Ribbon thickness -Transformer ratios - Cinemag 9887

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ciscan.81

Active member
Joined
May 26, 2012
Messages
43
Location
Adelaide Australia
Hi Marik, Stewart, CJ, rodabod, PRR and Co.

I'm hoping one of you have a minute or two to answer a couple of questions for me.

Background:

I have followed the threads by the group DIY ribbon community for a while, having undertaken a 'from-scratch' royer-styled ribbon mic build last year.  The mic turned out looking and sounding fantastic (think full gloss nickel plated royer 101).  I used a Lundahl transformer, 2.5um foil and silver plated clamps (thanks for the advice on that Stewart).  I didn't get around to posting some final pics etc, but will get around to it soon, as I am really proud of the build.

The point:

Anyway, I liked the mic so much (my first ribbon) I decided I wanted a pair for overhead duties...However, being a little time poor at the moment (too many build projects on!), I decided to just buy a pair of the ubiquitous Apex 205's to mod.

I have a couple of Cinemag 9887's (0.12 : 150 (1 : 35 ratio)) left over from the 'from-scratch' build, which I am hoping to use in these mics.

My questions are:

Would 2.5um or 1.8um foil be better suited to the above transformers in regard to impedance and ratios etc? (Given stated 50mm ribbon length x 3mm-ish width?)

I've never been exactly clear on this, but my rudimentary understanding is that thicker ribbon is lower impedance and so transformer ratio can be higher...? So a 2.5um could be better for the Cinemag 9887???? Yes, no?

Do you guys recommend removing the dampening from these mics as touted elsewhere? I gather the perforated plate is used to reduce  boominess from proximity effect and increase HF ala 4038 (in BBC monograph)? Others claim gain in HF clarity at the expense of increased "boominess" upon removal...thoughts?

Any advice would be appreciated.

Kind regards

Wayne
 
...a pic of my ribbon motor from the mentioned 'from-scratch' build.

I love the nickel plating! I found a local semi-retired electro-plater who did all my plating at a great price...

Anyway just wanted to share, I'll follow up at a later stage with some more pics of the build on another thread...

 

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Wayne, i'd have to know the ribbon geometry to answer the thickness question. We use 1.8u, but manufacture our own transformer.

As far as perforated plates, I don't know what is on the apex. But we use them as does Coles. It was patented long ago by Olson.
The pair of plates form a wave resonator...with a pressure minimum at the center and therefore a pressure gradient maximum. The one we use is tuned to a broad  +4 dB at 12.5kHz. The ingenious Olson "waffle plate" as we call it has reduced effect off axis...perfectly complimenting the
reduced front to back difference and improved HF the ribbon has off axis!

With the thicker ribbons a fine mesh damping screen is often used very near the ribbon. Without it one can often see FR effects of ribbon resonance harmonics. It does cause a loss in LF response. We (as well as coles) use an outer baffle in the plane of the ribbon (perforated metal with cloth) to bring the bass back up.

Les
L M Watts Technology
 
Hi Wayne,

I'm pleased that your first build came out so well!

The Apex style mics have big wide ribbons, and the Cinemags usually have plenty of inductance on the primary (arguably at the expense of a little noise, but no big issue). So they should work OK with the 1.8 micrometer foil in that respect, and depending on how you cut and corrugate your ribbon you can expect an output impedance of around 300 to 400 ohms - maybe a little more.

Personally, I am not a big fan of tearing out all the inner grills and so on as it leaves the ribbon exposed to dirt and wind blasts. It may sound nice but perhaps not for long.

If you look at a Coles 4038, for example, you'll find lots of protection! But let your taste decide on that one.

Hope that helps. With luck some of the other guys will chime in with their opinions!
Stewart

Edit - Les posted whist I was doing so!
 

Really nice work! It looks great.

Did you manage to find some stainless steel or brass screws? Fitting ribbon clamps near neodynium magnets is a real pain if the screws keep jumping into the gap!
 
Wow, that was quick! One of the few benefits of working night shift on an oil rig in the middle of the ocean in the southern hemisphere! (yes I am posting from work!) ;)

Thank you both for your replies...

Thanks for the info on the Olson waffle plate Les, I do remember reading about it in the monograph and some Olson patents during my build research, but couldn't quite remember what its exact function was...

Stewart, I'm feeling the same way about gutting the mics. I'll have a look at them when they arrive, and perhaps do some experimenting with damping but I figure I'll start with the transformer and ribbon, and perhaps improve the wiring and make a call from there...

Thanks again guys, I'll follow up with some pics and thoughts once I've got the mics... do you think 'the drawing board' is the appropriate place, or should I start a new thread  in the 'Microphones' section...?

Thanks again...

-W

Stewart, keep an eye out for my build post, I'll put up some pics etc in a couple of weeks when I get home...
 
yep, I found some M1.6 stainless screws and a tiny ceramic screwdriver on ebay...

tapping the holes in the silver plated copper wasn't a bad as I expected, I put the tap in my drill press to keep it at 90 degrees and just wound it by hand with lots of cutting oil...worked great...

 
Hey Les,

I am keeping my eyes wide open for your vari-pattern mic when it arrives. Do keep us up to date with progress!

Stewart
 
Will do Stewart. It's in initial production.
It's certainly the most complicated microphone I have ever designed...far more so than any of the condensers.
I wanted everything right though.

A bear to manufacture too.

Les
L M Watts Technology
 
while we are on impedance and transformers...

Is it possible to have a preamplifier input impedance that is too high for a ribbon mic..?

I know that it is recommended that the input impedance of the preamplifier should be 4-5 times the output impedance of the ribbon mic to prevent ribbon loading...

So for a ribbon mic with an output impedance of 200-300 ohms, the preamplifier input should be around 800-1500 ohms...? Yes?

What would happen if the preamp input impedance starts getting up around 9k...?

Just wondering, as I have built some 312 style pre's on Baby animal boards with the input trans ratio set to 1:3.5 ohms. I implemented variable impedance pots...I have the feeling I may have set my range too high on this pair of pres...1200-9k

I have another 312 build with the input transformer set to 1:7 with variable Z, range 300-1200 ohms [edit] sorry, it's actually 300-2400 ohms [edit], which works great...

any thoughts?

thanks

-W

ps. Les, the rig work funds my gear building habit for my home studio back home and also affords me lots of down time to read through patents and forums to research projects while on board...not a bad job really!
 
Guess i'll answer.
Most all ribbons are designed to work open circuit...i.e about 10 times their impedance.
If impedance is lower you lose output particularly in the bass.
If higher, there's no real advantage as far as bass response, but there are reasons not to make
preamp input impedance too high.

Ours work optimally with a 2K load...they'll work at 600 with a couple dB bass loss and slightly reduced level elsewhere.

Les
L M Watts Technology
 
cool, thanks Les...

all makes sense...

I've done some reading on mic/pre impedance and feel like I have a handle on it now, although it is a somewhat confusing topic...things keep changing at different frequencies!!!

thanks again

-W
 
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