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.

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>

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.

Hi Scott,

I'm a pretty serious pinout nerd in that world, if you need info
please feel free ask.

I'm going to take you up on that offer. I don't know if you are familiar with Sony UWP bodypack wiring (it's probably less popular than Shure/Sennheiser in the US, whereas in Europe it has a pretty fair share with Sennheiser in the TV studio market)

Sony UWP uses 1/8" (3.5mm) TRS jack with the following wiring:
Tip - audio and 7k/9k Ohm bias resistor to ground (varies a bit among generations, UTX-B03/B40 are 7K, old UTX-B2 is 9K)
Ring - 5V (through ~330 Ohm current limit resistor)
Sleeve - GND

*Now, knowing this pinout, how would you go about connecting a DPA 2-wire lavalier to Sony UWP? *

DPA mentions <>this: "DPA miniature and subminiatures should never be connected to a compensational circuitry as it may reduce the already optimized output. In the adapter connection scheme of DAD6010 (MicroDot to 4-pin connector) which is shown below, pin 4 (for active load) is not connected." -- so in the case of the Shure pinout you mentioned, they make a point about NOT connecting the load resistor.

Yet, their $105 adapter DAD6019, listed as compatible <>with Sony UWP, does exactly that, it's just a 3k3 ohm resistor from 5V to Tip (audio and 7K to ground). This means, that even under no load, the voltage at the microdot is ~3.3V = 5V*7K/(7K+3K3+0K33), as measured here <>.


What happens if the voltage is below 5V for these mics spec'd for at least 5V? DPA answers here <>: "As mentioned, the output signal depends on the supplied DC voltage. If it is too low, it may result in asymmetrical clipping of the audio signal above a certain level [see figure 4]", and "If the available bias voltage is, say, 4 or even 3 volts, the microphone still works. But the acoustical input must be reduced and kept below a limit, lower than the usually specified max SPL; this is to avoid distortion."


This is precisely what happens with the DPA DAD6019 adapter, although DPA mentions no max SPL limitations introduced by the adapter:


I've made my own adapter which biases the mic through a 3K3 (metal film) resistor between the 5V and the mic wire (as all the adapters for the 5V systems do), but then only AC couples it to the Tip (SMD tantalum, anode at mic), effectively removing the Sony's 7K resistor from the DC bias.


The build quality is not like the DPA, but it's not $105 either. Of course, this fixes the positive swing issue:


(Note, my "sound source", an iPhone loudspeaker, was distorting the sine, not the mic.)

I wrote all this to DPA ~1 year ago, they replied but only said they will investigate. They have not updated the schematic of the DAD6019 in the meantime.

*So, do you think there is another way around this?* My "A-frame" made of a cap and resistor is not the most mechanically sound solution, but it seems electrically correct.

Thanks for the answer.

I was going by Sanken’s terminology found at when I called DPA “2-wire” (and by that same nomenclature I’d call Shure three-wire).

Secondly, the DPA CORE variants of the microphones do not have a single JFET in the capsule, they claim that “five times the amount of silicon has been added to the circuitry and thereby extended the linear range of the electronics and diaphragm system” in the original press release:

I have a V(I) plot for a DPA CORE 4080, but it’s not a mic I can afford to tear apart to investigate further.

Based on that V(I) plot, what I can say is that it really wants to stay at a fixed voltage (2.6V for my unit) for a wide range of current (0.2-0.8 mA). With 5V and a 3K3 + 0K33 resitor places it at 0.66mA.

I know one two-wire topology with a similar V(I) curve that offers similar performance advantages to DPA CORE but it:
(1) is patented by Audio-Technica (for some embodiments at least)
(2) V(I) not quite as flat as the DPA, but still very flat (AT slope across 0.2-0.8 mA is like a PN junction, DPA’s is less)
(3) does not satisfy the criterion of “5 times more silicon” (but maybe that was just necessary improvements for not infringing the A-T patent).

I’ll post the DPA measured V(I) curve and the number of the Audio-Technica patent later today.

OK, so I happened to have a DPA 4066 headset capsule on my bench - I work for a company that rents out audio gear, including a lot of wireless with DPA mics. So this one showed up as unrepairable due to the wire connection failing.? The mic itself is still good (well, maybe not after this much disassembly) and I've rescued a couple others in the past as experiments.

This is the standard DPA omni lav/headset capsule - yes it's square!? The wavelength of 20k is longer than these dimensions so no issues with that.? What you can see on the bottom is a 3-pin SMD device, presumably a FET. Where the wire connects there is what should be a small capacitor to shunt RF, and on the other leg is a resistor - 33k ohms to ground.? The pin on the perforated side would then have to be the gate of the FET, connecting to the diaphragm. I'm not sure how biasing is done.



-Scott

On 6/12/24 08:21, sergio_logic via groups.io wrote:

Thanks for the answer.

I was going by Sanken’s terminology found at when I called DPA “2-wire” (and by that same nomenclature I’d call Shure three-wire).

Secondly, the DPA CORE variants of the microphones do not have a single JFET in the capsule, they claim that “five times the amount of silicon has been added to the circuitry and thereby extended the linear range of the electronics and diaphragm system” in the original press release:

I have a V(I) plot for a DPA CORE 4080, but it’s not a mic I can afford to tear apart to investigate further.

Based on that V(I) plot, what I can say is that it really wants to stay at a fixed voltage (2.6V for my unit) for a wide range of current (0.2-0.8 mA). With 5V and a 3K3 + 0K33 resitor places it at 0.66mA.

I know one two-wire topology with a similar V(I) curve that offers similar performance advantages to DPA CORE but it:
(1) is patented by Audio-Technica (for some embodiments at least)
(2) V(I) not quite as flat as the DPA, but still very flat (AT slope across 0.2-0.8 mA is like a PN junction, DPA’s is less)
(3) does not satisfy the criterion of “5 times more silicon” (but maybe that was just necessary improvements for not infringing the A-T patent).

I’ll post the DPA measured V(I) curve and the number of the Audio-Technica patent later today.