Apex 460 to ELA M probs

[Gilgamesh]

Hi guys,

I've done the apex 460 to ela m (style) mods pretty much exactly as is described here:

http://www.allthisnoise.com/All_This_Noise/Apex_mod.html

After I swapped in a Tim Campbell CT12 and changed the PSU zeners to get 114v I found the mic became very "full" or tubby.
I have checked the relevant schematics and it looks to me like I've wired the capsule correctly - I have the backplates wired together and wired to the junction of c4 and r10. I have one capsule side going to the junction c5 and r9 and the other side going to the circuit ground that meets r7. What is strange is that if I short the c5/r9/capsule junction point to the mic body it behaves properly - it loses the tubbiness and opens up to become very clear. I guess I could just wire that in permanently but I'm worried I've done something wrong somewhere and even when I do that its still not as bright as I imagined it should be. It's very natural and very nice sounding but seems to have less pronounced highs then my mk47 with a beezneez K7. I actually miswired the capsule initially, treating the two backplates as if they were the capsules and one of the capsules as the backplate so I'm hoping that didn't damage the capsule.

Anyone had a similar issue or might suggest where to start troubleshooting?

 

[pucho812]

well first off are you trying to mod it to work as a c12 or elam 250 or 251?

The differences between the C 12 and the ELA M 250 are small but important. The C 12 is remotely controlled for pattern by polarizing the rear diaphragm with a separate variable voltage from the power supply, while the ELA M 250/251 has switches in the capsule assembly that change its polarization pattern. The 250 has two patterns, Omni and Cardioid. The 251 included these patterns and also the figure eight.

The second difference is in the circuit. The C 12 and ELA M 250 circuits are similar but not identical. They both use the CK 12 capsule, T/14 transformer and either the 6072A tube (US version) or the AC701 (Europe), but the grid coupling, cathode bias and capsule polarization are different.

Third, the body style of the C 12 and ELA M 250 are not the same. Most important, the head grilles are different, so acoustics are also different.

I suggest you listen to some examples of a c12 so you can compare it with you have done and go from there. I have done that mod and although it's an improvement it's not a c12 nor will it ever be. But it will be close.

 

[Gilgamesh]

Thanks for your help. I have used a Bock 251 and listened to some samples of c12 examples. I hear the 251 sound as a bit fuller and not as bright and open as the c12. I am after the 251 sound. As I understand it the cathode bias is now like in the 251 circuit as I've swapped in a 1k8 resistor and it sits in parallel with a cap. As for capsule polarization I want to be able to use the stock power supply to change the pattern so need to ensure I have set up the circuit to do that properly.

I didn't think about the headgrill! I may temporarily sit a looser weave or single layer headgrill on for testing purposes, it may be that this is the reason I'm not getting that bit of extra "brightness" I believe I've heard in other 251 examples vs my modded mic. I should note though the other reason I think its not behaving properly is because the stock capsule was notably much brighter.

I have some reading to do, will compare all the relevant schematics and go through this again, thanks for the leads. Any idea what is going on circuit wise with grounding the junction of c5/r9/capsule - i.e. why shorting the circuit to ground there would open up and brighten the sound (in a very obvious, not subtle way)?

 

[Gilgamesh]

OK, bit embarrassing but figured that whole shorting thing - I was cutting the voltage and therefore switching it to omni and the thinner omni sound without proximity effect was what I heard as the brighter more open sound. So the mic is actually operating correctly but the sound is quite dark, not at all what I have heard in C12 or ELA M examples and for that reason I think there is still either an error in the circuit somewhere.

 

[baadc0de]

Does it have a high shelf at 3-5kHz descending slowly, but surely? I have a similar problem with my Apex460 conversion..

 

[Gilgamesh]

I don't think so but I'll put an analyzer on it and have a look...

 

[pasarski]

My CT12 has a huge proximity effect, very much of masking effect going on, and if used from the same distance will sound very different to stock Apex witch is crazy bright and thin. You have to have some distance (and a nice room). Or use it in omni from very close. There might be something going on with the electronics, better to double check everything, but if it's not the case and you still find the mic too dark, Tim will adjust it to your taste.

 

[Gilgamesh]

OK that makes a lot of sense. I was testing with headphones and holding the mic fairly close - I couldn't be bothered mounting it for every component change I made. When I tested again with Adam A7s I was amazed at what I hadn't heard before. I also read that the stock capsule had a 3k bump and sure enough when I notched out the stock capsule test with a little 3k I heard a similar frequency response but with the CT12 example sounding worlds better. I think when comparing to a crazy bright example you can lose a reasonable reference point and anything but crazy bright will sound a little dull. I'm pretty happy with this mod after testing under proper conditions and will play around with a few more circuit mods and headbasket options.

Sorry baadc0de I don't have appear to have a specific problem with descending brightness over time but will check back in if I identify something like that.

 

[david-p]

I didn't think about the headgrill! I may temporarily sit a looser weave or single layer headgrill on for testing purposes, it may be that this is the reason I'm not getting that bit of extra "brightness" I believe I've heard in other 251 examples vs my modded mic. I should note though the other reason I think its not behaving properly is because the stock capsule was notably much brighter.

I think the headgrille is a secondary effect.

Any idea what is going on circuit wise with grounding the junction of c5/r9/capsule - i.e. why shorting the circuit to ground there would open up and brighten the sound (in a very obvious, not subtle way)?

Shorting the polarizing voltages is not nice for them!  In your initial posting you mention shorting C5/R9 to the capsule backplate.  This cuts off the operation of the back capsule, and makes the polar diagram cardioid.

Then, quoted above, you talk of shorting C5/R9 to ground.  This makes the polar diagram omni.

You should get the same results by switching the polar diagrams at the power supply, so check first whether you also get the same sound qualities you describe.

If you are close to the mic, most of the sound of your voice will be picked up by the front capsule, even if you are in omni mode, with boosted bass because of proximity effect.  This is not a very good test of the C12 capsule, which is designed for distant as opposed to close use (the U47 with its top roll-off is better for this).  What I recommend is that you evaluate the capsule by putting the mic on a stand in the middle of a well-furnished room and walking around it at a radius of 18-24 inches, talking all the time, saying where you are with respect to the front.  Record this, or have an assistant do the talking while you listen.  If you get a good omni and cardioid polar diagram, proceed to fig 8.  This is the real test, as the output should be much lower and distant-sounding at +/- 90°.  The sound quality may vary a little between the polar diagrams, but not as much as you describe, and certainly not on the front.  I also use the sound of my grandfather clock ticking, and the sound of traffic outside to evaluate the sound subjectively.  I have also known respected engineers to use musical boxes, as they provide a repeatable performance.

If this checks out to your satisfaction, you should record instrumental music.  In the absence of live music, you can use music played through a good loudspeaker to compare the sound on different polar diagrams.

By the way, 114v is not too low, but I always aim for 117-120v for this capsule  I have never actually found 60v zener diodes, and sometimes the diodes used originally are not of the same voltage, and it is easier to change just one of them to get the voltage right.

David

 

[Gilgamesh]

Yes, I meant shorting to ground, I think I just described it wrong and yes it seems sonically identical to setting the pattern switch to omni. I will try the tests as you describe to properly evaluate the sound. What effect does the higher voltage have on the sound? I reduced the voltage so as not to damage the capsule. I see discussion about the benefits of differing voltages but I'm interested in what those benefits would be. Also what do you mean that the headgrill is a secondary effect - do you mean that it has quite a small effect compared to circuit variations? Thanks very much for taking the time to help.

 

[david-p]

Yes, I meant shorting to ground, I think I just described it wrong and yes it seems sonically identical to setting the pattern switch to omni. I will try the tests as you describe to properly evaluate the sound. What effect does the higher voltage have on the sound? I reduced the voltage so as not to damage the capsule. I see discussion about the benefits of differing voltages but I'm interested in what those benefits would be.

Pretty much every capsule that I have encountered, including Tim Campbell's, is intended to work at 60v polarization. If you set the HT at 120v and then use a potential divider (equal value resistor pair) to set half that for the capsule you are in good shape.  As to what difference a capsule polarizing voltage of 60 volts will make as opposed to the 57 volt that you have, I do not know fully as I have never tried to measure this. Beyond the fact that it will theoretically increase the output signal by about 0.5 dB, it may change the frequency response slightly.  Personally, I expect this change to be minimal; but I'd be interested to hear what others may have found.

Also what do you mean that the headgrill is a secondary effect - do you mean that it has quite a small effect compared to circuit variations?

I mean that it is small in comparison with changing the capsule type: probably it has a larger effect than changing capacitor types, which is itself a subtle effect in my opinion. I have also read that removing the inner mesh is not a good idea if you are using the mic for vocals, since the membrane is less well protected  from dampness and flying debris!

David