Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3200 on: June 21, 2020, 01:50:31 PM »
Does he have the bodies back in stock with the graphics for the switch patterns and such?

Yes, I got the [budget YT5][https://store.studio939.com/product/budget-yt-body-kit], which has the switch graphics, but I don't think the nice brass one is in stock yet.  The graphics are easy to print on waterslide decal paper, in a pinch.


Ricardus

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3201 on: June 21, 2020, 09:33:20 PM »
Yes, I got the [budget YT5][https://store.studio939.com/product/budget-yt-body-kit], which has the switch graphics, but I don't think the nice brass one is in stock yet.  The graphics are easy to print on waterslide decal paper, in a pinch.

Cool, thanks.

Yeah, I wanted the nicer brass ones when I did my build, so I ended up having to go elsewhere.
Audio mastering for hire..

Matador

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #3202 on: August 08, 2020, 03:58:25 PM »
Hey Matadaor! I hope this isn't a silly question, but how does one go about calculating Vgs at 1% Idss

I can do the Idss measurement no problem, but am not sure I understand the Vgs at 1%. 

Any help is much appreciated, thank you!

Best
J
IDSS is the saturation current when the potential between the gate and source are the same (e.g., both grounded).  We typically use 9V or 10V on the drain to measure this.  Let's say for example on a particular JFET this is 10mA.

VGS @1% IDSS means "What gate-source voltage causes the drain current to equal until it's 1% of IDSS".  So 1% of 10mA is 0.1mA, or 100uA.  You lower the gate voltage (down from ground, keeping the source grounded) until the drain current falls to 0.1mA.  This gate-source voltage measured when the drain current is 0.1mA is VGS @1% IDSS.  It's considered "reasonably close" to the VGS(off) for that JFET.


Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #3203 on: August 08, 2020, 04:31:49 PM »
IDSS is the saturation current when the potential between the gate and source are the same (e.g., both grounded).  We typically use 9V or 10V on the drain to measure this.  Let's say for example on a particular JFET this is 10mA.

VGS @1% IDSS means "What gate-source voltage causes the drain current to equal until it's 1% of IDSS".  So 1% of 10mA is 0.1mA, or 100uA.  You lower the gate voltage (down from ground, keeping the source grounded) until the drain current falls to 0.1mA.  This gate-source voltage measured when the drain current is 0.1mA is VGS @1% IDSS.  It's considered "reasonably close" to the VGS(off) for that JFET.

I think I'm beginning to understand, thank you.  But, how do I lower the gate voltage down from ground while keeping the source grounded?  When I measure the Idss, the instructions are to short the gate and source and then connect to the neg terminal of the battery.  Should I remove the short?  What is the connection scheme and do I use a adjustable resistor to lower the gate voltage?

Thanks and sorry for my lack of understanding! I've tried to figure it out on my own but it seems to be beyond my grasp of the concepts.

Matador

Re: Neumann U87 Vintage Circuit Build Thread.
« Reply #3204 on: August 10, 2020, 04:17:34 PM »
But, how do I lower the gate voltage down from ground while keeping the source grounded? 
You need to make it negative with respect to the source.  You can do this with a pot across another 9V battery (connect the second 9V battery + to the source (aka. 'ground')), and then put a 100K pot across the battery, and feed the gate from the middle of the pot.  This will let you dial between 0V and -9V on the gate.

Or, you can just put a meter, in voltage mode, between the source and 'ground', and you can use the high impedance of the meter to measure very close to VGS(off).  The meter is 'V' in the diagram below.



If the meter impedance is 10M, then the source will float up to whatever VGS flows approximately 9V/10M = 0.9uA, which is very tiny, and very close to VGS(off) (remember VGS(off) is the voltage at which no current flows, and 0.9uA is approximately equal to 'zero' current). :)
« Last Edit: August 10, 2020, 08:29:58 PM by Matador »

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3205 on: August 18, 2020, 07:04:37 PM »
Does anyone have any thoughts on Vishay vs TE resistors? It looks like TE offers some metal film resistors with .1% tolerances.  If there isn't difference in quality, the .1% tolerance for 2.2k resistors would make it pretty easy to match those resistors (i.e. no matching necessary).

Thanks!

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3206 on: August 18, 2020, 09:06:19 PM »
I've always considered Vishay to make very good sounding, very high quality resistors. I've never used or even heard of TE connectivity, which doesn't mean they're bad.

Ricardus

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3207 on: August 19, 2020, 01:05:02 AM »
I've used plenty of TE Connectivity resistors. They're good quality.
Audio mastering for hire..

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3208 on: August 26, 2020, 03:09:47 PM »
I have 2 lots of 20 2n3819s i bought from Mouser (Central Semi) and eBay (Fairchild... possibly),  the  Central Semis all measure between 16 and 19mA IDSS when  testing using a 9v battery and multimeter set to mA, and the eBay 'Fairchild' fets all measure between 13 and 16mA (marginally better but still nowhere near the numbers people have talked about in this thread....).

I tried the mic with one of the lowest IDSS values from the  fets from mouser and it seemed to work, although the gain required to get much volume out of it was rather a lot, and the lowest value from the ebay fets also seems to work, however when i popped a few of the fets into my transistor tester both reported as actually being jfets but the values reported by it were wildly different between the two sets of transistors - i have attached a picture of the transistors and the values the tester shows - could someone give me some insight into why the values are so different? Im assuming it might be because the 'Fairchild' fets are perhaps not what they are labelled as!

As far as the gain is concerned to get a usable level out of the mic at a regular speaking volume - i tried with two interfaces, a focusrite 2i2 - this needed the gain turned up full and still comes out low, and a Audient ID44 which gets about the same level at 3/4 way round the gain knob, does this seem right?


Ricardus

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3209 on: August 26, 2020, 05:03:11 PM »
I have 2 lots of 20 2n3819s i bought from Mouser (Central Semi) and eBay (Fairchild... possibly),  the  Central Semis all measure between 16 and 19mA IDSS when  testing using a 9v battery and multimeter set to mA, and the eBay 'Fairchild' fets all measure between 13 and 16mA (marginally better but still nowhere near the numbers people have talked about in this thread....).

 Those numbers seem incredibly high. I bought some really questionable 3819's from a supplier and these were the worst I had ever seen, but were still below those numbers. Check to make sure you're doing that test properly.

 I was thinking about buying some of the ones from Central Semi when Mouser had them in stock a few months ago (edit, looks like they have almost 9k in stock right now), but didn't. But I did buy some from On Semi last year, and while they don't test as well as my vintage Motorolas or Fairchilds (the Fairchilds regularly had IDSS's in the 2.3 to 2.9 range), they all were under 8 or 9 and would have been usable in this build (from what I have read).
« Last Edit: August 26, 2020, 05:18:14 PM by Ricardus »
Audio mastering for hire..


Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3210 on: August 26, 2020, 05:40:58 PM »
Thanks Ricardus,

Pretty sure i'm measuring right! I used this technique - https://io.groupdiy.com/file/gdiy-2017/u/39511/58d1402a02141.jpg

Ive attached a picture with a random pick from each batch, the fairchild version appears to be pin order SGD rather than DGS, hence why the left picture is pin 1 & 2 together, and the right is pin 2 & 3 together

Tried with a brand new battery as well, not much change in values!

Ricardus

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3211 on: August 26, 2020, 08:37:59 PM »
That does look correct. No idea.
Audio mastering for hire..

TillM

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3212 on: August 27, 2020, 12:58:37 AM »
Of course you can use FETs with higher rates.

Ricardus

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3213 on: August 27, 2020, 04:11:07 PM »
This thread has said you usually want an IDSS under 10, but I don't understand the theory.
Audio mastering for hire..

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3214 on: August 27, 2020, 05:11:14 PM »
I thought lower idss meant less gain, but less noise and more headroom, and a higher idss was the reverse of that, unless ive misunderstood somewhere

i still think even with the high idss fets i have the mic seems to be outputting way lower than what i think it ought to

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3215 on: September 03, 2020, 12:14:40 AM »
I'm new to the diy scene and i have two questions,

can the 330k ohm resistor (R13) be substitute for this:
https://www.mouser.com/ProductDetail/594-5043ED330K0F

i dont want to wait for them to come back in stock at mouser

and

does anyone know where i can get just one 63v 10uF cap (C12)
 i dont really want  to have to order 50 from mouser

Ricardus

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3216 on: September 03, 2020, 01:54:38 AM »
I'm new to the diy scene and i have two questions,

can the 330k ohm resistor (R13) be substitute for this:
https://www.mouser.com/ProductDetail/594-5043ED330K0F

i dont want to wait for them to come back in stock at mouser

and

does anyone know where i can get just one 63v 10uF cap (C12)
 i dont really want  to have to order 50 from mouser

 That resistor is fine, and try this capacitor:

https://www.mouser.com/ProductDetail/Vishay-BC-Components/MAL203038109E3?qs=4T%252B5yYZyZYee5Zk%252BsfMHmw%3D%3D
Audio mastering for hire..

Matador

Re: Neumann Vintage U87 Clone : Build Thread.
« Reply #3217 on: September 12, 2020, 02:38:41 PM »
This thread has said you usually want an IDSS under 10, but I don't understand the theory.
All other things being equal, gain will be maximized when the quiescent current is close to IDSS.  Since the U87 operating point is about 0.7mA, you want the lowest IDSS that comes close to this value (if maximizing gain is your aim).  There is of course a tradeoff between gain and input signal swing, so you need to choose between the two depending on how you'll use the circuit.


 

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