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

[PG_Sax]

PG_Sax: by bridging wire, are you referring to the hi-z wiring? Chunger sells a nice little package of small gauge Teflon (PTFE) coated stranded wire. I've also purchased some from Apex Jr but if you're not building a lot of gear, buying rolls of of different colours is way too expensive making Chunger's store your best bet.

Wherever you get it, get something small (26AWG or so), Teflon coated (not only heat resistant for soldering but also very hi-z) and stranded (less microphonic).

Cheers Delta!

Yeah the bridge wiring in HiZ section. I have some teflon coated wire but from Dan's pictures he seems to be using some non coated stuff?

I assume that there is no advantage to using coated or non coated?

 

[PG_Sax]

Also can anyone clarify does the Relay 1 need to be grounded (to shield on 7 pin)?

I guess what I'm asking is does any other pin need to be linked to ground other than pin 7? It looks like Relay 1 should be grounded also?

 

[Delta Sigma]

I getcha now. Looks like Dany used component leads to make the little "bridges" in his photo. This allows some structure when building the floating sections. Use your stranded Teflon to connect to the hi-z section (like the capsule leads).

Are you using Trans4funks1's schematic? You'll find it much easier to follow.

https://cdn.groupbuilder.com/groupdiy/u/39511/58d1402a06461.pdf

Voltage applied to the coil of a relay will "energize" it, causing the contacts to change state. The above schematic shows the relays in a non-energized state. When a pattern selection is made on the power supply it either adds or removes 6VDC to the one side of the coil (1) so for it to energize, the other side of the coil (16) is tied to 0V. IIRC this is done via PCB.

 

[PG_Sax]

I getcha now. Looks like Dany used component leads to make the little "bridges" in his photo. This allows some structure when building the floating sections. Use your stranded Teflon to connect to the hi-z section (like the capsule leads).

Are you using Trans4funks1's schematic? You'll find it much easier to follow.

https://cdn.groupbuilder.com/groupdiy/u/39511/58d1402a06461.pdf

Voltage applied to the coil of a relay will "energize" it, causing the contacts to change state. The above schematic shows the relays in a non-energized state. When a pattern selection is made on the power supply it either adds or removes 6VDC to the one side of the coil (1) so for it to energize, the other side of the coil (16) is tied to 0V. IIRC this is done via PCB.

Cheers Delta! Appreciate your help. I was looking at that Schematic, Dan just mentioned in one of the posts that Relay 1 (which to him was on Pin 3 on the PSU was tied to XLR shield)? But that isn't the case on the schematic?

 

[PG_Sax]

I always run my cable the same way on the xlr cable pinout,
1-audio +
2-audio -
3 shield up to xlr tab as well
4-heater
5 B+
6 Pattern control ( relay voltage )
7 circuit ground

For AMI T14 it will be :

Blue , Blue    will be primary + and Primary -
red , red      will be secondary + and secondary -

hope this helps ,
Dan,


I actually made a little mistake here as there is 2 wire for the relay control , i will be making a drawing in a short moment ,
to make this all clear , for the mean time i will dit this post to add the proper xlr config for this particular build,

edit:

1-audio +
2-audio -
3- (Relay control 1)
cable shield up to xlr tab as well
4-heater
5 B+
6 Pattern control ( relay control 2  )
7 circuit ground

I have highlighted in bold the one I am wondering whether it is tied to the shield of xlr?

 

[Delta Sigma]

Pin 3 is not tied to 0V or the shield! Pin three supplies voltage to pin 1 of K2 when supply is switched to omni.

I see the confusion. Dany uses the same pinout for his builds where pin 6 is pattern control, pin 7 is 0v and pin 3 is tied to the shield. He must've posted this in error as this build needs two pins for pattern control relays where others only needed one pattern pin. He then corrected it in the post you quoted.

Think of a variable pattern mic like the M49 or C12; changing of patterns in continuously variable pattern mics comes from a separate variable supply from the power supply. That variable supply is tied to the back membrane and when it reaches the same voltage as the front membrane you get omni, when it reaches the backplate voltage you get cardioid and when it becomes opposite of the front membrane you get figure 8.

In the 251, there is no separate pattern supply. The variable patterns are achieved by relay contacts changing how the membranes/backplate are tied to B+ and 0V. To achieve figure 8, pin 6 supplies voltage to K1 and K3 changing their states. To achieve omni, pin 3 supplies voltage to K2. With all three relays de-energized (as drawn in Trans4funks1's schematic) the mic will be in cardioid.

Review the schematic even if it is a lot to absorb - it's probably as clear as it can be in Trans4funks1's schematic - and try to get a grasp on the switching. Consider diodes as open. They reverse polarity so no current flows. They are there to protect the relays (flyback diodes).

 

[PG_Sax]

Thanks for your help Delta.

Thats what I took from looking at the Schematic was just confused by the post as he left in ('cable shield up to xlr tab in both posts'). Couldn't understand why you would ground that  pin. I have the pins organised slightly differently on this build. Was just trying to be super vigilant as am waiting on some valves to fire the mic up.

Hopefully will get it working later on today. Will post up some sweeps when its working as I have a couple of transformers/coupling capacitors/valves to try. Be interesting to see what the differences will be.

 

[rockinrob86]

I am also having the dreaded "white noise" issue.  It is at about a -50db level with the gain on my UAD 1073 at 45.

I tried 4 different 12ay7s with no appreciable difference in the noise.  I guess next step is ruling out the capsule?

The mic was perfect after I built it a year ago, first noticed the problem a couple months back, but just assumed it was a noisy tube and hadn't had a chance to check.

Will the backplates show continuity in circuit?  Visual inspection of the hi-z section does not show anything.  I sprayed contact cleaner on one of the tubes I switched out with no difference. 

 

[Tim Campbell]

Well there are still quite a few  things you could check before the capsule, especially if it has been quieter before.
Clean everything, the pcb, around the relays, etc.
How have you been storing it? It should be kept in a plastic bag with a desiccant pouch.
Does it make noise after switching?
Capsule noise usually sounds like an uneven paper rustling sound not a constant hiss but so does any dirt.

 

[rockinrob86]

Thanks Tim

I found Matador's useful test here:
Try to narrow down where the noise is coming from.  I posted this several times in the thread, but the basic steps are:

1) First make sure you have a low impedance ground:  with everything powered off, check the resistance between ground on the PSU PCB and all ground points in the mike body, including all parts of the outside shell.  You should have less than 0.1 ohms everywhere in the mike
2)  Make a small length of wire with two small alligator clips, one on each end
3) Clip one end to ground on the PCB
4) Starting at the 'output' hard ground different points in the circuit:  then measure noise
a) the output of the coupling cap (where it enters the transformer) - it should be dead silent
b) the grid of the tube - this should only output the self noise of the tube
c) the capsule connection - this should only be the noise of the tube + HiZ components

You should be able to see the noise contribution of each stage this way.
--------------------------------------------------------------------------------------------------------------

After running this (I went through a 500uf Electrolytic capacitor because I was under the impression dead shorting stuff to ground wasn't the best way to do this) and I get near silence with headphones on and the gain turned up at every step. 

It would describe this sound as a "SSSSSSSSSSSSSSHHHHHHHHHH" white noise

I have been storing the mic in the "fancy" box with the velvet lining in an air conditioned room, but no dessicant pouches.  I ordered some of those, but also scrounged around and found one.  Put the mic back in the box with the pouch in there. 

The noise is steady regardless of switch setting, other than the quick pop/freakout thing all mics do when you switch patterns for a second.

The circuit was very clean, but I cleaned it again with isopropyl just to be sure, and confirmed all ptp connections were truly floating.  Solder joints look good.

If the capsule has taken on moisture or something, can it be saved with the pouches, or is it toast?  I would be very surprised, as I have taken good care of this mic.  It has maybe been left out for a few days here and there on the stand, but it has not been subject to temperature changes, humidity, etc.  I don't even really use it on vocals, it is more a go to room mic for drums, acoustic guitar, guitar cabs, etc.

 

[Tim Campbell]

How are the connections at the capsule? Are they tight?
Do you have another capsule you could try? Or perhaps another mic to try the capsule in?
If it is the capsule and it's one of mine please contact me about how to ship it to me and I'll get it back to you asap.

 

[rockinrob86]

It's not one of yours, but thanks!  I am hoping to get one of yours in the near future for a different mic build, and appreciate the service on display here

 

[rockinrob86]

It works!  Desiccant packs saved the day!

Will this cause any recurring issues, or if I keep it stored with the packs going forward it should be good?

 

[Gustav]

Just finished a build for a friend who has been sitting on one of these for years, and dropped it off with me.

It has a pretty insistent, extremely loud hum going on.

No hum from the PSU with the mic unconnected.
Voltages seem fine.
Hum comes on with the heater, stays on as long as the PSU caps are charged.
I can wave a finger around the capsule/relay area, and it gets worse.
I can touch the base or the basket, and it lessens (still very loud, though).
I tried reversing the pins to the backplates, same same.
I haven't hooked up the relay switching yet, so just checking in cardioid (unless I am reading that incorrectly).
Can hear sound coming through when I scratch the head basket, so something is right.

Tim's capsule

(finished a C12 as well, seems the PSU is the same, except for  jumper added, using Tim's capsule as well, no problemo on that one).

Any pointers?

Gustav

 

[Delta Sigma]

Shielding? Ground loop?

I had grounding issues in an M49 build that caused a loud hum. I ended up changing my grounding scheme and that corrected the problem.

 

[Erik2345]

This may be a delicate operation: switch capsules and see if the problem moves to the C12

 

[Tim Campbell]

Capsules don't hum. If you're getting a hum it's either a bad sheilding problem or if it concerns the capsule it is a broken connection.
Make sure all the connections are good to the capsule. How does it behave when switching patterns?

 

[Gustav]

Thanks!

I wired up the selector.

I dont have an analyser to put on it, but I would describe it like I have two octaves of hum in cardioid, only the lower octave in omni, and only the higher octave in fig 8.

So, the switching is working, at least.



(Id rather not swap the capsule, since the C12 is working perfectly)

Ill go over the p2p assembly, give it a cleaning, and trace the ground through it - just wondering if there were any obvious caveats.


Gustav

 

[RuudNL]

Are you using a single layer headbasket?
Sometimes the openings in the mesh are big enough for a less than perfect Faraday cage effect. (Screening.)
I have heard from a lot of people who had hum problems with single layer headbaskets.
(It is not without reason that Neumann uses 3 layers of mesh in a lot of their microphones!)

 

[Gustav]

Are you using a single layer headbasket?
Sometimes the openings in the mesh are big enough for a less than perfect Faraday cage effect. (Screening.)
I have heard from a lot of people who had hum problems with single layer headbaskets.
(It is not without reason that Neumann uses 3 layers of mesh in a lot of their microphones!)

I doubt its anything that esoteric - Getting very loud hum, completely drowning out signal.

Just going over all the soldering, components placement and the p2p section.

C3 is oriented incorrectly on the pictures in the first post, right?

Gustav