Beginner here.

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I've been looking over the instructable for the True Condenser OPA and there are 2 things that I'm not completely understanding yet. I suspect they are related though. In other mics I've looked at (the MXL Schoeps-style, eg) the high voltage passes through a 1G resistor before being applied to the backplate and the polorizing cap and then to the FET gate past another 1G resistor while the capsule wire is connected to ground. I think maybe this is a typical setup for a pencil mic.

If I'm understanding the instructable correctly, the 80V (or so) is applied to the backplate and the capsules are coupled through the polarizing cap to the OPA gate past the 1G resistor. I think this makes sense (since what we're sending to the OPA is the difference between the 80V and the capsule) but I'm not 100% on this. I'm wondering why there is no 1G resistor between the 80V and the backplate. Is this not necessary since the 80V DC cannot pass through to the capsule side?

The bias generator actually has a 1M output resistor as part of the output RC filter. One other thing to know is the virtual ground is about 6V thus the actual bias voltage is about 75V?

Some of the designs you mention that have 1G resistors on both sides do some switching of bias voltages to change the mic pattern etc.?

Hope this helps?

Here's why some have two 1G resistors, some have one. It comes down to capsule mounting and designer choices.

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The capsule needs to have a polarizing voltage between diaphragm and backplate. Call it 60V for convenience. Shorter to write than "polarizing voltage".

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The input to the electronics needs to be at or near ground potential, but with a very high resistance to ground.

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In a pencil mic, the diaphragm is nearly always grounded because it is connected to the capsule body, which is screwed to the mic body, which is grounded. So a pencil mic has to have both 60V DC and signal attached to the backplate. Therefore, one big resistor to 60V, a coupling cap to keep 60V out of the FET, and another big resistor to hold the FET gate near ground.?

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The OPA Alice and other mics with a headbasket can have the capsule insulated from ground, so neither the diaphragm nor backplate need be grounded. In which case, 60V can be applied to one, and signal can be taken to the FET from the other. The capsule acts as its own coupling capacitor. Personally, after trying different setups, I settled on applying the bias voltages to the diaphragms and having them grounded through caps for audio. Better shielding for hum and RF noise. Take signal from the backplate with a single 1G to ground.

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The downside of connecting the electronics directly to the capsule is greater chance of DC leakage due to humidity, breath, or dirt. Some mics use a DC blocking input cap like a pencil mic just for that reason.

Thanks, Henry. This makes perfect sense. I figured that the second resistor was related to the signal coming from the backplate but you've explained why. I am interested in your comment about a capsule acting as its own coupling capacitor. I thought the coupling capacitor was necessary to polarize the capsule by maintaining a consistent charge.

Also, your preference for applying the bias voltage to the diaphragm and grounding through caps while taking the signal from the backplate is intriguing. I hadn't even considered that was possible. If you do this with a dual diaphragm with a single backplate, does that mean you can only do figure 8 and (if you cut the charge to one diaphragm) cardioid?

Ah! That all makes a lot of sense. I love that I learned a lot today. I realized that the TSB-2555 has a fixed static charge on the diaphragm and takes the signal from the backplate, similar to what Henry was suggesting except without the external bias voltage.

After a few months, things are finally making at least a little sense. Thanks for your help.

Not to toot my own horn, but schematics of many mics I've tinkered with are on audioimprov.com. Not well organized, but there is a list of topics.

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3-pattern electret made with 2 cardioid electret capsules (works great with TSB2555s):

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3-patterns with double side "true condenser" capsule (2 cardioids back to back):

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Variable pattern mic with one diaphragm grounded, backplate at 70V DC and signal taken there, other diaphragm variable 0-140V DC, grounded for audio:

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and the ever popular basic mic circuits:

I need to add the OPA Alice to the basic circuits.

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You have a right to toot that horn a bit. was the post that got me started with this whole crazy obsession back in November. I was looking for any information I could find about how to make my 603s sound better and this was the post that really lit a fire under me. Since then, I've completely destroyed a 603s, bought two more used and re-implemented them with better components and replaced and re-biased the JFETS. They now sound "better" but I needed more.

I found Scott Helmke's "Alice" mic through your blog and got super inspired. That led me to Jules' OPA Alice but I do things the hard way and wanted a front-address mic to stick under the bridge of my contra bass. I've been iterating on that for awhile and am narrowing in on a prototype using my own PCBs, 3D printed parts, brass tubes and copper mesh. It sounds pretty good but there is still measurable distortion in the mids because I was just guessing on the PCB design. It sounds better than the 603s mics I modded and I'm pretty sure this next PCB design is a lot better, after studying up on it. Now I'm thinking about building an OPA into an MXL 990 with an RK47C capsule. Like I said. I'm obsessed.

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I've come pretty far in 3 months but there is so much to learn. The breadcrumbs of information left by you, Matt and Jules are like beacons of light to me and I really appreciate it. There really are not many good resource for this information -- especially in the age of ChatGPT.