Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)

[cornell9]

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?

 

[Delta Sigma]

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]

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.

 

[cornell9]

Thanks e.oelberg and Delta Sigma; I will order a mic pre. 

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

 

[dapolk]

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!

 

[Tim Campbell]

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]

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!

 

[midwayfair]

I've got a weird problem that just started today with my 251 build. In cardiod and fig8 only, there's an occasional squeaky sound like someone letting air out of a balloon slowly. It lasts for a couple seconds then goes away for a minute or so, then it comes back.

My capsule voltage is 110 (it's one of Tim's). I cycled the power a couple times, double checked the cable, etc. It went away for about an hour and then it came back again.

I'm going through the diagram and schematic and I'm not really seeing a cap that's only involved when the mic's in cardiod or figure 8, even though the behavior seems like something capacitor-y. Any ideas?

 

[Tim Campbell]

I'd say to try all the advice I gave dapolk. Is it after speaking/singing into it?  Sounds like it could be dirt or moisture.

 

[midwayfair]

I'd say to try all the advice I gave dapolk. Is it after speaking/singing into it?  Sounds like it could be dirt or moisture.

I'll clean it. It was really moist yesterday since we had a tornado and a ton of rain, so maybe it was just humidity. Hasn't made the noise today.

I'm about 12 inches from the mic, off axis with a popguard, so I wasn't really considering any human-produced moisture.

 

[Tim Campbell]

Storing mics with a dessicant pack in a plastic bag is always a good idea.

 

[Delta Sigma]

Storing mics with a dessicant pack in a plastic bag is always a good idea.

I like these guys because you can tell when they need to be recharged in the oven.
https://www.amazon.ca/Dry-Packs-Indicating-Silica-Aluminum-Canister/dp/B00967JZY0/

 

[Purplenoise]

Hi, soon I’ll be starting my build so I try to find some parts closer to the original. I was able to find two wet tantalum caps 3.6uf 125v and 1uf 150v. Would those work in the build? I think I ve seen somewhere on this thread that 1uf is fine as long as I substitute the 30m grid resistor to a 250m one. Thanks in advance.

 

[Delta Sigma]

If you want to be close tot he original, maybe you should start with the designed values then worry about component types. Start with the film cap in the BOM then substitue your 3.6/125 wet tant and see how much practical difference it makes to you.

What power supply case are you using? If you have the real estate, install a standby switch or a Off-Stby-On switch (Carling makes one). That way, you can have your tube warm when the B+ comes on. This is to prevent the entire 120V B+ from going across the output cap.

Alternately, if you've decided the wet tant is worth it, you can shell out the big bucks for a brand new one.

 

[Purplenoise]

If you want to be close tot he original, maybe you should start with the designed values then worry about component types. Start with the film cap in the BOM then substitue your 3.6/125 wet tant and see how much practical difference it makes to you.

What power supply case are you using? If you have the real estate, install a standby switch or a Off-Stby-On switch (Carling makes one). That way, you can have your tube warm when the B+ comes on. This is to prevent the entire 120V B+ from going across the output cap.

Alternately, if you've decided the wet tant is worth it, you can shell out the big bucks for a brand new one.

Thank you so much, great idea about the standby switch. I’ll be probably be using the psu enclosure and I guess parts of the studio 939 mic body kit for 251...same as apex 460 I believe? On this build I try to source as much info before I start just to have a few options beforehand and get a better idea by more experienced builders. I already have three wet tantalums that I got for very cheap. The two I mentioned and a 22uf for c3. Since that’s what was used in the original from what I read I’d rather start with those than desoldering later. So you think aside from voltage tolerance 3.6uf is fine without further modification right?

 

[Delta Sigma]

Do you mean 3.6 vs 3.2 on the schematic? I doubt there'd be much audible difference. Do you have an LCR meter? Your cap may even be 3.4 or so; maybe 3.8.

I know pulling apart your newly built mic can be a pain but make sure you experiment with things like output caps. There can be a big tonal difference there.

 

[Purplenoise]

Do you mean 3.6 vs 3.2 on the schematic? I doubt there'd be much audible difference. Do you have an LCR meter? Your cap may even be 3.4 or so; maybe 3.8.

I know pulling apart your newly built mic can be a pain but make sure you experiment with things like output caps. There can be a big tonal difference there.

Thanks, yes for the 3.2. Actually I am hoping since it’s an older cap to have drifted down a little haha. I haven’t built the mic yet ( finishing up another 67 for now). I am just gathering info and parts to get an idea or a plan. It’s easier when you have options in front of you that you can swap right away and hear the difference. For now (although subjective sometimes) I try to find out about other people’s experiences with this build. Thank you again so much for your help.

 

[rmburrow]

Storing mics with a dessicant pack in a plastic bag is always a good idea.

Agreed, some of that dessicant can be heated to regenerate it.
Tim, are you doing any work with nickel membranes, Schoeps or AKG capsules (CK1)?

 

[Purplenoise]

So I was able to source 3 wet tantalums 1uf 150v. From what I understand I could connect them in parallel to get close to 3uf for the coupling cap right? Thanks.

 

[Purplenoise]

Do you mean 3.6 vs 3.2 on the schematic? I doubt there'd be much audible difference. Do you have an LCR meter? Your cap may even be 3.4 or so; maybe 3.8.
I know pulling apart your newly built mic can be a pain but make sure you experiment with things like output caps. There can be a big tonal difference there.

Measured it, it is 3.4 actually. If I am not mistaken I read in one of your posts that you used two 6.8uf 75v in series ( 3.4uf 150v sum). I found two Siemens on eBay with those values so I am gonna give it a try since it looks like a very simple and effective solution. How did that work out for you? Thanks.