> Given that the operating Id is selected to be 1.78 mA (and why was that value selected?)
That is NOT the designer's "selected current", it is what Gus computes from the facts already presented. Full analysis is tedious: I know because I did a partial workup before I saw Gus' math. My answer is 1.88mA, meaning Gus and I neglected different 5% negligibles. This may be more accuracy than Octavia's parts-pickers had.
The real "selection" is apparently that it obeys the old 2mA limit on Phantom draw. Allowing 10% for biasing that leaves 1.8mA for hard work.
But the recent raise to 10mA limit does no good, because we need "high" voltage to polarize the capsule. 2Ma in 3K4 is 6.8V drop from 48V un-loaded Phantom supply, leaving just over 40V for capsule polarization. That's OK and we don't want much less, so 2mA is an upper limit. OTOH, good JFET+BJT action into loads near 1K suggests more than 1mA for hard work. Hence almost-not-quite 2mA.
> my phantom power is different from my pre a mixer I have and a cake walk pre I have
With nothing plugged in, do you get +48V on the holes?
For this mike, you really should.
> is the bias points for the transistors critical ??
The more critical voltage is the +40V at the XLR pins. This is capsule polarization. Sensitivity will vary directly as this voltage varies. Significantly lower voltage, like 25V, will increase THD on loud sounds.
The transistors will self-bias with anything that makes the capsule happy and with almost any transistors.
See attached image. The voltage across the 84K is applied (with small shift) to the 7.5K. Transistor action levers the 159K/84K against the 7.5K, so the transistors together act-like 14K. The 159K divider makes this about 13K. The 2*13K power combining resistors plus the 2*6.8K Phantom resistors work out to 9.8K. So the JFET Drain should sit near 13K/(13K+9.8K) or 0.57 times the unloaded Phantom supply, or +27V. If JFET Vgs is about 0.7V more than BJT's Vbe then it works out to 26V.
This system IS "self-biasing". The 0.6V-0.7V and 0.5V-2V device voltage tolerances are SWAMPED by the ~~14V voltage between them.
The PNP only needs to have hFE over 50: from 50 to infinity makes less than 10% difference in current and this current is NOT critical except that we'd like to find over 30V at the "40V" point so the capsule gets polarized good.
The JFET nominally should have Vgs=1.3V at 1.9mA. This means Idss must be well over 2mA and Vth should not be large. Most jellybean JFETs will be fine. 2N3819 seems a good pick to me. 2SK170-BL is not an all-purpose device. It has large gate area, therefore large capacitance and leakage. It will work; it may not work any better (unless you believe it does).
> replace those 3 resistors with a 100k pot a 47k pot and a.......
NO. Leave them alone. They are correct.
As long as the JFET will pass 2mA, it should work. If you get a high Idss and high-Vth FET (source sits several volts higher than the 39K-84K junction), use a different FET. If you can't, then the voltage across the 7.5K will be large, current will be large, drop in 6K8 Phantom resistors is large, "+40V) will be somewhat lower. I can't find a reasonable JFET which won't bias acceptably.
The other mods.... don't make technical sense. But I respect Dorsey. And reading on, I realized that these mikes were assembled at low cost with odd parts. Changing a good 680Meg resistor to 1Gig will NOT change the top-end. However if Octavia was using bad parts (a good high-R resistor is not common nor cheap) then any good part may improve sound in unexpected ways.
It's a extra-clever circuit. Aside from being quasi-balanced (irrelevant in small studio work) I see no problem with the topology or the nominal values. If it was assembled with scrap parts, and you are in a position to invest, put good parts in. Certainly crappy electrolytics should be replaced before rot sets in. If Q1 is suspected of being east-europe reject stock, use a good new asian JFET. NPO or Silver-Mica cap at C1 is cheap. The 10nFd before R1 should be any *good* film-cap. C1 should not suck. C3 C4 should be good and should be reasonably well matched for best hum rejection.
Note that the JFET source is bootstrapped. Its source-induced THD is far under 0.05%.
There is another FET distortion: nonlinear Cgd. At high levels with large-gate FETs, this may dominate.
The emitter follower THD is tenths-percent near clipping and lower for all normal levels.
But excess "modding" just invites mistakes and accidents.
> 22.9V .... not 26V
You have very nearly 40V at the "40V" point, right? Then you are fine. Take out the trouble-prone pots and enjoy.
> I removed two layers of mesh and the resonator discs. That made the mics sound a little duller..... I think they have put some thought in
Yes. Over-diaphragm resonators are standard tricks of the mike-designer trade. They can bump a droopy top-end. They are not free; the fact they are used is a hint that they may be useful. The clearance tolerances may be microscopic.... getting it back just-right may not happen.
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