Another DIY Mic: Royer/SELA'sh with a 6AK5

[Blue Jinn]

Just thought I'd share this. Body: MXL V89MPB (broken=cheap)
Parts I had on hand including K47 style capsule and a KCM transformer (which are the one's AFAIK FoxAudioResearch uses.)



Smoke test sat,
Hello world test sat.







Schematic:
I didn't have a 1uF cap that would fit, so the output cap is a 2.2uF. Pretty basic circuit, but a fun build.



The 6AK5 specs are almost exactly the same as the 5840. So the circuit is basically the RoyerMod same values, with the added 2G Rg. I had a 2G with long leads so I used it.

The SELA T-24 is very similar with the magic tube:

 

[ecc83]

Hi Blue.
Nice bit of "shoe horning" there!

What sort of self noise are you getting? I know nothing of the design processes for cap' mic valve circuits, used to more "heavy" stuff!

I ask because I wondered why the anode load was 100k? If it was 10k you would get about 10dB less noise? A swift varder at the valve data seems to suggest you would lose little gain?

Dave.

 

[Blue Jinn]

Thanks Dave.

As the 6AK5 has the same specs as the 5840, I just used the plate and cathode resistor values as the ""RoyerMod," which was designed to be similar to the U47. The BigUgly, Royer, U47, and the SELA above are all very very similar.

The "BigUGly" is basically the RoyerMod with the G7 pattern switching scheme and a large Rg. I went just with an omni/card pattern, (like the U47) which is less complicated, and the mic body already had the SPST switch. The G7 is another DIY mic also based on the German designs.

I don't remember the reason the load lines are way down in the weeds, but for a microphone, IIRC that is desirable.

I haven't put it through any serious paces yet as to noise. ( I don't have the time, nor skill to do "scientific" measurements, (I'm not sure how either....)) It probably doesn't perform as well as if I had a "hand picked" tube, which like a Neumann . Also, I need to swap out transformers on my PSU, because I'm only getting about 60-70v under load, and it should be about 100v, although it works just fine with the lower B+. (I tried the two transformer trick but I think my tiny step down is too wimpy to handle the load )

I'm not that much knowledgeable obviously, but what I understand is you want a lot of gain from the capsule and you're "throwing away" a lot of that with the 11:1 transformer, because you need to match a nominal 600ohm microphone input. The G7 has an optional feedback scheme to lower distortion, but he ("gyraf") doesn't recommend using it.

The G7 can be found here: (I know you don't like links!) http://www.gyraf.dk/gy_pd/g7/gic.htm There are several discussions of the BigUgly on GrouDIY. I had a bunch of PCB's for that one fab'd, so I'm sortof the goto guy on the BigUgly....I think the schemo's for the BigUgly and the Royer are in another of my posts here. I can send you the schematic for the Royer and the BigUgly if you're interested. (I have to dig up on a DVD.) FWIW, the pattern scheme on the G7 PSU makes more sesne than the simple pot on BigUgly v 1.0. there is a v 2.0 that encorporates a more G7 like scheme, but I didn't fab any of those. The PSU board has a low current B+ and an LM317 regulated heater, so it is ideal for other tube preamp circuits.

 

[Blue Jinn]

Here's a link to the Royer.


The Royer Tube Mic Mod for the MXL 2001

Also, here is the M49:


here is the BigUgly

It has the pattern controlled by a pot. There is a PSU schemo with an incorrect input voltage, I don't have the correct one on this PC.