ELA M251 Inspired Microphone - Build Thread

[johnsonic]

Hello! Just built this mic and seems like the output is low - it measures about 8db less than the matachung c12 I built a few months back.

Here's what I've done so far:
- Confirmed all voltages stated on the schematic
- Swapped cables
- Used a different power supply
- Have a beesneez 13:1 torroidal installed, swapped for a cinemag 2048
- Confirmed the lower output happens in all patterns
- Swapped tubes

I see about 30V on the backplates, 14V on one of the capsules in cardiod if that helps. Polarity seems to be doing what it's supposed to.

Sounds great, no weird noise - just wicked quiet.

One difference between the mics is the capsule, the 251 has an OPR, the c12 has a beesneez. I'm under the assumption that they will both put out about as much signal - didn't want to dive into swapping capsules until I can verify there's nothing wrong with the circuit. Anything I should measure?

Oh - I also checked that there was no continuity between diaphragms and backplate.

Have pretty much A/B'd everything except the capsule. Anyone have any ideas?

 

[GeorgeToledo]

I’d double check the connector and make sure one side isn’t shorted on the mic with less output, and doublecheck that the output transformer is wired correctly.

 

[Matador]

I would confirm that the primary and secondary of the transformer is right.

 

[johnsonic]

Yup - high side is on the PCB, low side to XLR (for both transformers tested).

What voltages should I be getting to the backplate & membranes? 30V at the backplate seems low - shouldn't it be closer to 60?

 

[Delta Sigma]

Yup - high side is on the PCB, low side to XLR (for both transformers tested).

What voltages should I be getting to the backplate & membranes? 30V at the backplate seems low - shouldn't it be closer to 60?

Your DVM is loading the circuit. You cannot measure directly at the capsule with a normal DVM.

 

[johnsonic]

Got it. So I am stumped - if it were a couple db I'd understand it, but 8db seems excessive. I saw on the C12 thread that someone who built a 251 had the same issue, but unfortunately there was no resolution. Open to test anything for science and for future builders!

 

[Fairman]

I've experienced some C12 capsules were the backplate wire has no contact with backplate, cause by a too short screw. I use a capacity meter to check before installing. Typical a C12 is about 100 pF.

 

[johnsonic]

Thanks Fairman! Unfortunately, my cap tester doesn't go this low but can infer that this isn't the issue since all polar patterns work as expected, which would lead me to think the backplates are in the circuit.

I should mention that I did recheck all component values and everything seems to be in order. Also no errant solder traces, was a pretty clean build.

 

[Matador]

The only way to narrow down for sure is to inject a known signal into the grid of the tube on each mic, and measure the overall gain between that signal and what you see on the XLR pin 2/3.

Do you have a signal generator and a scope?

 

[johnsonic]

The only way to narrow down for sure is to inject a known signal into the grid of the tube on each mic, and measure the overall gain between that signal and what you see on the XLR pin 2/3.

Do you have a signal generator and a scope?

I do have a scope, do not have a signal generator — unless it's possible to use my DAW (could pick up a generator if this is not possible).

Would you be able to walk me through what I have to do to test - would be an awesome thing to learn! Thank you!

 

[johnsonic]

Can anyone help me out here? Would be an awesome skill to have! And would love to find out why this mic isn't as sensitive as it should be - was a really clean build.

 

[Matador]

Send a sine wave plugin through an unbalanced output on your audio device. Use the scope to trim it to a peak value of 500mV.

Attach the tip signal to the grid, and the sleeve to audio 0V signal on the microphone (aka. the ground node).

Measure the output across pin2/pin3 of the XLR output. You should see 5-10dB of overall gain.

 

[johnsonic]

This is helpful! So, set everything up, was measuring 520mV out of the interface, 440mv out of the mic with the tip wire connected to grid, and sleeve to ground. Checking from the tube forward now. Thank you!

 

[Matador]

This is helpful! So, set everything up, was measuring 520mV out of the interface, 440mv out of the mic with the tip wire connected to grid, and sleeve to ground. Checking from the tube forward now. Thank you!

440mV across pins 2-3? Is that peak?

 

[johnsonic]

Yep - came up as peak to peak, with -220/+220.

Definitely less than I was putting in.

 

[johnsonic]

Also swapped tubes and got the same result.

 

[johnsonic]

In today's edition of "What's wrong with my mic?" - traced the circuit and everything checks out. Also traced from the transformer to the xlr through a power supply, were both connected and not bridging to ground. Still stumped!

 

[Matador]

If you do the same test on your C12, what do you see?

 

[Delta Sigma]

Did you measure your plate/cathode voltages?

 

[Matador]

Did you measure your plate/cathode voltages?

That, and what frequency was your sine wav plugin set to? 1kHz?