MXL 2001 "U87 Front end mod"

[Blue Jinn]

I've been on a modding frenzy lately. This was relatively simple, and I modded it because it was there. The FET circuitry before the pnp section is pretty similar to the U87 amplifier section (no pattern, low cut, or pad), but has the feedback circuitry.

Anyway, did one of these, thought I'd share. Basically take out the pnp section, and replace the 2:1 transformer with a higher ratio.

Here's what I did:

Replaced the capsule with an OEM 34mm K67 style unidirectional capsule
Replaced C2 with a 470pF mylar
Remove R9, R10, and C6, and Q2 (labeled D2 on the board)
Replaced C8 with a 1uF poly one side connects to the new transformer the other to the junction of R8 and C6. One you remove parts. there a plenty of places to solder the transformer, and the new C8.
Replace R7 with a 5k pot and R6 with a jumper.
Replaced T1 with a 11.5:1 ratio. I happened to have a bunch of these. Probably a 10:1 would work better, e.g. an AMI T13 or a Cinemag 2480.
Adjusted R7 for 10vdc measured between drain and ground. I haven't measured the value of R7 now though, and left the pot in place.

Anyone tell me how to post pictures? Otherwise there are schematics for the 2001 floating around.

[Brett Siler]

Upload the pictures on your computer, then upload the to flicker and post the links on here and use the little "Img" html thingies to have them appear on this thread.

So, how did it sound after the mods?

[Blue Jinn]

Here are some in progress. It doesn't show the trimmer installed or R6 removed.

http://www.flickr.com/photos/87779587@N ... 028007004/

I don't have another 2001 at the moment to compare to, but I did compare it to a stock MXL 2006. I think the transformer is a bit too high a ratio, as the mic is a little less sensitive. However, the sounds is "fuller" and less "bright" sounding than the 2006. That isn't a fair test, as the circuits are different.

"Mellower?" if that makes sense. I will put it through some more tests and post back. I do have a brief "this is a microphone" spoken voice before, and I'll do a spoken word A/B to compare.

The transformers I got from ebay, although a 10:1 would probably work better. (and fit better too)

An upgraded FET might also help too and biasing with a scope (which I don't have.) I biased the FET strictly on voltage, which should be close enough.

I'll also shoot a final picture of the board and post it to that flickr site. Thanks, I didn't realize it was so easy to setup a flickr account....

[aamicrophones]

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

I've been on a modding frenzy lately. This was relatively simple, and I modded it because it was there. The FET circuitry before the pnp section is pretty similar to the U87 amplifier section (no pattern, low cut, or pad), but has the feedback circuitry.

Anyway, did one of these, thought I'd share. Basically take out the pnp section, and replace the 2:1 transformer with a higher ratio.

Here's what I did:

Replaced the capsule with an OEM 34mm K67 style unidirectional capsule
Replaced C2 with a 470pF mylar
Remove R9, R10, and C6, and Q2 (labeled D2 on the board)
Replaced C8 with a 1uF poly one side connects to the new transformer the other to the junction of R8 and C6. One you remove parts. there a plenty of places to solder the transformer, and the new C8.
Replace R7 with a 5k pot and R6 with a jumper.
Replaced T1 with a 11.5:1 ratio. I happened to have a bunch of these. Probably a 10:1 would work better, e.g. an AMI T13 or a Cinemag 2480.
Adjusted R7 for 10vdc measured between drain and ground. I haven't measured the value of R7 now though, and left the pot in place.

Anyone tell me how to post pictures? Otherwise there are schematics for the 2001 floating around.

[David_Kessler]

Not trying to completely hijack the thread but...

Dave do you have any mods to make the Studio Projects C1 a smoother more usable mic?

[aamicrophones]

Hi David, the C1 is a cardiod only with a transformerless schoeps type discrete output circuit.

Here are the specs:-

Pressure gradient solid state condenser microphone
Cardioid

14mV/Pa=-37dB(0dB=1V/Pa)
<200>1000 Ohm
131 dB SPL for 1% THD @1000Hz (0dB SPL=0.00002Pa)


Noise: 27dB (A weighted)---17dB S/N 77 dB
PAD: -10dB or -20dB
Low Cut:- 6dB/Octave at 75Hz or 150Hz
48 +/- 4V
<2.5mA
Gold-plated 3-pin XLR
JFET impedance converter, discrete balanced output

It has a economy 32mm K67 type capsule skinned with 3-4 micron mylar.

This results in a 4-6db rise in the 3-5khz range and early LF roll-off.

I find that the C1 works best with something like our 6 micron skinned AK47 capsule.

This is an identical reproduction of the original K47 capsule.

It has at least 1/2 an octave more low end before roll out over the stock capsule and it has only a 2-3db rise max in the 3-5khz range.

Since the C1 does not have a class "A" transformer coupled to "soften" the sibilance then the "darker" response AK47 seems to be a better choice for this circuit.

Your further option is to replace the circuit with a transformer coupled class "A" circuit like our CM47fet or CM87 but by the time you add up the cost of an Ak47 ($95), transformer ($45) new circuit board plus your time it might be better to sell off the C1 for $100 and buy our 2-pattern CM47fet for an extra $189.

However, with a capsule like our AK47 the microphone becomes quite useable in most recording situations. The AK47 is a double sided Large Diaphragm Capsule so if you install a SPST switch and pull the rear capsule to ground then you will have an OMNI/CARDIOD option.

I much prefer OMNI on acoustic guitars and other stringed instruments.

Cheers, Dave
www.aamicrophones.com

Not trying to completely hijack the thread but...

Dave do you have any mods to make the Studio Projects C1 a smoother more usable mic?