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Author Topic: White Noise from Lewitt 840  (Read 1540 times)

ahenry75

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Re: White Noise from Lewitt 840
« Reply #15 on: July 20, 2023, 03:25:10 PM »

OK, I wouldn’t call this a typical “tube mic”, but a mic “with tube”.
The high count of components would make drawing of a schematic a high effort.
You would also need to identify the 16-pin IC (probably a logic IC) and other unlabeled components.

If you are familiar with guitar amps I’d suggest another approach:
Let’s check the amp by feeding a signal of ca. 50-100 mV through a 50pF cap.

Then see if it comes out correctly.
Spec’ed 23 mV/Pa let’s expect about 6 dB of gain.

If it does, the amp is OK and the capsule is the culprit.

Thank you Kai. 

I am honestly not sure how to do that, and you have been more than generous with your time here, so unless it is very easy to guide me through that, I think it may be best to declare the mic dead and move on.  I yelled at Lewitt for their refusal to share a schematic.  They didn't give in, but did give me a discount code for a new mic, hahahaha.   Oh well.  Thanks to you and @Klaus.  Really appreciate the help from you both.

-Andy
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Kai

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Re: White Noise from Lewitt 840
« Reply #16 on: July 22, 2023, 09:48:23 AM »

Thank you Kai. 

I am honestly not sure how to do that, and you have been more than generous with your time here, so unless it is very easy to guide me through that, I think it may be best to declare the mic dead and move on.  I yelled at Lewitt for their refusal to share a schematic.  They didn't give in, but did give me a discount code for a new mic, hahahaha.   Oh well.  Thanks to you and @Klaus.  Really appreciate the help from you both.

-Andy
Really easy, no special equipment needed:

• Locate the amp input wire from the capsule, desolder it at the amp side.
• Find a capacitor, ca. 50pF 60V minimum rating, solder it to the input.
   Don‘t use more solder that necessary, to avoid contamination with flux at this sensible point.
• Solder a shielded cable to the other side of the cap, shield connected to any ground connection of the mic.
• Now you have a wired input you can play all types of signal into, music, sine waves from a generator (e.g. smartphone app) etc.
• Plug the mic in and listen or measure the output result.

Should be close in level and sound to what you put in.
This way you can determine if the amp is broken or not.
Typically the max. undistorted level a mic amp can handle would be somewhere about 200mV, proportionally more with the pads engaged.


If the amp is broken and the capsule is fine, part out the amp and built your own, simplified tube amp, based on basic schematics from the net, e.g. a U47 variant consists of very few parts.

You already have the tube, socket and transformer, and important, PSU, cable and housing .
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ahenry75

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Re: White Noise from Lewitt 840
« Reply #17 on: July 22, 2023, 01:52:29 PM »

Really easy, no special equipment needed:

• Locate the amp input wire from the capsule, desolder it at the amp side.
• Find a cap ca. 50pF 60V, solder it to the input.
Don‘t use extra solder to avoid contamination with flux at this sensible point.
• Solder a screened cable to the other side of the cap, screen to mic ground.
• Now you have a wired input you can play all types of signal into, music, sine waves from a generator (e.g. smartphone app) etc.
• Plug the mic in and listen or measure the output result.
Should be close in level and sound to what you put in.
This way you can determine if the amp is broken or not.
Typically the max. undistorted level a mic amp can handle would be somewhere about 200mV, proportionally more with the pads engaged.


If the amp is broken and the capsule is fine, part out the amp and built your own, simplified tube amp, based on the the basic schematics from the net, e.g. an U47 variant consists from very few parts.
The tube, socket and transformer, and important, PSU, cable and housing you already have.

Thank you Kai, I will try this in the next few days and see what I can figure out.  Silly question, I assume the amp input wire(s) to be the one(s) from the center termination of each side of the capsule, is that correct?  I am sure I can figure out the rest of all of the fairly easily.  Thank you!
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Kai

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Re: White Noise from Lewitt 840
« Reply #18 on: July 22, 2023, 04:01:55 PM »

... I assume the amp input wire(s) to be the one(s) from the center termination of each side of the capsule, is that correct?
Likely, but not necessarily.

Depending on the biasing scheme even the backplate can be the capsule output.
Dual-diaphragm mics with selectable pattern use this quite often, as this arrangement needs the least amount of parts, if the capsule capacitances have a doubled purpose as DC bias blocking caps.

You have to test this.
Remove the capsule, connect the mic and touch each capsule connection point with an isolated screwdriver - at least one should give sound.
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