How do I add a -10dB pad to the Banzai KM84?

[zarndyhoff]

Hi, as the title suggests, I have built two of the Banzai KM84s. I love them and they sound fantastic. I have A/Bd them against our real KM84s at work, and they sound the same. Yay me.

Trouble is, they clip on overheads and other loud sources. A pad is the answer!

Is there someone who can give me clear, detailed instructions on how to add in a pad? I don't mind whether it's switched or un-switched.

Attached is a shot of my build, so you know I'm not a complete n00b.

 

[zarndyhoff]

Before anyone rushes to reply, I have had an answer through private channels. Leaving this up in case anyone wants to reply and help educate others

 

[egroys]

Before anyone rushes to reply, I have had an answer through private channels. Leaving this up in case anyone wants to reply and help educate others

Would you be willing to post the answer?

 

[zarndyhoff]

I'm hesitant because I don't want to invite a storm of finger-wagging curmudgeons (I have just finished scrolling through a forum argument elsewhere on the site), but here we go:

If C2 is currently a 4pF, replacing C2 with a 10pF capacitor should reduce the sensitivity by about 10dB.

I haven't tested this yet, but I trust my source!

Cheers

 

[pasarski]

I don't want to be a finger-wagging curmudgeom, but the 10db pad is 15pF in paraller to the the 4pF and you can find that value from every KM84 schematic available in interwebs

And those look wonderful btw!

 

[zarndyhoff]

I don't want to be a finger-wagging curmudgeom, but the 10db pad is 15pF in paraller to the the 4pF and you can find that value from every KM84 schematic available in interwebs

And those look wonderful btw!

Thank you for the kind words. I loved building them.

I am a solid assembler, having worked in hi-fi audio manufacturing, where I learned to solder & assemble to a very high standard. However, I know very little about circuit reading. I have seen that schematic, but having someone say exactly what to do is very helpful!

I very much appreciate your input

 

[jp8]

The KM84 pad consists of a 15pF cap, indeed. Your circuit may require a slightly different value. The Banzai circuit may have different stray capacitances, maybe you used a different JFET and maybe the capsule capacitance differs from the original KM84 (e.g. 3U capsule measures 53pF, KM84 just 36pF. The 3U capsule would need ~40-50% higher pad capacitor). They all determine the required capacitance to obtain 10dB attenuation. And maybe you'll actually want more attenuation than just 10dB. So you'll probably want to do some experimenting and see what works for you, starting with a 15pF capacitor.

If you want to add a pad switch, the tricky part is to keep the wiring to the switch as short as possible. And the switch itself should be tiny and add little stray capacitance. Stray capacitance from wiring would also attenuate the unpadded signal and you would have to reduce the value of C4. Not to mention potential microphonics of the wiring (vibrating wires will have varying and uncontrolled stray capacitance to the mic body). To prevent this, you could choose to reduce the capsule polarization voltage instead of a capacitive pad by puttng a resistor across C6 as depicted in the atrached schematic. For a 10dB pad, choose 4.7M, for a 20dB pad 1.1Meg (or just 1M).

Jan