[rogs]

I've now built and tried out several of these mics, using different capsules and circuit variations.
I've decided that the 2mA version I described above is probably the best option for most situations...

After all, this type of mic really comes into its own outdoors, and is therefore more likely to be used with portable recorders.
It's then that current drain can be important.
OK, the noise floor is slightly higher, but is still way below any outdoor ambient noise level I've managed to record  so far ...

I've redone the schematic and parts list to allow for different capsule types, and have posted a combined document here:

 http://www.jp137.com/lts/RF.AMX10.v.2.3.2mA.schematic.and.parts.list.08.20.pdf

(There is a link to that document on the website Schematics page as well)

I'm not sure just how important this kind of mic is for indoor use? - but it has been interesting to listen to the same capsule, when used with  both  a 'standard' Hi-Z Schoeps type circuit and with this Lo-Z RF bias circuit.
It certainly sounds a little different - although it's difficult to qualify 'different'. ....Slightly less 'HF edge' seems to sum it up.
Understandable perhaps, when you change the  bias on the capsule from c.60V DC, to around 2 or 3 volts of RF AC bias.....

[EmRR]

I have a selection of MKH mics I use indoors all the time.   They are definitely among the quietest mics I own, and the most detailed.   

I suspect a lot of what I and others like about them has to do with the capsule design and the corrective EQ approach.  Is lack of an RF specific capsule a bottleneck here, the last 5%?  Particularly, I'm unaware of any push-pull capsule design as found in the MKH mics of the last 35 years, they sound quite a bit clearer and cleaner than the previous non-PP MKH types. 

I see 25pfd mentioned as the substitution cap value with the earlier MKH models for alignment.  It's been mentioned by someone who repairs those mics in America that a capsule that presents as shorted can be resurrected with a couple of good raps on a table(!). 

I would think it worth exploring SDC variants, and identifying capsules that are more 'in the direction of' the specs found in the Sennheisers, both LDC and SDC.   

Historically, the obscure AKG RF mic is interesting, it's covered in a BBC technical paper along with the MKH 404 (I think).  I find no mentions of that mic online.   

I'm tempted to try your circuit with a Neumann TLM67 I have, since I have a spare set of Neumann metalwork....but I owe someone a report on a U87 build first! 

[rogs]

I would think it worth exploring SDC variants, and identifying capsules that are more 'in the direction of' the specs found in the Sennheisers, both LDC and SDC. 

So far, I have only experimented with a variety of cheap Chinese 34mm capsules -- both centre and edge terminated.
The chance to try out something more appropriate to the concept -- like the Sennheiser 'push pull' capsules -- would be a fascinating next step to try. 
Sadly, the prices of actual Sennheiser capsules are WAY above my hobby budget, and I've not seen any others of that type offered. 
I'm guessing Rode must have their own 'RF' capsules for their range - for example - but I've never seen any offered for sale? (Understandably).

It has been interesting to note how 'standard' cheap capsules seem to react differently when biased with a few volts of RF - rather then 60 or 70 V of DC.....
Although I don't have test equipment sophisticated enough to be able to measure those differences objectively.

There is quite a variation in the sensitivity of the different types of capsule - with the 'K67' types seemingly more sensitive than the K47 types.  That presumably has something to do with membrane tension .
Sadly, discovering which cheap Chinese capsules are built with lower tension membranes seems to be largely a matter of luck... The published 'specs' usually appear to be just a bunch of numbers picked out by the marketing guys....
All enjoyable experiments though !
 

I'm tempted to try your circuit with a Neumann TLM67 I have, since I have a spare set of Neumann metalwork....

I'll look forward to reading about what you discover -- when you get the chance to try that...

[rogs]

I've just had the chance to try out an alternative inductor for this project.

The same UK supplier (Spectrum Communications) has introduced a new range of  'High Stability' IF cans (see here: http://www.spectrumcomms.co.uk/Components.htm?LMCL=si5NNn ) .
The version used for this project has the part no: '5u3HH' (as opposed to the original '5u3H' part no. - (which is still available)

The newer version works as well as the original -- in fact, I suspect slightly better....
It seems easier to 'tune' accurately -- although I've yet to have any actual figures on the differences.

Although the 'Q' is listed as being less, the practical results seem to be very similar to the 5u3H coils.

And it is the high 'Q' of the inductor assembly in this project that makes all the difference!

[gyraf]

    (still enjoying this from the side line - looking forward to having the time to dive into this.. thanks..!)

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