AdamFrisen
Well-known member
I found this from former technical director of Neumann in US:
from: https://repforums.prosoundweb.com/index.php?topic=36194.0
from: https://repforums.prosoundweb.com/index.php?topic=36194.0
Comparison of JFETs for mic applications
- Thread starter Voyager10
- Start date
Help Support GroupDIY Audio Forum:
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
Well-known member
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.
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.
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.
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.
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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
Well-known member
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.
(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.
