MXL 2001 "U87" front end mod question/help

[Khron]

PS. Quick question - can anyone measure the distance between the "lip" (lowest edge) of the MXL 2001 headbasket, and the centers of the mounting holes for the pcb's?

 

[micolas]

I totally understand. What then would be your ideal MXL 2001 mod(s) using the existing circuit board? Component and capsule changes are permitted. Name a few mods from minimal to maximal as you think of them.

My favorite, cheap mod:
Remove C2 and R4, connect the RED Capsule directly to the jFET gate. The phase changes, so you will reverse the ends of one of the windings of the transformer (either primary or secondary) or reverse XLR2 with XLR3
Remove R6, put it to GND (R6=0 ohm)
replace R7 with a 15..25 kohm semi-adjustable resistor that will bias the jFET)
Adjust C1=33...56nF for bass to taste
Eliminates: R15, R16, C13, C14.
Add a small DC/DC converter (powered from the connection point between R11 and R12) that delivers +63...65V to the capsule through R17. Because it will consume current from the Phantom, the voltage on the audio side will decrease, so we reduce the resistance R18 to 3.5...3.9 kohm, so that the voltages on the collector, base and emitter of the BJT are equal to the stock ones, before the mod.
Increase (if necessary) C2 , to taste, to further attenuate the high frequencies.
If you are not satisfied, you will change the stock k67 capsule with a better quality one.
Do not remove the inner mesh from the headbasket.

It will become another microphone:
Self-noise will decrease substantially
It will increase the output level of the microphone and you will get an earlier saturation of the transformer.

Or make it a U87 as Khron suggests. (You will need a more expensive transformer)

 

[micolas]

PS. Quick question - can anyone measure the distance between the "lip" (lowest edge) of the MXL 2001 headbasket, and the centers of the mounting holes for the pcb's?

Comparatively measuring, on the 1:1 scale pictures, I would estimate approximately 30mm

 

[micolas]

PS. Quick question - can anyone measure the distance between the "lip" (lowest edge) of the MXL 2001 headbasket, and the centers of the mounting holes for the pcb's?

I struggled through the boxes to find a Hansen microphone, a brand from a Nordic country, OEM twin brother to the MXL2001.
The distance is 32.6 mm.

 

[Khron]

The distance is 32.6 mm.

Thanks!

 

[Blue Jinn]

Reviving this old thread with an aggregation of info that could help others with any interest in the MXL 2001 as a decent platform for minimal mods (some circuitry changes and capsule) or conversion to OPIC/OPA and use with soliloqueen (Arienne Audio) flat 47. Difference in price points can be drastic depending on the mods, so that's why it helps to read around and mull over things.

That said, I've found you, @Blue Jinn, in about three different places asking this question and posting about a lack of success, but I don't know if you missed Dave Thomas' (Advanced Audio Microphones) reply to you on Tape Op some time after that? Would like other's opinions too, whom I've seen post about MXL 2001 (and others) mods (@Matador, @Gus, @Khron, @ricardo, @mjmajilton @MicMaven, @SparkleBear, @micolas ) similar to this topic, specifically regarding capsule and doing things affordably. This may also benefit @Arbolito based on some questions over in another thread regarding MXL 990 modding with Schoeps circuit.

I'll post Dave Thomas' reply below regarding the U87 front end mod for reference, and ask the questions: what do you guys think of his recommendation, and what other transformer (perhaps something from @Moby) and coupling capacitor choices would you suggest?

@soliloqueen 87 capsule also seems like the most logical capsule choice if one is after that sort of sound. But would the flat 47 also work well in this kind of circuit, or is that utimately best suited for a flat circuit like @rogs OPIC, JLI OPA Alice, etc.?

- - - -

Dave Thomas:

"Hi Guys, this is completely unecessary!!!!

The circuit in the 2001 is just like the original AKG 414eb which has at least 14db more headroom the poor U87.

I have some 2.25:1 dual bobbin transformer with bi-metal laminations that we use that are worth $45 each to replace the economy single winding transformer in the stock 2001.

This and replacing R7 with the 5K pot plus jumpering R6 will allow you to optimize the fet bias.

Do not remove the output transistor as it is dropping the ouput impedance so that a lower ratio transformer can be used.

I also increase the value of the output coupling capacitor from 1ufd to 16ufd.

You will also notice by looking at the original 2001 circuit that the output transistor can swing more voltage that the fet which is supplied with a lower voltage.


This makes much better sense from a designer's point of view rather than trying to copy the venerable old U87 circuit.

The difference in signal loss between a 10.5:1 and a 2.24:1 is 14db. This means when you decrease the gain in the fet stage by 14db then you have 14db more headroom.

The lower impedance output transistor dampens the Back EMF in the transformer much more quickly than driving the transformer from the fet.

So once the 2001 circuit is optimized it is superior to the old single stage fet circuit in the U87.

The AKG 414eb would handle higher SPL than the U87 in my experience recording everything from Punk to Jazz in the 70's and 80's.


I also find with the 6 micron skinned 35mm K67 type capsule based on the work of Verner Ruvald (retired Neumann Physicist) and John Peluso can be used without the de-emphasis circuit when coupled with the 2 stage 2001/414eb circuit and produce a response nearly indistiquisable from a U87 in blind fold listening tests.

Cheers, Dave Thomas
www.aamicrophones.com"

Hi,

Yes I did see Dave's answer in TapeOp. I actually have transformers from him to do the Apex 460 (HST-11) modifications, just haven't got round to finishing that project.

I'd like to try that sometime, I just don't have another 2001 laying around.

 

[svyet]

10:1 transformer, remove the PNP emitter follower, possibly tweak the HF feedback cap, optionally replace capsule?

Going further in, add a DC/DC converter board & pattern switch, to fully utilise the new capsule (most likely dual-diaphragm)?

My favorite, cheap mod:
Remove C2 and R4, connect the RED Capsule directly to the jFET gate. The phase changes, so you will reverse the ends of one of the windings of the transformer (either primary or secondary) or reverse XLR2 with XLR3
Remove R6, put it to GND (R6=0 ohm)
replace R7 with a 15..25 kohm semi-adjustable resistor that will bias the jFET)
Adjust C1=33...56nF for bass to taste
Eliminates: R15, R16, C13, C14.
Add a small DC/DC converter (powered from the connection point between R11 and R12) that delivers +63...65V to the capsule through R17. Because it will consume current from the Phantom, the voltage on the audio side will decrease, so we reduce the resistance R18 to 3.5...3.9 kohm, so that the voltages on the collector, base and emitter of the BJT are equal to the stock ones, before the mod.
Increase (if necessary) C2 , to taste, to further attenuate the high frequencies.
If you are not satisfied, you will change the stock k67 capsule with a better quality one.
Do not remove the inner mesh from the headbasket.

It will become another microphone:
Self-noise will decrease substantially
It will increase the output level of the microphone and you will get an earlier saturation of the transformer.

Or make it a U87 as Khron suggests. (You will need a more expensive transformer)

Thank you both for these suggestions. I like the idea of cheap and characterful as I'm not necessarily looking to replicate the U87 sound. However, that may change as I continue mulling over things. Thank you again!