ELA M251 Inspired Microphone - Build Thread

[kimothebeatmaker]

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]

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.

 

[kimothebeatmaker]

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]

"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.

 

[kimothebeatmaker]

"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]

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)

 

[kimothebeatmaker]

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. 

 

[garp]

I have the exact same problem ... rumble is far but audible and if i shut down the power supply it simply disapear and for five seconds till power chute there is not a single noise. I tried a lot on the ground job with little success. Any idea ?
a few moments later ... problem solved !

 

[ripemedia]

I’m encountering a problem with the mic sounding thin on any setting but Omni. I would like some advice about where to look to fix this.

In Omni, both front and rear capsules sound great. As soon as I switch to any pattern other than Omni, the mic sounds thin/trebly and there is a significant drop in sensitivity. I swapped the 7 pin cable and that didn’t make a difference. I also tried a different PSU and the results were the same. I also swapped the tube and that didn’t fix it. So, it seems something has happened in the mic circuit or maybe the xlr connector.

I will go through some troubleshooting steps but it would be great if I knew which components are in the path that would affect this, then I could look specifically at those.

Thanks!

 

[dmp]

Capsule polarization. Do you have 60 vdc at the junction of R12, R13 and R14?

 

[Matador]

Was it fine before and suddenly broke?

 

[ripemedia]

Yes that's correct. I haven't used it in a while, and this issue wasn't present previously. But then over the weekend I was doing some recording and tried this mic and discovered the issue. I've checked the circuit of the mic and everything appears to be in order.

 

[ripemedia]

I took some voltage measurements and heater voltage was at 6.4v. This is higher then when I assembled the mic. I adjusted it down to 6.3v.

B+is is at 119v when switched to omni, but it drops as soon as I change the pattern. It was 107v when switched to figure 8. Heater voltage remained at 6.3v. Could the voltage drop be causing the loss of gain and thin sounding signal? Or, perhaps indicative of the underlying issue?

I checked some recordings I did with the mic prior to experiencing the issue and they sound fine. So, something has happened but I'm not sure what. I re-soldered some of the joints and that didn't make a difference.

 

[ripemedia]

Capsule polarization. Do you have 60 vdc at the junction of R12, R13 and R14?

Thanks for this, dmp. Sorry I missed it before. Yes this sounds like a good place to look. Here are my readings:

At the junction of R12, R13, R14

• 50v @ omni
• 46v @ cardioid
• 45v @ figure 8

At the plate

• 60v @ omni
• 49v @ cardioid
• 36v @ figure 8

This seems to indicate there is voltage drain somewhere, but I don't know. Would love some pointers on where to look to try to resolve this. Thanks!

 

[dmp]

Yes, there's definitely a problem. The voltage at R12,R13, and R14 should be 60v for all positions.
Is the voltage at R17, R12 correct (120v)? I'm guessing it is also dropping when you change positions, which it shouldn't.
Take a look at the polarity switch in the PSU. Are the resistors the correct values? Wiring mistake? Seems like the polarity switch is lugging down the B+

 

[ripemedia]

Is the voltage at R17, R12 correct (120v)? I'm guessing it is also dropping when you change positions, which it shouldn't.
Take a look at the polarity switch in the PSU. Are the resistors the correct values? Wiring mistake? Seems like the polarity switch is lugging down the B+

Yes. B+ is 119v when switched to omni, but it drops as soon as I change the pattern. It was 107v when switched to figure 8. I'll check the polarity switch when I can later today, but I did try a different PSU a couple days ago (one that works fine with a C12 style mic) and it was the same issue; although I did not take voltage measurements with it, so I'll try that again, too. But, I'm wondering, could anything in the mic circuit affect the voltage when the polarity changes?

 

[dmp]

Sure - if the polarity connection is not to R16 and C5 only, as it should be, it could be a problem.

For omni the polarity is sending 0v. For the other positions the polarity is 60-120v. If the polarity connection in the mic is taking some current, it could lug down the voltage as you are seeing. The polarity connection should see a very high impedance. If there is a path to ground (i.e. C5 is shorted) it will draw current and mess things up.

 

[ripemedia]

Thanks, dmp. I will check those things today. This gives me hope that the issue can be located and fixed. Cheers.

 

[ripemedia]

Sure - if the polarity connection is not to R16 and C5 only, as it should be, it could be a problem.

For omni the polarity is sending 0v. For the other positions the polarity is 60-120v. If the polarity connection in the mic is taking some current, it could lug down the voltage as you are seeing. The polarity connection should see a very high impedance. If there is a path to ground (i.e. C5 is shorted) it will draw current and mess things up.

Ok I tested the voltage from the PSU and it seems to be in-line with what's expected. And, I tested at the end of the cable and it was as expected; so, cable seems fine. But, with the mic plugged in, polarity at the mic is half what's being sent from the PSU. So something is definitely not right.

I'm checking the connections for polarity on the mic and I'm a little confused. I see that it connects directly to R16 and C11. I do not see C5 on the circuit, though. But I do see it in the schematic. Let me know if you can help me locate it on the mic.

Thanks!

 

[Matador]

Definitely there is an extra DC current drain which is dragging down B+, which in turn drags down everything else. The polarity voltage should *only* connect to R16, not C11. But a mis-wiring seems unlikely given it was working before.

I would first suspect C1 / C5. You can lift them out and re-test, because they are there for noise reasons: the mike will completely function without either of them.