[Quote]What I want to focus on is whether it would be advantageous to use an Audio isolation transformer. I also use the SimpleP48 circuit (I used it for two theater events at our school). But although it depends on the type of Audio interface used, there is a 60Hz hum in the recorded sound. I wonder if we can eliminate this by using an isolation transformer? [/Quote]
In most cases, hum is due to improper shielding of the capsule. Using a transformer? doesn't solve the issue.

Hello Adam,

Maybe the hum is related to your recorder/audio interface?
I built several mics with AOM5024 in different casings and experienced no issues. Even when I was, recently, recording trams really close to their power lines. With rather poorly shielded capsules. No hum at all.

A capsule (for Takstar CM-60) I was experimenting with, developped an exemplary 50Hz spike, with harmonics, after I slightly enlarged the vents but AOMs were dead quiet in the same environment.

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You can try reducing the value of the 2.2uF capacitor, but the cut-off frequency depends on the actual input impedance of the preamp it is connected to.

You need to experiment. Trial and error.

Hello Everyone,
My question is how can add 80Hz LF to SimpleP48 circuit? Mainly, I'm using PUIAUDIO AOM-5024P-HD-MB-R and Panasonic WM-61A.
Many thanks.

Hi Mauricio,
Yes, WM-61A is excellent and tiny, but I prefer AOM-5024P-HD-MB-R :)

Hi Kenn,
Yes, I absolutely agree with you, I have about 30 AOM-5024P-HD-MB-R and 200 Panasonic WM-61A (bought about 10 years ago, only used about 12).
What I want to focus on is whether it would be advantageous to use an Audio isolation transformer. I also use the SimpleP48 circuit (I used it for two theater events at our school). But although it depends on the type of Audio interface used, there is a 60Hz hum in the recorded sound. I wonder if we can eliminate this by using an isolation transformer?
My another question I would like to ask is what is the function of the 2.2uf capacitor in the SimpleP48 circuit?
Finally, how can I modify the SimpleP48 circuit to cut below 80Hz?
Many thanks for answers.

The Panasonic WM-61A was good in its time... but there are now much better omni electret mic capsules, that will give you better sound with less noise.

Consider the PUI AOM5024L, or the Primo EM272 electret capsules. Electrically similar, but better. Also, on this group, read up on the simpleP48 circuit for using such electret capsules with +48v phantom-powered XLR mic inputs. Easier to build, and better performing than Mr Shin's transformer -equipped design.

Hello everyone,
Are you aware of this website? I have been following it for a while and I would like to make this microphone .
What are your opinions about this microphone??
Mr. Shin used SMD ED8 5P 600¦¸£º600¦¸ Audio isolation transformer and Mic capsule Panasonic WM-61A. I ordered 15 of them and I want to experiment with them. Since I can't find a topic about this DIY microphone in this group, I wanted to start a topic and ask for your opinion.

Many thanks, and I am waiting for your opinion.

The Audio-Technica is very much in force (anticipated expiration 2038).

However (with the disclaimer that this is not legal advice and I'm not a legal professional), here is some color:

• AT patent is filed in 2017, DPA already had lav mics on the market using this topology
• the one independent claim of the AT patent requires:
  • "a constant current diode configured to supply a current to the FET" (not the case for how DPA mics are biased)
  • a second BJT, "a collector grounding second transistor" (Q2 in the diagrams)
• the next claims are dependent and do not remove the two requirements I mentioned (that would not be proper form for a dependent claim anyway).

In my unqualified opinion, even if a company would build a lavalier microphone with the JFET-PNP topology today, it would not infringe on the AT patent (and DPA never patented this topology, or the CORE preamp for that matter).

Again, take this with a grain of salt, the closest I've ever been to this stuff is being an inventor on one US patent, but a very good patent attorney, paid for by my employer, wrote that patent based on my technical docs and explanations.

Sergio,

Thanks for the report!? Very interesting, and I'm assuming that the Audio-Technica patent is long expired at this point. The outside transistor on my sample also had "3FN" on it.

-Scott

--
---- Scott Helmke ---- scott@... ---- (734) 604-9340 ----
"I have ceased distinguishing between the religious and the secular,
for everything is holy" - Joe Henry

So here is the full non-CORE DPA 406x teardown.

Scott Helmke already kindly posted a couple of photos of the capsule here, I am reproducing them below for full context (and because my own teardown wasn't as clean). The outside shows a 3-pin SMD device (turns out it¡¯s a PNP), a 33K resistor and a RF blocking cap.





I went further and tore down a defective one (¡°thin tinny sound¡±) I scored on eBay (it had dirt and even a droplet of blue glue on the membrane, the presence boost cap was glued on with the same blue glue and maybe this was done carelessly).

The outer device from Scott¡¯s photo is a PNP transistor, code ¡°3F¡±, likely some flavour of BC857. You can't see the "3" in his photo, but the rest of the marking matches my unit.





The JFET is deeper inside the device, potted to the ¡°lid¡± behind the back plate of the electret. Its marking has been removed (laser). The gate is connected to the back plate of the capsule using a bondwire.



The ¡°toy¡± component tester identifies it as a JFET, but the values for |Vgsoff| and Idss are wrong. I have measured |Vgsoff| = 1.77V and Idss = 14.8mA.



This is not the little league JFET with 100-300uA Idss typically found in electret lavalier mics. It doesn¡¯t have integrated gate diodes either. Because my DPA 4080 CORE U(I) curve also has a ¡°knee¡± at about 50uA, I believe in both cases the JFET is selected such that it will bias at 50uA with the 33K resistor placed externally. Versus a low Idss JFET, this configuration offers higher transconductance even at a lower drain current because the device itself has a much higher Idss (see below for why).



Since the JFET doesn¡¯t have integrated gate bias diodes, the potting compound also holds a back-contact chip resistor (this is very likely the high value resistor). ¡°Back-contact¡± meaning one of the resistor¡¯s terminals is on the back, and the top one is supposed to be connected with a bondwire. You can see where this back-contact pad was soldered to the case, making the electrical contact to ground. The resistor looks like laser trimmed thick film.





Another component in the potting compound remains unidentified (and I cracked it while removing the potting compound). Could be a ferrite bead on the gate (Schoeps has one). If it¡¯s a capacitor, I have no idea what it would be doing on the gate side, so I don¡¯t think it¡¯s a cap.



To summarize, this is it, plus the mystery component which I left out:



As I eventually suspected above, it¡¯s the same topology from the Audio-Technica , except that one depicts JFET with integrated gate bias diodes so it has to be a low Idss device. The topology keeps the JFET¡¯s VDS constant at ~0.6V (within the variation of the PNP¡¯s VBE), which bootstraps both Cgs and Cgd, so the capsule sees a very low input capacitance (which is a big advantage for tiny capsules, DPA¡¯s is probably around 5-6pF). The output is a PNP emitter follower, which means it has very low output impedance (tens of ohms) compared to a JFET source follower.

The LSK189B model in LTSpice has just the right VGSoff and Idss to bias correctly in the DPA topology (I¡¯m not saying the JFET actually is a LSK189, but that is a good model to simulate with, if someone wants to try out the topology in LTSpice), while BC857B is a likely candidate for the PNP.

It's really nice that in Jan's charge amplifier topology the BJT is actually doing useful work at AC. For the Schoeps phase splitter we really want to keep it quiet at AC (hence turning it into a Miller Integrator). But yes, at DC they are using the exact same biasing principle.

Funny that sometimes people at different places on the world have the same ideas at the same time.

For me, in this case it's a validation that the idea is good, and that really makes me happy. I am certainly not nearly as experienced and/or qualified in this field as Jan.

OK, so I decided to publish my impedance converter circuit as well. As depicted here, it operates from 16V. It looks quite similar to Sergio's circuit, doesn't it? However, in this circuit, there's also AC feedback from the output to the JFET Gate, which makes this essentially a charge amplifier, like the KM84. The circuit, and some other circuit features not depicted here, resolve several of the KM84 shortcomings, so I named it KM84+++ (I also designed a KM84+ and KM84++ btw, which stay closer to the original KM84 design). Output can be a single-ended, transformerless impedance-balanced circuit, or through a 1:1 transformer. I designed a PCB for the Takstar CM-63 with the Lundahl LL1968 transformer. PCB is ready and waiting to be populated. If I'm satisfied with the results, I will make the PCB and complete schematics available through PCBWAY.

Vbe of T2 and R16 define the JFET bias current. R2 provides DC feedback, like in Sergio's circuit. I had ~3.5mA bias current available for this circuit, which must be shared between JFET and BJT. In the design depicted here, they have approximately the same current, but you can play with other current distributions, depending on the JFET, noise requirements, and output impedance requirements.

This circuit enables the use of more different JFET types, because it depends less on high gm values than e.g. a KM84 circuit does. Not having to adjust the bias may not be such a huge benefit for the average DIY, but I've read enough posts from people who do seem to experience troubles. Small trimmers are also not very reliable and take a lot of board space. "But what's that P1 doing on your PCB?", I hear you saying. Well, that's optional trim pot to adjust the polarization voltage, so you can easily match the gain between two mics. It's not for JFET bias adjustment.

Another advantage of this circuit, or charge amplifiers in general, is that you can select the closed-loop circuit gain within a reasonable range by changing the value of C2. With Schoeps, it is more or less fixed and depends on the JFET chosen. Low-end roll-off is defined by R2 and C2 and to prevent noise, you want to increase R2 to higher values (e.g. 2, 3 or 5G) if C2 is decreased to obtain a higher gain. Due to the additional open-loop gain provided by the BJT, distortion is much lower compared to the KM84 and output drive capability has increased.

Enough for now. Once I have assembled the circuit, I will share the measured specifications (THD, Ein, etc.)

Jan
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The bottom looks like a common pad that three wires use. Could be for ground. ?

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You just hit the nail on the head with availability of good FETs. TI has a really nice one that is extremely low noise but is SMD is a weird package?

On Tue, Jul 2, 2024 at 11:28 AM, sergio_logic wrote:

is there a significant drawback that I¡¯m missing here?

It looks interesting. I have the following questions:

• One of the biggest hurdles to making FET impedance converters is the scarcity of suitably good FET transistors. Does this circuit/technique help to make more FETs "suitable"?
• Being a DIY group, adjusting the trimpot to set the bias on a pimped Alice board isn't a big drawback for most of us. Does the servo approach offer other advantages, like superior audio performance?
• Conversely... would the active servo itself be a potential source of additional noise?

Anyway, this looks intriguing, and something that could be beneficial in a mass-produced generic head amp for electret and condenser mics. I look forward to your results when you test some prototypes.

In case anyone is interested, I'm selling a few microphones:

Crown PZM30FS 400€
MTG M300 600€
2xMTG M294 750€ each