geekmacdaddy

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1020 on: August 08, 2012, 05:18:27 PM »
+1 For Geekmacdaddy,  i specially like the place when we hear " Its Hotter Now"  8)

Good Job ,  thanks For you nice Picture,

Dany,
Best,

Thanks to you, my solder sniffing brother, for all of your hard, brilliant, wonderful work. The feeling of accomplishment in orchestra with beating the system by about 2,800.00 USD is quite envigorating. I'm all over the '67 when it's ready. 2 probably. Sniff on!


geekmacdaddy

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1021 on: August 08, 2012, 07:44:50 PM »
Went back in, Swapped out the 220pf (C6) for a Wima 100pf, and hopefully biased at 13.7 volts and here's a clip

https://dl.dropbox.com/u/87086401/Clone1stTweak.aif


wave

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1023 on: August 09, 2012, 12:52:43 AM »
Geek,
You made the right move. It's totally sounding better and I don't hear any more distortion. What do you think?

Dave

geekmacdaddy

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1024 on: August 09, 2012, 07:45:54 AM »
I did think I heard something towards the end...back down to 10v....I needed to do this to match the real U87

« Last Edit: August 09, 2012, 08:25:00 AM by geekmacdaddy »

tskguy

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1025 on: August 09, 2012, 08:23:04 AM »
That's strange, I really don't hear the bump in the 1k that you do.
I also liked the way your tweak sounded better than the original its smoother. And as for biasing  the scope its really the best way to be sure!

sr1200

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1026 on: August 09, 2012, 10:26:40 AM »
Hmmmm i have some 100p wima's layin around.... suggested mod?  I like the way your clip sounded.
MEI Studio - Long Island, NY: http://www.meirecords.com

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1027 on: August 09, 2012, 11:25:36 AM »
Hi Guys,
Ok so I have not used my Mic in a bit and when I took it out today I noticed that the capsule was leaning to the left quite a bit because of the way it was in the case and how the case was stored. 

My question is what kind of mounts are you guys using in the aurycle body?  Obviously this plastic mount that comes with it bend easily and doesn't hold its form.

Would I be able to get one of Peluso's mounts in there?

What are you guys using?

-Scott


wave

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1028 on: August 09, 2012, 03:02:28 PM »
Hi Scott,
I'm using the stock Aurycle mount but I noticed mine wasn't molded correctly. One half of the mold was a bit higher than the other half so I took my Dremel to it with a sanding drum. After I sanded it flush, the mount holds the capsule correctly.

Dave

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1029 on: August 10, 2012, 01:15:06 AM »
Can a noob ask for a little troubleshooting help? I got the mic all put together but I'm just getting a faint sound. I have to turn up the gain all the way before I hear anything. And then it's just handling noise. Or I can tap on the head basket and hear the tapping sound. I checked all of the voltages before I hooked up the trafo and capsule and everything looked great. I had trouble biasing the Fet since I had never done it before but after several tries I finally got even clipping on the scope. So now I don't know where to take it. Any ideas?


sr1200

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1030 on: August 10, 2012, 11:20:21 AM »
im gonna guess the capsule is not wired to the board correctly.  I did this the first time too with the same results :)
MEI Studio - Long Island, NY: http://www.meirecords.com

HellfireStudios

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1031 on: August 10, 2012, 01:01:52 PM »
Can a noob ask for a little troubleshooting help? I got the mic all put together but I'm just getting a faint sound. I have to turn up the gain all the way before I hear anything. And then it's just handling noise. Or I can tap on the head basket and hear the tapping sound. I checked all of the voltages before I hooked up the trafo and capsule and everything looked great. I had trouble biasing the Fet since I had never done it before but after several tries I finally got even clipping on the scope. So now I don't know where to take it. Any ideas?

What capsule are you using? Does it have the isolated dual backplates?
"Tommorrow is just yesterday in reverse. Just like friends are enemies in reverse." -Gary Busey

Matador

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1032 on: August 10, 2012, 03:51:14 PM »
Here is some data I took on a few FET's.  The plan is that I'll mock up the front end of this circuit so we can see how these parameters play into the operation of the circuit.

I started by breadboarding up a simple test rig:



You first dial in the pot (a multi-turn precision type works best for this so you can dial in voltages accurately) to show 0V on the voltmeter.  This should happen roughly half way through the range of travel.  You can then connect the JFET and read the IDSS on the ammeter directly.  Note this value down.

Now we want to figure out VP:  dial the pot so that the voltage becomes negative.  As you make the voltage more and more negative, you'll see the current drop as the device becomes more and more pinched off.  What you want to do is lower the voltage until the current drops to 10% of it's IDSS value.  Note this negative VGS value down as well.  Repeat these two measurements for all the devices you want to measure.  Here are results for the first three JFET's I had in my stash:



Now VP is difficult to measure exactly, as it's defined as the gate-source bias that results in zero drain-source current.  It's difficult to measure no current, but we can use the characteristic equation of the JFET to get very close:

ID = IDSS * [1 - VGS/VP]2

If you solve for VP, you get:

VP = VGS/(1 - SQRT(ID/IDSS))

So you have all the above information from your measurements, which results in the following data for these three devices:

Code: [Select]
IDSS ID VGS VP
-----------------------------------------------------
10.79 1.079 -3 -4.387425887
13.51 1.351 -3.41 -4.987040758
9.33 0.933 -2.45 -3.583064474

This way of calculating VP is not perfect but I've found it gets pretty close.  In general, IDSS and VP track together very well, meaning that the higher the current, the more negative the pinch-off voltage.

So I'm going to pick through 20 or so 2N3819's and 20 or so 2SK170BL's, and find one of each at the high and low ranges of these parameters, then I'll construct the front end of the circuit so we can see what the gain and input ranges look like.

As a primer:  gm (or transconductance) is defined as the delta change in ID with a delta change in VGS.  Mathematically, this is just the derivative of the ID equation above with respect to VGS.  The result is this equation for gm:

gm = | 2 * IDSS/VP * [1 - VGS/VP]

The first interesting result is that gm is a linear function of VGS (and the other parameters as well, which are generally constants).  People tend to think of transconductance as a constant but it isn't in this case!  It also means that the transconductance is maximum the closer VGS is to zero.  These equations can be found in any standard text on amplifier circuits BTW.

For the U87, the basic gain equation for the front end (assuming bypassed source resistor) is:

AV = - gm * RD

So gain is maximized when gm is maximized, which means we should keep VGS as high as possible and operate the JFET as close to IDSS as possible.

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1033 on: August 10, 2012, 04:16:45 PM »
Hey Mike , this is awesome ,  i have to say i do like the Math and derivative, 
can't wait to see Gm versus Gain and go back in the lab with this ,
Thanks For taking the times to reveals such a wonderfull door as this one ,  8)
Best ,
Dan,

wave

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1034 on: August 10, 2012, 06:48:04 PM »
I agree with Dany that this is awesome. Honestly though it's a bit beyond me. Is there a simple explanation of what we want to
look for? That's some complicated math for me  :)

Thanks for doing this! Hopefully the penny will drop for me at some point.

Dave

tskguy

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1035 on: August 10, 2012, 06:58:20 PM »
ummm.....what number makes it sound better  :o


Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1036 on: August 10, 2012, 08:23:16 PM »
Can a noob ask for a little troubleshooting help? I got the mic all put together but I'm just getting a faint sound. I have to turn up the gain all the way before I hear anything. And then it's just handling noise. Or I can tap on the head basket and hear the tapping sound. I checked all of the voltages before I hooked up the trafo and capsule and everything looked great. I had trouble biasing the Fet since I had never done it before but after several tries I finally got even clipping on the scope. So now I don't know where to take it. Any ideas?

What capsule are you using? Does it have the isolated dual backplates?


I'm using Dave Thomas' AK67 capsule

tskguy

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1037 on: August 10, 2012, 08:51:50 PM »
If your using a 67 style capsule it will only work in this mic as cardiod, How do you have it wired?

Eric

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1038 on: August 10, 2012, 09:31:42 PM »
If your using a 67 style capsule it will only work in this mic as cardiod, How do you have it wired?

Eric


Well I actually thought I could wire it as a multi pattern so that's where my stupidity lies... I added a fourth wire and wired it like the multi pattern should've been. Capsule wires in the appropriate places and the body wires in the appropriate places but reversed for the figure 8 pattern. How would I wire it for cardioid only?

Marcocet

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #1039 on: August 10, 2012, 10:35:44 PM »
Thanks Matador! Extremely well put. I can't wait to see what your results come to.


 

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