Measurement mic/preamp questions

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omba

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Joined 2003
http://www.linkwitzlab.com/images/graphics/microph1.gif

You need a negative power supply to do this 2 wire connection.
Simply changing connection will not work as intended.

Fig.(a) shows how the power supply is connected to a normal mirophone.
Your microphone preamplifirer is probably configured like this,
power supply(+) resister (drain FET source) ground.

Looking at Fig.(c) the FET is connected like this,
gound (drain FET source) resister (-)power supply.
 
wintermute

wintermute

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omba said:
http://www.linkwitzlab.com/images/graphics/microph1.gif

You need a negative power supply to do this 2 wire connection.
Simply changing connection will not work as intended.

Fig.(a) shows how the power supply is connected to a normal mirophone.
Your microphone preamplifirer is probably configured like this,
power supply(+) resister (drain FET source) ground.

Looking at Fig.(c) the FET is connected like this,
gound (drain FET source) resister (-)power supply.
Click to expand...


Yes it was that diagram that got me wondering!! I also note that he uses a 9V battery to drive the mic, whereas the wallin preamp provides about 4V to the mic (I suspect this is to limit the output voltage to something most sound cards can handle).....

anyway I've found that I have to use excessively high volume levels to overload the mic even when not modded so I'm thinking I'll stick to unmodded for now :)

Also re-reading the linkwitz article, I'm guessing the original problem I was seeing with the modded capsule was because I didn't put the silicon on the back of the capsule.... I was seeing the freq response roll off starting at about 200Hz.... I think the break in the lead and resoldering was the biggest mistake I made though!!

Tony.
 
soongsc

soongsc

Member
Joined 2005
wintermute said:



Yes it was that diagram that got me wondering!! I also note that he uses a 9V battery to drive the mic, whereas the wallin preamp provides about 4V to the mic (I suspect this is to limit the output voltage to something most sound cards can handle).....


Tony.
Click to expand...


Could you actually measure 4V to the mic, I've done two boards and none of mine gives 4V to the mic.:confused:
 
wintermute

wintermute

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I made it on verro board... see here --> http://www.diyaudio.com/forums/showthread.php?postid=554438#post554438

I do seem to recall that I had a problem at one stage where the voltage was low, I had to add in a resistor to get the circuit going, here is an extract from an email I sent...

Finished preamp II this afternoon, and it didn't work
> :( used an
> ic experimenter board from jaycar, double checked everything but
> everything looked fine, voltage reference was out though.....
>
> I tracked down Jasons schematic and he had a 100K resistor
> between +9V and
> the LM4040 (I actually used a LT1009CZ because that was what
> I could get)
I checked your pcb layout and sure enough there was provision for this
> resistor.... thought I'd let you know so you can update the
> schematic, may
> save someone else some greif :)
>
> I added the resistor and the preamp works fine :)
Click to expand...

edit: This was an email to Eric Wallin, he responded and said the problem was probably that the LM4040 was having problems self starting. He also gave this alternative suggestion.

Another way to fix this is to remove R1, and install it in the position
of the resistor you had to install. That way the regulator would be
biased only from +9V, and would be guaranteed to work every time.

If you were to do that, however, you would probably want to recalculate
it to give the correct current to the regulator. Do this by finding the
current across R1 in the position it is in in the schematic:

I = V / R = (4.17 V - 2.5 V) / 15k = 0.1113 mA

Then find the resistor that gives this same current from 9V:

R = V / I = (9 V - 2.5 V) / 0.1113 mA = 58k

Thanks very much for the tip!
Click to expand...

as a quick fix try putting a 100K resistor from +9v to the LM4040 and see whether it improves things :)

Tony.
 
soongsc

soongsc

Member
Joined 2005
wintermute said:
I made it on verro board... see here --> http://www.diyaudio.com/forums/showthread.php?postid=554438#post554438

I do seem to recall that I had a problem at one stage where the voltage was low, I had to add in a resistor to get the circuit going, here is an extract from an email I sent...



edit: This was an email to Eric Wallin, he responded and said the problem was probably that the LM4040 was having problems self starting. He also gave this alternative suggestion.



as a quick fix try putting a 100K resistor from +9v to the LM4040 and see whether it improves things :)

Tony.
Click to expand...


While I was posting, I removed R1 and pulled a 15K to the 9V power, but I could only get about 0.9V at LM4040 and 1.5V at the phantom power output. Lowering the this 15K does not help. I was originally planning to get a bias current of aroun 1.6mA through the LM4040 by lowering the resistor to around 3.3K, still the same 0.9V at the LM4040.
:(
 
soongsc

soongsc

Member
Joined 2005
wintermute said:
I know that I looked at mine multiple times before I installed it :) and even then I was hesitant!

It's usually the simplest thing that fixes the problem :)

Hope you are now getting some more voltage!

Tony.
Click to expand...

Got the voltage right now with LM4040 hooked directly to the 9V power via resistor. Do you think I'll still have startup problems if I put R1 back where it was?
 
wintermute

wintermute

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Hard to know. If it is not to much trouble to desolder it and put it back the way it was, then try it. If it stops working put it back :)

I was using a different voltage reference chip to the LM4040 so it may have been the reason I had that problem, don't really know.... probably best to try it in the original config and if it works leave it that way :)

Tony.
 
Vikash

Vikash

Ex-Moderator
Joined 2003
vikash.info/audio/
I have tried it and (after resoldering the LM4040 pins correctly) there is correct voltage at the LM4040 and mic without changing anything else.

And just for reference in case anyone's wondering why Wallin did this:
Note that the supply for this reference is itself derrived from the actual regulated output of the op-amp, which increases the regulation effect. That really isn't necessary for the high-quality reference used, but hey, its essentially free.
Click to expand...
 
wintermute

wintermute

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Vikash said:
Note that the supply for this reference is itself derrived from the actual regulated output of the op-amp, which increases the regulation effect. That really isn't necessary for the high-quality reference used, but hey, its essentially free.
Click to expand...

When I was trying to debug mine that detail really messed with my head ;) I just couldn't work out how it could work, as you can see from the excerpt to my email to Eric, I thought he had left out the resistor (in jason neal's version) in his schematic :)

btw Vikash, did you base your kit on Jason or Erics pcb/schematic... did you do a new PCB layout?

Tony.
 
wintermute

wintermute

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