It does start at 2-3K, but it's very shallow. It's only down <0.5dB at 20K. At 10K, it's down less than 0.1dB. You must have better ears than me to hear it.
M49C circuit details / questions
- Thread starter Khron
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pasarski
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I definitely couldn't hear that. But it introduces some distortion, especially at high frequencies, that could be heard.
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I’ve experimented a lot with this. Our ears are very sensitive. (BTW I’m not a hifi golden ears guy. I’m very critical and I know that our brains can easily fool us).
I agree, a low pass filter is not the only thing that is going on.
Roman Beilharz
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Interesting thread, as I intend to start building a M49c clone right now and I don't want to implement the calibration input, either. Still I don't see one of Khron's original questions answered here - or maybe I didn't catch it?! It was:
"...would the 2meg R12 still need to exist, and be connected to the pattern adjust voltage?"
So: Do I have to throw in 2M between POL and the bottom of C3 regardless or not?
Thank you in advance for clarifying!
"...would the 2meg R12 still need to exist, and be connected to the pattern adjust voltage?"
So: Do I have to throw in 2M between POL and the bottom of C3 regardless or not?
Thank you in advance for clarifying!
I did not include that in my mic (I essentially built Oliver's version in post #12), and I am very happy with how mine turned out.
I owe @tomas.borgstrom an apology. I thought the effect of this cap would be inaudible due to the shallow nature of the curve, but I now think I was wrong. I tried it out in my M49 (built around a 5718/EC71, based on Oliver circuit), and I found the differences to be very audible. I made two recordings. While I refer to them as "Microphone A" and "Microphone B," they're actually the same microphone through the same preamp and cable, in the same position, etc.
Obviously, this is not a scientific comparison, since it's (by necessity) a recording of two different takes, but I think you'll find the difference is pretty clear (listen to how "sharp" or "crispy" they sound). Lets see if everyone can guess which is which.
Again...the only difference is the presence or absence of "C6" from the original schematic (a 600pf - or 560pf in my case, from plate to ground). Side note, the noise/hum in the background is an air purifier I didn't bother turning off, not microphone noise.
View attachment Mic A.wav
View attachment Mic B.wav
Last edited:
garp
Well-known member
A is brighter ... but not only ...
What else?
garp
Well-known member
The medium around 2k is also affected and despite the background noise, noise floor (S/N ratio ) seems also a bit different.
Personally, I have not had good experiences with the Oliver M49 circuit. It was noisy like hell. I have tried tons of different 5840 tubes, from Thompson, Raytheon Philips, tubes from Royer... tried different capsules, from 3U M7 to various Chinese capsules... I would definitely lean M49 builds on the original M49c circuit and use that. The noise level is not worth mentioning anymore, and a much better, fuller sound. The few components less are really not worth not building M49c compared to the Olivers. I have modified all my 4 mics to M49c. They all sound just super now. By the way... the sound changes dramatically better and really open if you turn the 8pF "balancing" capacitor down to a 2pF Glimmer Capacitor. Regards
I don't have any noise issues with mine. The main differences are just the lack of feedback resistor (which cuts bass), the lack of the "level setting" capacitor, which really just lowers the output, and the lack of calibration input. The Oliver circuit also doesn't have the "treble cap," which I do like though (at least, with this Thiersch Blue Line M7 capsule).
Granted, I used a 5718/EC71 instead of a 5840...but I don't think there's anything wrong with Oliver's circuit. Certainly nothing (outside of a noisy tube) that would lead to any more noise, and if anything, it should actually have a fuller sound (do to the lack of feedback cutting low-end).
Oh, also, I used @Moby's Bv.11 instead of the AMI. I'm not crazy about how a lot of the AMI transformers sound.
Oliver Archut’s version is interesting.
I made 2 prototypes of the M49C version and followed Rudd’s advice on the second post of this thread:
« Forget C8. Connect the 'bottom' of C3 directly to ground if you don't need the calibration input.
C4 ("abgleichen") is used to adjust the output level (Higher value of C4 = lower output. It is a form of feedback.
I suppose you don't need a fixed mV/Pa. ratio. In that case simply omit C4. »
And...
« R3/C3 form a low-pass filter. (More feedback for low frequencies.)
The presence of R4 (200 ohm) in series can be neglected compared to the 5M.
C2 is only there to isolate the AC feedback from the polarisation voltage»
They came out really really nice!
The thing that really made a big difference is the transformer !!!
Big core and 6.5:1 ...? Big sound !!!
More to come, watch this space
M
I made 2 prototypes of the M49C version and followed Rudd’s advice on the second post of this thread:
« Forget C8. Connect the 'bottom' of C3 directly to ground if you don't need the calibration input.
C4 ("abgleichen") is used to adjust the output level (Higher value of C4 = lower output. It is a form of feedback.
I suppose you don't need a fixed mV/Pa. ratio. In that case simply omit C4. »
And...
« R3/C3 form a low-pass filter. (More feedback for low frequencies.)
The presence of R4 (200 ohm) in series can be neglected compared to the 5M.
C2 is only there to isolate the AC feedback from the polarisation voltage»
They came out really really nice!
The thing that really made a big difference is the transformer !!!
Big core and 6.5:1 ...? Big sound !!!
More to come, watch this space
M
