Re: What is with this trace? (Lavalier Teardown: Shure WL185 with RF CommShield)
Digression on wireless beltpack connectors...
The Shure TA4F (aka mini-XLR) wiring scheme is pretty clever - you can wire it for different topologies as the audio and power pins are separate. The 20k resistor pin is there to provide a DC return path when needed.
The basic pinout is:
1 - ground
2 - DC +5v
3 - Audio
4 - 20k resistor to ground
Note that the mini-XLR pin #s are not the same as regular sized XLR!
A lot of other wireless manufacturers put both power and audio on the same pin, as do a lot of the lavalier mics.? Sennheiser does that on the high-end LEMO connector versions, which means that if you want to use a guitar cable you have to add a DC-blocking capacitor.? Their cheaper Evolution line uses 1/8", and the ring contact is an audio-only input with no power.? Shure's implementation of the same 3-pin LEMO connector is compatible but slightly more complex.
I'm a pretty serious pinout nerd in that world, if you need info please feel free ask.
-Scott
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On 6/9/24 21:43, sergio_logic via groups.io wrote: Thanks for the answers!
The the Shure bodypacks have a 20KOhm bias resistor to ground where I wrote ¡°BODYPACK_20K_BIAS_AND_AUDIO¡±. What difference does 100Ohms make? It¡¯s a 0.5% difference, probably less than the tolerance of that 20K resistor. This is why I doubt it has any relevant impact on the Q-point of the FET.
--
---- Scott Helmke ---- scott@... ---- (734) 604-9340 ----
"I have ceased distinguishing between the religious and the secular,
for everything is holy" - Joe Henry
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I updated my hydrophone instructables with a new one. The biggest change is input protection for the OPA1642 high impedance buffer with a pair of?Schottky diodes. I stress?tested this big time after implementing.??
-- Best Regards,
Jules Ryckebusch
214 399 0931
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Re: [allowed] Re: [MicBuilders] True tree ears or DYI binaural dummy head.
Bamboo, like elephant grass(miscanthus), has a very high silica content, 10x higher than barley straw and 5x ¨C 10x higher than rice straw on a dry matter basis. This blunts tools very quickly. ?
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Re: What is with this trace? (Lavalier Teardown: Shure WL185 with RF CommShield)
Update on the FET: I removed the capsule from another one and it's marked "IA". I believe it's a Toshiba TTK101MFV, datasheet states it's for "compact ECM" and the equivalent diagram shows a bias diode (and resistor, but that's not real, its "equivalent", it's not possible to integrate large value resistors).  
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Re: What is with this trace? (Lavalier Teardown: Shure WL185 with RF CommShield)
The effect of R1 on the bias point is indeed negligible compared to the 20k source resistor load. I think R1 is there to avoid ringing or even parasitic oscillations caused by the capacitive load of C1 and the parasitic inductances from the PCB tracks and capsule. Similar to emitter followers, which you want to decouple from capacitive loads through a resistor for the very same reason.
Jan
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Re: What is with this trace? (Lavalier Teardown: Shure WL185 with RF CommShield)
Thanks for the answers!
The the Shure bodypacks have a 20KOhm bias resistor to ground where I wrote ¡°BODYPACK_20K_BIAS_AND_AUDIO¡±. What difference does 100Ohms make? It¡¯s a 0.5% difference, probably less than the tolerance of that 20K resistor. This is why I doubt it has any relevant impact on the Q-point of the FET.
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Re: What is with this trace? (Lavalier Teardown: Shure WL185 with RF CommShield)
R1 is to bias the Fet properly. Also there is no 1G resistor as that Fet has an internal diode designed to ¡°leak¡± current and somewhat act as the 1G resistor. Most electrets with built in FETs have that too.? Best Regards,
Jules Ryckebusch?
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On Jun 9, 2024, at 13:19, sergio_logic via groups.io <sergiu757@...> wrote:
?When cell phones became common, Shure lavalier microphones were picking up GSM noise. Shure changed their FET circuit, added a dot on the lavs so you'd know they're the new type and marketed them as having " Technology for RF Filtering". I was curious about the circuit so I opened up a Shure . The family of lavs (WL185, WL184, WL185) have interchangeable 10mm-ish capsules (cardioid, supercardioid, omnidirectional) and are very popular for sound reinforcement applications. Here is the circuit I traced: a few caps (not 100% sure of values, but they should be close to what is below), a SMD ferrite bead, no real surprises in the schematic apart from the 100 ohm resistor which actually increases the output impedance of the mic. I believe R1 is to allow the caps to kick in from a lower frequency, any other ideas? <dummyfile.0.part> The real surprise, however, is the PCB layout. The source terminal of the JFET is routed as a ring trace all around the gate terminal capsule connector, on both sides of the board! Is this FET circuit using PCB losses to bias Vgs instead of a large value resistor (or, as would be expected for a small electret, back-to-back diodes)? Or is it RF magic? To me it seems that the first one is more likely: the trace is a full ring around the capsule terminal, routed with minimal clearance. Any other explanation? Identifying the FET would help, but it has no marking. <dummyfile.1.part> <dummyfile.2.part> <dummyfile.3.part>
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Re: What is with this trace? (Lavalier Teardown: Shure WL185 with RF CommShield)
It's sometimes called electrostatic guard. A very common
technique in RF and very high impedance circuits.
The parasitic capacitance is used profitably to bootstrap the
gate. It shields the gate as much as a ground pour but without the
detrimental effects of parasitic capacitance.
Le 09/06/2024 ¨¤ 20:06, sergio_logic via
groups.io a ¨¦crit?:
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When cell phones became common, Shure lavalier microphones were
picking up GSM noise. Shure changed their FET circuit, added a dot
on the lavs so you'd know they're the new type and marketed them
as having "
Technology for RF Filtering". I was curious about the circuit so I
opened up a Shure .
The family of lavs (WL185, WL184, WL185) have interchangeable
10mm-ish capsules (cardioid, supercardioid, omnidirectional) and
are very popular for sound reinforcement applications.
Here is the circuit I traced: a few caps (not 100% sure of values,
but they should be close to what is below), a SMD ferrite bead, no
real surprises in the schematic apart from the 100 ohm resistor
which actually increases the output impedance of the mic.
I believe R1 is to allow the caps to kick in from a lower
frequency, any other ideas?
The real surprise, however, is the PCB layout. The source terminal
of the JFET is routed as a ring trace all around the gate terminal
capsule connector, on both sides of the board!
Is this FET circuit using PCB losses to bias Vgs
instead of a large value resistor (or, as would be expected for a
small electret, back-to-back diodes)? Or is it RF magic?
To me it seems that the first one is more likely: the trace is a
full ring around the capsule terminal, routed with minimal
clearance. Any other explanation?
Identifying the FET would help, but it has no marking.
 
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What is with this trace? (Lavalier Teardown: Shure WL185 with RF CommShield)
When cell phones became common, Shure lavalier microphones were picking up GSM noise. Shure changed their FET circuit, added a dot on the lavs so you'd know they're the new type and marketed them as having " Technology for RF Filtering". I was curious about the circuit so I opened up a Shure . The family of lavs (WL185, WL184, WL185) have interchangeable 10mm-ish capsules (cardioid, supercardioid, omnidirectional) and are very popular for sound reinforcement applications. Here is the circuit I traced: a few caps (not 100% sure of values, but they should be close to what is below), a SMD ferrite bead, no real surprises in the schematic apart from the 100 ohm resistor which actually increases the output impedance of the mic. I believe R1 is to allow the caps to kick in from a lower frequency, any other ideas?  The real surprise, however, is the PCB layout. The source terminal of the JFET is routed as a ring trace all around the gate terminal capsule connector, on both sides of the board! Is this FET circuit using PCB losses to bias Vgs instead of a large value resistor (or, as would be expected for a small electret, back-to-back diodes)? Or is it RF magic? To me it seems that the first one is more likely: the trace is a full ring around the capsule terminal, routed with minimal clearance. Any other explanation? Identifying the FET would help, but it has no marking.   
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Re: True tree ears or DYI binaural dummy head.
Ha! This is some interesting stuff! I didn't know bamboo can be so hard it requires metalworking tools. Great reading, thanks for interesting and extensive information :) Didn't know your pole measures 5 ft. It definitely is unwieldy to carry around :-D
?
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Re: AOMs-5024 in enclosures from China
I opted for the felt-covered version in the hope that they would be slightly better at rejecting dust etc in the field. Also, they were a few cents cheaper. I recall seeing SONY and other pro lavalier mics with the perforated front, so maybe this design favours voice pickup?
I doubt that my old ears would pick out any difference, but it would be interesting to hear back from any one who has detected a difference between the two versions, or can explain it.
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Re: True tree ears or DYI binaural dummy head.
On Sun, Jun 9, 2024 at 08:25 PM, pmfalcman wrote:
Bamboo too heavy?? Seriously? It's empty inside. Or is there something I don't know?
You are right. Many bamboo species are light. It depends on the type of bamboo. This is a so-called Moso bamboo (Phyllostachys edulis). The pole with the headphone is dry, hollowed and about 5 feet in height. The weight is still 5.8 kg. It's like wood, very dense and not so easy to work with. It is best to use tools that are normally used in metalworking. In spring, the shoots grow very quickly. In comparison to wood, Moso bamboo can be harvested after just 4 years. Here's another photo I took. 
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Re: True tree ears or DYI binaural dummy head.
Heinz - what a splendid setup! :)
Bamboo too heavy?? Seriously? It's empty inside. Or is there something I don't know? Compared to my head it should weight next to nothing :)
Mine's probably around 2 kgs.
Not to mention the 3,5 kgs of Neuman's head...
But, true, ours are not something easy to carry around on long distances.
As one of famous photographers (he worked with large format cameras) said:
"If it's more than 150 m from the car, it is not photogenic".
In our case: "it doesn't sound interesting" ?
Thanks for the kind words.
Near field sounds result obviously in much more interesting spatial effecs.
Once I snuck up on a log-with-lavs setup and reloaded a gun. A metal replica. But it being a real world sound, not Hollywood SFX, it wasn't recognisable when I listened to it few days later. True, nice spatial positioning but just a metallic clicking.
Cheers
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Re: True tree ears or DYI binaural dummy head.
On Sun, Jun 9, 2024 at 03:47 PM, pmfalcman wrote:
BTW your (lovely) bamboo housings prompted me
Thanks for your kind feedback! I tried something similar with the Victor HM-200 Binaural Microphone Headphone. But it turned out that the bamboo is much too heavy to carry it around. Anyway, it was fun and I will continue working on the idea. :-D Btw: The sounds interesting. Especially at the end, when you walked around the True Tree Ears Head. 
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Re: True tree ears or DYI binaural dummy head.
Yup! One's portable binaural tree ? BTW your (lovely) bamboo housings prompted me to move my lazy ass and finally make this True Tree Ears version. Beforehand I just fastened home-brewed lavaliers to a log. It worked fine, I made few nice recordings of crickets by placing the log on the ground in the grass. But I wanted ears! Greets
Pawe?
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Re: AOMs-5024 in enclosures from China
Vladimir - I also got the same version: no felt, perforated front (from TME in Poland, much lower shipping than from Mouser).
Official PUI datasheets of both versions show no differences. Theoretically the shouldn't be any and even if - a minuscule one.
I hope so anyway ?
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Re: OPA Alice: can 1 gig resister be replaced with a 750M ?
Oh. would you please tell a bit more about 'fun stuff' you mentioned. It is interesting.
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Re: True tree ears or DYI binaural dummy head.
So cool! One should start a true binaural forest soundscape project. :-)
Which reminds me to continue my bamboo microphone housings.
Many thanks for sharing!
Heinz
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Re: OPA Alice: can 1 gig resister be replaced with a 750M ?
The one gig is pretty much ¡°industry standard¡±. The capsule capacitance swamps the LF noise it can generate. You could easily go to 2G at 10G you can run into humidity issues and other fun stuff. ? Best Regards,
Jules Ryckebusch?
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On Jun 8, 2024, at 19:06, Vladimir Matveyev via groups.io <depeschzeu@...> wrote:
?It's going to work, but not as good. I have a friend who is an electronics expert who said that even 1G resistor is actually not too good and it is recommended to use even higher value resistor. It has something to do with overal noise etc. I'm not sure as I am no expert, just that I've been told.
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Re: OPA Alice: can 1 gig resister be replaced with a 750M ?
It's going to work, but not as good. I have a friend who is an electronics expert who said that even 1G resistor is actually not too good and it is recommended to use even higher value resistor. It has something to do with overal noise etc. I'm not sure as I am no expert, just that I've been told.
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