MXL 9000 design quirks. thoughts on it?

Blue Jinn

Blue Jinn

Rider of the ARPocalypse
I picked one of these up on the cheap, w/o a PSU. So I haven't been able to test it out, but did some research about it and found a schematic on GS and a discussion of mods to it on GDiy. I also went and bought a used V76t which uses the same PSU (and AFAIK is the same mic on the inside.)

So, first, if anyone has used one, opinions? And, looking at the schematic, it has a parallel triode section, a fairly high B+ and a weird coupling capacitor set between the tube and transformer. Also, there's a lot of presumably RFI filtering on the output.

The coupling capacitor is really baffling,: a small value high voltage, in parallel with a lower voltage (considering the B+ of 200v) higher value electrolytic. As soon as I get the V76t I'm going to try it out, but have in mind replacing the coupling cap with something more conventional, 1 or 2 uF high voltage cap, and perhaps strip out the RFI section and use that part of the PCB to add pattern switching (after swapping capsules). Once I commence that I'll post a DIY thread, but thought I'd start here to get people's opinions overall of the stock mic.
 
Got the V76t to compare. Have not tried either out just yet, but looking at the circuits, they are slightly different. Also, the electrolytic referenced in the first post, actually is rated on both at 250v. These are pretty small tube mics, so everything is crammed in tight. It may be the .1uF it is in parallel with that is lower voltage. Hard to tell, but it is pretty small (physically) considering the value. Most reviews indicate an unbalanced tone in both these microphones, so replacing this capacitor combination seems to be in order regardless. There are also some decent quality parts in the V76t.
 
xaudia said:
>And, looking at the schematic...

Can you please link to the schematic? ;-)
Click to expand...

I pulled this off GS. It's also up on GroupDIY. It doesn't clearly indicate the parallel triode sections. Also, the V76t is different. I haven't looked at it close enough to determine how different, but the two appear "close enough" The GS thread has a picture of a modded circuit board also. It isn't clear from the posting if the mods in the picture also included removing several parts. Here is that thread: Finding the M49/ M249 in the MXL 9000/V76t - Gearslutz.com

@moresound: It is my understanding that the V76t is derived from the V77. I've not seen confirmation of that though. The V900 does look like a cool platform to mod, 414'sh????? BTW, I don't own anything special. The most expensive mic I have is an older Audio-Technica electret condenser.

And here is the schematic for the 9000:
 

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Thanks for that.

C6A (and C41) are likely to be higher quality film or ceramic caps which are used in parallel with a cheaper electrolytics. The idea is to improve the performance / sound, particularly of high frequency / transients etc. Both should be rated to the full voltage.

I think usually it is better just to use a good quality higher value film cap instead, if there is space. If not then you are stuck.

33uF looks big for C6 / C6A. You could try replacing them with a good 1uF or 2u2 film cap and seeing how it sounds.

These capsules can be rather bright. C3 looks like capsule correction, feeding a little of the very high frequencies back to the tube grid to tame the top end. There is also some trickery around C1/C2/R2/R6 that I don't quite understand. Again, could be correcting the response. It would be interesting to dump R2 and jumper across R6 and see what happens!

Finally, try removing C5. The mic will get quieter by a few dB, but you might prefer the sound. You might not!

Have fun, and report back!

Stewart
 
33uF looks big for C6 / C6A. You could try replacing them with a good 1uF or 2u2 film cap and seeing how it sounds.
Click to expand...

I have a 2u5/250v that ought to fit there with some contortions to the leads.

There is also some trickery around C1/C2/R2/R6 that I don't quite understand. Again, could be correcting the response. It would be interesting to dump R2 and jumper across R6 and see what happens!
Click to expand...

The GS thread (or another GS thread) talks about that as a feedback network to tame the brightness of the capsule. That may be a good thing, and the 67 style capsule may not be a bad thing to leave in or maybe replace with a different 67 (or c12 style--which I'd though of doing as well)

Finally, try removing C5. The mic will get quieter by a few dB, but you might prefer the sound. You might not!
Click to expand...

Funny, after I posted the schematic here I started to look at that, with the thought of eliminating it. The Dave Thomas mods to the Apex 460 (I have a thread going on that, and got all the parts in, just need to clean off the bench and my calendar....) eliminate the cathode bypass caps as well. I don't know the ratio of the transformer in this mic, but the amplification with two sections of 12AT7's (and that's assuming the stock tube isn't really a 12AX7, it's OEM'd and unmarked....) should be plenty to begin with, so a loss of a few dB's shouldn't hurt much. The Apex 460 with one 12AX7 section is already plenty if not too much.
 
Blue Jinn said:
The GS thread (or another GS thread) talks about that as a feedback network to tame the brightness of the capsule. That may be a good thing, and the 67 style capsule may not be a bad thing to leave in or maybe replace with a different 67 (or c12 style--which I'd though of doing as well)
Click to expand...

That makes sense - I just can't work out in my head which frequencies it is attenuating! Probably a job for Spice or something similar! If you do swap to a good C12 style then you could/should get rid of R2 and replace R6 with a bit of wire so that it is just a filter stage. (And lift C3 too)
 
Blue Jinn said:
@moresound: It is my understanding that the V76t is derived from the V77. I've not seen confirmation of that though. The V900 does look like a cool platform to mod, 414'sh?????
Click to expand...



I've heard the same about the relationship between the 76 and the 77.

If you haven't seen a V900 in person you will be surprised when you do. Pictures just doesn't do it justice. For it is as big as your head!!!
 
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