Another ribbon mic question.....

[orange]

I've just finished replacing the ribbon on the top section of a BM-5, the old one had sagged to such and extent that is was dragging on one of the poles. My hamfisted attempted to tighten it destroyed it, mind you it was never gonna be possible to adjust it, as the ribbon had corroded/stuck itself onto the mounting clamp.

Phew, it was a fiddly job. I used some 'silver' leaf stuff that I found in the drawer - I'm guessing it's what people are suggesting is .6 micron (?). I couldn't find any suitable gears so corrugated the ribbon around a hosepipe 'thingy'. Took several attempts but eventually plugged the thing in, it worked and more amazingly sounded pretty good. BUT It's about 4 or 5 dB down on the other half of the mic.

SO:

I assume I've lost level because I made the ribbon a littler thinner (widthwide), so the ribbon to poles gap is larger than it was before ? FWIW the old ribbons were Duralum - and have a slightly different shape to other ribbons I've seen. They were corrugated for about 1cm at the ends but the center section was slightly curved.

OR:

is the new ribbon foil too thin ? too high a resistance (it's about 5cm long) ?

I also think that the foil I used is too flimsy to hold it's corrugations - so the bends aren't nice and crisp like pictures of other ribbons I've seen. Would randomly pounding the foil with a hammer halp introduce some springyness ? It would have been great to have one of those RCA corrugating machines in the photo that rodabod posted. With better corrugation there would be more mass of foil inside the poles - how would that affect the output ?

any advice gratefully received.

 

[rodabod]

[quote author="orange"]I'm guessing it's what people are suggesting is .6 micron (?). [/quote]

For some. Usually smaller ribbons. I think I'd go for 1.8u or 2.5u for the B&Os though.

I assume I've lost level because I made the ribbon a littler thinner (widthwide), so the ribbon to poles gap is larger than it was before ?

Yes, and in particular, bass response.

They were corrugated for about 1cm at the ends but the center section was slightly curved.

This is the piston type. Mainly seen in Beyer mics and some STCs.

is the new ribbon foil too thin ? too high a resistance (it's about 5cm long) ?

Possibly. It works ok with some mics (giving higher output even), but 5cm is quite long. Even worse if you made it too narrow.

With better corrugation there would be more mass of foil inside the poles - how would that affect the output ?

It may help, but I'd say that trying thicker ribbon first may be your best bet.

Roddy

 

[orange]

Thanks for the reply Roddy,

I noticed that Tremaine mentions the mic in the audio cyclopedia and says it has a .0001" (2.5 micron) ribbon - As it's him, I'm assuming that's gospel.

Any thoughts on a cheap source of 2.5 micron ribbon in the UK - the only place I've seen is about £60 a sheet. It would work out about the same to send the mic to ENAK and get it done properly. Chewing gum / fag packet perhaps ?

RE: the corrugation
I noticed you mention something to do with a telescope lens in another post - looking at my telescope I noticed the motor drive gear...PERFECT.

RE: the low output
I didn't leave a huge gap between ribbon/poles - just a fraction more than the original (cos the ribbon was so blooming fragile) so I'm not sure if that totally accounts for the low output. Might just be incorrectly tensioned (too tight - low bass ?)

I think you're right - I'll try 2.5 micron first and take it from there. I'm getting quite good at making ribbons with the .6 micron so the thick stuff should be a breeze.

si

 

[Marik]

[quote author="orange"]Thanks for the reply Roddy,

I noticed that Tremaine mentions the mic in the audio cyclopedia and says it has a .0001" (2.5 micron) ribbon - As it's him, I'm assuming that's gospel...

RE: the low output
I didn't leave a huge gap between ribbon/poles - just a fraction more than the original (cos the ribbon was so blooming fragile) so I'm not sure if that totally accounts for the low output. Might just be incorrectly tensioned (too tight - low bass ?)
[/quote]

Not sure about how universal that gospel is for every single ribbon mic, but for that particular B&O 2.5 micron will work much better.

What happens is that with 0.6um stuff the impedance becomes way to high and the input loads the signal. The B&O transformer is just not designed to work with high ribbon impedances.

Also, what is in your understanding "a fraction more"? How much do you allow for the gap?

Do you have means for measuring tuning resonance?


Best, M

 

[rodabod]

Si,

I can send you some suitable foil. That offer is not open to everyone else unfortunately...

I'm away for the next few weeks doing training, but I will be back weekends and will see what I can do.

Roddy

 

[orange]

[quote author="Marik"]

What happens is that with 0.6um stuff the impedance becomes way to high and the input loads the signal. The B&O transformer is just not designed to work with high ribbon impedances.
[/quote]

thought that might be the case - I don't have a simple way to measure impedance (especially as low as .2ohm)

[quote author="Marik"]
Also, what is in your understanding "a fraction more"? How much do you allow for the gap?
[/quote]

I'd say the gap is less than .5mm or so (each side), really not much more than the original. The problem comes when you corrugate the ribbon by hand. Due to slight innaccuracies in the corrugations you impart a very slight curve to the ribbon - so you are forced to use a slightly larger ribbon/pole gap (to allow extra clearance for the curve). I think with my new corrugation technique I can improve this.

[quote author="Marik"]
Do you have means for measuring tuning resonance?
Best, M[/quote]

....mmm... I have a scope and a signal generator ? can you point me towards a way of doing this ? Can I apply a very small AC voltage and watch for resonance ?

[quote author="Roddy"]]I can send you some suitable foil. That offer is not open to everyone else unfortunately...

I'm away for the next few weeks doing training, but I will be back weekends and will see what I can do.

Roddy[/quote]

That's a very gracious offer...PM sent


PS - Watching the royer 'tour' I notice that they use a lightbox to align/tension their ribbons. I think I might have one somewhere - that should make the job easier...and next time I'll take/post pictures.

 

[Marik]

[quote author="orange"][quote author="Marik"]
Also, what is in your understanding "a fraction more"? How much do you allow for the gap?
[/quote]

I'd say the gap is less than .5mm or so (each side), really not much more than the original. [/quote]

:shock: :shock: :shock:
I knew something is going on there. To waste 1mm of ribbon area just for a gap is WAY too much.
I allow 100um each side. Anything bigger than that is prone to losses.

Best, M

 

[orange]

I'll give it a try and report back.......

thanks for the help

si

 

[orange]

here's a quick update.

firstly thanks to roddy for the ribbon and much advice....... :sam: :guinness: :sam:

here's the first .6micron ribbon that I installed.....

and a close up so you can see the gap between ribbon and poles.

here's the 2micron ribbon cut and crimped....

and here it is installed (sorry about the focus...shaky hands !)

well the 2micron stuff was MUCH easier to work with than the .6micron - I'm sure that the corrugations are a bit more robust too.

well I think it would have been possible to get the gap a little smaller BUT it's a bit hit and miss and I could have been there forever making ribbons by trial and error. The gap doesn't look too much bigger than the original ribbon. In the absence of a decent resonant frequency test system I tensioned it by eye.

Output is still down about 2 or 3 db on ther original B&O ribbon but slightly higher than the .6micron - sound pretty good too....I'm considering this JOB DONE!!!

PS

In conversation with Roddy we talked about the various corrugation systems in use - some are cross-corrugated (lengthwise) etc...I wonder what effect they have on the output/frequency response. The ribbon manufacturers (beyer/coles etc) must do this for a reason ?

 

[rodabod]

Looking good, Simon.

The gap you have doesn't look too bad to me.

[quote author="orange"]
In conversation with Roddy we talked about the various corrugation systems in use - some are cross-corrugated (lengthwise) etc...I wonder what effect they have on the output/frequency response. The ribbon manufacturers (beyer/coles etc) must do this for a reason ?[/quote]

I'm still wondering about this. We do know that the "piston" ribbons are probably more rugged, but as you say, what happens to resonance and output?

We have less mass for a given foil thickness with the piston, and obviously it is more stiff so that the whole ribbon will shift rather like a dynamic diaphragm. The standard ribbon may tend to bow.

For resonance, Simon and I were agreeing that we'd see less (magnitude) of the upper modes of resonance for the piston ribbon, although I wonder if these modes would be more complex. I generally consider the standard ribbon as being like a guitar string.

In my experience, I have found the original piston ribbons to give greater output than what I can achieve with standard methods, but I also found the sound a little different. I think the high end possibly sounds different.

So, why have STC (Coles), B&O, RCA and Beyer all used this method at some point?