[HellfireStudios]

Here is a the direct Sine wave into the iPad.

[HellfireStudios]

Here is the Sine wave sent thru the mic into the iPad at minimum volume on the I/O Dock.

[HellfireStudios]

Here it starts to oscillate as the volume increases a little.

[HellfireStudios]

Finally, here is the full on oscillation at any healthy volume level. BTW, the signal doesn't seem to clip. It just gets louder.

[mukole]

I think that looks like 50 or 60 hz interference, i had the same thing when i put the high impedance side of the mic face down on the table,
it would lessen if i picked it up, and dissappeared when i put the mic body on

Finally, here is the full on oscillation at any healthy volume level. BTW, the signal doesn't seem to clip. It just gets louder.

[mukole]

Hi Hellfire,
Your voltages on r8 and r13 are much lower than mine,
what are your voltages on r15 and r16?

Have you double checked the 2 electrolytics  c11 and c13 are installed the right way around?

Also I'm guessing you may need to rotate your pot anticlockwise to increase the voltage depending which pins you
used on the pot.

cheers

top of L1 - 47.2v
top of L2 - 47.2v
top of R18 - 47.2v
top of R19 - 46.6v
bottom of R17 - 23.4v
top of R17 - 23.4v
Zener 23.4v
FET drain pad 6.7v
FET source pad 2.8v
top of R12 6.7v
top of R13 21.3v
top of R14 20.2v
top of R8 11.5v

Some of my readings don't seem to be right. Any suggestions as to what could be causing these unsettling problems?

[poctop]

Hi Hellfire,
Your voltages on r8 and r13 are much lower than mine,
what are your voltages on r15 and r16?

Have you double checked the 2 electrolytics  c11 and c13 are installed the right way around?

Also I'm guessing you may need to rotate your pot anticlockwise to increase the voltage depending which pins you
used on the pot.

cheers

top of L1 - 47.2v
top of L2 - 47.2v
top of R18 - 47.2v
top of R19 - 46.6v
bottom of R17 - 23.4v
top of R17 - 23.4v
Zener 23.4v
FET drain pad 6.7v
FET source pad 2.8v
top of R12 6.7v
top of R13 21.3v
top of R14 20.2v
top of R8 11.5v

Some of my readings don't seem to be right. Any suggestions as to what could be causing these unsettling problems?

FET drain pad 6.7v
 this seems to be a bit on the low side for sure i would expect it to be arround 11-13V after the bias,

DAny ,

[sr1200]

Hey i finally found out what happened to my mic!  It wasn't a miracle from the gods afterall... seems i had the pad switch wired backwards... when i recorded with it last, i must have inadvertently switched it by accident.   

[kante1603]

Hey i finally found out what happened to my mic!  It wasn't a miracle from the gods afterall... seems i had the pad switch wired backwards... when i recorded with it last, i must have inadvertently switched it by accident.   

Aaaaaaaaaahaaaaaaaaaahhhhh,
Cool you keep us posted!
Yes,it's easy to reverse switch connections on toggles,glad you got it sorted,

Rock on,

Udo

[HellfireStudios]

FET drain pad 6.7v
 this seems to be a bit on the low side for sure i would expect it to be arround 11-13V after the bias,

DAny ,
[/quote]

There was no biasing that could be done on the scope. As I said earlier, no matter how much signal I pumped into the mic, it would not clip. It would just start to look wobbly until the wobbles started to create their own wobbles (the last screenshot). So I'm nowhere near biased at this point.

Mukole, I checked all my electrolytics and tantalums. They are all oriented correctly. I even went as far as to recheck all my resistor/capacitor values to make sure I didn't make a stupid mistake. Cold solders are a thing of the past for me (I use silver-bearing solder to make my life a lot easier). None of my switches are hooked up. Just the transformer. Could my very low IDss FET be the culprit (3.8mA)? Thanks for your help so far.

[kante1603]

Hi Hellfire,

Even if you have other problems the biasing must be done accurate first.
Some questions to your procedure:
Is the capsule disconnected?
Where do you insert your test signal (sine)?
Is the transformer connected?
From which point do you pick up the signal to the scope?

Cheers,

Udo.

[mukole]

As i couldn't get anything useful on the scope i set the voltage on the drain to 10.5v as dany suggested to do all my testing. This was pretty close to the final bias settings anyway. It wasn't until i disconnected the primary connections to the transformer that the scope started showing me anything useful.
Have you tried scoping without the transformer connected?

None of my switches are hooked up. Just the transformer. Could my very low IDss FET be the culprit (3.8mA)? Thanks for your help so far.

[HellfireStudios]

Hi Hellfire,

Even if you have other problems the biasing must be done accurate first.
Some questions to your procedure:
Is the capsule disconnected?
Where do you insert your test signal (sine)?
Is the transformer connected?
From which point do you pick up the signal to the scope?

Cheers,

Udo.

The capsule is disconnected. I insert the positive of the sine wave at the top of R9 (which is in direct contact with RT). The negative goes to ground via the negative leg of C11. The transformer IS connected, and I use an XLR cable to hook it to my I/O Dock's mic input straight from the mic's XLR.

As i couldn't get anything useful on the scope i set the voltage on the drain to 10.5v as dany suggested to do all my testing. This was pretty close to the final bias settings anyway. It wasn't until i disconnected the primary connections to the transformer that the scope started showing me anything useful.
Have you tried scoping without the transformer connected?

None of my switches are hooked up. Just the transformer. Could my very low IDss FET be the culprit (3.8mA)? Thanks for your help so far.

I may do the biasing-by-voltage technique first to get the circuit close to where it should be. Then, retest my voltages.
When I do go back to the scope, where would I tap the signal from without the transformer connected? Also, if my scope is showing possible 60HZ hum, how can I tweak the pot with the mic's body in place?

[mukole]

You need to scope at the drain of the fet, once I disconnected the transformer I could see a much greater signal at the drain of the fet and the interference was no longer noticable on the scope.
I Hope this helps

[HellfireStudios]

You need to scope at the drain of the fet, once I disconnected the transformer I could see a much greater signal at the drain of the fet and the interference was no longer noticable on the scope.
I Hope this helps

I appreciate the tip. I'll try it tonight, and let you guys know if that does the trick. I don't know where I got the idea to hook the mic's output to the scope... Thanks again.

-James-

[kante1603]

James,

best to do this is the matador method.
What you have on your scope is a heavy oscillation,I ran in the same thing when I did it the first time-with a real scope!
We want to bias the fet,therefore it is best not to have too many surrounding parts here influencing the measuring.
Capsule and transformer should be disconnected first.
Next step is to insert the sine signal at the right point.For us it is not "rt" and "sw",it´s the junction between R6 and C4,the cap that is leading to the fet´s gate.
For picking up the signal for the scope you attach your probe to the fet´s drain connection.
Follow matador´s instructions (you find it in the other thread,first page).
It´s easy to understand and also alot of fun.
This should work now.

Hope to have helped,

cheers,

Udo

[Matador]

I cannot claim credit, I got the method from Zapnspark on micbuilders.

You should be able to bias the FET with the transformer connected.  But try to load the secondary with an appropriate resistance if you leave it in circuit.  A 1k resistor across pins 2 & 3 is ok to represent the inputs to a mike pre.

However you STILL always measure the drain AC coupled in order to do the biasing:  don't measure on the secondary side of the transformer.

[HellfireStudios]

Alright, I disconnected the primary side of the transformer, hooked a 1/4" guitar plug to two gator clips (+ hooked to "flipped" R6, Gnd hooked to ground), and ran a sine sine wave at 1000Hz through said cables. Then, I hooked another plug to my mic/guitar input (+ goes to drain of FET, Gnd goes to ground). This cable goes to the oscilliscope. I now get something resembling a sine wave, but I'm still unable to get any clipping from the mic no matter the level (when line outs are at maximum, the positve wave stays the same, while the negative side widens considerably). I can get the mic pre on the I/O Dock to clip, but that's it. Rotating the pot clockwise raises the amplitude, and counter-clockwise the opposite. No clipping regardless of the pot's setting, as well.

I may have inadvertently shorted some FET leads while phantom power was applied. Could this cause a fault?

[tskguy]

You are feeding it 48volts correct??

[wave]

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

Dave

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