Jim Williams: I recently went through my set of original NT-2's. I found a few things to fix up insides.
RF/stabilisation
- I replaced the 2 output ferrite beads with 22.1k Dale CMF60 resistors, that solved some loading problems.1
- I added a pair of 56.2 ohm output series resistors to help stabilize the mic on long cable runs.2
- ...the inductors off the output leads need to have a series resistor installed, I use 56.2 ohms. That stablizes the mics driving long lines and rejects rf.3
- I...removed the 2 ferrite beads on the outputs and replaced them with a pair of Dale 22.1 ohm CMF60 resistors, that solves some loading problems.5
- I lifted up one end of the original ferrite beads on the outputs and added 56.2 ohm series resistors. That solves loading and rf pickup problems.6
Coupling caps
- The .15 uf Wima interstage coupling caps were changed to small 1 uf stack mylar film caps to extend the low end to below 20 hz, I added .01 uf polyprops underneath as bypass caps.1
- ...the .1 uf signal coupling Wima MKP-2 caps are bypassed with .68 uf MKS-2 to extend the low end, it's thin to me...On these two NT-2's I rewired the capsule so the polarization voltage is applied to the backplate, that eliminates any caps in the signal path, the capsule is wired directly to the low noise jfet gate...Another error are two 1000 pf Wima polyprop film caps in the signal path, only one needs to be there with that wiring.2
- Other mods include removing the input coupling caps3
- Other fixes are re-setting the polarization voltage to +62 volts and re-wiring the capsule/front end so the back plate is polarized. That removed the input 1000 pf blocking cap. The sonics really open up if that cap is removed.6
Bias
- The bias polarization voltage was a bit high. I added a 1.5k resistor in series with the voltage feed to that pcb. I removed the 6.2V zener diode on the main audio pcb to provide a higher voltage to that oscillator pcb. I then added a 220 uf/25V cap after that 1.5k resistor and placed a 6.0~6.2 V zener acorss that cap to limit the voltage. The 1N4148 rectifier diodes were changed to 1N4934 rectifier diodes. The end result is to get that polarization voltage to +60 volts, measured in front of the 1 meg resistor on that pcb.1
- ...the polarization voltage is too low, I rebuilt those cards to deliver 60 volts.2
- ...the capsule polarization voltage is set too high. It needs to come down to +60 volts. To do that, I install a series resisor (1.5K) from the + V line feeding the oscillator pcb. Then a zener diode is fitted to clamp that voltage down until +60 volts is reached. Anything from 6.2 to 8.5V zeners will get there, select imperically.3
- The 6 k psu feed resistor was changed to 3.9k to allow richer biasing2
- ...the polarization voltage was a bit high, like 70 volts. I removed the DC converter pcb and added a series 1500 ohm resistor to the + voltage input and added a large 330 uf cap with a 6.0 V zener clamp. That along with some fast recovery rectifiers set the polarization voltage right at 60 volts.5
- Other fixes are re-setting the polarization voltage to +62 volts6
Backplate polarization
- On these two NT-2's I rewired the capsule so the polarization voltage is applied to the backplate, that eliminates any caps in the signal path, the capsule is wired directly to the low noise jfet gate.2
- ...wiring the capsule switch directly to the jfet gate. That requires polarizing the capsule back plate instead of the diaphram. Then the output wires are reversed from pin's 2+3 on the XLR to retain phase.4
- Other fixes are re-setting the polarization voltage to +62 volts and re-wiring the capsule/front end so the back plate is polarized.6
Capsule brightness compensation
- On one I kept the K67 capsule. There are a pair of Wima FKP-2 470 pf green box roll-off caps fitted. I increased those to tame the tops of that capsule. I ended up with Wima MKP-2 3300 pf, it sounds very much more like a U-87 curve, the screaming 12k hz peaks are gone. On another I used a C-12 capsule. I left the 470 pf caps in place. It sounds very good, but the headbasket design adds some low mid resonance that I don't like. That quality is gone in other mics I've installed those in.1
- As to "tuning" the k-67 style capsules, I changed the 470 pf roll-off caps to 5600 pf, that smooths it all out very nice. On these I'm replacing those original 32 mm capsules with new Chi-com 34 mm k-67 copies, those sound very good.2
- The circuit was designed flat to 50k hz. If you use the original or a k-67/87 capsule, those have a large hf lift that needs to be attenuated. Replace the 470 pf green Wima FKP-2 caps with 5600 pf to smooth those out, including the original capsules.4
- ...to cure the excessive k-87 capsule's 12k hz peaks, I changed the 470 pf roll-off caps and ended up with 3300 pf. That tamed the 12k peaks and gave the mic a very U-87'ish quality I'm enjoying. Replacing the capsule is another option, one I fitted with a C-12 capsule.5
- Those designs used an early version of the 32 mm k-67 capsule design. The Microphone Parts k-67 or k-87 is the best replacement choice to retain the original sonics. Those are accurate 34 mm designs, like Neumann. In mine I replaced the 470 pf roll-off caps with 5600 pf, that smooths out the tops. If you use a k-47, the entire sound will change with bumps at 3~5k hz added. Not a bad sound, but not what you are used to.6
Oscillator filtering
- One mistake is the oscillator pcb, the .47 Wima polycarbonate cap is wired from the oscillator output to the + 13 volt rail, it should be cut from that and soldered to ground to filter out the ocillator waveform from the capsule.2
- I also screened the oscillator pcb, that eliminates the huge amount of oscillator leakage into the audio path, look at the output of these mics on a scope to see that, it's present in most of the Schoeps knock-off circuits. If I don't do that, the large oscillator waveform can exceed the audio waveform in quiet situations, run that into my wideband (40 mhz) mic preamps and the oscillator waveform silently clips the mic preamp!2
- There is also a lot of oscillator leakage into the outputs, you can see that on a scope. A piece of copper foil across the rear of the oscillator pcb stops that.4
Headbasket reflections
- Other fixes include raising the capsule post a bit to eliminate the dome reflections that creates a bump at 200 hz.7
