For anyone not familiar with the Neumann SM69FET, this mic was introduced in 1970 as a solid-state Series80 update for the valve-based SM69. It has since been superseded by the USM69 which is still available at the time or writing (£5,460) and it uses the same capsules. The electronics have been updated to give more headroom (10dB greater max SPL).
The main operational difference between the SM69FET and the USM69 is that the power supply and polar pattern selectors have been built into the USM69s mic body, instead of being housed in an external control box.
Related Neumann microphones include the Neumann SM2 stereo mic which used small-diaphragm capsules (from the KM56) and the later SM23, but these mics have long since been obsoleted.
AKG also made broadly equivalent stereo mics. The C34 used small diaphragm capsules, for example, but the closest equivalent to the SM69FET was the C422 which came out in 1979 as a solid-state replacement for the valve C24. AKG's marketing claimed both of these models employed the 'CK12' capsule, although the C24 and early C414s used the brass version, while the C422 and later C414s used the 'teflon' version. The C422 was was superseded by the C426.
All of these stereo mics have two separate dual-diaphragm capsules, mounted one above the other. The lower capsule is fixed to the body of the mic while the upper one is free to rotate over 270 degrees (usually -90 to +180).
Each capsule can be configured independently to provide any desired polar pattern from omni through cardioid to figure-8. Consequently, the rotating head and adjustable polar patterns together allow any stereo XY or MS coincident configuration to be created.
The K870/67 capsules used in the SM69 FET are exactly the same as those used in the previous SM69 model, as well as in the U67, U77 and U87ai (and the M269 and TLM67 mics).
However, the impedance converter circuitry in the SM69FET is slightly more complex than that in the U87ai, , and gives a marginally brighter high-end as a result — more like the original U67 and U77, actually — although the core character and midrange punch are much the same.
The SM69FET is a fabulous mic for orchestral and choir recordings (especially if rigged as a Blumlein pair), but also works well as an overhead on a jazz drum kit (max SPL is 123dB, so its a little risky over a loud rock kit). It's also brilliant as a vocal mic where you can set the two capsules with different gain structures for gentle/loud sections in songs, or for dramatic voice-over work.
Another useful technique is to set one capsule in omni and the other in fig8 and record them to separate tracks. They can then be combined to taste which has the effect of allowing the polar pattern to be varied in post-production — which can be really handy for tackling varying proximity effect.
Circuit Technicalities
The SM69FET requires fixed DC voltages to bias the capsules (60V) and power the impedance converter circuitry (33V), as well as variable voltages to adjust the polar pattern (0–120V). It is worth noting that both capsules and both channels of impedance converter circuitry are completely separate, even down to receiving power via different connections. This ensures that a failure in one channel of the microphone can not affect the other output.
Each of the capsule back plates are held at 60V, with the front diaphragm at 0V, providing a cardioid pattern facing forwards. Biasing the rear diaphragm to 60V essentially turns it off (backplate and diaphragm at the same potential), leaving just the forward facing cardioid response.
Alternatively, with the rear diaphragm biased to 0V a second cardioid pattern faces rearwards, and combining the two outputs together gives an omnidirectional pattern overall.
Raising the rear diaphragm to 120V provides a cardioid pattern facing backwards but with the opposite polarity to the front pattern, the combination resulting in a figure-of-8 overall polar pattern.
Intermediate voltages provide intermediate polar patterns. The SM69FET offers nine polar pattern positions selected on rotary switches.
Neumann made a variety of power supply/pattern controller modules, some free-standing and others designed to be incorporated into mixing consoles. The most common freestanding types are the CU48 and the NS69.
The NS69 is a mains-powered unit which provides fixed 120V outputs on pins 3 and 10 of the 12-pin mic connector. This voltage is halved inside the microphone via a resistive potential divider to 60V, and used to bias the capsules. It is further reduced through resistors and zener diodes to provide 33V for the impedance converter electronics.
Variable voltages ranging between 0 and 120V are provided on pins 9 and 11 for the rear diaphragm polarising options. Again, these are derived via simple resistive potential dividers.
All of these DC voltages are referenced to the audio ground on pin 4, while the chassis screening ground is via pin 12. The transformer-balanced audio outputs are on pins 7/6 and 1/2, and these are passed directly to the audio output connectors in the NS69 or CU48.
The CU48 power unit is a phantom-powered equivalent to the NS69 and is built into the same sized box. It contains a DC-DC converter inside a sealed metal module to generate a 120V supply from the incoming 48V phantom (required on both channels).
This 120V supply feeds the polar pattern selectors to provide the variable polar pattern voltage for the capsule rear diaphragms, and is also halved inside the unit to provide a 60V supply for the capsule backplates. The variable pattern voltage is applied via pins 9/11 again, while the 60V capsule bias supply is connected via pins 5/8.
The DC-DC converter used in the CU48 has quite limited current availability, which is why it is only used to bias the capsules and not to power the microphone's electronics.
Instead, the impedance converter electronics receive power directly from the 48V phantom supply, which is passed through to the mic, and it is routed to the 33V zener diode from a pair of 6k8 resistors across the output transformer.
However, it has been suggested that this phantom powering arrangement is not able to supply as much current to the electronics as the NS69 system, and that this may have an impact on the sound quality.
Also, since the CU48 requires phantom power, the audio outputs have to be routed through DC blocking capacitors on their way into a connected mic preamp, and (in theory) these capacitors may also degrade the microphone signal...
Of course, most modern preamps use DC blocking capacitors of good quality so this is unlikely to be a real-world problem, but a transformer-input preamp may avoid the use of capacitors altogether (although not all do!) and thus a transformer-coupled preamp might be a better option when using the CU48 to power the SM69FET.
I have and use both the NS69 and the CU48, and I can't honestly say I've noticed any reliable difference (assuming the phantom source is up to proper spec!)
Here is the SM69 FET microphone schematic, complete with expected voltages from the various power unit options:
This is the inside of the NS69. Note: there are two fuse holders, but only one must be used at a time. They are marked and swapped for 230 or 115VAC inputs (and different fuse values are required for the different mains voltages).
The metal earth bars at the bottom of the picture allow the 0V audio reference ground to be linked directly to the mains safety ground, or not. As supplied from the factory, the audio ground is NOT linked in the NS69 (but the audio reference ground and chassis screening ground are linked at the microphone itself!).
The trimmer at bottom right allows the 120V supply to be adjusted, but this must only be done with the microphone attached as the actual output voltage is dependent on the current flow.
And finally, this is the inside of the CU48 phantom power unit. The DC-DC converter resides inside the metal box. The trimmer bottom centre again adjusts the 120V supply, but must only be adjusted with the microphone attached.
