"guerilla" omni nic?

[gravy boat]

Can anyabody elaborate on the making a omni mic article on page 31 of the new TapeOp? It talks of "filling the tiny holes at the rear of the capsule assembly". From the picture, I couldn't really see what he meant. Say I wanted to do this to a 57, what holes would I be filling? Anybody else tried this? How about doing this to a cheap ldc like an Oktava 319?

[eeldip]

this will be near impossible on a side address mic like the 319.

you can experiment on a cheap mic by just using duct tape and covering up the rear ports of a cardiod mic... it might be harder to do it on a 57 than other mics actually.

the 58 is probably a better one to play with, cause it is easy to pop that shield on and off.

anyway, when you get the ball off, there is gonna be a diaphram at the business end, and some more business behind it. see if you can wrap all that business up in tape and see how that changes the response... i think it will pretty much omni-ate the mic.

i could be wrong though.... careful not to wreck the mic unless you are rich.

anyone else.?

the funny thing about this process is that it might just be better to plunk down the $40 and get a cheap EV omni mic.

[MichaelJoly]

Oktava MK-219 / 319 omni conversion instructions -

This is a really great mic to convert to omni.

Take the mic apart. Really. Just figure out how to take it apart by looking at the thing and figuring it out. The 319 is trickier than the 219 but you'll feel good in a DIY sort of way after you do.

Once you've got the circuit board with attached capsule free of the housing here's what you do...

...remove the clear membrane on the back side of the capsule - an xacto knife works well. Notice the grey, curved channels and holes in between them. This is the back wave labyrnth that produces the cardioid directionality. Get some 5-minute expoxy. Mix it up good. Wait about 3 minutes for the epoxy to start to solidify. Then, quickly fill in the labyrnth with the epoxy. The goal is to fill the channels between the curved grey partitions but not allow the epoxy to flow through the perforations in the back plate. Relax. Don't worry about it. It will be OK.

Put the whole thing back together after you do this. Oh, by the way. Take off the freak'n HF boost discs too. (forgot to mention that before) This will improve your transient response significantly by eliminating group delay. You didn't need that faux 4dB HF boost at 10kHz anyway did you?

After you do this, you might consider puting a 5840 tube cathode follower in there instead of the orginal cheap ass FET circuit.

Oktava MK-219 / 319: Rule the world if you're willing to DIY

cheers,

MJ

[eeldip]

i stand corrected.

different technique of course...

[gravy boat]

Oktava MK-219 / 319 omni conversion instructions -

This is a really great mic to convert to omni.

Take the mic apart. Really. Just figure out how to take it apart by looking at the thing and figuring it out. The 319 is trickier than the 219 but you'll feel good in a DIY sort of way after you do.
Oh, by the way. Take off the freak'n HF boost discs too. (forgot to mention that before) This will improve your transient response significantly by eliminating group delay. You didn't need that faux 4dB HF boost at 10kHz anyway did you?

After you do this, you might consider puting a 5840 tube cathode follower in there instead of the orginal cheap ass FET circuit.

Oktava MK-219 / 319: Rule the world if you're willing to DIY

cheers,

MJ

Allright, I just scored a 319 for $40 at a pawn shop. Let the games begin. "Take off the freakin' HF boosts, too." Please explain. I think you mean the little capacitors that are shaped like discs. Not even looking at the mic, just guessing. After I unsolder them, what do I connect in their place? Nothing? A little jumper wire? I'm an electrical dolt. Please take mercy on my unenlightened soul...

[MichaelJoly]

I've posted some really long and detailed instructions on rec.audio.pro about modifying Oktava mics.

Curing body resonance problems, head grille analysis and mods, 5840 tube conversion, cardioid to omni conversion etc

Search "Oktava Joly" on Google Groups to find the whole collection.

cheers, MJ

[gravy boat]

Thanks fo the info. Definitely gonna go omni on the 319.

[MichaelJoly]

Gravy -

if you get stumped and need some help just shout.

Hint...the switch panel on the back of the 319 has to be disconected in order to remove the circuit board. I usually remove the -10dB pad and LF roll-off wiring permanently - makes it a whole lot easier to get back into the mic for future mods.

Plus, I've never felt low-cut has any business being part of a mic circuit when it can be done at the pre or console. Depending on your application, the -10dB pad might not be needed either. Room mic or vocals for example.

Oh, let's see...something I didn't mention in the rec.audio.pro posts - I always install a little conical-shaped foam peice that extends from the bottom edge of the capsule to the floor of the capsule mount. (narrow at the top, wider at the base) This absorbs and deflects HF energy inside the head assembly. See Geffel's web site where they talk a bit about the benefits of the conical shape of their M7 capsule mount.

The head grill mods are discussed pretty thoroughly in the rec.audio.pro post - roll your own grille from a single layer of bronze window screen. This will be fine for RF sheilding for room mic use and you can always use a popper-stopper in front for close-mic'd vocals. (which the omni MK-319 is really sweet for)

later, MJ

[Family Hoof]

Can anyabody elaborate on the making a omni mic article on page 31 of the new TapeOp? It talks of "filling the tiny holes at the rear of the capsule assembly". From the picture, I couldn't really see what he meant. Say I wanted to do this to a 57, what holes would I be filling? Anybody else tried this? How about doing this to a cheap ldc like an Oktava 319?

An omni directional mic capsule is omni because sound only reaches it from the front. I know this sounds stange but that's how it works and I'm not going to go into a physics lesson right now. All other polar patterns use a combination of frontal pickup and delayed sound enering ports at the sides or rear to cancel out a portion of the pickup and create directionality. The Tape Op articles suggests that you plug the rear ports of a cardioid mic capsule to make it behave like an omni one. An excellent example of this sort thing is the Shure KSM141 small-diaphragm condenser that changes from cardioid to omni when you twist the capsule casing causing a ring of metal to block off the vents on the side. I have used these mics faily extensively and I don't know how they do it but this thing sounds great in either configuration.

However, in my limited experience it is rare that a mic cardioid by design, dynamic especially, will sound good when plugged for omni. In theory, most of them shouldn't. You see, cardioid mic capsules are designed with a tuned frequency response. It usually starts out with one big resonant frequency peak that's slowly canceled out and turned down by variuous other resonances achieved through the placement of small ports and chambers behind the diaphragm. In example, I have a pair of Rode NT5 small diaphragms which resemble the Shure KSM design quite closely except no twistable omni switch. When i cover their vents with gaffer's tape they do become fairly omni except for the high end but also have highly exagerated bass and low mid response and sound pretty awful (colored in a bad way).

My two cents, feel free to dispute. I hope somebody learned something.