DIY RCA BK5 Ribbon Mic- small signal output

a soBer Newt · Mar 14, 2023

jannae66 said:
there´s also a small hole under the bigger hole in the KU5A, but I don´t know what advantage that would have. I don´t have a development team to figure that out.

I couldn´t exactly figure out how it is made on the original. Is there a huge gap between the magnets only covered with cloth? What would be the benefit? I thought that it´s too much of a open area, that would be too sensitive for incoming noise.

I was lead mechanical designer on the KU5A project let me know if you have questions.

jannae66 · Mar 17, 2023

a soBer Newt said:
I was lead mechanical designer on the KU5A project let me know if you have questions.

Hello soBer,
Thanks a lot for your help. I honestly have a lot of questions. I hope they don't concern any stuff you're not allowed to talk about.
Have you tried to build these coiled lybyrinths at AEA or do the classic labyrinths in RCA design work well?
What are the advantages of dividing the labyrinth into two parts like in the KU5A?
Are the phase shift openings on the original BK5 just holes or are they open across the entire gap between the magnets?
Am I correct in saying that the phase shift openings on the KU5A each consist of a large hole and a small one? What are the advantages?
Thank you very much and best regards!
Jan

jannae66 · Mar 17, 2023

Marik said:
As mentioned earlier, first try to fill the labyrinth much looser and without the filling at all in the elbows. Then try the straight tube coupled to the motor loosely filled (and experimenting between filled/not filled) and see if there are any changes. That will give you an idea for the optimal arrangement.

The spiral labyrinth you mentioned has the same 'elbows'. Earlier I mentioned coiling the tube exactly for the reason to eliminate those elbows.

The main purpose of the labyrinth is to provide an infinite load to the ribbon to get an optimal output and uniform frequency response. Obviously, since the output is low the load is not optimal.

Best, M

So I tried to implement all the tips as best as possible. I had to cut and sand the return circuit all by hand, but the parts all fit together very well with no gaps. All of this was held together very well by the magnetic force even before it was glued. I also revised the blast filter and filled the labyrinth very loosely with the acousta stuff. Below the ribbon I now have more distance to the mesh and a bit of acousta stuff. It was a real pita to fit the bigger motor into the headbasket, so I had to rework that also. I hope to test soon and that everything worked out. The Lundahl transformer isn't coming until next week.

Thanks and best regards!
Jan

a soBer Newt · Mar 20, 2023

jannae66 said:
Hello soBer,
Thanks a lot for your help. I honestly have a lot of questions. I hope they don't concern any stuff you're not allowed to talk about.
Have you tried to build these coiled lybyrinths at AEA or do the classic labyrinths in RCA design work well?

We played around with a horn shaped labyrinth at one point, something that George Augspurger suggested but that never really went anywhere.

jannae66 said:
What are the advantages of dividing the labyrinth into two parts like in the KU5A?

The KU5A take a lot of design cues from the NHK/Toshiba model H, that is where the divided lab comes from. There is a triangular structure in the magnetic return path that started just below the back side of the ribbon. We did a number of listening tests and decided that the model H out preformed the BK5s so that is why we went with the dual path.

jannae66 said:
Are the phase shift openings on the original BK5 just holes or are they open across the entire gap between the magnets?

Its been a long time since I looked into a BK5, I am no longer at AEA, I do not remember at this point.

jannae66 said:
Am I correct in saying that the phase shift openings on the KU5A each consist of a large hole and a small one? What are the advantages?

Yes there are two holes, It had to with controlling the polar pattern at higher frequencies if I remember correctly. They are covered with Saati Tech to help control the impedances of the leak at frequency.

jannae66 said:
Thank you very much and best regards!
Jan

The biggest thing that I can tell you is any leak what so ever with completely throw off your polar plot, make sure you have a very good seal on the labyrinth, and any other point that the backside of the ribbon "sees"

jannae66 · Mar 23, 2023

a soBer Newt said:
We played around with a horn shaped labyrinth at one point, something that George Augspurger suggested but that never really went anywhere.

The KU5A take a lot of design cues from the NHK/Toshiba model H, that is where the divided lab comes from. There is a triangular structure in the magnetic return path that started just below the back side of the ribbon. We did a number of listening tests and decided that the model H out preformed the BK5s so that is why we went with the dual path.

Its been a long time since I looked into a BK5, I am no longer at AEA, I do not remember at this point.

Yes there are two holes, It had to with controlling the polar pattern at higher frequencies if I remember correctly. They are covered with Saati Tech to help control the impedances of the leak at frequency.

The biggest thing that I can tell you is any leak what so ever with completely throw off your polar plot, make sure you have a very good seal on the labyrinth, and any other point that the backside of the ribbon "sees"

soBer,
Thank you very much for your insights!

So I have now tested the microphone very unprofessionally with a signal generator from my cell phone. You can read the results in the photo. The numbers are of course negative dB. I am very satisfied with the front to back difference of approx. 15dB. Here are two measurements. The second one is a bit different and theres´s a comparison with the Beyer M500.

However, the difference of approx. 7dB to the M500 and 11dB to a SM57 is very high. Is it because the ribbon (approx. 2mm) and the resulting wide magnetic gap element is too wide? Would I get even more output with a narrower one? AEA KU5A has got a 1,27mm wide ribbon. Or should I use even bigger magnets?

I would love to have the magnetic return steel parts on the sides made of one magnet like in the RCA BK5/AEA KU5, but unfortunately I can't find magnets with this orientation anywhere... Or I just glue several magnets together in the size I have.
after a few tests:
I liked more output, cause there was a notable noice in the background if I do vocal recordings, compared to the M500. And I also tried the Klark Teknik CT1 (Fet Head Clone) with no success. BUT: I figured out that the Behringer UMC 404HD was the Problem. After I put the mic into my modded M Audio DMP2, there was no background noice. Plenty of clean gain with it.

Maybe I will have the time to post a few recordings today.
Best regards, Jan

jannae66 · Mar 23, 2023

Here are finally the recordings. The mics: modded tBone RB100 (1.8um ribbon and Artur Fischer Transformer)

Beyer M500 (actually a BM85 with XLR), BK5 (the DIY one, of course) and Shure SM57. I chose the SM57 because almost everyone has it and therefore it's quite good for comparison. Acoustic guitar: Furch G20 CM with 60cm spacing to the mics. I know the placement of the mics will result in different sounds, but I've tried to line it up as accurately as possible.

Unfortunately, the vocal recordings aren't that great because I've got a cold. Sorry. Once from a distance of 20cm and once directly in front of the microphones.

And finally, the electric guitar. Fender Strat with twang banger in the bridge and leosounds neck/middle pickup-> DIY Nagaviper (silicon Rangemaster)-> DIY Marshall 20Watt Bass Plexi->DIY open back 2x12 with WGS Reaper HP- right next to the dust cap. Clean sounds and distorted sounds achieved only by turning the guitar volume knob.

I really like the sound of my DIY BK5 on the electric guitar. If it sounds a little bit like a real BK5, I understand why it´s a favorite one on guitar cabs. A completely different timbre compared to the other mics. I didn't use the medium ribbon on the electric guitar because it has too much proximity effect. I need to take another shot with my other ribbons.
But so far I´m really happy with my DIY BK5.

Just for fun I made a 1-inch ribbon mic on the fly, without a labyrinth, only with 2 magnets without magnetic return, but almost the same blast shield and a china transformer, the grip of the microphone is filled with the acousta stuff and towards the end it is filled a little more densely. The foam around the motor is only for holding the motor in place ,but maybe has a negative impact on the sound. Although the ribbon size is the same as my DIY BK5, it actually sounds quite different. (see soundfiles: DIY small ribbon) The rear sound rejection is not so good without a labyrinth (see photo), but acceptable. And approx. 7dB less output than my DIY BK5.

jannae66 · Nov 30, 2023

Hello again,
it´s been a long time I switched the transformer to a FAB USA Microphones transformer, but I wanted to wait how it will sound in a recording and now it´s finally online.
I have to honestly say that Chris from FAB Microphones is a very friendly and helpful man, who cares deeply about his customers' concerns.
I bought different transformers from him and they are really exactly what I was looking for. I tested all and finally installed the FAB USA in my BK5 clone which gave me around 5dB more output with a great sound for acoustic guitar and vocals- besides electric guitar. I thought it was a little more forward sounding in presence region compared to the Lundahl. I liked the change. The USA had a tiny bit more output than the other FAB transformers. Maybe only in specific frequencies. I put it into a Mu Metal can and no noise here.

I´m really happy with my BK5 clone now.
Here are two songs online which have around 75% of the BK5 and the rest is the Sennheiser e906 on the electric guitar:

Sondebris - Remembering
Soundebris - Grinding Sorrow

Next song should come out on 16.12. which I mixed a little better I think. Also with mainly the BK5 on the guitar. It has become my go to mic for electric guitar.
The lundahl is now working with another one in my Stereo-Ribbon i made out of two t.bone RB100.

The Drum Room in the recordings is this mic in the hallway of our rehearsal room. No aritficial reverbs were used for the drums except for the snare only.
On the next song I wanted to test the BK5 on vocals in a recording. But that will be next year I think.
The bass in the songs is recorded with a modified t.bone RM700 with the FAB XL transfomer. Great sounding mic. Would also be nice as a vocal mic.

My next project will be to make the DIY small ribbon seen in the older post work better. I have also made a magentic return path and built a small labyrinth which gave me a big 500 Hz bump, which is maybe caused by the short labyrinth. So i make a 100cm coiled labyrinth for this one. 3D-printer would be handy, but I have none, so I make it with concrete and hot glue

But I start another thread for this one.

Best regards,
Jan

aprats · Dec 11, 2023

jannae66 said:
Hello again,
it´s been a long time I switched the transformer to a FAB USA Microphones transformer, but I wanted to wait how it will sound in a recording and now it´s finally online.
I have to honestly say that Chris from FAB Microphones is a very friendly and helpful man, who cares deeply about his customers' concerns.
I bought different transformers from him and they are really exactly what I was looking for. I tested all and finally installed the FAB USA in my BK5 clone which gave me around 5dB more output with a great sound for acoustic guitar and vocals- besides electric guitar. I thought it was a little more forward sounding in presence region compared to the Lundahl. I liked the change. The USA had a tiny bit more output than the other FAB transformers. Maybe only in specific frequencies. I put it into a Mu Metal can and no noise here.
View attachment 117850 View attachment 117851
I´m really happy with my BK5 clone now.
Here are two songs online which have around 75% of the BK5 and the rest is the Sennheiser e906 on the electric guitar:

Sondebris - Remembering
Soundebris - Grinding Sorrow

Next song should come out on 16.12. which I mixed a little better I think. Also with mainly the BK5 on the guitar. It has become my go to mic for electric guitar.
The lundahl is now working with another one in my Stereo-Ribbon i made out of two t.bone RB100.

View attachment 117852
The Drum Room in the recordings is this mic in the hallway of our rehearsal room. No aritficial reverbs were used for the drums except for the snare only.
On the next song I wanted to test the BK5 on vocals in a recording. But that will be next year I think.
The bass in the songs is recorded with a modified t.bone RM700 with the FAB XL transfomer. Great sounding mic. Would also be nice as a vocal mic.
View attachment 117853 View attachment 117854 View attachment 117855

My next project will be to make the DIY small ribbon seen in the older post work better. I have also made a magentic return path and built a small labyrinth which gave me a big 500 Hz bump, which is maybe caused by the short labyrinth. So i make a 100cm coiled labyrinth for this one. 3D-printer would be handy, but I have none, so I make it with concrete and hot glue
But I start another thread for this one.

Best regards,
Jan

Where did you find that mu metal can for the fab transformer?

k brown · Dec 11, 2023

jannae66 said:
soBer,
Thank you very much for your insights!

So I have now tested the microphone very unprofessionally with a signal generator from my cell phone. You can read the results in the photo. The numbers are of course negative dB. I am very satisfied with the front to back difference of approx. 15dB. Here are two measurements. The second one is a bit different and theres´s a comparison with the Beyer M500.
View attachment 106881 View attachment 106882
However, the difference of approx. 7dB to the M500 and 11dB to a SM57 is very high. Is it because the ribbon (approx. 2mm) and the resulting wide magnetic gap element is too wide? Would I get even more output with a narrower one? AEA KU5A has got a 1,27mm wide ribbon. Or should I use even bigger magnets?

I would love to have the magnetic return steel parts on the sides made of one magnet like in the RCA BK5/AEA KU5, but unfortunately I can't find magnets with this orientation anywhere... Or I just glue several magnets together in the size I have.
after a few tests:
I liked more output, cause there was a notable noice in the background if I do vocal recordings, compared to the M500. And I also tried the Klark Teknik CT1 (Fet Head Clone) with no success. BUT: I figured out that the Behringer UMC 404HD was the Problem. After I put the mic into my modded M Audio DMP2, there was no background noice. Plenty of clean gain with it.
Maybe I will have the time to post a few recordings today.
Best regards, Jan

Care to share the deets on your DMP2 mod (for higher gain, I assume)?

jannae66 · Dec 13, 2023

aprats said:
Where did you find that mu metal can for the fab transformer?

Hi,
I found it on ebay: 1x A262CAN MU-Metal screeningcan für AF Transformator | eBay

Regards,
Jan

jannae66 · Dec 13, 2023

k brown said:
Care to share the deets on your DMP2 mod (for higher gain, I assume)?

Hi,
Of course. It was not due to higher gain, cause it's got plenty of it. The mod has made the bass tighter, better depth in sound, smoother highs and stabilized the power supply. But rahter subtle differences. I changed the caps in the signal path to Wima MKP and the big ones to ELNA Silmic II and in the power supply for bigger caps, 1000uF.

And installed a pad switch and a low Z switch.
Greetings,
Jan

Biula · Jun 1, 2024

The mild steel magnetic "return" block is essential in the ribbon mike magnetic motor: the magnetic circuit must have closed force lines.

Deadly Mix · Jun 3, 2024

Man... The RCA BK5. Wes Dooley offered to rebuild my old Grampian GR1 as a BK5 many years ago. I wasn't familiar with the BK5 at the time and I didn't know what a legend I was speaking to. I ended up passing, thinking "Forget it. I'll just do something better rebuilding it myself". I hit up AEA years later, after I knew what an ***** I'd been to pass on his work and the BK5 design, and he wasn't interested in doing it anymore

.
Sorry, not relevant. Just wanted to praise your choice of mic to model after.

DIY RCA BK5 Ribbon Mic- small signal output

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