TwentyTrees
Well-known member
I've a quick question about biasing these mics - specifically, while 10V drain voltage is noted on the Neumann schematic and referenced several times throughout this thread, is it always the right point? I'm asking as my testing suggests looking at a slightly higher voltage, and I'd be grateful for a sense check.
Test conditions
Thanks for any help and insight!
Andy
10V drain:
12.2V drain:
Test conditions
- I've injected a 1kHz sine at around 200mV direct to the sprung contact with the capsule removed.
- I'm monitoring drain voltage at the drain/R4 junction using the multimeter; while observing the both the waveform shape on my Picoscope, and the harmonic distortion profile using Voxengo's SPAN plugin.
- JFET is a Motorola 2N3819, IDSS is 5.4mA and Vgs(off) is -1.81V.
- I've taken measurements at 10V drain (R3 = 5.3k) - however, increasing source resistance to 6.7k (12.2V drain) led to a dramatic decrease in 3rd harmonic distortion, by almost 40dB. 2nd harmonic increased by about 2dB, and the 1kHz tone dropped by about 1dB. 4th, 6th and 9th also dropped (though 7th and 8th increased).
- When increasing the test tone amplitude, I was similarly able to dial in a significant reduction in 3rd harmonic by further increasing the source resistance.
- I've not been able to get a 100% symmetrical waveform at any value on the trimmer, or symmetrical clipping.
- The same pattern is apparent across all the KM84 kits I'm building.
Thanks for any help and insight!
Andy
10V drain:
12.2V drain: