may very well apply to line-level circuits powered from the mains grid, that interact with other equipment powered from the mains grid. Some appropriate context for that quote would have helped..?

Found @ricardo 's quote on this forum as well, but as expected from the wording, it refers to preamplifiers, NOT microphone impedance converters...

https://groupdiy.com/threads/ina134-and-drv134-supply-bypassing.56079/post-715735

Unless you have special mic cables that have a separate shield that connects ONLY to the mic chassis, and the circuit ground of the circuitry connects ONLY to XLR pin 1.

As far as I'm concerned, a phantom-powered microphone might as well be a "floating" system, since it ONLY connects to the preamp you plug it into (tube mics could have a ground-lift on the output XLR, but that's a different issue).

Really no idea how and what "ground loops" you can achieve inside a mic, but i haven't managed that yet, despite my "careless" and "unwise" use of ground planes

PS. I would also slightly object to there being multiple "earths" there - i've been taught "earth" is (strictly) mains earth; anything else is a (circuit) ground. If only to try to avoid confusion...

 

[Juan Vargas]

hi guys at the moment I got this layout that will fit into this cheapo Temu mic that please don't laugh at the tiny PCB , ok so I add an opening for the NTE10-3 transformer that can be secure with zip ties the PCB size will fit only on this type of body that looks like those real Telefunken microphones body style I want it to add this data here so the screws mounting for the frame top PCB fastening are about 28.5 mm wide the next screws down are about 28 mm and from this screw mounts down is about 52 mm PCB size is 37 mm x 90 mm I'm gonna continue checking the layout to make sure that there are no bugs I've been doing this for many years but there is always one "oh no I forgot to..." you know what I mean the only thing different that I add to the layout is another R* so to combine different value resistors after FET is adjusted and get the trim pot resistance value to R* final value good day guys

​

 

[Korablev]

hi guys at the moment I got this layout that will fit into this cheapo Temu mic that please don't laugh at the tiny PCB , ok so I add an opening for the NTE10-3 transformer that can be secure with zip ties the PCB size will fit only on this type of body that looks like those real Telefunken microphones body style I want it to add this data here so the screws mounting for the frame top PCB fastening are about 28.5 mm wide the next screws down are about 28 mm and from this screw mounts down is about 52 mm PCB size is 37 mm x 90 mm I'm gonna continue checking the layout to make sure that there are no bugs I've been doing this for many years but there is always one "oh no I forgot to..." you know what I mean the only thing different that I add to the layout is another R* so to combine different value resistors after FET is adjusted and get the trim pot resistance value to R* final value good day guys

​

Dear Juan Vargas.
Are you planning to make the gerber file available for free use or sell it so that I can order a PCB of your version on Aliexpress?
The thing is that the bolt pattern of your board version is ideal for my case, which I recently purchased on Aliexpress.
https://aliexpress.ru/item/10050064...l.0.0.20204aa6XU8jLw&sku_id=12000037309239239
I would like to take this opportunity to say a big thank you to everyone who actively worked on this project. This is simply a wonderful opportunity to try to assemble a clone of the U87.

 

[Khron]

Are you planning to make the gerber file available for free use or sell it so that I can order a PCB of your version on Aliexpress?

Do none of the mounting holes on the ORS87-Plus on PCBway fit your intended mic body?

 

[Juan Vargas]

I just check the link and I don't think this mic body will fit the ORS 87 the V1.1 that they do have the link to order on PCBWay


oh by the way I don't sell things I just do for personal use and learning new things, OK, so if you have that mic body then measure with the vernier caliper you have top screws distance in millimeters then the metal frame width then front top screws to the last screw mounting also how long the PCB can be with that frame there are other option to fastening the tiny transformer PCB openings but for the mic body I have only I can use the NTE10-3 from Mouser the Ali one will not fit the mic body I have not sure about your mic body you got that one look like a really nice heavy duty mic shell

 

[Khron]

I just check the link and I don't think this mic body will fit the ORS 87 the V1.1 that they do have the link to order on PCBWay

Do none of the mounting holes on the ORS87-Plus on PCBway fit your intended mic body?

It helps if you read (all of) what is actually written, not what you might imagine is written...