U47 Clone Distorts Easily

[granger.frederic]

misha ,
i don't want to reproduce a "war thread" with you again...
i'll tell you that kindly !
you have to learn more about electronic tube applications
if you want  , i can PM you some good books ...

thanks for the EF12 datasheet (i have it since 20 years..) , it tells exactly what i said : Ri=10K with 100V at anode...
but Ri at 75V is way higher.
A tech has already measured the EF12 output impedance for us :  http://www.moxtone.com/mU47_U47_part3.html

 

[Phrazemaster]

OK I've  made a few tests according to everyone's suggestions.

First of all, Fred, you said maybe the transformer was shorted due to overheating or underwinding. I measured the resistance for pirmary/secondary, and came up with: 733R : 17.6R. Very close to the datasheet. So I don't think it's shorted. I do trust this ratio transformer is fine simply because Andreas suggested it as a working design, and others here have had good success with it.

Pip, to the DC on the primary: nada, zilch, zippo. We're cool in that regard.



Fred you suggested I pump a 1K sine wave into the grid and measure the output, if I understood you correctly. I did that, at 1V RMS. Output across XLR pins 1-3 was 1.6V; across XLR pins 1-2 it was 1.3V. Not sure why the differential; I would have thought they would have been the same. Not sure how to interpret the numbers either.

O3m, I played with the bias point, paralleling a variety of resistors across the 15K top voltage divider resistor, and found that at -2.23V bias it sounded the least distorted and the best.

But

I've worked with many microphones, and this still has a weird buzzing sound that is unpleasant, and noticeable. 2 other singers commented on it, liking my "other" mic better (M49 clone). Indeed that mic takes much higher SPL's and it sounds gorgeous.

So I'm not sure what to do from here now. Andreas thought I was simply overloading the mic pre. He was right; it does go into clipping on my preamp before it clips in my DAW, but a lot. So I turned down the gain on my premap. Voila, no distortion.

HOWEVER

The sound is dull, lifeless! Not what I would expect from a clone of the "world's greatest microphone."

I will probably redo the rest of the resistors, and also sub in a new resistor for the 15K to get bias to -2.23V, which is an equivalent of 14K. Not too crazy, and and easy fix too. But it's not sounding like a $10K mic, or even a 2K mic.

Exasperated. I may have to send this to someone, unless you gents have other good suggestions. I am replacing the capsule - getting it reskinned, since I biffed the back side. Ever so possibly it's the capsule, so I'll know about that in a few weeks after getting that going (only sent an email to Thiersche, so it' gonna be a while).

I'm exhausted today and not sure I'm up to subbing the capsule as Matt suggested; I may try. Just dunno of I wanna solder anymore.

Thanks everyone; I'll keep you guys informed as to progress/lack thereof, and thanks for any more thoughts. At least I think we can rule out the transformer (?).

Best,

Mike

Edit: I checked. I do have a cheapie chinese clone capsule, but it's too big to fit the M7 holder. Not even gonna go there.

Hey, anyone in the Southern CA area feel like hanging out and helping a buddy fix up some flubbs?

Thanks guys,

Mike

 

[o3misha]

Fred, moxton is selling PSUs? Period.  We discussed this article with AG : people there were measuring the weather in Africa, not what you need for U47-sh mic building.  You're basing your argument on amateurish article( your link), which does not hold water. I trust Andreas more. I'm sorry. Please argue with someone else. For example : with A.Grosser or G.Wagner or with experts from Flea  They all dropout    is that what you want to say?  All wrong- you are not?  Thank you for touching care that you showed to me and my knowledge in each topic where I appear. Ok: EF14 is the best lamp in the world, buddy. Satisfied? Period

I hope, our topicstarter will solve his problem, and I hope he will report here what was that. I am sure that it is not wrong transformer, but lets see...
best
ps : Phrazemaster, could you upload clean voice as link to mp3 or wave file, please? I need to analize the signal more carefully. Thank you.
Pps: I used -2.3-2.4 bias- it sounds more interesting and soft for me.  Feeding 125-130VDC alows to record louder signals without distortion. 60 VDC polarisation gives less sensitivity but you can work with louder sources. 0.5uF instead of 1uF gives less low freq-s, but more "live" sound in the middle and high freq-s. Thats my 50cents.

 

[Phrazemaster]

Hi O3m

I'm happy to post more; I just left the studio for the day but I will do so in the next few days when I go back to the studio again.

I really appreciate your time and thoughts.

Best to you,

Mike

 

[Phrazemaster]

Also, O3m, thanks for the detailed real-world information on changing the voltage to 125-130v and the other suggestions. Invaluable!!!

Thanks!!!

I'll report back here when I have more to say. Stay tuned!

Mike

 

[granger.frederic]

voltage across both output transformer secondary coils should be the same !
the ratio you measured is 3:1 !
i think you have a defective (and mismatched) transformer !
you'll get advantage to put a 10:1 or more with an EF12, but if Mr G and misha say it works ....
my opinion : with a higher ratio the mic will sound better...
i suggest an AMI T49 or equivalent.

http://www.tab-funkenwerk.com/id53.html

cheers
Fred

PS: Important : i see that Mr G U47 schematics are floating around, so be careful at the diode across the lm7906 !On mine, it is inverted and produce a short circuit like.VERY DANGEROUS.You could have a firing PSU quite quickly !

 

[o3misha]

Neumann BV12 has ratio 6.9:1.

 

[granger.frederic]

i've forgot Mike :
when you measure static resistance of a transformer , always demagnetize it after, with the process i've told before...
try the mic with another transformer
even a 6:1 should work quite normally with your design, but this tube will perform optimally around 8-10:1 in a U47 original design
remember that the EF12 gain and Ri is higher !
i've built several mics U47 with EF12 and it appears to me that 110v HT/Ra= 100kohms/60v pol capsule/H=-6v/bias -1.3v and a transformer 10:1 , was optimal...
0.5 or 1uF sounds the same in the mids if the both caps are in the same family, only change: -3db 30hz or 15hz

 

[Phrazemaster]

Thank-you both Fred and O3m. I appreciate the detailed information.

Fred, did I measure this correctly? Injecting 1K sine at 1V RMS into a 75pF capacitor in place of the capsule, then measuring across output XLR Pins 1-2 and 1-3? I just wonder if I did it wrong?

Thanks much.

Mike

 

[granger.frederic]

don't need 75Pf ...???
inject 1V 1Khz pure sinus  in the input of the transformer and measure between hot and cold (+ and - ) XLR or between the output wires of the transformer , with a 1k load at the output.
VAC RMS of course
if you have 0,3V , there's something wrong
If you have 0,15V , then it's OK
you have to check on the scope, at what input voltage the transformer saturates ...
report back asap 

 

[Phrazemaster]

Thanks Fred for the detailed description. I'll see what I can do. It's very odd; I've built mics before, and I'm fairly certain I've built this correctly although this is my first PTP mic.

I have also contacted Andreas about it, and after a few back/forths it's agreed he's going to take a look at it. In the process  convert it to using his VF14ER tube and swap the PSU. I'm excited about it! Although, it pretty much kills figuring this out!

But before sending it off to him I'm going to do the test you suggested Fred; could well be a defective transformer or perhaps in some way I did damage it, although I don't think so.

The mic is so dead quiet, however - I did star ground just about every connection within the mic, with a dedicated run back to a star ground in the bell of the mic...sigh...happy and sad at the same time here.

Anyway I'll keep you guys updated and try to do the test you suggested Fred, so we'll at least know something more.

Thanks many thanks to all who have contributed, and I'll keep you all updated on what happens.

 

[o3misha]

Wow! Congats!
Guy, don't even hesitate - send the MIC to Andreas. I am convinced that Andreas will bring your MIC to perfection. What about his VF14er lamp : it is a stunning lamp. The microphone with VF14 er better than with most of  VF14s,  in my subjective opinion. This lamp has a large overload capacity and incredibly dense sound in the low middle. And very musical, juicy tops.

 

[Phrazemaster]

I know I know I'm so excited man!

And bless his heart what he is asking is beyond fair, bordering on charity. Except shipping is lunacy - $224 one way!!!

He's obviously got a soft spot for DIYers or he's just incredibly nice or both...but we're lucky to have him around! Such a pleasure to talk to him.

Not too many true gentlemen left in the world, present company excluded of course!

Thanks man; I'll keep you posted!

 

[Phrazemaster]

Actually he told me he has a VF14EF which is FET based and a VF14ER which is tube based.

He said 80% of his customers use the FET but his US customers like to have a tube so he sells the ER version as well.

Do you have any thoughts between the 2?

I told him to pick the one that sounds most like a real VF14.

Thx!

 

[o3misha]

Yes, postage costs are pain in the ***. But you will never regret, trust me. I know of no better master who knows more about microphones in Europe. From Russia ( I am) it is cheaper to fly to Germany, which I did. Inform us about your mic.
I, personally, have both Vf14EF( it is working in my real old Neumann U47 at the moment) and VF14ER.  Vf14EF is closer to VF14M ( same k2,  k3 distortion, dynamic, tone , especially in low freq-s). VF14 EF can work in mic 50000 hours vs. 5000 VF14 or EF12. VF14EF is a champion of overload capacity- you can record drums or guitar cabs. But the jucy highs of VF14ER are killing me on vocals,- its a real tube. I think, you should start with VF14er( I started with VF14ER), but you should choose your self. Its hard choice...
P.S: Yes, Andreas is awesome, responsive and ...trusts people because he believes that most people initially decent ( a true gentleman's belief). I ask everyone who comes into contact with AG to be honest with him, because such people should be protected.
Best

 

[Phrazemaster]

Such beautiful words O3m. Thanks. Agreed. We should protect kindness, innocence, and trust from others when we try it. And you are a kind gentleman too; thanks!

I think I will go with the EF, but also ask him if I can buy an ER as well to be able to swap out in the future. It sounds like he'll go for it.

I will certainly keep you all informed. This was an unexpected turn of events!! He's even going to fix my capsule gaffe.

Thanks,

Mike  ;D

 

[Phrazemaster]

Hey all, just updating. I sent the mic off to Germany today...cheapest shipping? Guess...

I did try to do the test that Fred suggested prior to shipping it. Put a 1K 1V sine wave across the transformer primary. I just got weird results. Measuring XLR Pins 1-2 or 1-3 I kept getting all kinds of different readings. I'm not sure if it's because Andreas' power supply has a capacitor across ground, or if my connections weren't tight enough. But I got everything from 8V from pins 1-3 to 120mV between the pins.

So, inconclusive, and I wasn't about to start desoldering to make a proper better test. Sorry.

Will update this thread with any further news.

Thanks,

Mike

 

[granger.frederic]

isolate the transformer from the rest
inject 1VAC primary
measure around 0,14VAC secondary (ratio 7/1)

 

[tmuikku]

Shouldn't you measure between xlr pins 2-3? pin 1 should be cable shield.

 

[Phrazemaster]

Yes I did try measuring between pins 2-3 as well. I just got weird readings.