Any new ideas for the MXL 991?

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HellfireStudios

Well-known member
Joined
Oct 17, 2011
Messages
513
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The Sunshine State
I was given a 991 as payment for a bass rig setup. I've already done the Royer mod (stereo), and a point-to-point KM-84 mod (also stereo) to these mics already. I'm not really interested in optimizing the original circuit (like the Dorsey mod).
Does anyone have any new ideas for this mic? I don't want to put this mic in the back of locker to never be used again. Any ideas are much appreciated.

-James-
 
I have been bouncing around an idea similar to the Royer SDC mod. I actually got the idea from Marik. Instead of just the CF add a gain stage. The tube is a 6111 subminiature. The circuit topology is very similar to an Apex 460. It is a gain stage DC-coupled to a cathode follower. With a change to a 4:1(or 5:1) output transformer, you should be able to use the Royer mod PSU. Gain stage, plate: 100k,  cathode:1k      CF, cathode: 22k~100k
It should have a simple front end with minimal parts on the circuit board in the mic. The circuit is simple enough to build using just perfboard. I would say that though seeing as most of my projects wind up being built on perfboard! I hope this helps. All I have is a hand drawn schematic that is missing some values or I would post it. Good luck!
 
Which Royer mod PSU will work with it (SDC or LDC)? I'm assuming the LDC PSU is the one your referring to.
As for the perfboard, I'm an advocate of that stuff myself. Let me know if you need info on how to fit a Cinemag transformer in the MXL-style SDC (it was one of the more "interesting" parts of my perfboard KM-84 build). BTW, the Cinemag CM-24110 (4:1) should work for that circuit, and fit in the mic.
I'd be interested in that schematic, but I intend to use the 991 as the "before" mic in a test amongst my other modded SDC's, so I need to find time to do a shootout before I can mod the stock mic.

-James-
 
Actually I was referring to the Royer SDC mod PSU with the output transformer off the circuit board/ outside of the mic. I am actually in the process of building such a PSU. It is for two mics and has removable transformers, octal-style. For the Royer SDC mod, I have a pair of Dukane 2:1 transformers. To extend the PSU's usefullness, other transformer ratios can be installed for different purposes such as a tube DI, the previously mentioned SDC mod, the DC-coupled CF mic, ect. The PSU is also going to use a zener follower for the B+ which will give near battery quiet operation, the FIL a more standard regulator supply.

If the Cinemag trans will fit, this DC-coupled CF SDC mic could very well use the Royer LDC PSU. That would be an awesome build. I am trying to get a layout of inside the mic done. That, coupled with the schematic, should give you an idea of what I am thinking. I will scan both drawings and post them ASAP.
 
Ok, is this the Royer mod you guys are talking about?

            http://messageboard.tapeop.com/viewtopic.php?printertopic=1&t=53130&postdays=0&postorder=asc&start=0&finish_rel=-10000


It looks like quite a bit of work, with the external power supply, etc.

How are you liking the sound of converting this mic to a tube?  How is the
frequency response different?  Is the mic less harsh on the high end?

Are you keeping the same stock capsule?  How about doing the capsule mods, and removing the
gill slots, and using that with the tube?

 
Yeah, that is the Royer mod. I would recommend using perfboard for the whole job. It is much easier to cut a piece of perboard to size to fit in the mic, than to pull all the components off the PCB.
As for the PSU being difficult, the only hard part is getting the holes for the jacks in the case. I went with the onboard 24VAC transformer, but the external wall wart now seems much easier to implement.
I have not done the capsule mods (I'll be using Marik for that), I did cut out the slits on the side supposedly to reduce off-axis coloration, and remove a 9kHz peak caused by the thin slits. My first impressions were good. I have not had a chance to compare it to the original so I won't venture to make a comparison. I hope to be able to shoot them out in the near future.

-James-
 
The stock Royer mod PSU is crap with the voltage tripler. I am not going down that road again. What it needs is a zener follower PSU which easy to build and damn near battery quiet. As soon as I get this scanner up and running, I will post the schematic.
 
mabell313 said:
The stock Royer mod PSU is crap with the voltage tripler. I am not going down that road again. What it needs is a zener follower PSU which easy to build and damn near battery quiet. As soon as I get this scanner up and running, I will post the schematic.

It is not a crap circuit. 

It works fine with a simple cathode bias circuit. 
It slow starts the heaters with the RC network. 

It needs a good transformer.

I have used an adjusted version of it to power a fixed type bias microphone without any noise issues.
 
Most of the values are wrong for the application. There is also some weird series resistors in some of the schematics. I will make corrections and repost the schematics with them. Take it for what it is.
 
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