Tube mic quieter in Fig-8?
« on: February 09, 2017, 11:26:31 AM »
Quick question... I've been modding a used Apex 460, nothing crazy, just the Brian Fox mod and I'm pretty happy with the way it sounds, even with the stock capsule. I noticed however, that the noise floor is significantly lower when its in full Fig-8 than when it is in straight up Cardioid, or even in any of the detents in between cardioid and fig 8. In cardioid it's much noisier than my FET mics, which is to be expected I suppose, but in Fig-8, it's the same noise level. The tube is a fairly new EH6072, I've been burning it in for just over a week.

Is this usual or unusual?

Also, is this an indication that possibly the backside of the capsule is better, or in better condition, than the front side and maybe I should reverse the capsule?

I really like how it sounds in fig-8, so whatever is going on, it's working for me, but I thought I'd ask in case reversing the capsule could further improve the sound.

Thanks in advance!


Re: Tube mic quieter in Fig-8?
« Reply #1 on: February 16, 2017, 08:52:33 PM »
It's unlikely that reversing the capsule will improve the sound if you mean flipping the front and back diaphragms within the circuit.
Fig-8 is achieved by attempting to use both diaphragms equally (with some intentional cancellation between the two for side-rejection, as opposed to Omni). Turning the capsule around will probably result in exactly the same outcome.

The lower noise is probably a result of the side rejection circuitry that the other patterns don't have.

Here's a little explanation of the concept. I don't know the actual circuitry in your mic that accomplishes these patterns.

https://ehomerecordingstudio.com/microphone-polar-patterns/

abbey road d enfer

Re: Tube mic quieter in Fig-8?
« Reply #2 on: February 17, 2017, 02:10:16 AM »
Quick question... I've been modding a used Apex 460, nothing crazy, just the Brian Fox mod and I'm pretty happy with the way it sounds, even with the stock capsule. I noticed however, that the noise floor is significantly lower when its in full Fig-8 than when it is in straight up Cardioid, or even in any of the detents in between cardioid and fig 8. In cardioid it's much noisier than my FET mics, which is to be expected I suppose, but in Fig-8, it's the same noise level. The tube is a fairly new EH6072, I've been burning it in for just over a week.

Is this usual or unusual?

Also, is this an indication that possibly the backside of the capsule is better, or in better condition, than the front side and maybe I should reverse the capsule?

I really like how it sounds in fig-8, so whatever is going on, it's working for me, but I thought I'd ask in case reversing the capsule could further improve the sound.

Thanks in advance!
How much difference is there between the fig-8 noise and the other positions?
What about noise in omni?
Is that with the mic in a soundproof box or just in the room?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

Re: Tube mic quieter in Fig-8?
« Reply #3 on: February 17, 2017, 12:03:36 PM »
How much difference is there between the fig-8 noise and the other positions?
What about noise in omni?
Is that with the mic in a soundproof box or just in the room?

It's about 6db more noise in cardioid, measured in the VO booth I use for work every day. I did take a listen in omni and can't recall at the moment if it was noisier or the same. Dave Thomas at Advanced Audio mentioned on RGO that he felt it was the stock capsule that was the culprit so I have the capsule out of it at the moment but I'll try to pop it in there at some point soon and check it again. I've tried 3 different tubes with pretty much the same results so I'm inclined to try a different capsule next.

abbey road d enfer

Re: Tube mic quieter in Fig-8?
« Reply #4 on: February 17, 2017, 01:06:56 PM »
It's about 6db more noise in cardioid, measured in the VO booth I use for work every day. I did take a listen in omni and can't recall at the moment if it was noisier or the same. Dave Thomas at Advanced Audio mentioned on RGO that he felt it was the stock capsule that was the culprit so I have the capsule out of it at the moment but I'll try to pop it in there at some point soon and check it again. I've tried 3 different tubes with pretty much the same results so I'm inclined to try a different capsule next.
Have you tried replacing the capsules with two capacitors of similar value (I would think 60-80pF)? Then you would have only the electronic noise.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

Re: Tube mic quieter in Fig-8?
« Reply #5 on: February 17, 2017, 02:16:14 PM »
Have you tried replacing the capsules with two capacitors of similar value (I would think 60-80pF)? Then you would have only the electronic noise.

That's a great idea. I'll get a couple of caps that size and try it.

Re: Tube mic quieter in Fig-8?
« Reply #6 on: February 18, 2017, 12:15:01 AM »
Use good quality silver mica or mylar caps rated for 150 volts.   Even at 48 volt polarizing, some low quality "run of the mill" capacitors can fail.  Sub out each section of the capsule with a cap and compare the noise level.  You could have a noisy resistor in the anode (or cathode) of the tube circuitry and the capsule could be perfectly good.  It is unlikely the high impedance resistors (probably 200 megohm or higher) are bad, since virtually no current flows in these.

I have to assume that in figure 8, both capsule sections receive full polarizing voltage.  In cardioid, one section of the capsule may be polarized and the other section "floats".  The noise source may depend on how the polarizing voltage for the capsule sections is derived.  In the event the cathode resistor is high value (47 k or higher), and the capsule voltage is derived from the IR drop across that resistor, any noise generated by that resistor would go right into the grid of the tube and be amplified.   If the polarizing voltage is derived by voltage dividing resistors off the HT anode supply, those may be suspect but unlikely.    I would need to see a schematic of the mic to offer any other opinions...

Re: Tube mic quieter in Fig-8?
« Reply #7 on: February 18, 2017, 02:04:49 PM »
So I'm pleased to report that it would appear the capsule was the culprit. I had another identical dual-sided, generic 32mm K67 type capsule that I swapped in and the noise level dropped dramatically to the point where I feel that I could use this mic for VO if I wanted to. Given just a quick eyeball reading of VU meter readings it actually might now be about the same noise level as my main axe, the Sennheiser 416.

It's still slightly quieter in Fig 8, but only a couple of dB as opposed to 6dB. From what Dave Thomas said on RGO, in his experience this type of disparity between cardioid and Fig 8 isn't uncommon.  This 460 was purchased used so it could be the result of condensation on the capsule over time so that, and of course manufacturing tolerances probably accounts for why the nearly new capsule I swapped in fixed the problem.

As far as I'm concerned, this issue has been solved for me. Thank you very much to everybody who took a moment to give helpful suggestions. I definitely came away from this with a lot more mic knowledge and trouble-shooting ideas than I had before! :)

Re: Tube mic quieter in Fig-8?
« Reply #8 on: February 19, 2017, 08:51:37 PM »
Glad you found the noise problem....it's fortunate you had a capsule on hand to substitute. 
The usual LF "rumble" or sputtering noises usually points right to a bad capsule section.
Considering the cost of a K67 capsule (around $600 or more from Neumann),  spending a little time eliminating the tube or any noisy components in the mic first may be cost effective compared to dropping big money upfront....