Hello Guys, my apologies!!! All the best for 2017 in your moddding and building projects.
I forgot all about sending out circuit info on our CM87. I seldom get the chance to check into Forum's much lately.
Its been a very busy time for AA and I had the privilege last year of working with my old friend Malcolm Toft on a new transformer coupled microphone preamp circuit. It was just released and is now on our AA site as the MT8016.
I also designed a new tube microphone in 2016, the CM48T and used the CM87/48 body with a 47 type headgrill, AK47 capsule with a traditional U48 circuit designed around a GE/JAN 5654w 7-pin miniature tube. I am really happy with this new design and in my experience it compares more than favourably to an original U47 which we put it up against in London. It has very easy access for "modders" to the transformer, capsule, capacitors and tube but in my humble opinion it only needs some capsule and tube selection. The tube is socketed and they are very economical as is the CM48T.
CM87:-
I am including the schematic for the CM87 that folks were asking about and some notes for "modders".
The schematic looks a bit complex but most of the circuitry is to provide a 3-pattern microphone circuit that will double the polarization voltage on the rear diaphragm much like the early C414 or the U87AI.
T3 is configured as a 400khz oscillator that provides enough A/C voltage when rectified with a voltage quadrupler diode circuit to provide +/- 5% of 105v dc to the rear diaphragm in Fig 8 and 52.5 v to the back plate of the AK67 capsule. The voltage quadrupler circuit is quite effective because the capsule is not drawing any current and only needs to be polarized. However, this is why it takes 15 seconds to fully polarize the capsule when you change patterns.
The dc to dc converter should provide 105v dc +/- 5% at the positive side of D6.
The circuit looks complicated but there are over 20 components in the DC to DC converter circuit. There is also a RFI filter circuit that has 6 components and this rolls out the HF response past 40khz to reduce the chance of Radio Frequency Interference when used near Broadcast transmitter locations.
The audio portion of the circuit is a T2- 2SK170 impedance converter driving T1- A1015 as an emitter follower through capacitors C7, C5/C6 and C4. C7 and C4 are 22ufd tantalum capacitors which have a lower ESR than electrolytics. C5&6 are metalized poleyster film which have even lower ESR ratings than electrolytics or the faster tantalums. Neve uses tantalums in the 1073, AKG used tantalums in the C414 and Neumann in the U87 for coupling to the output transformer. They are not cheap.
If you look at the spec's of a U87 you will see a maximum output from the microphone of 390 mv which is -6dbu. This is what you get with a single high gain FET driving a 10:1 ratio output transformer.
The CM87 and CM87se can deliver +10dbu. In the CM87se we replace the economical single winding transformer with a dual bobbin transformer having more complex bi-metal laminations and this reduces distortion of the low frequencies with very transient high level sound sources like drums, percussion and grand piano.
The majority of our CM87 microphone are sold for voice work but they can easily handle the level in front of a kick drum, bass amp or percussion.
Like the original C414, the CM87 uses a emitter follower circuit that can drive a 2:1 ratio transformer yielding 16db less loss. So, the CM87 circuit requires 16db less gain from the input FET and can be a 2SK170.
Capacitors C19, C8 and C17 are in the capsules audio path are spec'd as metalized polyester film capacitors with 5% tolerences in the CM87. In the CM87se they are custom fitted 1% polystyrene capacitors and C17 is a 5% tolerance polystyrene.
Capacitors C5 and C6 are metal film and provide a 6db/ octave HP filter when C6 is switched out of the path.
Capacitors C7 and C8 are 22ufd tantalum capacitors. C7 bypasses the input fet and needs to be 22ufd to provide a good bass response while C4 is also a 22ufd/35v so that any resonance between the inductance of the output transformer and C4 fall below the audio range down in the sub-sonics.
The AK67 has an OD of 35mm and is skinned with 6 micron mylar it should be flat out to 3khz and only up <2db at 5khz then rising up 5 db at 12khz. We do not use any de-emphasis in our circuit as the AK67 has a smoother response to our ears than an original K67 (87) capsule. If you fit the microphone with a true K67/87 capsule you will need to add de-emphasis.
It there is excessive rise in the 3-5khz range then the capsule is compromised. With the natural rise at 12khz a simple HF shelving EQ can easily give the CM87 a more vintage sound if required. We have had great success with the AK67 capsule in both our CM87 and CM67se microphone. We have very few problems with this capsule and get less than a 3% failure rate over time. However, we do see the odd failure from time to time.
If you require a more "vintage" curve our AK47 and saddle will fit on the post and you will have a microphone that sounds like a solid state M49. We also use this circuit with our AK12 capsule in our CM414.
I am just working on a modification to the polarization circuit so that the AK67 capsule is passive in cardiod. Which means no voltage is required on the rear diaphragm except for Fig 8. This would also reduce the input circuit by one capacitor. We use this method in our CM48T as its more traditional. However, it will take some time to change over the CM87 as we have 200 sleeves engraved with Cardiod on the right and FIG 8 up the middle as the circuit I attached shows and I would have to re-design the circuit board if we like the sound. Since, cardiod would have to be in the center with a passive cardiod position.
Cheers, Dave Thomas
aamicrophones.com
