In this installment of As the Ribbon Microphone Gets Modified, we'll replace the ribbon itself.
You may recall from the last post that I removed the factory ribbon. The stock ribbon in these mics is relatively thick - between 4 and 5 microns (μm). That's thick for a microphone ribbon. The modern practice is to put a thinner ribbon in - meaning between 2 and 3μm. The thickness of the ribbon, and its tension have a critical effect on sensitivity and frequency response. So I'm making a new ribbon in a quest for improved performance.
You can get thinner ribbon foil - down to about 1.2μm, but it's much harder to work with. A thinner ribbon should have better response and a more extended high end, but I haven't seen any side-by-side comparisons.
I should also note that the most interesting document about ribbon mics that I've seen - the famous 1955 BBC Monograph No. 4, The Design of a Ribbon Type Pressure-Gradient Microphone for Broadcast Transmission, states that the famous BBC/Coles mics used 0.6μm silver foil for ribbons!
Anyway, let's get on with making a ribbon and installing it in our Apex 210.
To cut the ribbon, you'll need a good straightedge, a sharp utility knife, and a good cutting surface. I bought a cutting mat for this very purpose and am finding it useful for other projects too.
The foil is extremely thin and you must handle it with care. It will crinkle, fold and do other nasty things if you look at it wrong. It's also super light, so don't sit near a fan and do this work.
This was my first time doing this. It took me 4 tries to get one good ribbon. Now that I'm experienced, I'm sure my good-to-bad ratio would be much better. But be forewarned.
You'll also need a lot of patience and care when doing this. It's a simple process, but the challenge is handling the delicate foil.
You will find you can't just pick the foil up and stick it in there. I wound up using tweezers to sort of slide the foil across the mat and maneuver the foil sheet into the paper.
To give you an idea of how thin the foil is, ponder this: a standard piece of aluminum foil is about 14μm. Our microphone foil is 2.5.
I measured the gap between the magnets in the ribbon motor. I wanted to try and get the ribbon as close to the magnets as possible without touching. (Sorry, I didn't write the measurement down).
Then I made pencil marks on the mat the same distance apart as the gap distance.
I used a section of foil that was 25 or30mm longer than the length of the gap in the ribbon motor. It's good to have some extra length to work with.
I lined up the edge of the paper fold with one of my 'gap' marks, and lined up my straightedge with the other mark.
Then I cut the ribbon/paper sandwich along the edge of the straightedge.
Cutting the foil is the easiest part of this. With a new blade, the foil cuts like...well, a sharp blade through very thin foil.
Now the fun starts.
If you look closely at the strip I cut, you may be able to see that I used a piece of paper a bit longer than the foil. That extra width of paper will help down the line.
But the number of corrugations could be an area for experimentation later. IF I were to do this again, that is.
To corrugate the ribbon, just hold the paper square and run it though the gears in the crimper tool. Keep a small amount of tension on it and keep it on the bottom edge of the crimper.
If you pull too hard, the foil will break or tear.
This part caused me the most grief.
You cannot simply pull the ribbon out. It will tear. I tore 2 pieces trying to get it out.
Finally I came up with a crude method using a small screwdriver to gently open the paper and release it from the foil. Corrugating it really makes it stick together.
This is how I wound up making the paper a bit longer than the foil - that extra bit of paper on each end allowed me to open up the paper easier.
Just be patient and don't force it is the best advice I can give you.
It's easy to tear the ends of the foil here too - see the tear on the right side in this picture. A tear may not be a showstopper, but it's something to avoid. So again, having some extra length of foil is A Good Thing.
Believe it. The ribbon wants to stick to any metal surface, and the alcohol acts as a lubricant of sorts.
This is what I wound up with. Not as hard as you'd think.
Then you put the contact bars back on the ends of the ribbon. I didn't tighten them down all the way.
With the bars on, I got one end lined up so it passed evenly through the magnet gap, then I put some gentle tension on the ribbon and did the other end and tightened it. I used a flat toothpick to gently move the ribbon around on the contacts.
There is a ton of information on the interwebs about tensioning the ribbon. I tried to keep the corrugations from being too flat (stretched), or sagging too much (not enough tension). One suggestion I read is that the ribbon is under the correct tension when it moves about 0.5mm (I know, right?) in the center when you speak into it (don't blow, you may tear the ribbon). That's about where I left it tensioned. My ribbon moves the tiniest amount I can see when I spoke near it.
If you take this on, look for info online - there is more than I can tell you here.
Now we'll put the motor back in and install our wonderful new Lundahl transformer.