ELA M251 Inspired Microphone - Build Thread
- Thread starter Matador
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Sorry, a bit slow in my mind on a saturday evening
026.5 ohms, dances around a bit but mostly around that figure!
Matador
Well-known member
That seems quite high. You should only see an ohm or two between the mike shell and PSU ground. And even then, 0.2A across 26.5 ohms is only about 5V, not 60V.
Thank you for helping!
Yeah that sounds strange. Any ides what to check first?
So strange it reads a perfect 6.3V when checking at the PSU.
Time to book another date with my tech guy!
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Matador
Well-known member
You should read the heater voltage direct across pin 4 to pin 9 (or pin 5 to pin 9, depending on which side of the tube you are using). All the tube cares about is the voltage directly at the socket pins.
It sounds like there may be an issue between the mike shell, and pin 7 (the wire that carries 0V from the PSU to the mike). Although in your picture I see a dedicated wire soldered to the rail I can't really tell where it's going.
It sounds like there may be an issue between the mike shell, and pin 7 (the wire that carries 0V from the PSU to the mike). Although in your picture I see a dedicated wire soldered to the rail I can't really tell where it's going.
Sorry I figured that I measured from the wrong pin so the heater voltage is actually fine now !
My very high noise still exists unfortunately but the search goes on!
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ChrisMM
New member
Hello all! I think I may have discovered an unintended design quirk in this microphone that could degrade sound quality. I'll describe the problem and how I fixed it in my build.
The problem: At audio frequencies, the capsule drives an impedance of ~15M(Ohms), instead of the intended 30M, which loads down the capsule and reduces low end (among other potential degradations). This happens because C11 and C5 end up providing an additional AC path from grid/capsule to ground, through R16. So at audio frequencies, both R15 and R16 (at 30M each) end up combining in parallel, making a 15M load. I don't think this is supposed to happen because in the original Ela M 251, there is only one 30M resistor connecting capsule/grid to ground, making a 30M load for the capsule.
My proposed fix: Replace R15 and R16 with 60M each. That way, they will combine in parallel to be 30M at audio frequencies.
To test this hypothesis, I temporarily desoldered C11, taking the rear diaphragm and R16 out of the audio path entirely. The capsule/grid to ground resistor then becomes R15 only, 30M, as in a real 251. When I did this, the low end finally became what it should be! I could sing a foot or two away from the microphone and still get the full character of my voice, whereas before, it would sound too thin unless I got right up on the grille.
With that test being a success, I decided to replace R15 and R16 with 60M resistors each (in my case I made 60M by putting two 30M resistors in series, since that's what I had on hand), and reinstalled C11, bringing the rear diaphragm back. Now the microphone finally sounds the way I imagined it should! I haven't experienced any problems after the mod so far, though I haven't spent a ton of time testing it either. That being said, I really recommend doing this mod, especially if you want more low end.
I absolutely love this microphone, and greatly appreciate all the design effort that went into it. It's so cool to have a 251-sounding thing but with polar patterns controlled by the PSU. Works great with my Microphone Parts VPS1 PSU.
Please correct me if I've missed anything or spoken in error--I am a total design noob.
Chris
The problem: At audio frequencies, the capsule drives an impedance of ~15M(Ohms), instead of the intended 30M, which loads down the capsule and reduces low end (among other potential degradations). This happens because C11 and C5 end up providing an additional AC path from grid/capsule to ground, through R16. So at audio frequencies, both R15 and R16 (at 30M each) end up combining in parallel, making a 15M load. I don't think this is supposed to happen because in the original Ela M 251, there is only one 30M resistor connecting capsule/grid to ground, making a 30M load for the capsule.
My proposed fix: Replace R15 and R16 with 60M each. That way, they will combine in parallel to be 30M at audio frequencies.
To test this hypothesis, I temporarily desoldered C11, taking the rear diaphragm and R16 out of the audio path entirely. The capsule/grid to ground resistor then becomes R15 only, 30M, as in a real 251. When I did this, the low end finally became what it should be! I could sing a foot or two away from the microphone and still get the full character of my voice, whereas before, it would sound too thin unless I got right up on the grille.
With that test being a success, I decided to replace R15 and R16 with 60M resistors each (in my case I made 60M by putting two 30M resistors in series, since that's what I had on hand), and reinstalled C11, bringing the rear diaphragm back. Now the microphone finally sounds the way I imagined it should! I haven't experienced any problems after the mod so far, though I haven't spent a ton of time testing it either. That being said, I really recommend doing this mod, especially if you want more low end.
I absolutely love this microphone, and greatly appreciate all the design effort that went into it. It's so cool to have a 251-sounding thing but with polar patterns controlled by the PSU. Works great with my Microphone Parts VPS1 PSU.
Please correct me if I've missed anything or spoken in error--I am a total design noob.
Chris
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I think you’re right, but an even easier fix (that would be closer to the original circuit) would be to just connect C5 on the other side of R16, which is where it should be anyway if the purpose it to reduce ripple on the pattern voltage to the capsule.
Drosselmeier
Well-known member
I have made many microphones using this design. I thought they were too noisy. I replaced R15 with 460 MOhm.
ChrisMM
New member
In this circuit, wouldn't that just tie the capsule/grid directly to ground? It looks like audio would pass through C11 and then C5 straight to ground with no impedance. Unless I'm missing something.
Matador
Well-known member
I think the vast majority of people prefer 1G for the grid resistor, as noise is lower and the full bandwidth of the capsule is preserved. I don't know why such a low value was in the original, however I assumed it was because less bass response was important.
Doesn't work, because it will pad the capsule output severely (by over 98%). Some people just omit C5 and given the polarization supply is already well filtered, doesn't add any perceptible noise.
Good point. I was thinking of C4 in this version (which is where it looks like C5 in this version maybe originated from?):
But, since this is a single-backplate version with C11 for variable pattern voltage, there's a direct AC path between capsules, it won't work here.
