MXL 9000 (Frankenmic) assistance please....

[illacov]

Hi folks after getting my 3 thread hit on the search function, I have a problem that's purely aesthetic with my MXL 9000.

Its fully functioning, has a MK319 capsule, Groove Tubes ECC81, Cinemag transformer a 5:1 ratio, grille mod and a replaced 1000pf capacitor 630v.

I used to have a Jensen trafo in there that was a 12:1 ratio, it sounded very clean, very articulate but warm (more like the way the capsule sounds under optimum) the only thing was the level was very very low and required the kind of preamplification you would use on an SM7 or a ribbon. I wasnt pleased, but did enjoy the sound.

I went to Cinemag and they recommended a 5:1.

Immediate jump in level, sounded like we have a winner. The only problem is the mic has alot of proximity effect now and some kind of really old school almost lo fi business in the low mids. So here's the issue.

What would be the culprit?

I think its the trafo ratio, which is fine, because I like the level out of the mic.

Do you think that maybe I should change the tube from an ECC81/12AT7 to a tube with less gain? I'm thinking maybe the trafo is soaking up level from the output of the tube??

Maybe there's a circuit mod that can be done to reduce the output from the tube to the trafo? Change a cap value? Or a resistor?

I'm PMd Gus and he is obviously busy with something because I haven't heard back from him.

Anyone care to chime in??


EDIT!! Samples posted.

Here is the old example of the mic before the mods you guys recommended.

Recorded about 6 inches back, no compression, into the line input on the EMU0404, right out of the output of the MXL9000 power supply.

http://www.gearslutz.com/board/attachments/rap-hip-hop-engineering-production/110952d1235059836-mxl-9000-dark-mics-hip-hop-sound-samples-mxl-9000-neg-1pnt7-n-proce.mp3

Here is the latest mods (C3 removed, C6 is now a 1uf 250V, R7 is 2.2k 1/2 W 5%)
Same deal as the previous mod. Notice the noise level has dropped.

http://www.gearslutz.com/board/attachments/rap-hip-hop-engineering-production/111311d1235262284-mxl-9000-dark-mics-hip-hop-sound-samples-mxl-9000-latest-mods.mp3

Peace
Illumination

 

[rodabod]

Have you tried changing the value of the output coupling cap from the plate?

 

[illacov]

The schematic is on the board here somewhere, do you recall what capacitor that would be and which direction would we be going in value wise?

I'm no expert but if you can say c3 or something like that and change it to a 220uf or whatever then we're in business.

Thanks for your quick reply

Peace
Illumination

 

[rodabod]

I don't have time to look for a schematic. It wold be around 1uF and connects from the valve's plate to the transformer primary. I would maybe try reducing its size.

 

[illacov]

OH I wasn't saying get the schematic for me man

I was looking at the schematic saying WTF?? if its around 1uf then thats a clue right there.

Thanks.
When I happen upon some candidates, I'll ask some more neophyte questions! 

Thanks again

Peace
Illumination

 

[illacov]

Ok I got it.

Its labeled C6A and it says .1uf? I don't remember if thats the right rating due to some errors in the schematic the OPs tracing but that is the actual cap thats going to the primary of the transformer from the tube.

When you say reduce it, what rating are we talking?? like .01uf?? The schematic might be labeled wrong but either way a little more help when you got a second.

Im going to look at the mic itself and see what we can find.

Thanks
Peace
Illumination

 

[rodabod]

0.1uF sounds wrong. When I say reduce it, I might suggest halving its value and then listening. The Oktava capsule has quite a solid low-end (it can be great on bass guitar by the way).

Can you post a schematic?

 

[illacov]

Ok the cap is reading 250v 100n k. I thought that was .01 uf??

That indeed is the cap betweenst the trafo and the tube plate.

Heres the schemo

Peace
Illumination

 

[rodabod]

The usual cap is actually two here in parallel; C6 and C6a. The smaller cap is there to bypass the electrolytic to improve sound quality.

Try replacing the 33uF electrolytic cap with maybe a few other values, say 0.33uF and 0.47uF. May as well try 1uF too. This should help to roll off a little at the very bottom end which may clear things up (assuming this is what is causing the Cinemag to sound "boomy" or however you describe it).

It's actually a slightly odd circuit that, because the Chinese-built mics usually follow a fairly similar design to each other.

 

[Rossi]

Is that the original MXL9000 schematic or is this a modded version?

There are two caps from plate to trafo: C6 and C6A. Replace both by one cap between say 2.2u and 330n rated 250V, preferably a film cap, although an electrolytic will do (mind the orientation, + must go to the plate). This cap forms a hi pass; more capacitance will give you more bass. Basically. A small cap can procuce a resonance in conjunction with the transformer inductance. So a smaller cap can sometimes produce more bass than a larger one. Try some values and listen.

The schemo looks weird in a Chinese way.n Somehow it looks a lot like a KM84 circuit with a triode tube replacing the FET and a typical Chinese output circuit (R20, R21, L1, L2, C20, C21).

C6 seems very large, I don't know what R40 is doing (why not have R41 be 30k and drop R40?)
The whole output stuff behind the tranny secondary (R20, R21, L1, L2, C20, C21) is probably unnecessary. In most "real" tube mics the secondary goes directly to the output pins.
Not sure C41 has an audible effect, but it doesn't hurt, either

Maybe the tube gurus have an answer to this: Why R30 and R31 instead of dropping R30 and change R31 to 10 ohms. Or maybe a little less; 5.36V may be a little lower than optimal. Gefell use 5.8V for their tube mics, which use an EC92 (which is said to be a good tube for underheating; I don't know if the ECC81 likes more underheating).

Another question for the tube guys: What about C3 good idea or bad idea?

 

[Rossi]

Hey Roddy, we were typing at the same time, it seems.
Funny how we both found this circuit typically Chinese. ;D

In fact, I wonder what the original trafo ratio is. 33u seems very large for a plate to tranny cap. Plus the extra output R. So maybe it is the usual 2:1 tranny, they use in their FET mics?

 

[rodabod]

True!

I think R40+R41 plus R30+R31 are just typical examples of supply filtering where you create a sort of "RC ladder" of filtering. Nothing special really. In the case of R40+R41, there is further filtering stages going on as you approach around to the capsule.

I agree the netwrok on the secondary is probably unnecessary as usual.

The operating point of the valve is not typical. That and the fact that it is really quite underheated too.... Could be interesting.

 

[rodabod]

In fact, I wonder what the original trafo ratio is. 33u seems very large for a plate to tranny cap. Plus the extra output R. So maybe it is the usual 2:1 tranny, they use in their FET mics?

33uF was probably just cheap. Anything over a few uF and electrolytic would have been good enough for them I'd guess. 2:1 would provide too high an output impedance from the 12AT7 configured like that.

 

[Rossi]

33u 250V is not that cheap and quite large, too. IMO it only makes sense if the trafo ratio is fairly low, which, yes, means a fairly high output impedance. But maybe it's a typo (3.3u?) The whole output circuit, too, makes sense only if the secondary is very low R. But then again, Chinese mic circuits don't always make sense.

With regard to filtering: wouldn't we get better filtering if R31 was a higher value (e.g. 10 ohms) and R30 was dropped?

 

[illacov]

Ok got a 1uf metal film cap 250v. Dumb question time. This cap isn't  marked like its polarised but one leg does appear longer so I would guess that's the + leg?

If the cap is not polarised will it matter?

Peace
Illumination

 

[Rossi]

Film caps are not polarize, you can use them either way.

Electrolytics including tantalums are polarized and will blow up if inserted the wrong way.

 

[illacov]

ok I was hanging oc703 on my ceiling.

I can wash my hands (glove funk) and see about soldering this new cap in!

Will report back hopefully by tonight with my findings.

Peace
Illumination

 

[Gus]

What are you trying to do with the microphone?

Ask your self what is C3 about? why did you change the stock transformer?
  GT ECC88 means nothing to me is it an NOS or a china one?

I did not reply to the PM because I don't feel like giving out free design work anymore.

 

[illacov]

Ok thanks!

How you been?

Peace
Illumination

 

[illacov]

I changed the transformer per Dave Royer recommending it. I figured he knew his mics and he was specific about the mxl 9000 taking a jensen direct box jtdbe 12:1. It didn't work for me gain wise so I tried a different tranny. The cinemag does sound a lot better than the stock tranny, as did the jensen, the orig tranny was screechy under loud sources. I don't miss it or the screech.

I put a Groove Tubes ECC81 in there as an improvement over the stock tube.

The capsule needed a home. I like it overall and am trying to put the finishing touches on the mic.

My only concern is the low end being tad bit too much. Eq is a true possibility but it seems to be beyond normal proximity effect so even eq will not be enough.

I apologiE for disturbing anyone with my questions. I only pmd you Gus because you were one of the few people involved in the mxl 9000 exposed thread, it kind of stopped before it started so I thought you have held some interest.  I wasn't trying to panhandle or harrass anyone per se, if I came across that way Im sorry. On a side note I really liked your mod for the mxl 990/603/991. Its very nice and articulate. I used a Peluso CEK12 with it and it sounds nice.

Ill be installing the cap tonite so hang tight folks!

Peace
Illumination