Comparison of JFETs for mic applications

[thor.zmt]

Is that Vgs of M1 in your circuit?

The Threshold voltage of 1.7V is a datasheet value. My BSS84 Model seems to indicate 2V Vgsth.

What voltage do you sim at the Drains of M2 & 3?

Especially for the Guru, a sim with all DC voltages and currents probed and spelled out:



Incidentally, I made up an A-Weight + 5th order 20khz lowpass network (all passive, VCVS & Ohm level resistance's) which suggests an 89dB(A) SNR @ 94dB SPL (or 5dB(A) SPL Self Noise), which is pretty Horrorshow in my books, except capsule noise scotches this big time.

HD @ 94dB SPL & 1kHz at 0.006% (or -83dB of pure H2) is also pretty pukka, innit?

Again, the goal was/is to reuse all of the BM-800 China Mike (except the saddle & Capsule where we go 26mm or 34mm Electret or 32/34mm LDC) including the PCB, mostly use commodity SMD Parts that fit the PCB.

And to make a studio recording Mic that betters relatively affordable (sub 500 USD) China made "recording microphones".

Not to make some supa dupa world record setting something. I leave that to Guru's.

I'm too pragmatic to be a guru.

I will show in a separate the thread the LDC Capsule version, with Bias from P48. It looks....



Thor

 

[Matador]

I for one appreciate the back and forth, even accounting for the snark.

Especially for the Guru, a sim with all DC voltages and currents probed and spelled out:

Why are the independent voltage source(s) added? Is it to monitor branch currents?

I'm curious what would change if you abandoned P48 power, and were free to set the MOSFET currents to whatever "optimum" you felt was best?

 

[thor.zmt]

Why are the independent voltage source(s) added?

Voltage source? Only 48V is source. You mean the Schematic symbol for a Meter with a "V"?

These are virtual volt meters. This is what I like about TINA, I can throw on Meters and probes galore to my desire.

I'm curious what would change if you abandoned P48 power, and were free to set the MOSFET currents to whatever "optimum" you felt was best?

Ok, take a step back.

The point of this design is to reuse existing PCB's and plug into a mixing desk or ADC/Pre with P48.

Once we abandon these limitations, we have other options.

A lot then will depend on the design goals again.

Say we want a "colour Mic", use a 6AK5 without Gigaohm resistors and with a decent but small output transformer (Neutrik has one). There is a thread on such a DIY Mic. It will saturate early, may need a switchable C-Pad, but it will generally give "good sound".

As alternative for "super low noise, super low distortion, super high SPL" (let's set the goal to beat anything else with a similar capsule on all metrics), perhaps a 2SK2145 paralleled plus Op-Amp as charge amp with flyback diode for bias and +/-18V rails, with enough gain to go directly to a 10k line level input on the AD. Say 146dB = +20dBu out and < 4dB(A) SPL self noise. Who has a 146dB Dynamic ADC in their pocket?

Or just go fully digital with a large (say 64pcs) array of PDM output MEMS mic elements and a suitable DSP, direct output to Wifi/USB, nothing analog whatsoever. DSP for directivity, frequency response and emulation of classic Microphones (distortion/noise etc. vs level vs frequency on top of directivity vs frequency).

In 2025 we can make almost anything, but still one person cannot make everything.

Thor