Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #660 on: September 13, 2019, 08:07:34 PM »
Low Sensitivity Issue?

I was wondering if this 251 behavior was considered normal:

I have to add 45db of electronic gain in my audio editing program (on top of the 13db gain from my Lavry AD10 ADC) for spoken voice recordings within 6-12 inches of the microphone.  Same issue with my other ADC with built in gain.  This has been the case since building the mic about two years ago - it did not worsen over time.  Otherwise the audio quality is pretty good - a bit noisy due to the low signal but not too bad.

For those who built a 251 and are not using additional gain stages in between their mic power supply and ADC , how much total gain are you adding to reach good recording levels?

Transformer seems to be correctly wired.  Resistance at the end of the blue wire (primary resistance) and ground is 965 ohms - close to the 1k needed there.  S+ from the transformer goes to pin 1 of the microphone which goes to XLR pin 2 and S- goes to 2 of the mic which goes to pin 3 (bottom pin) of the XLR connector, per the wiring diagram.
Secondary (between two red transformer wires) measures 21 ohms - close to 25 needed.

Components measure within tolerance for all the ones I tested, except one:  137pf for c2 should be 100pf - this is a 2.5% tolerance cap - would this be worth replacing for the 251 circuit?
« Last Edit: September 14, 2019, 12:43:28 PM by cornell9 »


Delta Sigma

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #661 on: September 16, 2019, 09:48:22 AM »
What are you using for a preamp? 13dB from your ADC isn't enough to bring mic level signals up for recording on its own. If you've already added gain before your ADC and still requite 13dB from your ADC and 45 digitally then there is something wrong with your mic.

C2 @ 137pF will not cause your level to be low, it'll just take slightly more high end off - but probably not noticeable. Are you measuring components with a component tester or a DMM? Your DMM probably won't be accurate in the 100pF range.


e.oelberg

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #662 on: September 16, 2019, 10:14:11 AM »
Hi cornell

I suspect you are not using a mic pre. Although a tube mic has a PSU, which looks similar, you will need a preamp with Phantom power switched off. With something between 30 and 40 dB gain on the mic pre you feed the adc. Than you should have a nice spoken word amplification with still maybe 10dB headroom on the digital side and this is what you are aiming for.

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #663 on: September 20, 2019, 09:57:38 AM »
Thanks e.oelberg and Delta Sigma; I will order a mic pre. 

Am using the DER EE DE-5000 LCR meter for testing.

dapolk

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #664 on: October 19, 2019, 05:16:38 PM »
Hi guys,

I built this mic about 3 years ago, it always sounded great and quiet, no issues of any kind across many sessions. Haven't used it much in the last year and just noticed a lingering noise issue that occurs when I make certain low vowel sounds and/or plosives. Here's  a short recording where you can hear it :

https://drive.google.com/file/d/1Lbm5J1s50biGCBkLHFdw3MCJZSk4OEjb/view?usp=sharing

(I was monitoring through speakers thus the slight feedback/ringing)

It's a CT12 capsule, NOS GE 6072, AMI T14. I went for a couple of the LF mods discussed earlier in the thread, though I can't remember exactly which ones. The output cap is a Sprague 3.6 uF, 125V wet tant.

Has always been stored in a zipped pouch with a dessicant pack.

Any help would be much appreciated!
« Last Edit: October 19, 2019, 05:19:54 PM by dapolk »

Tim Campbell

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #665 on: October 20, 2019, 07:10:53 AM »
does this happen only when speaking or singing into the mic at close range?  How about if you just move the mic through the air or play an instrument into it?

Try making sure all switches are clean by mechanically manipulating them and make sure the connections to the capsule are tight. examine to make sure there are no cracked or cold solder joints.

dapolk

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #666 on: October 20, 2019, 04:45:51 PM »
Thanks Tim - I worked the 3-way switch with contact cleaner, and touched up the solder joint going to ground in the mic's output section that looked a little suspicious. The joints w/ leads coming from the capsule all looked good.

It's dead quiet again now, even if I give it some strong plosives with my mouth touching the grill. Much appreciated!