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Re: offcenter virtual ground, opamp mic with transformer

 

I agree that SOIC? - and especially? VSSOP - make guard rings very difficult - if not impossible!
I've used 'air mounting' and scrupulous cleaning of the mounting adaptors - together with isolating pin 3 from the stripboard - and have had some success with my simple hobby mics .?
The photos near the top of ?show how I 'air mounted' the input coupling components for the multi pattern mic I built.
That required the capsule to be referenced to real gnd, and not half rail, to allow symmetrical opposite polarity capsule polarisation voltages to be applied to the capsule.
So the input needed to be AC coupled, using 2 x 1G¦¸? resistors.
As I mentioned in an earlier post, I'm still using 'real' solder, and generous IPA cleaning of the complete assembly has proved to be successful so far.
I built my first mic using that construction over 3 years ago, and that is still performing perfectly.
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My Rode NT1 reference mic utilises a guard ring around the JFET input, and that seems to work well. ... I did however haves some serious noise problems with that mic caused by dirt elsewhere.....? Once again, it was copious quantities of IPA that came to the rescue!
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I agree that guard rings would make life easier - epsecially with all 'on board' component assembly.?
However, the temptation to use a reasonably specified op-amp, instead of endlessly trying to find the 'sweet spot'? when biasing discrete JFETs was too great! :)?
OK, there maybe marginally higher noise levels than you can get with the very best JFET inputs, but the distortion figures are way better.
As is the headroom available from a rail to rail output op-amp supplied with c.24v!
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I tend to build 'one off' hobby mics, so often use stripboard instead of creating PCBs.... The exception was my ?which did require a PCB.
However, as that is a low impedance condenser mic project (the primary purpose of RF bias mics) much of the very HI-Z protection techniques become redundant!
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On Sun, May 4, 2025 at 09:06 AM, @jp8 wrote:

"With regard to the gap between the op-amp pins, I think we have to assume that the gap is sufficient to allow the specified JFET impedance to be utilised?
Otherwise there's no point in Texas supplying these devices in the SOIC or VSSOP format."
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If that were true, then why would we still want air gaps, air connections, guard rings, turrets and you name it in the high-Z nodes of impedance converters in general, where most of the time the clearances between pads and tracks are mostly even greater?
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These opamps are not only used in GOhm impedance circuits. So SOIC and even VSSOP can still be acceptable in those circuits. But for "our" circuits, especially if manually soldered, TI could have done us a great service with a different package design that would make it easier to add gaps and guardrings.
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Jan
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