Ela-M 251 Body from Aliexpress

[Murdock]

What about insulation resistance? I would think the switch would need a pretty high insulation resistance as it is part of the very high impedance area.

Edit: I just checked. Most of these small toggle switches have an insulation resistance of only 100M ohm. That seems kinda low... or isn’t it that important in the Ela M251 circuit?

 

[ElSmurf]

After some digging I realized that even though the basic idea of the switch is very simple, trying to recreate it with store bought parts is proving to be a difficult undertaking. Especially considering the limited space in this specific body. The tube and transformer take up 2/3 of its length, which leaves little room for a clunky rotary switch.

Looking at how the original is constructed, I'm wondering if there's a way to create one from scratch.



Anyone know a manufacturer that sells loose switch parts? If there's a way to get hold of these insulated contact pads and wipers, I think it should be possible to come up with something that takes up less than a cm in height, provide great insulation, and still allow cables to run through.

Maybe even 3D print a complete replacement for the metal base plate, including the cap, as in the original. Since there's no need for drop-in replaceability, there are some options. Would be cool to have a relatively low cost solution that actually looks great.

 

[RuudNL]

What about insulation resistance? I would think the switch would need a pretty high insulation resistance as it is part of the very high impedance area.

It depends. In my situation I use the switch to select the polarisation voltage on the rear membrane of the capsule.
I take the signal from the backplate. The front membrane is always at 0 Volts.
For the switching of the polarisation voltage, the insolation resistance of the switch is not critical.
In fact it can be any switch.

 

[ElSmurf]

In my situation I use the switch to select the polarisation voltage on the rear membrane of the capsule.

That looks like how my micparts V251's pattern switch is implemented, only there the switch is on the PSU. Out of curiosity : am I correct in assuming that your switch is just putting a resistor in the path of the polarisation DC to cut it in half, or switching it to ground for 0VDC?

Something that could really simplify things for me is to build a 250 instead of a 251. I don't see myself using figure of 8 anytime soon (I mostly do vocal recordings) and it seems that is what makes the pattern switch mechanism so complex in the 251. Where the original implementation of the switch in the 251 requires a 3P3T switch, the 250 only needs a simple DPDT switch. Insulation resistance would be a non-issue as well since the switch would only have one side of each pole connected - if I read the original schematics correctly. And in case I do end up using a rotary switch, it would also solve the problem of finding one with a 22.5 index angle. 45 degree angle rotary switches are also more easy to come by it seems.

 

[RuudNL]

The switch in my microphone switches the rear membrane between 0 V (Omni), +60V (Cardioid) or +120 V. (Figure-8). The backplate is at +60 V.
There is an R-C filter, to prevent loud 'thumps' when switching.
It takes a couple of seconds to completely change the directional pattern.

 

[ElSmurf]

This one should work well I think. 45 degree index angle, less than 20mOhm contact resistance and 1MOhm insulation resistance. It's also very small physically. Is there any reason this wouldn't work?

Cosmetically, it would also allow me to put the pattern symbols next to the actuator, instead of below, so I wouldn't have to cut and file out the T-shape in the body.

 

[gyraf]

You need really high insulation resistance here, iirc. This why the original had this elaborate construction of the switching.

 

[Khron]

You need really high insulation resistance here, iirc. This why the original had this elaborate construction of the switching.

I'm still ever so slightly confused about the necessity (?) of such overly complicated and demanding/sensitive switching solution in the original. That and the need for the insulated-backplate capsule...

 

[ElSmurf]

OK, I think I found a solution. In Dany's switch implementation, the three relays are powered by the heater voltage, which runs through a simple switch on the PSU that sends 6V or nothing to the mic down on of two leads, depending on the switch position. My current idea is to butcher Dany's D251 pcb and use only the relay part, and move the switch inside the mic, and build the visible part of the switch with acrylic. The rest of the mic I'll do with acrylic and turrets.

I know this is a rather convoluted way of getting the switch looking original, but this is mostly a case of "because why the hell not."

 

[RuudNL]

Personally I don't like the relay solution. Especially because it makes things unnecessary complicated.
If you follow my suggestion, you only need to switch a single voltage on the rear membrane of the capsule.

 

[Khron]

If you follow my suggestion, you only need to switch a single voltage on the rear membrane of the capsule.

... But that's TOO DIY...

 

[ElSmurf]

Personally I don't like the relay solution.

Any technical concerns besides the complexity?

 

[RuudNL]

It will probably work, but I don't like the idea of three relays inside a microphone when there is no need to have them there.

 

[Khron]

Any technical concerns besides the complexity?

It will probably work, but I don't like the idea of three relays inside a microphone when there is no need to have them there.

... Especially when one or several are barely mounted to the PCB with only the coil pins. You won't wanna be shaking that mic around too much for too long...

 

[paulcheeba]

Just about to start my build and I bought mint C12a’s with the same capsules as my 251 OG capsule in a C12. I bought Beesneez as back up as I was gonna harvest 414eb’s brass ck12 but Bens sound exactly the same as the 1960’s C12a which are the same as the 251.
The man has nailed it imo!

 

[paulcheeba]

Hey H, so just checking, can I use the Retro tube PSU as is or does it still need Poctops PCB population?

 

[Mikael-ange]

Hey H, so just checking, can I use the Retro tube PSU as is or does it still need Poctops PCB population?

Hi Paul
You need to use Dany board for the psu. The stock one on the retro psu need some redesign if you want to use it with the 251 build, so don't bother.

 

[ElSmurf]

The stock one on the retro psu need some redesign if you want to use it with the 251 build, so don't bother.

Could someone confirm that the retro psu uses the same transformer (175V+6.5V secondaries) as the SCT2000 and similar PSUs?

 

[paulcheeba]

Hi Paul
You need to use Dany board for the psu. The stock one on the retro psu need some redesign if you want to use it with the 251 build, so don't bother.

Thanks for sharing this information. I keep reading different things.

 

[Mikael-ange]

Thanks for sharing this information. I keep reading different things.

Ok so let's list what you need voltage wise:
B+ at 120v (stock psu need some resistor change for dropping the voltage down)
Heater at 6,3v (better design and noise filtering on Dany psu)
Relay voltage (in theory you could find a way to derive it from the heater but some serious thinking and redesign need to be done for it).
So since you have Dany psu board, why bother.