RCA/GrundigGRM /ribbon mic **thanks PRR/Marik/xvlk*IT WORKS!

[ToobieSnack]

hey all,
finally got around to uploading my current mic project pics
my previous mic worked well but needed minor improvements to boost the output. while I waited on magnets I started this mic.

The goal here was to see if these ceramic magnets could be optimised for use in a decent ribbon mic.

i used my RCA 74b as a guide figuring that RCA did thier homework and this was an opimum arrangement. you can see it in the first pic.

As it was stated by PRR iron is 1000 times more reactive to magnetic fields than any other object on earth (with some small exceptions).
For this reason i used iron straps, some small angle iron and some zinc coated square iron rod all intended for iron railing construction. this material is readily available at any hardward store.

the peices were made as close as possible to my 74.

the magnets will be removable for future changes according to the outcome.

I will maintain the ribbon size and spacing so that i will be able to compare the output increase to my previous mic....ther will definitely be an increase in output due to the focus of the magnetic flux through the angle iron pole peices. Additionally the strapping between the magnets (on the back) will aid in this inreased output.

As you will see in the photo's there are 2 different mounts for the motor

this was done per PRR recommendation. the RCA used brass for this item and I used aluminum. these materials are alike in the fact that they do not respond to magnetic flux.Magnetic material outside your magnet structure will decrease you max flux at the ribbon gap. height was also increase to accomodate the tension assymbly and the trafo.


the bottom ribbon mount was made using a track used for mounting a light to a camcorder. it is modified to allow tension adjustment to the ribbon.
I will make the top ribbon mount the same but no adjustment will be available or needed for the top ribbon mount.
the ribbon mount was recently removed and will be relocated.

I will be updating and adding info to this post as I progress.
for now here are the pics of my current progress

as always your kind comments help and suggestions are welcome
enjoy
ts

later
ts

 

[ToobieSnack]

ok i have decided to go with a different body style....
the guts will still be the same of course....as close to identical to the RCA as possible with readily available materials....
my frame turned out so nice and slim...this is the reason for the design change...
actually another reason is the grill work seemed a bit of a challenge..... both in skill and available materials

it will be similar to the Grundig GRM ribbon mics

the base will still be flat and rectangular but... above the base structure will be wire mesh wrapped over the top lined with cloth...the side will be copper made like a u channel ...rounded at the end(top) with a lip around the front edge to hide/attach the grill material.

this will give a nice clean,slim look... I have mocked it up and it looks really cool....forming this "lip" on the copper may prove to be a bit of a challenge ...I'll probably wait until another sunday metal work session at my brother-in-laws house. the copper man...he worked up a base for me in twenty min..... i really wish I didn't have to tell him there is a design change...crap...i'll probably have to pay him this time...:sad: hehe :grin:

anyway as alway your kind comments , help and suggestions are always welcome
later
ts

 

[ToobieSnack]

also this mic is going a bit faster than the first one...
i have mounted the ribbon clamps and I am ready for wire and a ribbon
the body will take time but I beleive it will be worth it. :wink:
later
ts

 

[PRR]

A magnet has two sides.

Your L-poles get good contact on one side.

But you have no good contact on the other, back, side of the ceramic. Just a poor contact to the edge, and contact with the edge of both faces of the magnet.

Have you ever worked on a car starter and battery wires? The battery is the magnet. You want most of the voltage to arrive at the thin wire inside the starter. You have a fat red wire from the battery to the starter, AND some fat path through engine block, jumper-strap, frame, and back to the other battery terminal.

In that analogy: you don't have the return path, except poor edge-contact (and in effect to both battery terminals!) and through the air. The neo-magic-ium magnets can actually work with just air return, but classic ceramics will need a shorter return path through iron.

 

[ToobieSnack]

PRR 1000 thanks
I know you are very busy and thanks for taking the tme to review this.
your comments are alway helpful and I always learn something.

So How can I fix this?
If i understand correctly I have no outside iron...(to the outer magnetic poles)
My square that I copied from the RCA is ONLY edge connected as you have stated...
If i were to add iron to the outside and connect it to the square added to the back would this solve my problem?
this would make a C shape of iron when view from the top..rather than a line across the back.
ASLO: does the outside iron absorb the magnetic flux and keep it from wasting into the air?

as always much thanks
ts

 

[Marik]

Toobie,

Does the square constructed of iron strips? If so, once you place the magnets on the square, the iron will short N and S, reducing the energy. If you make outside U- looking structure out of thick iron (even better like O--looking), it will work nicely as a magnetic return circuit, and as a supporting structure.

 

[ToobieSnack]

thank marik

well I'm kinda cloning the RCA so whatever RCA did ...I did...

One small note

RCA used a bolt to attach the magnets and pole peices
it travels in a grove "in the magnet" at this outside area you and PRR speak of (polar opposite of the ribbon gap)
this is the only metal at this location on my RCA. of course RCA is using alnico...but...

is this bolt enough/any help to the return circuit?

Does the fact that RCA put this bolt through a groove IN the magnet make a difference?
(as opposed to just laying down the side of the magnet like mine)

Is there a flaw in RCA's design?

***AS A FIX***

waht if I put a 1/32" iron plate on the polar opposite (from the gap) side of the magnets (outside) and touching the back square iron plate...?

this would effectively make a U (or as i previously referenced... a C...this is like tom-a-to tom-ah-to...U...C...its the same shape ..right? :grin: Well block letter c would be)

ASLO: if i had the room...would the actual...C...shape (looking down from the top of the magnets) be better since it can be more precisely aligned with exact N/S magnetic energy?
kinda like the O pattern you referenced...
but without anything in front of the ribbon...
the ends of the C's at the exact polar opposite if the ribbon gap?
(centered on the magnet)

Im i getting this?...i think so... let me know.....



thanks soo much
ts

 

[El Fito]

Toobie, good progress! I'm keen to hear the results when you are done. Sorry I actually posted my impressions about your work on the other thread. It looks seriously promising though :grin:

 

[ToobieSnack]

NP nice to dsee your comments always fito.
did you enjoy the reading material I have recently discovered here at the forum?
lots of cool info in that NYD post/.pdf...:grin: thnx agin NYD....

I was recently lucky enough to recieve some info from xvlt ...thanks xvlt...I will share this very excellent info....

I have numbered the info for easier discussion...feel free to expand on any of these individual quotes or the info as a whole...:grin:

1

Hi, Toobie.
I am not much diplomatic, but I think, that your
design is some obsolette to use neodymium.

You can be correct with ferrites. You can use pole pieces
made from ARMCO steel to concentrate field to gap.
But you need some flux frame to form loop.
For ferite or ALNICO it is no problem, and you can use
Bang/Olufsen or Royer type of flux frame.

2

But if you want to se neodymium, you must use generally
different design. You must use magnet as pole pieces
(as this solved Anderson in his RCA KB3). You can
not use frame for flux loop purpose, because of saturation-
you must use magnetic leak in the air to enclose flux
loop. In that ways Marik is O.K.
but you must also have correct shape of Neodymium
magnets (this shape was developed by Gayford for formerly BBC,
now Coles velocity microphones, but shape from Anderson s KB3
would be O.K.

3

And you can generaly do better magnetic-circuit if you
use enclosure (mic body) as flux path. This way was done
by E.Beyer in his Beyerdynamic mic. I mean, that his
helical is obsollete and better is to use many of ARMCO rings with
two neodymium magnets inside it. Rings forms
body, shield, screen and flux frame altogether.
And you can combine all this ways and have 0.8 T induction in a gap with
magnet width 0.5 cm and superior frequency and directional response.

The only one reason why I not did that was technological, price
of elements to complete this my mic will be higher than
tio buy Royer. And better parameters can not that investition
gap balance. And I need only model for measurements in vacuum,
not whole microphone.

4

That is all about modern unit. You can build, you can only
ARMCO cylindrical rod with 2.5 cm diameter,
neodymium rods customer done and time.
... And money and factory to do custom Neodymium in 10 pieces for
prices lower than dollars 100 000...

5

Ribbon dimensions: 2 inches length and 1:8-11 width ratio.
Diferen dimensions are hard to assembly.

6

After this unit you need transformer. 0.25 to 200 ohms.
I spend one year to design ribbon transformer.
I studied amplification part (noise of ribbon - contact - transformer)
rather than acoustic design, and still this was hard work,

7

Good luck for You, development of ribbon mic is hard work,
and after you do magnets there ale twice problems at electrical part.
It is low impedance low noise circuit and every Schottki contact destroy
your previous work. It is not mistake, that Schottki diode
was only garbage in the ribbon mic development.

Michal Vlk, M.Sc
TESLA, division of AM transmitters, Prague.

great info
thanks again xvlt

As always your kind comment,help and suggestions are sincerely appreciated
later
ts  

 

[ToobieSnack]

OH BTW...marik i now see waht you were talking about with consideration to the O shape magnetic return...
well I am definitely learning...
later
ts

 

[ToobieSnack]

Ok update:

last night I went out and bought some 1" iron strap...
cut it to the 1 7/8" lenght to match my magnets...
sawed..grinded...rounded with a drill... a grove down the middle across the metal strap to accomodate the passage of my magnet mounting bolts...

now I have this U shape marik, PRR and xvlk have advised (thanks..ya'll)

now questions

should I "double up" this iron with a second peice?
I have the room if i slot the second one as well...(like a hole drilled into two peices of strap metal ...1/2 (U shape) through one...the other 1/2 (u shape) through the other...like a slot...

also should I continue this metal onto the front of my magnet and join to my pole peices?

It seems that this is how RCA did it.


to further explain****
this will mean that my magnets will be "wrapped" on three sides ....with only the small amount on the fourth side where the iron L poles don't cover the magnet in the center (back...behind the poles) ...well also FWIW they have no iron on their extreme top/bottom..the ends of the magnets...which i think...is no concern to me...N/S is what we are after...right?

as alway your kind comments, help and suggestions are sincerely appreciated
thanks
ts

PS does anyone have exterior/interior pics or technical data on these grundig GRM ribbon mics?

 

[ToobieSnack]

Another Cool Sunday trip to my sisters... where... my bro-in-law made me some :
WAY EXCELLENT" body peices from copper.. :grin:

Square base box like the RCA but thinner (3"w x 2 1/4"h x 1 3/4"d )and....
i made side peices to attach / build the screen.
the sides are u shaped about 5" long 1 3/4" wide with a 1/4" lip on either side...and arched over top.
the top of this side peice is arched (from a side view of the mic)... so a portion of the lip was left long..... to bend over and cover the top of this arch....

I will solder the arch tabs.... and solder the screen to the 2 lips

from the front it will look square
from the side it will resemble an ice cream stick...flat on the bottom then straight up ...with the top arched...ALL COPPER!!! :shock:
the screen is the same screen i used on my first mic ...brass...so..

Copper with a brass screen.

IT LOOKS BEAUTIFUL!! :grin:

I'll post some pics as soon as I can

I think you will all like it. :grin: I know I do.... :grin:

later
ts

 

[ToobieSnack]

here's a pic update of my iron changes and my body

there are 2 new peices of iron here....on the front connection to my pole peices and the polar opposite from my gap and pole peices... my magnet is "Wrapped"

this pic shows the iron connecting to the back square...
QUESTION: Marik, (or others) should I insulate the connection between the magnet and the back square?
Only allowing contact on the sides and front to the pole peices...and a bridge (square) accross the back?

this is the body we did at my bro-in-laws house... this is just held together by the elastic bands for now and not fully representative of the finished product..one of the sides will be re-fabricated due to a slight error..the right side is beautiful and only needs soldering...what do you think :grin:

how is my iron now? magnetic return circuit ....?

do you like the body style?

as always your kind comment, help and suggestions are always welcome
and appreciated.
later
ts

 

[El Fito]

Mate, I love the body style! It reminds me of an old Remington shaver my dad used to have - most cool indeed! Thanks for letting me know about the pics. I'm keenly following your progress :grin:

 

[Marik]

[quote author="ToobieSnack"]here's a pic update of my iron changes and my body

there are 2 new peices of iron here....on the front connection to my pole peices and the polar opposite from my gap and pole peices... my magnet is "Wrapped"
this pic shows the iron connecting to the back square...

QUESTION: Marik, (or others) should I insulate the connection between the magnet and the back square?
[/quote]

I am kinda uncertain what is your arrangement, but it seems that you short N and S of the magnet. If that's the case, you loose a lot of magnetic energy.

 

[ToobieSnack]

just ribboned her up and did inital testing ,,,,and.... HEY.... HORAY IT WORKS

TONS of output... the output is quite wonderful...well above acceptable

I have not tested for frequency .... just initial power output listening test

my preaamp is pretty noisy

I'll see if I can get a better solution and post some audio clips

I tested without any body, cover or damping material
I was getting some odd sounds I assume are directly related to all the hard surfaces immediately next to the ribbon and some damping should take care of this..

I'll add an update soon

later
ts

 

[ToobieSnack]

update:

the odd sound I was hearing was:
my ribbon was too tight...it was acting like a string on a guitar or piano.
vibrating after the initial energy was applied.
I have been experimenting with some treatment with my ribbon so that it will have maximun ribbon pattern "memory/tension" and i guess it's working better than i expected .... :wink: :wink: :grin:

having adjustable ribbon tension mount ROCKS! :grin:

this mic..... :shock: SMOKES :shock: ......my RCA
!!!!!:grin: better everything :grin:!!!!!!

anyone want to buy an RCA 74b? :grin: ... hehehe... lol ...

later
ts

 

[El Fito]

Come on, Toobie, I'm all wound up now! Good job! Let us hear the clips and see some pics of the finished-finished product!

 

[ToobieSnack]

:grin: ...lol... ...haha... :grin:

yeah I know fito... beleive me iit's killin me too! :sad:

you can see at the bottom of my post (i need help with ...blah blah blah) that my weakness is my compressor/mic-pre part of the audio chain... :sad:

I really don't want to represent my mic through a noisy preamp/compressor... if you want I will sen YOU a clip via PM since we are workin so closely together.. and you would understand my position and take that into account....since you have similar obstacles...(vintage tube preamps)

AT the same time I nees 48v phantom for my NEW JZ-1 mic.

Also I am currently trying to get an identical trasformer so I can do comparisons of the two designs I built ...using the same magnets,ribbon,and trafo's and then... also comparing it to my RCA.
So you see this is no simple undertaking.

If I can't get another identical trafo there will be some soldering and un-soldering involved which will make the process even more tedious... :?

then obviously I would blend in the new JZ-1 to this mixture and see what total sound I could acheive....beleive me ...when it happens it will be a ..beautiful thing.....I gotta make sure I have new guitar strings too.. hehe

I have a lot of loose ends here...and i love the forum so much because it is my savior :grin: ....haha..... technically speaking ...lol

anyway ... thanks for expressing your enthusiasm and confidence in me!! :grin:

and rest assure ...clips are coming... and you will be the first to know.. :wink: :grin:

later
ts

BTW ..fito..my ribbon material is radically excellent !!!...still ... :wink: :wink: all proceeding as anticipated :wink: :grin:

 

[Guest]

[quote author="Marik"]I am kinda uncertain what is your arrangement, but it seems that you short N and S of the magnet. If that's the case, you loose a lot of magnetic energy.[/quote]
... And I can not see anything. Much magnetic energy eats that bad pole
pieces...

But this work is mainly pre- Olson ribbon mic. Some year 1924,
Schottki and Gerlach etc.
Yes, you can use it for nice, welvet sound, but (because of very
wide magnetic part) you will have 5 kHz cutoff.
Yes, good ribbon telephone.
It is anachronismus now, if modern ribbon mic is very high quality
studio equipment.
Yes, I agree, My mic was not superior, but I used it only for experiments
mainly in vacuum, where magnet geometry are no interest.
But there was some corrections (horn etc). Only what you can do is to plate magnets and pole pieces by welvet and hope that it helps. Some
mics of pre - Olson design have it. But I can not explain, if it will work,
OR to make magnets porous... There are much ways that do ribbon mic with modern materials.

xvlk