LDC with KM84 Circuit

That looks like an all around good time

Welcome, marten

Really? That would make me trust them even more...

After Bob Ohlsson talked about motown using km84s on vocals I started messing around with one of my km84 clones on vocals and liked it. First thought was why there wasn't a km84 voiced ldc. This looks awesome.

didn't know that. Everytime I hear those they sound dark to me?

How about a SDC kit of this with the same thread and pin as a KM 84 so I can buy it and a new KM84 Capsule from Neumann and have what we really want?

I didn't do the design on those but they do use my "selected components" for the old KM84 design. Matt sent me one of these kits about a year ago. It came with their standard 1" rk-12 capsule. I haven't used it yet on a session as I have other stuff I favor. Compared to my rk-12 loaded MCA mics these have a thinner low end and a less open top end. In that regard it's been passed over for now as I do love the sound of the rebuilt MCA mics more.

A U-89 capsule would probably sound good in these as I put one in a rebuilt MCA mic for Ty Ford, it sounded amazing, far better than a TLM-170 to me. Then again, a KM84 capsule with that transformerless Schoeps variation circuit would also probably work for me here.

I believe the missing low end depth of the Microphone-parts KM84 circuit is caused by the output transformer. I sent it to Dave Hill at Jensen Transformers to run it on their Comtran system to quantify it. Jensen is now making replacement transformers for common Neumann mics, I'd love to get one to replace this one, I suspect it would solved the sonic issues I have.

Hmmm...replacement transformers for U67s?

sll

If anyone could do it right, Jensen would be on the top of the list.

Those cheap chinese SDC preamp bodies are not worth the crushed aluminum cans it took to make em. It would ruin a KK84. Plus the threads don't match.

I ordered one of these the other day. I have a stash of decent capsules of a few different varieties that I want to stick into some clean mic amps circuits. Low noise, useable mics for general purpose stuff. I also, have a stash of DIY and donor bodies around here. One of them needs multiple patterns, so I grabbed one of these to try in my donor body. So, this kit seemed like a reasonable thing to try.

I question the TX as well, very few TX companies make good TX's anymore. I asked Matt about it but I didn't get much info, just that it is better then the other OEM km84 TX's that the manufacture makes. I plan on sticking a Redline M7 in this mic. I am sure I will tweak a little more and probably look for a better TX.  These mics are gonna just be for extra channels and I'm not looking to spend a ton on them.

I'll let ya know how it turns out. I am sure it will be decent.

I use a single end class A transformer based mic with Thoersch blue line cap and fet electronics for vocals with great results.

Hi Guys, the AA CM1084 actually uses a class "A" C451 2-stage type circuit which is a medium gain fet, coupled into a single silicon transistor configured as an emitter follower.

It is very important to check the curve of the FET in the 84/87 circuit and you can see the bias resistor in each U87 is custom selected and I am sure this would be the same for the KM84. In the KM84 they also use a negative feedback capacitor to trim the gain of the FET.

The 451 circuit is the AKG414eb circuit without the pattern change circuitry.


The CM1084 circuit drives a 2:1 output transformer not a 10:1 like the KM84 required. The low frequency distortion and response gets much better when you drive a transformer from the lower impedance of the emitter.

Now, in theory you can use anything between a 5:1 ratio and 9.5:1 ratio in a KM84/U87 whose circuits are identical except for the pattern circuitry and there is no de-emphasis in the KM84 circuit. Matt said, "he found a 6.5:1 transformer seemed to be optimum".

With the 5:1 ratio you get a 200 ohm output impedance and with the 10:1 you get a 50 ohm output impedance. The 200 ohm impedance will yield 6db more output into a 1K ohm load or greater than the 50 ohm setting.

The difference between a 5:1 transformer and a 2:1 transformer is 8db. The difference between 10:1 and 2:1 is 14db. The 10:1 have 14db greater loss.

This means the class "A" emitter follower circuit has to do between 8db and 14db less work plus you have the advantage of driving the transformer from a much lower drive impedance.


The lower the transformer drive impedance the lower the LF distortion. Take a Cinemag CM2510 which you can find all the spec's for on the Cinemag site. The CM2510 has an 8:1 ratio just for example.

When you drive the Cinemag from a lower impedance (600 ohms) not only is the phase response at the high frequencies better but the distortion is lower in the low frequencies.

When the CM2510 is driven from 600 ohms there is .6% distortion at 20hz with a level of +18dbu. When you drive the CM2510 from 20K ohms the the distortion rises and +12dbu will produce .4% distortion.

So, you get almost a 6db improvement in low frequency distortion by driving the transformer from the lower impedance.

A LDC capsule also has much more output than a 22mm SDC capsule but the single FET circuit in the U87 has been successful. However, in my experience I would like to see a bit more headroom from an LDC circuit than a single fet provides.

The maximum output of a U87 is -6dbu. The maximum output from our microphone is +14dbu minimum. The KM84 circuit is very simple but adding a class "A" emitter follower circuit adds one capacitor, one transistor and one resistor for a 14db improvement.

Cheers, Dave
aamicrophones.com