MXL 2001 "U87" front end mod question/help

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Blue Jinn

Well-known member
Joined
May 28, 2009
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406
Location
USA Northwest
Well,

Decided to do this for fun, changed the capacitor between the two 1 g resistors to 470pf, removed the emitter follower, changed the coupling cap to 1uF, and swapped transformer. Signal is very very low, and there is a bit of hum and noise. Noise I think is partially dirty fingers etc, but I'm not sure about hte hum and hte low sensitivity. I didn't use the Cinemag though, I used the KCM transformers I have which i used for the Royer. The published specs are 11.5:1, which is one turn higher then the published spec for the Cinemag, (EDIT) which itself is one turn higher then the AMI T13, so 11.5:1 vs 9.5:1. That doesn't seem an extreme difference, or does it? I'm tempted to swap out the transformer for the Cinemag, (or maybe the 8.2:1 Edcor (????) and see if that makes a difference. As always, very much open to suggestions.

EDIT: I also swapped the stock 32mm capsule for a 34mm K67 type capsule. I replaced two capacitors and removed everything after the  FET,  coupling the FET via a 1uF/100v poly to the 11.5:1 transformer.  HOwever, I did not attempt to replace the FET or re-bias the FET.  (Something I'm reluctant to do just yet, until I have a 'scope and setup a better bench environment.)

This was a late night glazy eye project, so I haven't cleaned it up at all, which I'm sure will help the noise level.  I would expect some loss of gain with teh higher ratio transformer and the single amplifier stage, but the mic is basically unusable at the moment from and S/N standpoint.
 

Attachments

  • U87 FET Frontend.pdf
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Did you possibly get solder flux on the high impedance side of the circuit? That can cause all kinds of noise problems (clean the solder side of the PCB to remove dirt, oil, and solder flux). Also moisture/humidity at the capsule can cause low output (keep in a dry, dust-free place for a few days to a week). There is some info on the vintage U-87 build thread about how to "bias" the FET by ear. That may help optimize gain to some degree. Good luck in you troubleshooting journey.

-James-
 
Thanks, I knew the board was pretty dirty (came that way, and my manipulations weren't exactly "sterile" to say the least. I did clean both sides of the board w/ 99% isopropyl, which did help with the noise and hum. It still has a distinct lack of gain (slightly worse than a dynamic) and with a k67 clone capsule, a decidedly unbright sound. I think I'm going to pick up some 2n3819 from Mouser, and pop that in and also a lower ratio (and better fitting trafo) and see if that helps. I may just go for the "close enough" on the FET bias, I could probably squeeze a 5k pot in there and just wing it on voltage if I can remember how (or find notes) on how to measure that. Matadors Aurycle mic thread also talks to this, and I think he did the "U87" front end w/ the cinemag trafo. The Edcor is intriguing at lower turns (also has a center tap) but is 5-6 weeks (according to the website) made to order. Although about US$20 cheaper too.  Worst case, I have an excess of Royer mod PCB's and one I'm also currently putting together. I have a 2006 on the block for a mod too, just waiting and waiting, been waiting (aaaaaargh) for parts ...

More to follow....
 
The vintage U-87 build thread has a lot of info about biasing FET's with a scope, by ear, and with some sort of software, I believe. It also has plenty of useful info for your particular endevour (like styrene caps don't like alcohol or excess heat, etc.). Check it out.

-James-
 
You description makes little sense even to someone who is familiar with U87.  Can you post the original as well as your modified circuit WITH component values?

Was it working OK before your mods?
 
Thanks.

The mic was working ok before I started to mess with it. I followed the pdf file in the top post, except I changed C2 to a 470pF instead of leaving it as 1000pF based on recommendations in another thread. I have some J305 FETs coming in the mail. Also, R8 is about twice as large in the U87 schematic, so wondering about raising that. I'm assuming that raises the output impedance somewhat.

The original schematic is attached.

Except for C2 and the transformer, I did not change any component values.
 

Attachments

  • mxl2001-xaudio-redraw.JPG
    mxl2001-xaudio-redraw.JPG
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I don't understand.  I assume you don't have a real U87.  Also they don't use 2sk170 so the pdf isn't a real U87.  Where is this pdf from?  Was it ever tested? ie working?

So which schematic represents your original working circuit and which one your present non-working circuit.  Measure & put measured voltages on the present circuit.  Also mark the components you changed.

To trouble shoot an untried mod with several changes, you change each (one by one) back to the working original until the circuit works again.

Did you use a polystyrene for C2?  These are unreliable and microphonic in this critical position.  NPO/COG ceramics are best here.
 
Thanks, I do not have a real U87. I do not recall where I obtained the first schematic. The top schematic represents the present circuit, except for C2, which is 470pF instead of 1000pF, and T1 which is a generic 11.5:1 transformer. Otherwise the component values are all the same as on the original circuit.

However, I'm measuring 24v at the drain, and about 3v at the source, so something is likely amiss. (I don't have a stock 2001 to compare to at the moment)

I'm going to replace the FET as soon as some new ones arrive, and then double check my work and then make adjustments.

 
Update: I noticed I had left R6 and R7 in place, where the mod schematic calls for a ~3k resistor alone, with the 1g and capacitor all sharing a common ground at that point. So I removed R6 and replaced with a jumper (only pathway to ground on the pcb) and changed R7 to a 5k pot, set initially at 3k. At 3k, got 24v. Not sure if that is in the ballpark for a 2sk170, but now it sounds even worse... the sound actually is weak, distorted, and ripply, if description makes sense.

Damn, I thought this was going to be an easy one... Now however, I am pretty sure the circuit looks like the pdf file.
 
Looking at this further (and reviewing some other threads, including the "zapnspark" direct connect) why do I even need C2 if this is a cardioid only microphone? Why can't I connect the capsule directly to R5 and Q1? If I'm reading the schematic correctly, the polarizing voltage is applied to the backplane of the capsule. That would at least eliminate C2 as a problem source. (I'd be deviating from my "ersatz U87" build, but from a troubleshooting perspective seems worth a try.)
 
OK  :-[ :-[ :-[ :-[ I found my error. I had the coupling capacitor C8 installed at the wrong side of R8. I must have followed the traces wrong at some point. Moved the input side of C8 and viola, works now. Needs some tweaking, and I'm still tempted to bypass the capsule coupling capacitor, the mic isn't nearly as hot as some of the other MXL condensers I have. Going to put a 34mm capsule back in. (I've got to stop working on these things at midnight....)

but I certainly feel silly now....
 
I put in a pot from the source to ground and adjusted for 10v on the drain, and replaced the 32mm capsule with a 34mm. (4 micron per the manufacurer.)
 
I know this is old thread now just wondered how you got on Blue Jinn..... I have been messing with very similar circuit its working realy good learning lots too...
 
gary o said:
I know this is old thread now just wondered how you got on Blue Jinn..... I have been messing with very similar circuit its working realy good learning lots too...

I wasted a lot of time troubleshooting and hsould have double checked my wiring...Although, I have a 1956 magazine article and same thing the authors spent a whole bunch of time tearing apart the circuit, and then swapped a tube, and sure enough, one of hte tubes was bad...

It works great, and I like the way it sounds, although I'm sure the way it sounds is influenced by "I modded it, therefore it is an improvement..."

I would like to try the direct connect also, as indicated above. I did that with the 2006 I modded, actually that *was the mod* (and capsule swap.)

 
Cool beans Im glad your enjoying it ..... I think we all suffer from the I modded it syndrome ha ....I made very similar ( think its exactly the same) circuit but from scratch copied back end of U87 & capsule connection of MXL, I only just found your old thread & schematic you posted loving the sound although lots to do with the capsule.
 
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"
 
Last edited:
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"
I respect Dave's point of view, but I don't want an additional 14dB headroom, crystal sound, (for that I use TLM103 or Rode), I want harmonics, transformer saturation and overdriven FET (among semiconductors, the FET has the best sound close to the tubes), character, distortion, that vibe, even when I sing intimately, almost in a whisper.
Dave has a business to run, to make money.
I'm interested in intimate interaction with my microphone, to be an emotional extension of myself.
More important than the technical specification of the microphone is
psychoacoustic, the artistic aspect.
It's just my opinion.
 
I respect Dave's point of view, but I don't want an additional 14dB headroom, crystal sound, (for that I use TLM103 or Rode), I want harmonics, transformer saturation and overdriven FET (among semiconductors, the FET has the best sound close to the tubes), character, distortion, that vibe, even when I sing intimately, almost in a whisper.
Dave has a business to run, to make money.
I'm interested in intimate interaction with my microphone, to be an emotional extension of myself.
More important than the technical specification of the microphone is
psychoacoustic, the artistic aspect.
It's just my opinion.
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.
 
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)?
 

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