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.