Topic: MXL 990, 991 and Oktava 319 Mods

[Roger Gustavsson]

I wonder if it's because I put 150 ohm resistors in for R4 & R5, rather than 150K ohm resistors 

It would certainly result in a brutal roll-off at about 2.2 kHz! C4 and C9 could have smaller values if space is limited. Schoeps use to set their low frequency corner between 20 and 30 Hz.

Roger

[Chuck]

The wrong resistor values was the problem. When I removed the 150 ohm resistors and inserted the correct 150k ohm resistors the mics came to life. The modified mics sound much better now.

[chris319]

The wrong resistor values was the problem. When I removed the 150 ohm resistors and inserted the correct 150k ohm resistors the mics came to life. The modified mics sound much better now.

Good to hear. Do you now have a pair of $50 Schoepses?

[bugg100]

Chuck,

Have you recieved your KAM mics yet?

Way to go for hanging in and finding your mistake!

[Chuck]

I haven't gotten the KAM mics yet.

[Chuck]

I received the KAM i2's last night.

I've attached photos of the internals.

These bodies are about a 1/2 inch longer than the CAD GXL-1200 bodies. The KAM mics have the low cut and -10db pad switches on them. Also, the ceramic capacitors are green and all of the components match perfectly between the microphones. There is what I assume to be a quality control sticker on each mic circuit board.

The circuit boards look much better and there is no residual flux on them as there was with the CAD mics.
The electrolytic capacitors are smaller in size (physically) than the caps in the CAD mics too. This will make them easier to modify.

I did do some tests on them and they match pretty well.

I bought these on eBay, as B-stock mics because of the paint job. The flat black paint is already flaking away from the holes where the switches are. I'll probably sand down the tubes, install jumpers in place of the switches, fill the switch holes and re-paint them. As I don't have any use for the switches.

Also note that one of the XLR ends is brass and the other is (pot metal?). Also, the painted markings on each mic are slightly different, which may mean that they came from different production batches. 

Overall, I think these will be much easier to work on than the CAD mics, because of the longer body and smaller electrolytic capacitors.

[chris319]

So Chuck, what do you have in mind with the KAM i2s? Don't forget to make before and after recordings!

[Chuck]

I partially rationalized buying another pair of these mics to get the omni-capsules that were included with the KAM i2's. I'll probably experiment with putting different components into the new pair to see if it makes much of a difference in the way they sound compared to the pair I've modified already. If I end up liking one pair more than the other, I'll probably donate the other pair (with just the cardioid capsules) to the club I do most of my recording in.

Looking at it with hindsight, I would have made out better just getting one pair of the KAM mics that have both sets of capsules. They have a lot more room in them for larger capacitors and I got them for a good price.

[bugg100]

Don't forget to make before and after recordings!

dude. relax.....

[chris319]

Don't forget to make before and after recordings!

dude. relax.....

Butt out.

[bugg100]

i don't think so.

[Chuck]

I stumbled onto this Scott Dorsey circuit recently. It would not be too hard to adapt it to the i2 microphones. It's simpler than the stock circuit and all the transistors are the same.
It might be cool to use this circuit on the new pair to see what the differences are

[Chuck]

So, there's no reason to try that circuit unless I want less sensitivity and more noise. Thanks, Jon.

[Chuck]

I had some time to check and document all the components in my KAM i2 microphones and revise zapnspark's schematic for the Generic China Mics with the component values and circuit board labels I found.

I'm not 100% sure that D3 is a 6v zener. zapnspark didn't have that one on his original schematic.

Also, zapnspark suggested that I add capacitors C17 & C18.

This is what he said about those capacitors:

Those 2 caps across the 150k resistors actually perform a number of functions.
Mainly, they roll off the mic. response above 20 kHz.
* This kills any 2 mHz leakage signal from the Hartley oscillator capsule polarizing supply from reaching the XLR pins 2 and 3.
* They add stability to the 2 PNP emitter follower transistors. Especially when driving capacitive/inductive loads. (e.g. long cable runs)
* They aid reducing external RF interference that might enter the body of the microphone from the outside world.
There should be no audible effects from adding these roll-off caps.

So, I figured that I'd add them.

[Chuck]

So, adding 47 ohm resistors will make it better, I can do that. If it's a best practice.
Do they go here? (marked in red)