Re: ELA M251 Inspired Microphone - Build Thread
« Reply #60 on: April 02, 2021, 02:50:53 PM »
Let me quote myself from the other C12 thread, regarding isolating noise problems in this circuit:

A noisy tube will be revealed when you get to step 4b, and if it's coming from contamination of the HiZ portions of the circuit you'll hear it in 4c.  If it's the capsule itself, you can substitute two 68pF caps for the capsule and see if the noise changes/goes away.

Hi matador maybe you can help me...

Im having some hum/buzz going on with my setup.  I tried your troubleshooting steps and they were helpful. 

With PSU, XLR3 going into the interface there is no noise.  Once the cable is connected, there is some buzz, im guessing from the cable picking up interference.  Once the mic is connected, there is a different hum/buzz, low, but still there.  Once i turn off the PSU, the buzz goes away, and the mic stays on for a few seconds and the signal is clean.  Im getting 120-125V / and 6.3v. 

The transformer is a Chinese 115v / 115v : 175v / 9.5v

So if the buzz goes away once the PSU is turned off...is it the PSU?  But the PSU is dead silent when I disconnect the mic...so im confused? 


Matador

Re: ELA M251 Inspired Microphone - Build Thread
« Reply #61 on: April 02, 2021, 05:20:07 PM »
So if the buzz goes away once the PSU is turned off...is it the PSU?  But the PSU is dead silent when I disconnect the mic...so im confused?
It doesn't mean it's the PSU:  by itself, the PSU just passes signal from the 7-pin to the 3-pin with two wires and nothing else, so with only the PSU connected I would expect no/little noise.

Can you go through the troubleshooting steps you referenced in my previous link?  Do you know the frequency of the noise?  You can record the noise, and use an FFT plugin to get a hint if you don't have a scope.

Re: ELA M251 Inspired Microphone - Build Thread
« Reply #62 on: April 02, 2021, 07:20:37 PM »
It doesn't mean it's the PSU:  by itself, the PSU just passes signal from the 7-pin to the 3-pin with two wires and nothing else, so with only the PSU connected I would expect no/little noise.

Can you go through the troubleshooting steps you referenced in my previous link?  Do you know the frequency of the noise?  You can record the noise, and use an FFT plugin to get a hint if you don't have a scope.

https://www.dropbox.com/s/fq3zh38912v1d2a/psu%20noise.wav?dl=0
Yes here is a link to the sound file.  It starts off by me turning on the PSU, then you hear the hum noise, and then I turn off the PSU, and you hear the hum noise disappear. 

1) First make sure you have a low impedance ground:  with everything powered off, check the resistance between ground on the PSU PCB and all ground points in the mike body, including all parts of the outside shell.  You should have less than 0.1 ohms everywhere in the mike

I did this, and I have .5 ohms on the mike body, sometimes even as high as 12 ohms on the outer metal shell, which is covered in some sort of shine. 

These steps I didn't understand how to measure the noise.  I have alligator clips, and I clipped one to the ground on the PSU pcb, but not sure where to put the other end.  What do you mean by 'output'? 
__________________________________________________________________________________
2)  Make a small length of wire with two small alligator clips, one on each end
3) Clip one end to ground on the PCB
4) Starting at the 'output' hard ground different points in the circuit:  then measure noise
a) the output of the coupling cap (where it enters the transformer) - it should be dead silent
b) the grid of the tube - this should only output the self noise of the tube
c) the capsule connection - this should only be the noise of the tube + HiZ components

Matador

Re: ELA M251 Inspired Microphone - Build Thread
« Reply #63 on: April 03, 2021, 01:01:31 AM »
"Output" means the other end of your wire that has the clip on each end.  Start at the point where the output cap connects to the transformer (4a), etc.

Re: ELA M251 Inspired Microphone - Build Thread
« Reply #64 on: April 03, 2021, 01:08:03 AM »
"Output" means the other end of your wire that has the clip on each end.  Start at the point where the output cap connects to the transformer (4a), etc.

Measure the noise with what?  An oscilloscope? 

1 end to ground on PCB (mic or psu)? 
The other end to output cap - transformer, on the mic pcb?  The erse cap? 

TLRT

Re: ELA M251 Inspired Microphone - Build Thread
« Reply #65 on: April 03, 2021, 05:16:36 PM »
Hi matador maybe you can help me...

Im having some hum/buzz going on with my setup.  I tried your troubleshooting steps and they were helpful. 

With PSU, XLR3 going into the interface there is no noise.  Once the cable is connected, there is some buzz, im guessing from the cable picking up interference.  Once the mic is connected, there is a different hum/buzz, low, but still there.  Once i turn off the PSU, the buzz goes away, and the mic stays on for a few seconds and the signal is clean.  Im getting 120-125V / and 6.3v. 

The transformer is a Chinese 115v / 115v : 175v / 9.5v

So if the buzz goes away once the PSU is turned off...is it the PSU?  But the PSU is dead silent when I disconnect the mic...so im confused?


Check if your XLR connectors are grounded to safety earth point or the PSU body(remove paint from PSU box where they connect, or ground the XLR body points)

Re: ELA M251 Inspired Microphone - Build Thread
« Reply #66 on: April 04, 2021, 08:22:47 PM »

Check if your XLR connectors are grounded to safety earth point or the PSU body(remove paint from PSU box where they connect, or ground the XLR body points)

They were wired to each other, 7pin ground to 3pin ground, then to the pcb ground.  I moved it from the pcb to the chassis bare metal, and it helped with the noise a little...but its still there.   


 

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