KMV254c: Gefell MV691 to KM54c modification.

[SparkleBear]

Hey!
Thanks for Your hard work!
I would like to buy 4 PCBs for the Mic Body and 2 PCBs for the Power Supply!
I have 3 691 amd one 750 which i like to convert!

Very nice project!

Bye Friedemann

That’s great! I’ll get those out to you when I’m back from tour at the end of April! I’m stoked you’re gonna get into this project.

The pm750 has a capsule with the same element as the m94 but it does not have the 10m dropper resistor so it is not swappable with the other capsules in the system. If you want to use the capsule with the KMV you will have to set the capsule voltage to 40v. The kmv has the 120v split in half by two 300k resistors. If you swap one of them for 600k (as in the the original km54 circuit) you will get the 40v you need. But make sure you swap the correct resistor because otherwise you’ll get 80v at the capsule and will likely cause collapse and damage due to arcing.
I’m working on making a mod for the capsule to turn it identical to the m94 by installing a precise ring to allow the 10m to join the circuit.

 

[adamasnan]

Just discovered this wonderful thread!

I would be interested in 2 mic PCBs and a PSU PCB please, am based in Berlin, Germany

Have DM'ed aswell

Thanks!

 

[SparkleBear]

DM'd In return

 

[el_rhodes]

DM'd In return

Hey man, I'd be very interested in buying a set of 4 pcbs for the mic bodies and psu. can you send me a DM? I cant as I'm still a "new member" here! Looking forward! Best! El

 

[madreza]

Hey man
Congrats for this project. Sounds awesome
Can’t wait to mod some and try
I d be interested in 4 mic pcb and 2 psu pcb pleaaaaaase

 

[donivan]

2 mic pcb 2 ps pcb
are they still available?

 

[Leo Schaal]

Hey Guys,
adamasnan told me about this project a while ago, I took a look at it and liked it very much. But I instantly had concerns about the use of the BV130 transformer in a cathode base tube setup. I have a vague feeling this transformer has a very low ratio, but do not remember exactly. I knew I have one of these lying around, but I have moved this summer and most of my stuff was still packed in boxes, so I did not find it and then forgot about it. But the use of 10µF and 22µF coupling cap in the MV692 and MV691 makes a low ratio quite probable. Also the circuit used in the MV691 mics has a very low output impedance. I think this might be a 2:1 to 4:1 transformer. Usually triode tube stages in microphone use 7:1 to 13:1 ratio. Too low ratios are like a short circuit for the sound, you loose a lot of level, deep and high frequencies.
It is possible to double the ratio by wiring the secondary transformer windings in parralel.
I will measure the impedances the next days and get back.
Greetings, Leo

 

[Leo Schaal]

I measured it, its ratio is 5:1. Sorry, I should have done the measurement before spreading rumours

So this is not that bad, even if not optimal. I would recommend wiring it in a 10:1 ratio. I try to built up a test tube the next days and measure the frequency response of both versions.

 

[SparkleBear]

Hey @Leo Schaal , thanks for chiming in on the thread. It is not posted in the documentation schematic but if you dive deeper into the PCB layout pictures, you can see that it’s not parallel windings but serial so it gives us a 9:1 … these mics definitely don’t sound anemic. They have a nice balanced bottom. Maybe I have my semantics wrong about parallel versus serial… But I am getting 10:1 ish.

I don’t have a fancy test setup so I’m definitely curious about your measurements.

And thanks for all the lovely binder connectors for this project.

 

[shabtek]

It would be great if someone still made an xlr insert for these mic bodies

 

[SparkleBear]

I wish I had metal manufacturing tools. I would totally develop something like that. It wouldnt be too hard now with Send Cut Send and JLC CNC. It would take some prototyping and 3d cad work... it wouldnt be such a complex mechanical part and I also have the XLR insert version of the MV692 which would basically just be caliper work to transfer to 3d model. I just dont have the time right now and i actually prefer leo's beefy binder VS the XLR.

 

[Leo Schaal]

"And thanks for all the lovely binder connectors for this project." And thanks a lot for ordering them.

At present in your schematics and the PCB layout, the input and output windings are wired in series. In this configuration I measure a 5:1 ratio. To increase it to 10:1, I had to wire the output windings in parallel.
You can measure this without fancy equipment. Just feed a sine signal into the transformer, this can come from your soundcard or even your smartphone. Measure the voltages at the input and the output of the transformer with a multimeter in AC mode. A lot of multimeters can measure AC relative exactly up to a frequency of 400Hz, so I would use a sine of 200Hz. I put a 1k resistor to the output to simulate the load of a preamp. I measured 110mV at the input and 23mV at the output. 110mV divided by 23mV =4,8 this is the ratio.
Anyway, wiring the secondaries in parallel doubles the ratio, but the primary inductance remains the same. So the change may not have the increasing effect on the impedance, we would like it to have. I try to attach this to a tube and make a frequency analysis of both versions the next days.