Neumann Vintage U87 Clone : Build Thread.

[HellfireStudios]

You are feeding it 48volts correct??

I am using a standalone phantom power supply at 47 to 48 VDC.

Hellfire,
Can you set the drain to 11.5V and take another screenshot and post it?

Dave

I set the drain voltage to 11.5VDC by setting the positive probe of my DMM on the drain lead and the negative probe to ground. The screenshot is of a healthy signal before it starts to act funny (when more gain is added).

 

[HellfireStudios]

With these next screenshots, I took 8 successive shots while the signal went haywire.

 

[HellfireStudios]

It basically cycles between positive and negative clipping ina chaotic manner. Here is some negative clipping.

 

[kante1603]

It basically cycles between positive and negative clipping ina chaotic manner. Here is some negative clipping.

This cycling behavoiur could be something with your program you´re running on the iPad I think,maybe a sync issue.
Anyway,the waveforms the pics are showing are exactly what we´re looking for,meaning the sine flattens out.
Try adjusting the pot and see if the displayed waveform follows the pot movement.
You must work with the input level from the generator too,there´s an interaction between level,the resistance from the pot and on which side the sine flattens out.The goal is to get the flattening out on both sides at the same time and the same amount.
If you´re absolutely unsure about your fet I would replace it (just a few cents),this way you can shoot out if the cycling behaviour is related to the fet or the ipad/Scope program.
Some posts ago you wrote that you weren´t able to get a distorted on your scope-now you are!So there´s a little success in debugging your build,no?
You´re close to it!


Cheers,

Udo.

 

[HellfireStudios]

Well, not quite. The way it seems to be working is:

- I'll adjust levels until a stable sine wave appears.
- Then, I zoom in on the waveform to get a closer look.
- Following this success, I turn up the master output on the I/O Dock.
- The once-stable sine wave starts to fluctuate, and then clip wildy and erratically, until the volume is brought down, and a stable sine wave appears again.

Due to this erratic nature (the clipping seems to be at constantly changing points), I can't decypher when it is clipping, which side of the swing is clipping, or to what degree it clips. I wish I could show you guys video of this stuff.

As far as the iPad scope program goes, I was able to get a clean sine wave when plugging direct from the signal generator to the oscilliscope. The signal stayed clean until the I/O Dock's input clipped. Could my scope still be the problem?

Just to put my mind at ease, did I test the drain voltage correctly ( DMM + to drain, DMM - to ground)? Thanks again. One of these days, I'll get an amazing mic.

-James-

 

[mukole]

Hi James,

I totally undersand how impossible it is to perform this procedure when you can't see what your supposed to be seeing.

I think your definitely on the right track, I was looking over your first lot of pictures again and in the first pic (Direct Sine Wave) you can clearly see a 1 khz sinewave, the following picture (Sine thru u87 (low level)) is in fact showing a 50-60hz sinewave if you check the timescale along the bottom.
The 2 pictures after that show the 50-60hz sine mixed with your 1khz tone which seems unusally low in level but is what appears as "oscillation".

Comparing these to your second lot of pictures you are getting far more level now (97-100pa vs 4.4pa), I don't understand how the pa reading relates to mV as i use a stand alone oscilliscope which measures mV. But one thing i did notice is that the "pa" scale on the left varies over your last few pics. Are you manually zooming in and out or does the software also automatically zoom?

I think udo may have a point about you having a possible sync setting.
Do you have any option in your software to manually set the "trigger" or "trigger lock"?
If you do, then adjust this setting with your 1khz tone running into your scope input until you can clearly view the sinewave, then try rebiasing.

I 'm not familiar with your software at all and this is just a guess but maybe its possible that the trigger is losing its sync as you increase the signal, when i was biasing if i turned the signal down too far i would lose sync and see my waves scrolling past on the screen.

Another thing i would check is that your cable connections are good and solid, they can cause all sorts of erratic images on a scope if they are at all compromised.

I noticed you said you zoom in to get a closer look, have you tried not zooming in and just pushing your level up higher until you see the wave?
is it possible your not using enough level to cause viewable clipping?

Have you any way of listening to whats coming through the scope so you can get more idea whats going on?

I'm only guessing but i think there maybe some minor thing your overlooking.


Mick

 

[HellfireStudios]

I thought by "sync" you meant like A/D sync issue. It seems like it may be losing sync (the trigger threshold is set to 0.000 Pa) as I increase the volume. The wave begins to shift up and down a little. Then, it begins to shift out of control, clip wildy, and move accross the screen. The best way to describe the chaotic nature of the movement is to equate to Bugs Bunny, after he sees a monster, when he walks away muttering nonsensicals and generally freaking out. Spastic comes to mind. I'll give it another go soon.

I'm going to spend some time learning exactly what all those little menu options mean in SignalscopePro. Can anyone recommend a good place to learn the intricacies of the oscilliscope? Thanks.

 

[kante1603]

@ Hellfire,

the more I follow your posts the more I bet it's just your scope showing weird things.I also don't get these "pa" scalings same as mukole.As I don't know your program,can you say it is approved by the manufacturer that it is running stable on an alesis io dock?
I have read somewhere that it tends to do some nasty things when running different programs on it,for example randomly swapping inputs ans so on.Maybe it is really only a sync/trigger issue.
Can't you borrow a scope from a friend or so?

Just an idea,

Udo.

 

[HellfireStudios]

@ Hellfire,

the more I follow your posts the more I bet it's just your scope showing weird things.I also don't get these "pa" scalings same as mukole.As I don't know your program,can you say it is approved by the manufacturer that it is running stable on an alesis io dock?
I have read somewhere that it tends to do some nasty things when running different programs on it,for example randomly swapping inputs ans so on.Maybe it is really only a sync/trigger issue.
Can't you borrow a scope from a friend or so?

Just an idea,

Udo.

Unfortunately, I'm the only person I know who does this type of stuff around me. So no scope to borrow, that I know of.
On the other hand, Signalscope Pro is not a cheap program by the Appstore's standards ($75).  It has calibration controls that haven't been set beyond default. There are probably a lot of little things that need to be set appropriately before the scope will work correctly in my case.
FWIW, I use the Pa setting on the scope because it gave me a sine wave on the direct in test. There is also a FS setting, but I remembered seeing mV/Pa used for mic sensitivity, so I used it on the scope. I feel like I have a lot of reading to do before the scope will be my friend.
Which test is 2nd on list for effective biasing, and where amongst this plethora of information could I find the specifics? I'd like to test the mic audibly to make sure the electronics work before I go further with the scope. Thanks in advance. You have all been so helpful and quick with responses.

-James-

 

[kante1603]

Hi James,

I can not find a manual for signalscope pro,so it´s hard for me to help you.
At least you should try to change the y-axis to read mv.
"Pa" means "Pascal" and is relating to the sound pressure level through the air,"Pa/mv" is the result in Millivolts on the output of the measured microphone in relation to a defined acoustic signal source in a defined distance given a defined sound presure level and so forth.
We don´t need it for this measuring,just the amplitude on the y-axis in Millivolts and the time on the x-axis,that´s all.
Next I would check is the trigger modes,I´ve seen from the program´s description there are several options like these:

Auto, normal, and single-shot triggering
Trigger slope and threshold options

Just try these settings,it is nearly impossible to explain all the functions of a scope in a short way.
Swapping the settings on the fly doesn´t hurt,so it´s "trial and error-time" now,hahahaha........

Biasing should be done first,then give it a listen.You can try the mic for functionality of course,but depending how far the exact bias point is off to what it should be you might end up in pretty bad results.

Good luck,

Udo.

 

[Matador]

What kind of gain are you seeing?  Before it goes crazy, what are the input and output amplitudes?

 

[HellfireStudios]

I would have to perform the tests again to be sure, but it is just a tiny bit above the solid sine wave pic from the mic. I'm going to try using the I/O Dock as the signal generator, and run the oscilliscope program from the iPod Touch via my iRig "interface." We shall see if Alesis Dock is to blame for my strange readings. I'll report back ASAP.

 

[tskguy]

Hi all,

Doing a small run of capsules, thought you guys might enjoy a picture.

 

[HellfireStudios]

After trying to run the scope again last night, the readings were stranger and totally different. The sine wave looked "fuzzy" at normal levels and oscillated at higher levels. My points of contact were correct in this test, and it acted similarly to when I was doing it wrong earlier. If my scope doesn't give me consistent output, I'm afraid I will have to find some other way to bias it. Does any one know where in Central Florida I could find a scope? Is craigslist a feasible place to ask about using someone's scope to test some stuff? Would an electronics repair shop have a scope on hand I could rent? I'm so close to a completed mic I can taste it (electricity is tangy).

It can't hurt to put it out there. If anyone on this thread is in Florida, and has a scope, I'd be willing to drop a few dollars to "rent" it long enough to bias this mic and two preamps (Neve 1290 clones). PM me. Thanks.

 

[kante1603]

James,

Shops for electronics repairs like radio and tv certainly have scopes.
What just came to my mind was:why not ask a teacher at a lokal school,normally they have one for physics lessons!Would be perfect during the holidays,no?

Cheers,

Udo.

 

[HellfireStudios]

I know that the high school I went to had no scope to speak of (and the school was fairly well equipped), but it couldn't hurt to ask. If schools in your part of the world have scopes, it shows just how sad American/Florida education have become. I'll check around for a scope soon. Thanks again for all the help.

-James-

 

[Matador]

After trying to run the scope again last night, the readings were stranger and totally different. The sine wave looked "fuzzy" at normal levels and oscillated at higher levels. My points of contact were correct in this test, and it acted similarly to when I was doing it wrong earlier. If my scope doesn't give me consistent output, I'm afraid I will have to find some other way to bias it. Does any one know where in Central Florida I could find a scope? Is craigslist a feasible place to ask about using someone's scope to test some stuff? Would an electronics repair shop have a scope on hand I could rent? I'm so close to a completed mic I can taste it (electricity is tangy).

It can't hurt to put it out there. If anyone on this thread is in Florida, and has a scope, I'd be willing to drop a few dollars to "rent" it long enough to bias this mic and two preamps (Neve 1290 clones). PM me. Thanks.

It's possible the scope probes themselves are causing the problems, especially if they aren't high impedance.  Since you are looking at the drain, it's possible the scope may be putting hash into the ground connection which feeds back to the gate causing the oscillation.

I just purchased a Rigol DS1102E from Saelig for about $400, which is good to 100MHz.  If you plan on doing a lot of stuff like this in the future it's money well spent (along with a decent function generator that can do sine and square waves).

Aside from that, you should be seeing a gain of about 6-9 with reasonable input levels (say starting at about 100mV peak-to-peak).  After biasing, you should be able to put in nearly a volt peak-to-peak before you clip both sides of the waveform (assuming a normal gm FET, of course).  The output should be able to swing 7V or so peak-to-peak before you start pinching off the FET (again, assuming a FET in the middle of the specs).  Higher gm FETs will clip with less input signal, however will be "louder", all other things being equal.

And tskguy, stop showing those capsule pics!  I'm nearly to the point of replacing a laptop after all the drool in the keyboard! 

 

[wave]

For the record Matador, Eric's capsules definitely have a high drool factor!
I have 2 RK-87s and one of Eric's K87s that I have swapped through one mic. The RK-87s sound good and have a nice quality on the high mids that brings a lot of presence.
That being said, Eric's K87s have an incredible midrange response as well as a very nice hi end (we're talking in the 16K-20K range).
I have spent a lot of time recording different vocals through the capsules and the RK-87 is a great capsule, especially for the price range.
Eric's capsules are worth every extra $115. I have tried different combos of caps (styrenes and pulse films) and Eric's K87s sound great through either. I think if one wants to go the RK-87 route, using the styrenes is what I would do. I notice an openness and depth with the styrenes but it comes with a cost. They seem to be a bit noisier than their pulse film counterparts and as has been documented by myself and saxmonster, they are very sensitive when it comes to being installed. I had to replace the 2 470pF and the 10pF today as I think that I applied too much heat when I initially installed them. You really gotta be quick with the iron. I also changed the way I installed them from the orientation like the resistors to bending the leads down and in and orienting them horizontally.
When I first built the mic, it sounded great and in hindsignt, I think I would have left it alone. However, since I'm building them for other people now I wanted to fully educate myself as to what the sonic differences are with the different component combos.
The only thing I stillneed to try is the combos with a Cinemag. I have been doing all my experimentation with the AMI T-13.

Sorry this post was so long
DAve

 

[HellfireStudios]

Well, I went to my local TV repair shops today (all two of them), and was graciously turned away by people who seem to have less electrical knowledge and more oscilliscopes than I do (which is none). I don't know if was fear of "new" 1967's technology or my long hair that did me in with these backwoods types. Unfortunately, this leaves me without a cheap scope option yet again.
When/if I ever get this scope problem sorted out, I'll post pics of the finished mic (at least I know the outside looks pretty. Now, if only the mic sounded as pretty as it looks...).

-James-

 

[tskguy]

Thanks for the kind words guys! Interesting suff Dave, I am glad you figured out the issues with the styrenes.  I have to admit i am glad I went with the pulse films on my mic. ;D