Banzai's KM84 DIY Body & PCB kit build thread

[FarisElek]

Having a strangely hard time biasing these. I don't have a signal generator and when I send signal out my DAW with a 1000pf cap in the way, I just get high pitched screeching with the signal on or off. Not sure why. Getting good clean signal through both mics, thought they're both really soft signal.

Is there a test point with a ballpark voltage i can set these until I get my hands on a signal generator?


Ryan

 

[totoxraymond]

What are you using for software?

I used Room Eq Wizard which is free and has a signal generator plus a rta with real time distorsion display.
I also had my scope hooked up as well.

Start with a signal at 200mV pp and push it a little until it clips. The adjust ofr hot signal and lowest distorsion (you won't be at 0 unless your input signal is too low).

If the signal is really soft, your pot setting is probably too small.

I ended up with something around 18k if i remembered correctly.

Cheers,

Thomas

 

[FarisElek]

Is it normal for them to be very sensitive to air?

 

[RuudNL]

Is it normal for them to be very sensitive to air?

Yes, just like the Neumann KM84

 

[Strat96]

Is there any reason not to use electrolytics in place of the tantalum capacitors?

I had my reservations until I had a deep conversation with someone about them. They swore by them and said the sound you get from them is amazing. Electrolytics aren't the best sounding in the audio path, but I know of circuits that do use them in the audio path. The problem with tantalum is if you don't use the electronics for a long time they can dry up and short out, but the fail rate is still pretty low according to the conversation I had.

Thanks for your reply.
It would be great to do some comparisons on the different transformers.
GZT-84 seems cheap Chinese, have some doubts on how good are those.
3U Audio, although Chinese maybe it's good quality since they make some nice stuff.
Cinemag I can only say good things about them in general.

It's a shame that Moby doesn't make this transformer. Moby transformers are really high quality and he gives a lot of attention to detail and correct materials, that would probably be the best transformer for this project

I hope they don't sound cheap. I was kind of going off of that they were offered as an extra with the kit and they sold out.

 

[TwentyTrees]

I had my reservations until I had a deep conversation with someone about them. They swore by them and said the sound you get from them is amazing. Electrolytics aren't the best sounding in the audio path, but I know of circuits that do use them in the audio path. The problem with tantalum is if you don't use the electronics for a long time they can dry up and short out, but the fail rate is still pretty low according to the conversation I had.

Tantalum capacitors have a number of weaknesses - as well as your point about drying out, they are very intolerant to reverse or excess voltage (to the point of exploding, by all accounts!), and there are some ethical concerns about sources of tantalum.

I agree with your point about the sound though, when specified appropriately. I've done blind A/B tests with a Helios preamp clone, and with a Neumann V472, and consistently preferred the sound of tantalum to aluminium in key coupling positions. That's just two datapoints of course, and I've not tested this in a mic - and of course what sounds good to one person might not to another. I no longer default to replacing tant with alu, though.

 

[FarisElek]

Yes, just like the Neumann KM84

Great! It's my default to think i did something wrong. Sounds great. Finally got it calibrated. Was testing it near an AC vent and it caused much worry.

 

[fsalter]

I was missing one of the Tantalums for some reason (C4), and used a Nichicon electrolytic instead (4.7uf, 35V). Do you think it matters?

 

[FarisElek]

I was missing one of the Tantalums for some reason (C4), and used a Nichicon electrolytic instead (4.7uf, 35V). Do you think it matters?

Probably not. Modern electrolytic are pretty good. Some might say they hear a difference, but I think you'll probably be okay.

 

[midwayfair]

I'm planning to use cinemag's 8:1 (2510) instead of their BV107-alike. Is there anything majorly different except the copper tape to ground the laminate stack and the ratio? I know my output will be slightly lower, but I plan to use these for drum overheads almost exclusively so that's a feature not a bug.

https://cinemag.biz/mic_output/PDF/CM-2510.pdfhttps://cinemag.biz/mic_output/PDF/CM-5722-Data-Sheet.pdf
Is the copper tape literally just a piece of tape stuck on the laminate? Any reason I can't just sick a piece on there myself?

(I'd prefer not to have to order the "right" transformers, Cinemag is slammed right now and it took me a very long time to get the last few I bought.)

EDIT: Eh, soldered them up. Somehow managed to kill two FETs! Or maybe they were dead to begin with, they were a matched pair I had in my own stash. I replaced them with the pair that came with my kits, which actually turned out to be very very close. I don't know if that was intentional but it saved me having to measure a bunch looking for a pair. These sound really nice, too bad I finished too late to do some drum takes.

 

[stokesmusic]

Thank you everyone for all the knowledge you're sharing, and thank you Graeme and Bonzai for this kit! I just received two from Wouterplop, and I can't wait to start building. This will be just my fourth project, so I'm a total novice, but obsessed!

I've ordered the BOM from Mouser, but the WIMA .1uF cap, Vishay 10Mohm resistor and TE 1GOhm resistor are on back order for 18,19 and 22 weeks! Maybe normal in the DIY world? But if feels brutal.

I'm living in Berlin, but working often in the US and around Europe. Does anyone have any ideas on where to find these components in the US or Europe? I've checked Conrad Electronics in Berlin, and I am about to email Segor here in Berlin as well.

Last question - I picked up a handful of WIMA MKS 4 0,1/100 caps, but they have 10mm spacing between the legs. Is it possible to use these by bending the legs in, or is that asking for trouble?

Thank you all!!

Colin

 

[stokesmusic]

I realized after posting that there was already a conversation about the 1gig resistors here over on the original KM84 thread.

Neumann KM84 DIY Body & PCB kit – CLOSED

 

[RuudNL]

1 G.ohm resistors:
https://nl.rs-online.com/web/p/through-hole-fixed-resistors/1372742

 

[FarisElek]

Also, Mouser has some good 1g ohm resistors in stock, they're just called 1000m ohm instead for some reason.

 

[johnchantler]

Hello Colin,

I’m about to embark on this project too.

Ordered the following for the 10M resistors: https://www.mouser.se/ProductDetail/71-CMF5510M000FKEA

I’ve also ordered 63v versions of the out of stock WIMA film caps, but not sure if they will work. Is it just that they will have shorter expected life due to less headroom?

 

[stokesmusic]

Thank you for your replies Ruud, Faris and John!

Interesting that 1G ohm is listed as 1000m ohm on Mouser.

In case anyone else is having trouble, I ordered some Wima caps [MKS02-63 100N] and Vishay 10 M Ohm [VI MBE04140C1005] resistors at Reichelt Elektronik. I bought 30 of each to justify shipping cost, so if anyone needs any, I'm happy to share!

 

[stokesmusic]

Hello Colin,

I’m about to embark on this project too.

Ordered the following for the 10M resistors: https://www.mouser.se/ProductDetail/71-CMF5510M000FKEA

I’ve also ordered 63v versions of the out of stock WIMA film caps, but not sure if they will work. Is it just that they will have shorter expected life due to less headroom?

Hi John! Thanks for the info. I think yours is closer substitute than the one I ordered? If mine doesn't work, I'll order your suggestion!

I also ordered 63v versions of the WIMA caps. I'm interested to hear what others have to say about this substitution.

 

[FarisElek]

63v caps will be fine, just over- rated. The only problem I think thay might arrise is that they won't fit close to the transformer. I used a much smaller ceramic cap right next to the transformer since it was in stock and should work well according to an earlier post on this thread.

 

[CalGTR]

OK, so I'm having some issues, maybe, with biasing. I'm ending up with some strange results, seems to me. I'm basically seeing max headroom with max source resistance (20k). I am never able to see symmetrical clipping, regardless of trimpot settings. It's always the bottom that flattens out first, and backing off the source resistance only makes the bottom of the wave worse, all the way until the FET shuts off.

If I back the trimpot off to about 18.2kOhm, I can improve the distortion such that for a 200-300 mV input signal, or thereabouts, I see below .5% THD on the output, which seems reasonable enough to me (but then again I'm a bit clueless as to what to expect). Moving off of this particular bias point it either direction results in more distortion. The source-ground voltage I'm seeing at these higher source resistances is only about 4.5V, which is miles off of the 10V or so I thought I should be expecting.

Any of this seem reasonable to anybody, or should I be hunting for problems somewhere in the build? Should I seek a different FET that might be closer in spec to what one might usually expect? Any comments/advice will be appreciated.

Thanks,

Calvin

 

[totoxraymond]

It seems to me that i had the same experience. Just set for minimal THD as you did before and you should be good.

Thomas