[poctop]

Just tought i giving some figures regarding the bias of my new pair of U1621, suprisingly or not  the Iddle current sit exactely at the same level for both mic after bias procedure ,   

Mic A  Drain Voltage 10.3V   source Voltage 2.77V    iddle current trough R11 12.2K =  0.226 ma

Mic B  Drain Voltage 10.1V   source voltage 1.86V    iddle current trough R11  8.2K   =   0.226ma ,

results from this JFET below


cheers,
Dan,


 

[Blue Jinn]

Maybe this is obvious, but I've not seen this in the thread. For a single backplane dual diaphragm K67 style capsule (at some point I'll save up my pennies for a Peluso, or maybe look at the RK87 when it comes out, but I can only live so far outside my means...) is the backplane connected on the PCB to front body, rear body, or both? Both doesn't seem to make sense, and tracing the schematic it doesn't seem to matter, but I thought I'd ask anyway.

I'm tentatively planning on putting this in a MXL 2010 which has all the switches, and I have a two sided 34mm 67 style capsule on order.

[poctop]

Maybe this is obvious, but I've not seen this in the thread. For a single backplane dual diaphragm K67 style capsule (at some point I'll save up my pennies for a Peluso, or maybe look at the RK87 when it comes out, but I can only live so far outside my means...) is the backplane connected on the PCB to front body, rear body, or both? Both doesn't seem to make sense, and tracing the schematic it doesn't seem to matter, but I thought I'd ask anyway.

I'm tentatively planning on putting this in a MXL 2010 which has all the switches, and I have a two sided 34mm 67 style capsule on order.

Do you means using a K67 ,only on one side for cardiod pattern ?

[poctop]

Maybe this is obvious, but I've not seen this in the thread. For a single backplane dual diaphragm K67 style capsule (at some point I'll save up my pennies for a Peluso, or maybe look at the RK87 when it comes out, but I can only live so far outside my means...) is the backplane connected on the PCB to front body, rear body, or both? Both doesn't seem to make sense, and tracing the schematic it doesn't seem to matter, but I thought I'd ask anyway.

I'm tentatively planning on putting this in a MXL 2010 which has all the switches, and I have a two sided 34mm 67 style capsule on order.

Do you means using a K67 ,only on one side for cardiod pattern ?

if it is the case then connect the backplate to section FC pad (bdy)  and then pad (cap) to the diaphragme, you can leave the other diaphrgame wire unconnected,

hope this helps,
Dan,

[Blue Jinn]

Do you means using a K67 ,only on one side for cardiod pattern ?

No, I mean for a two sided capsule, (omni/card/8) but with a single backplane (three wires only, not four like the Peluso) I get the front/back connection, but which 'body' pad does the single backplane connect to front, back or both? or does it matter? Or do you *have* to use a dual backplane capsule?

[poctop]

Do you means using a K67 ,only on one side for cardiod pattern ?

No, I mean for a two sided capsule, (omni/card/8) but with a single backplane (three wires only, not four like the Peluso) I get the front/back connection, but which 'body' pad does the single backplane connect to front, back or both? or does it matter? Or do you *have* to use a dual backplane capsule?

you actually need a double-isolated backplate like the original circuit K87 Style  ,   the K67 will work fine in cardiod mode and you would connect it to the FC section,
D

[kante1603]

@Blue Jinn
Hi,excuse me for chiming in.
Maybe you are swapping this build with the U87A,in which a capsule with a single backplate does work.
This is the circuit of the U87 which is the predecessor of the U87A.
Looking at the schemo you can see why it must be a dual backplate when looking arround the switch arrangement.
Neumann decided to go back to the single backplate design (like in the U67) in the mid 80s since the dual design wasn't a real improvement,both K67 and K87 are nearly identical soundwise.Also they changed the supply voltage filtering as well as implementing a dc converter giving a higher polarisation voltage;this way they achieved a slightly better s/n ratio and a higher output level at some plus 10 to 11 dBu (after the math).

So in this (vintage) circuit a dual backplate is a must-have.Only if one decided to just use the cardiod pattern the K67 capsule will work.The back diaphragm wire can be left isolated then.In case of the front diaphragm ever gets damaged you can use the left-over back diaphragm as a spare part-cool.

Hope to have helped,

Udo.

[poctop]

@abechap your inbox is full so cant response 

D

[Blue Jinn]

Thanks Dany and Udo for explaining this. Mmmh. Looks like I'll need a Peluso at some point.

[kante1603]

Thanks Dany and Udo for explaining this. Mmmh. Looks like I'll need a Peluso at some point.

Well,don't have my T13s yet but I do have my Peluso capsules,hehehe.....

Udo.

[HellfireStudios]

Did that situation, in which the switches were acting as antennae, ever resolve itself, or do I need to take some precaution against it? I believe sheilded wire between the PCB and switches was suggested. Could this help? Thanks.

[berkleystudios]

i cant bias without a transformer can i? boards are stuffed and waiting on the RK87 and CM2480 to come in. looking for some thing to do ha. i have the tranny from the a460. could i bias with this and then swap to the cm2480? or would i need to re bias?

[poctop]

Did that situation, in which the switches were acting as antennae, ever resolve itself, or do I need to take some precaution against it? I believe sheilded wire between the PCB and switches was suggested. Could this help? Thanks.

I have built 2 Aurycle body and i did wire the switch directly to the pads and it work fantastic, i did not even bother to twist the wire, 

So yes ,please Keep us posted on this issue, 

Dan,

[poctop]

i cant bias without a transformer can i? boards are stuffed and waiting on the RK87 and CM2480 to come in. looking for some thing to do ha. i have the tranny from the a460. could i bias with this and then swap to the cm2480? or would i need to re bias?

you sure can start to bias if you would like ,  but first thing that i would do is to measure my initial voltage the Zener reg and the 2K2, and backplate
in the other hand, i have good reason to believe that the stock transfomer is not compatible with this project. some says it is a 2:1 traffo but it has not been confirmed yet ,  it is more suited i think in the PNP follower topology circuit from the stock circuit.

but this does not prevent you from trying it and see what kind of output i has , still would be nice to evaluate the traffo dB loss to find out a bit more,

Hope this helps,

[berkleystudios]

my drain does not go above 7.3v? what am i doing wrong?

[kante1603]

Did that situation, in which the switches were acting as antennae, ever resolve itself, or do I need to take some precaution against it? I believe sheilded wire between the PCB and switches was suggested. Could this help? Thanks.

Hi,
This happened to one guy and was related to a B2 mic.
He wrote that he'll take care later,meanwhile he's using the jumpers.
I allready posted possible solutions,please read the thead.

Udo.

[kante1603]

my drain does not go above 7.3v? what am i doing wrong?

Hi,

This is not enough information.
Did you do Dan's initial voltage check?
Have you checked the IDSS of your fet?
Did you swap it?
Are you sure you measured right?

Etc.,etc,..........

More details please,

Udo.

[poctop]

my drain does not go above 7.3v? what am i doing wrong?

I know Xqtion had the same issue here ,

I'm running into one biasing problem though. The highest I can get the drain volt is 7.43V.
And potentiometer resistance is measuring about 26.5k.

Please help?

Does it feel the same way ,   Perhaps Xqtion can shed some light here ?

[xqtion]

Yeah I had the same issue but I couldn't figure out what the problem was.

I just ordered one more pcb to try to build a new one from scratch but didn't get to it yet.

Well if you ever solve the issue please let me know I thought I was going crazy trying to figure it out 

[poctop]

  Attention to all completed project    your are very welcome to post in this thread to let us know how you are making out, 

Regards,

Dan,

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