Comparison of JFETs for mic applications

[AdamFrisen]

I found this from former technical director of Neumann in US:



from: https://repforums.prosoundweb.com/index.php?topic=36194.0

 

[abbey road d enfer]

I found this from former technical director of Neumann in US:

That's a very interesting post, however there is an element of confusion, which has been noted by Klaud Heyne.
The text refers to THD being lowered by decreasing the capacitive load, but improved by the presence of a NFB capacitor. This is all true, but slightly confusing, because the mechanisms must be considered as two totally different sources of distortion.
Distortion caused by parasitic capacitance is purely physical and does not pertain to the head amplifier, whether tube or FET. It acts on the global transductance of the system.
NFB capacitor reduces the THD of the amplifying stage, but, as it increases the apparent parasitic capacitance, it increases the capsule's transductance THD, so there are two antagonistic effects at work.
In most head amps, amplifying stage's THD is dominant, so using a NFB capacitor ("charge amp") improves THD at high spl, which is what most users are concerned with.

 

[Voyager10]

The ORS87 measurements to date have used a fixed 68pF input capacitor (for comparison with other circuits). We should have figures for 100pF (original U87) and 50pF (U87A in cardioid), as this will affect the gain and therefore THD.

If the U87 output transformer is 9.26:1, we'll need the FET gain to be about 5.8 to match the U87 numbers. With a 100pF inout cap that should be close. For the U87A the FET gain should be 9.26, which is some way off from my numbers.

Re. Abbey's point - yes, that's confusing. The capsule generated THD will go down, but that's not what is being measured when applying signal to the calibration input.

 

[kingkorg]

FWIW. By modern standards both u87, and km84 have mediocre THD performance. Just like most commonly replicated tube mics. However people seem to like that, as most modern low THD mics are frowned upon.

So do we want more of that "vintage THD", or less, or the same exact amount, or maybe focus on harmonic profile throughout the spectrum instead of TOTAL harmonic distortion, mostly expressed as a meaningless figure at 1Khz?

Or maybe just let it go, as most voices and commonly recorded instruments are never going to reach that SPL level, nor trigger any meaningful harmonic generation?

EDIT:
Here's u87ai 2nd and 3rd profile at 1% THD @1K. Clearly, simple 1%THD @1K figure is deceiving. Harmonic content generated by 1K content, will sound significantly different than 10K. Cymbals, sibilants? And C12 circuit, at slightly higher THD, all 2nd harmonic, except at low end where 3rd is generated by the transformer. Basically, total opposite of solid state. Ignore SPL axis, this is injection test.

 

[abbey road d enfer]

Re. Abbey's point - yes, that's confusing. The capsule generated THD will go down, but that's not what is being measured when applying signal to the calibration input.

So do we want more of that "vintage THD", or less, or the same exact amount, or maybe focus on harmonic profile throughout the spectrum instead of TOTAL harmonic distortion, mostly expressed as a meaningless figure at 1Khz?

It seems (correct me if I'm wrong) many prefer charge amp types over unloaded capsules.
A big difference is that capsule distortion is essentially 2nd harmonic, when head amp is mainly third.
Will it fuel the debate over "euphonic distortion"?
I fully agree with weak significance of THD at 1kHz; same for noise, for which spectrum is even more important.

 

[Voyager10]

OK, perhaps I've been misleading people here - the "THD" numbers here are largely a stand-in for headroom. For all these measurements, generally THD increases gently with signal level until it hits 0.5% or so, then all hell breaks loose:



(I didn't publish this chart earlier, it didn't seem to show anything exciting...)

All the THD numbers are telling you is how close to this threshold you are. Perhaps I should be reporting "max signal level for 0.5% THD" as with earlier posts - it's more time-consuming to measure though.

Certainly this one number won't tell you how a mic sounds when it's in its comfort zone, please don't read significance into the difference between 0.14% and 0.15% - that really is meaningless.

 

[Voyager10]

NFB Capacitor selection for ORS87Plus​

I've plotted the gain and maximum input/output levels for a variety of FETs in the ORS87+ circuit. A couple of changes were made to make it closer to a real U87 microphone circuit:

• Input capacitor now 47pF, close to the 50pF for a single-sided capsule
• Load resistor now 82K, representative of a 1K preamp input through a 9:1 output transformer

All measurements made at 1KHz, using 8pF, 5pF, 3pF (2 x 6pF in series) and no capacitor for C4.

Gain vs NFB cap​

Max input level vs NFB cap​

This is the maximum input level (V RMS at 1KHz) for 0.5% THD, as measured by REW.



I've also attached a plot of maximum output level (i.e. input level multiplied by gain) for reference.

Observations​

• The 2SK170, much used in "ADK" / MXL2001-style mics, has very low gain in this circuit, most likely because of a high input (Cgs) capacitance. Probably not suitable for charge-amplifier applications.
• With no C4 ("0pF") all other FETs have higher gain and lower max input level than is reported for the U87A. It's possible the actual U87A has more parasitic capacitance than the ORS87+, due to the latter's compact PCB layout.
• The J113 doesn't do particularly well at high gain / low C4 values.
• The J305 (InterFET, from Mouser) comes out best overall, it matches or exceeds the U87A figures for C4 in the 2.5-3pF range.
• OnSemi BF256B is a very close second, for a fraction of the cost of the J305 (11p each, 10 off, from CPC!) and also meets the U87A figures, for C4 ~ 2.5pF.

 

[Voyager10]

Charge amp noise measurements​

These measurements made with Cin = 68pF, and C4 = 10pF, which is comparable to the circuits used in earlier posts, especially the 'ADK/MXL2001' circuit. These aren't necessarily the right figures for a U87 application (Cin = 50pF).

The graph looks like this:



The A-weighted numbers for equivalent input noise are:

Device	Ein (dBV)
J113	-123.7
2N3819	-123.0
BF256B	-122.2
J305	-122.8
2SK117	-118.2
2SK30A	-121.4

These are looking noticeably better than the ADK circuit (post #33), which suggests removing the 2K2 source resistor and controlling gain with a feedback capacitor is a good move.

The 2SK117 has noticeably worse LF noise than the others; I've tried with two different devices, checked the bias, recalibrated REW, cleaned the board etc., but the same results. Not sure what's going on there.

(The 'effective noise floor' is not from some crazy liquid-nitrogen-cooled preamp, it's because I'm measuring the output noise (at the FET drain), and the circuit gain is between 4 and 5.).