ELA M251 Inspired Microphone - Build Thread

[johnsonic]

I'm back with a photo essay!

But before I write anything else - just want to thank you all that are helping me out. The kindness of strangers is a pretty awesome thing!!

@Delta Sigma - looking at the below
62.1v on the plate
1.04v on the cathode

@Matador
Sine was 1khz

And for clarity on my hookup - my xlr connection to the out of the mic PSU is ground on Audio-/Pin 3, signal on Audio+/pin 2.
Calibrated from the DAW with ground to sleeve, signal to tip.

Also - I did switch the power supply out between the 251 & c12 since the c12 has a negative bias. But did make sure both supplies were calibrated on the microphones before measuring.

Here's the scope straight out of my DAW - seeing 480mV


Scope from the 251 - seeing 440mV:



Scope from the c12 - seeing 1.12V


Hope this is helpful and thank you again!

 

[Delta Sigma]

This is a frustrating one, because it really looks like you're doing everything right troubleshooting wise. Just keep in mind that you will figure it out and you will end up with a great mic. I dealt with a long term troubleshoot on my M49b and what a relief when I finally figured it out!

 

[Matador]

What kind of gain do you see immediately before the transformer, but after the coupling cap (like probe the minus end of the coupling cap, where the transformer primary lead attaches).

 

[johnsonic]

OK! Numbers from today. Compared the C12 and 251 with the appropriate calibrated power supplies, used the same tube in both mics. C12 still has more output.

520mV: DAW sine output
5.76 V: 251 transformer/output cap connection
12.4 V: C12 transformer/output cap connection

Looking upstream, I confirmed all component values when putting them in and can confirm R1 is 1.8k and C2 is 22uF. Reconfirmed R1 in-circuit. All of the voltages at test points on the schematic check out. Still stumped!

Hope this is helpful and THANK YOU!!!

 

[vmanj]

Greetings to all.
What cathode bypass capacitor (electrolytic, tantalum) was used in the original ELA M-251e ?

 

[Purplenoise]

Greetings to all.
What cathode bypass capacitor (electrolytic, tantalum) was used in the original ELA M-251e ?

I have seen originals with electrolytics and also Siemens tantalums in metal casing that look like electrolytics. Try both…different sound.

 

[vmanj]

I understand, thanks.

 

[Matador]

Hope this is helpful and THANK YOU!!!

What if you temporarily remove C4?

 

[johnsonic]

OK! In tonight's edition of "Troubleshoot the mic"!

Disconnected C4 (100pF styro cap coming off the plate turret for avoidancxe of doubt). While it was out of circuit, I reconfirmed capacitance - came to 107pF. I picked up a LCR meter this week and can measure low capacitance now, and believe it can test caps in circuit? It's this one if it helps in any tests: https://www.amazon.com/East-Tester-Handheld-LCR-Meters/dp/B08FQSBWKN?th=1

I also need to mention that the capsule is disconnected - I don't want to risk damage. I can't see how it would affect these tests though.

I did not do a comparison to the C12 since we're deeper in the circuit and think we're in a place where the topologies start to differ.

On to the test:
488mV: Starting voltage straight form the DAW
500mV: Output from XLR. Super noisy.
5.32V: Output from junction of output cap to transformer

Doesn't look like that's the culprit, results look pretty similar to before.

Also - I learn by troubleshooting! Hoping some of us can learn along the way. Judging that removing C4 made the XLR output super noisy, am guessing that it has something to do with noise. I also see that it's a low value, non-polar capacitor bridging both sides of the transformer. If it were a wire, we would get no output. Can I ask how it does this? I know it's a dumb question but I honestly want to understand it.

Let me know if I should reconnect C4 before moving on.

@Matador you are a saint. Thank you!!!

 

[Delta Sigma]

@johnsonic, is the value and type of your coupling cap (C3)? It's not NOS vintage something-or-other, is it?

It seems strange to me that removing C4 made the output noisy. Mic should sound fine without C4; just more high end.

 

[johnsonic]

All components are straight from the Chunger kit, so probably not vintage. I didn’t actually hear that it got noisy, but the waveform on the scope was much less of a straight line showing the wave and more of a scribble showing a sine wave, if that makes sense. I’m away from home right now, but the value of the output cap is what’s defined in the schematic.

Thank you!!!

 

[Matador]

If I remember correctly, that particular cap was included on the original to filter out 18kHz broadcast signals (I don't recall their name), so removing it shouldn't have made a large impact on the sound (perhaps cut some high end by a db or two). In my prototypes, it needed to be at 820pF to 1nF range for that cap in order to start hearing a noticeable difference.

488mV: Starting voltage straight from the DAW
500mV: Output from XLR. Super noisy.
5.32V: Output from junction of output cap to transformer

You should expect to see 25-30dB of gain from the tube, and 22db of loss from the transformer, for 3-8dB of gain overall. I can't think why the C12 would be any different, especially with the same tube, as the plate and cathode resistances are almost the same. Other than the location of the biasing resistor, the designs are equivalent (in fact, I actually expect the 251 to have more gain, as it's cathode is directly bypassed).

Are you sure you're scoping across pins 2 and 3 of the XLR (as in, tip of the probe goes to one, and ground of the probe goes to the other), and not from pin 2 (or pin3) to audio 0V?

 

[johnsonic]

If I remember correctly, that particular cap was included on the original to filter out 18kHz broadcast signals (I don't recall their name), so removing it shouldn't have made a large impact on the sound (perhaps cut some high end by a db or two).

Yep, I didn't listen to the output, it was more like the the waveform went from being super crisp on the scope to being a sine wave drawn by scribbles if that makes any sense (wonder if it's the interference C4 filters out?). Am away from home but can take a photo when I get back if it's helpful.

Are you sure you're scoping across pins 2 and 3 of the XLR (as in, tip of the probe goes to one, and ground of the probe goes to the other), and not from pin 2 (or pin3) to audio 0V?

100% positive - actually soldered up a socket - the shield isn't being used in hooking it up to the scope. Used it to test both the c12 and the 251 and showed that the c12 had twice the voltage output.

Thank you for your explanation on what C4 does!

Still stumped here.

Is there a way to test the tub output directly? Was outside the range of my scope - would testing a 1000hz 500mV input on the output side of the tube using my DMM work?

Thank you all!

 

[Matador]

Yep, I didn't listen to the output, it was more like the the waveform went from being super crisp on the scope to being a sine wave drawn by scribbles if that makes any sense (wonder if it's the interference C4 filters out?). Am away from home but can take a photo when I get back if it's helpful.

Yes, unless you have bandwidth limiting on your scope, it will happily measure (and display!) ultrasonic frequencies that perhaps C4 is removing/reducing. Those transformers (like the CM-2480 and the T14) normally have bandwidth up past 50 kHz so it's entirely possible.

Is there a way to test the tub output directly? Was outside the range of my scope - would testing a 1000hz 500mV input on the output side of the tube using my DMM work?

Probing the connection between the output cap and the transformer is (in effect) testing the output directly. However you can also probe the plate directly ("before" the coupling cap), and you should see the same signal just superimposed on the DC quiescent point of the plate connection (40-60V). If you set your scope to AC coupling you can measure the plate directly (most stand-alone scopes will tolerate a few hundred volts on the probes, however check yours to make sure!) and you will see a ground-referenced signal (that goes up and down around the reference signal of the probe, or ground).

You can also disconnect the transformer entirely, and substitute in a 100K resistor for the transformer primary and remeasure to eliminate the transformer as a culprit.

 

[johnsonic]

Awesome. Plan for when I get home is to trace the signal there. The problem I think is somewhere between the tube and the output, since the c12 is much hotter off the output of the output cap, with the same specific tube and same input right on the grid/ground.

 

[MicMaven]

If I remember correctly, that particular cap was included on the original to filter out 18kHz broadcast signals (I don't recall their name),

Um ... er ...
18 kHz is a very low frequency ... I kinda, sorta doubt it was to avoid interference from broadcast station ... I believe 18 kHz is allocated to various industrial and commercial products which propagate various type of RF noise, for example ballasts for sodium vapor lamps and commercial induction range elements. See the list FCC site: https://fccid.io/frequency-explorer.php?lower=0.018&upper=0.018&exact

Perhaps it was installed to avoid interference from industrial and commercial electrical products producing deleterious EMI/RFI in that low range.

Just saying ... James - K8JHR -

 

[mad.ax]

For stereo FM broadcast, there's a 39KHz subcarrier for the L-R diff, and a 19KHz pilot.
Principle of RDS broadcasting

So Matador is right, indeed "that particular cap was included on the original to filter out 18kHz broadcast signals"

Axel

Edit: actually to filter out >18kHz broadcast signals

 

[mad.ax]

On many vintage HI-FI equipment, you'll find an MPX filter that notches out the 19KHz pilot

MPX filter - Wikipedia

Axel

 

[mhelin]

How about the R8 / R9, is it even possible to use the wrong heater half?

 

[johnsonic]

How about the R8 / R9, is it even possible to use the wrong heater half?

Super good guess! I'm pretty sure it's on to the right pads. When I get home I'll make sure there is no resistance there, but I do get a continuous circuit from the heat pin on the 7-pin XLR and ground. It was making my meter beep with very low resistance earlier, and saw the same thing on the c12 with the same tube for testing - so probably not suspect.

Feels like the fault is right around the tube - can't wait to get home this week and investigate!
Thank you!