Need help with a TLM 102 sized PCB for DIY LDC mic
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Not quite there yet, still working on the positions of things on the PCB itself, but I think I fixed the errors in the schematic, but judging all I've missed so far I wouldn't be all that surprised if I missed anything huge.
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C3 is supposed to connect ONLY to R3 & R8 (positive terminal) and ground (negative terminal), and NOT between R2 and C2. Just move it "south" of where it is and flip it upside down, to reduce the risk of further confusion.
And I'd also add another 1meg resistor between the R2-C2 node and R8-C10.
And regarding the board, well...
Yeah, I noticed this right after sending, thanks
And yes, I'm still learning where to position things on PCBs properly, it's rather unintuitive to the inexperienced
Positioning is only one thing, but try re-reading another couple times what i ACTUALLY wrote, hmm?
(regarding the PCB traces)Sorry, I should have specified those two statements were entirely unrelated, my bad!Positioning is only one thing, but try re-reading another couple times what i ACTUALLY wrote, hmm?
I added the 1m resistors where you said, fixed the C3 connection, and have redone the positioning of the PCB. I'm hoping I'm moving in the right direction and am SO sorry you've had to reply so much to address my mistakes
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![Soldering Iron Kit, 120W LED Digital Advanced Solder Iron Soldering Gun kit, 110V Welding Tools, Smart Temperature Control [356℉-932℉], Extra 5pcs Tips, Auto Sleep, Temp Calibration, Orange](https://m.media-amazon.com/images/I/51sFKu9SdeL._SL500_.jpg)
rogs
Well-known member
The addition of R11 will be fine - in reality it probably wont make much (if any?) differnce.
R10 is different. That will simply stop the circuit from providing any output. Adding 1M in series with R2 (47R) doesn't make sense.
R10 is different. That will simply stop the circuit from providing any output. Adding 1M in series with R2 (47R) doesn't make sense.
rogs
Well-known member
Khron - I think you probably mean 'R3 - C3' node and not 'R2- C2' ?
Nope - R8 already existed between R3-C3 and C10. I just suggested adding ONE more 1meg, from the R2-C2 node, to the R8-C10 node. Isn't that what i previously wrote?
MidnightArrakis
Well-known member
>> What is the diameter of your PCB? Inquiring minds want to know!!!
I am guessing that your "EasyEDA" PCB-design program doesn't have a feature called "Dynamic Reconnect", right? If it did, you would be able to perform a whole lot better components placement. Before you get too far along here with your project, I would strongly suggest that you switch over to the -- KiCAD -- PCB-design program instead. In fact, I was just on the telephone earlier this morning with a company asking me to perform some PCB-designs for them because they had also been using "EasyEDA" and they had finally decided to stop using it because of all of its shortcomings!!! What's that tell ya!!! As an example, any schematic program "worth its salt" would have flagged some of the errors that you had made in your schematic. Doesn't "EasyEDA" have any manner of "DRC" checking? It should!!!
>> Can you post an updated schematic that includes your recent corrections?
>> KiCAD is also -- FREE!!! -- just in case you needed to know that.
https://www.kicad.org/download/
https://groupdiy.com/threads/thinki...anel-manufacturing-service.86050/post-1133167
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MidnightArrakis
Well-known member
[it's rather unintuitive to the inexperienced] -- Helpful hint.....ALWAYS ROUTE TO CAPACITORS >> FIRST!! << for best circuit performance!!! (A point -- DRILLED -- into my head while designing countless numbers of PCBs for defense contractors and other military-related companies used in various "covert intelligence-gathering, surveillance and tracking operations" projects)!!!
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rogs
Well-known member
Ah -- I thought you were suggesting an additional 1M resistor in series with R8, to further improve the effect of the LPF formed with C10.
What is the purpose of adding an additional 1M resistor between the R2-C2 node (which is at half rail) and R8, which is at the full potential of the phantom power on pin3?
Both sides of the phantom power are used to power the op-amp, to help keep the DC feed balanced .
R3 and C3 decouple the 'cold' side, to help keep the impedance balanced.
There is effectively no further current drawn through R8, so it allows for the maximum remaining phantom power voltage on pin 3 (c.42V) to act as the polarising voltage...... That single sided high impedance feed doesn't affect the balance significantly.
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The diameter of the PCB is 36mm, it has to fit inside the TLM-102 body I'm using which I measured to have an internal diameter of just about 38mm (I gave an extra bit of space for mounting, I'll be 3D printing a "bed" of sorts for the PCB to sit in)
I'm fairly new to all of this, so luckily I haven't had time to grow stubborn in regards to PCB design software, I'll check out KiCAD and see if it works out any better for me compared to EasyEDA, I just used EasyEDA since this circuit is fairly simple and it has good integration with JLIPCB.
I think right now I'm just a tad confused about all of this, I'm still super new to the world of PCB design. But what I need to do to get this circuit working is rather unclear... I know there are "best practices" for circuit design, but as I haven't really done anything past a little bit of Arduino, so a lot of this is very foreign to me, I really apologize for the huge oversights I've made in this design so far. I'm very appreciative of all the help so far, it's just a lot of this is very vague to me
I know, blame it on some level of "OCD" or something... A personal preference, if nothing else.
rogs
Well-known member
I too have preference for symmetry normally, but in the case of differential audio vs. single sided audio with passive impedance matching, I had to admit that the extra headroom, plus reduced noise and current drain won the day.
Neumann and Rode both seem to prefer single sided audio with passive impedance balancing for some of their mics as well...
I originally had one side of the phantom power supplying the op-amp and the 'cold side' reserved just for polarisation voltage.
Actually the imbalance was not a problem with real world testing, but I decided that a symmetrical balanced feed from both sides for the op-amp was better.
I did stick with a single sided feed from the unused 'cold' side for the (non current drawing) polarisation voltage.
As the 'hot' side node has audio on it as well, I decided to give it a miss in this case!
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Okay... SO, I'm really not sure what else to do for this board, I feel like there's still problems but I obviously don't have enough knowledge to address them on my own, so I'm sort of at a crossroads.
If anyone would like to, let me know if anything is wrong - and specifically how I can address it if so, I'm not getting any DRC errors and can't figure out what else is going to be problematic, let me know in any case
if there are any other better views or screenshots I can give of the board, lmk!
If anyone would like to, let me know if anything is wrong - and specifically how I can address it if so, I'm not getting any DRC errors and can't figure out what else is going to be problematic, let me know in any case
if there are any other better views or screenshots I can give of the board, lmk!
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rogs
Well-known member
To be brutally honest, trying to do a decent PCB layout for an LDC condenser mic is not really a very good choice for a first project!
The input impedance required for interfacing a true condenser capsule is incredibly high..... In the order of 1000 million Ohms!
That kind of impedance is rarely encountered elsewhere in electronics, and to get it to work well requires some special attention.
It's really not a good 'starter' project.
If your layout now matches your corrected circuit, it should work .... but for how long, it would be difficult to say. The track spacings around R1 and R9 are likely to give problems, with noise from dirt and flux contamination, within a fairly short period.
The highlighted part of the attached image shows how Rode address the high impedance part of their circuitry. The highlighted flat white resistor, with what looks like a stripe, is the IG resistor. Note that it is surrounded by a pcb track which passes through it's terminals.
That's known as a 'guard ring', and is included to surround the input signal with a much lower impedance signal path, to minimise 'leakage' problems.
But it's a specialised subject (just google 'pcb guard ring' to get some ideas on the subject )
The alternative to that is to stand the appropriate terminals of the 1G resistors off the board (as I described in my earlier posts). That technique is adopted by many commercial mic manufacturers.
Don't want to seem negative, but continuing with the layout as you have it may not give you very good results.....
The input impedance required for interfacing a true condenser capsule is incredibly high..... In the order of 1000 million Ohms!
That kind of impedance is rarely encountered elsewhere in electronics, and to get it to work well requires some special attention.
It's really not a good 'starter' project.
If your layout now matches your corrected circuit, it should work .... but for how long, it would be difficult to say. The track spacings around R1 and R9 are likely to give problems, with noise from dirt and flux contamination, within a fairly short period.
The highlighted part of the attached image shows how Rode address the high impedance part of their circuitry. The highlighted flat white resistor, with what looks like a stripe, is the IG resistor. Note that it is surrounded by a pcb track which passes through it's terminals.
That's known as a 'guard ring', and is included to surround the input signal with a much lower impedance signal path, to minimise 'leakage' problems.
But it's a specialised subject (just google 'pcb guard ring' to get some ideas on the subject )
The alternative to that is to stand the appropriate terminals of the 1G resistors off the board (as I described in my earlier posts). That technique is adopted by many commercial mic manufacturers.
Don't want to seem negative, but continuing with the layout as you have it may not give you very good results.....
I see you're very stubbornly refusing to read, and/or comprehend and/or accept my tips. Sorry to be so blunt, but that board is a bit of a dog's breakfast, as they say... And you can't say I didn't try to help
And that's even before the high-impedance concerns that @rogs is referring to...
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