Ribbon Resonant Frequency Tunning/Tensioning - Ribbon Microphones

[Whoops]

Hello,
I have some Re-Ribboning to do on some mics and I would like to ask you guys what are the technics you use to tune the Resonant Frequency of the Ribbon while Tensioning?

I read a lot of stuff and it seems most Ribbons are tuned in the region between 16Hz to 45Hz.
AEA tunes all their ribbons to 16.5hz because there seems to be the frequency of the old RCA 44 and 77 mics.
Other mics are tunned between 20hz and 44hz.

I don't know what that means, to be honest, or how could I know or measure the resonant frequency of the Ribbon I'm installing.

I have Signal Generator, Osciloscope, RTA analyser, plus all the other main gear.
Could this be useful to measure and tune the Ribbon?

I also don't know how can I measure and tension it at the same time before clamping it down.

Thank you so much for your help

 

[emrr]

If you look at response in real time and turn speaker gain up, you'll see the resonant point. 

 

[abbey road d enfer]

I don't know what that means, to be honest, or how could I know or measure the resonant frequency of the Ribbon I'm installing.

I have Signal Generator, Osciloscope, RTA analyser, plus all the other main gear.
Could this be useful to measure and tune the Ribbon?

Resonance can be evaluated by looking at the impedance curve. Resonance results in a slight increase of impedance. Note that because the ribbon is heavily damped by air and electrical losses, it is not a large hump.
Your equipment is adequate for the task. Feed the mic via a resistor that's about 10-20 times the nominal impedance. The signal should be about 0dBu. Measure the signal across the mic (actually the xfmr's secondary). You should hear the tone from the ribbon. Of course you must shield the cables in order to minimize hum and interferences. I recommend putting the mic and resistor in a large open tin box earthed to the measurement system. I used a metal coffee tray to do that.

I also don't know how can I measure and tension it at the same time before clamping it down.

I'm afraid you can't. Minimizing contact resistance is essential for making the impedance hump visible.

 

[Whoops]

Feed the mic via a resistor that's about 10-20 times the nominal impedance.

Thanks for the reply Abbey,
How do I know the nominal impedance?

Thanks

 

[Marik]

I wish I could talk on this one... Unfortunately, even my employees do not know the details of our ribbon tuning process and method. It allows us to get some +/- 0.5Hz consistency unit to unit. When you do not need such tight production tolerances the "impedance tuning" will be sufficient. The problem there the peak is quite small and wide, so the precision is probably some +/-3Hz, which still should be quite adequate...

As for the tuning frequency--I would not go anywhere below 30Hz. The problem--the loose ribbon will be just flipping around with no much energy going there, just adding distortions. That is, because of air viscosity in the gap (between magnet/pole pieces and ribbon itself), as well as air loading, the ribbon goes from mass controlled into stiffness controlled system, so its work there becomes totally inefficient anywhere below...

Hope it is of help.

Best, M   

 

[Whoops]

I wish I could talk on this one... Unfortunately, even my employees do not know the details of our ribbon tuning process and method. It allows us to get some +/- 0.5Hz consistency unit to unit. When you do not need such tight production tolerances the "impedance tuning" will be sufficient. The problem there the peak is quite small and wide, so the precision is probably some +/-3Hz, which still should be quite adequate...

Thank you so much for your input Marik,
I think for my needs +/-5HZ is pretty good, I just would like to get in the ballpark if I put Ribbosn on 2 similar mics.
If I want to achieve 40HZ, I just want to be sure that I'm not on 60Hz or 90Hz

Thanks

 

[abbey road d enfer]

Thanks for the reply Abbey,
How do I know the nominal impedance?

Thanks

Should be written on the box 
Beware that, although most ribbon mics are spec'd at about 150-200 ohms, some are wildly off; I checked a Thomann ribbon mic at 2 500 ohms!

 

[Marik]

Thank you so much for your input Marik,
I think for my needs +/-5HZ is pretty good, I just would like to get in the ballpark if I put Ribbosn on 2 similar mics.
If I want to achieve 40HZ, I just want to be sure that I'm not on 60Hz or 90Hz

Thanks

Here is the process of making and installing ribbon which should help you to get into ballpark. First, with .001" graduation calipers measure the gap. Then pre-cut ribbon leaving some 0.010"--0.020" pole to ribbon gap per side. The tighter gap you can achieve the better lower end response you get.

Corrugate, pre-stretch (those steps are widely described). For installation use at least 99.99% pure alcohol to 'wet' the ends and make ribbon to get stuck to the frame. Any less purity will result in residue after drying and possible resistance (=noise) increase.

Once the ribbon is more, or less centered in the gap start stretching it on one end (usually, lower) until you get just very slight sag. Now (remember to wet the ends every some 15 seconds) take the frame up in the air and turn the ribbon sideways perpendicular the table. If too loose then the ribbon edge will go way too 'sable' and touch pole piece. If that happens little by little tighten it so that it 'sables' about half way. IOW, if the gap is 0.020" then it should 'sable' half of that-- 0.010".

At this point take the frame (ribbon is flat parallel to the table) and go up down. The movement is steady and not fast. If you see the ribbon deflecting way out of gap--it is a good indication it is too loose. Deflecting to the edge of the pole piece should be indicating the tension is pretty good.

When you want to make a second ribbon somewhere close in tuning--stick both together side to side, move up down and watch for the same excursion.

Best, M

 

[Whoops]

Thank you so much for the explanation Marik

 

[12afael]

Resonance can be evaluated by looking at the impedance curve

i have been using REW to measure impedance of cabinet speakers. Someday I will try with a ribbon.

 

[Whoops]

i have been using REW to measure impedance of cabinet speakers. Someday I will try with a ribbon.

Yes, that I what I want to do.
A way to measure and tune the ribbon with equipment.

 

[abbey road d enfer]

Here is the process of making and installing ribbon which should help you to get into ballpark. First, with .001" graduation calipers measure the gap. Then pre-cut ribbon leaving some 0.010"--0.020" pole to ribbon gap per side (say, if the gap is .200" then make the ribbon width 0.196"--0.198").

I believe you have your math mixed up. 
For 0.010 clearance, I get a width of 0.180
How do you cut the ribbon with such accuracy?

 

[Marik]

I believe you have your math mixed up. 
For 0.010 clearance, I get a width of 0.180
How do you cut the ribbon with such accuracy?

Abbey,

That what happens when you type it fast at 3am)))

We have a fixture with .001" graduation micrometers on each side to set depth of the cut. The ribbon is sandwiched in tracing paper and for the blade we use barber shaving blade in a custom holder. The reason for that style of the blade--it is much thinner (=much less tracking distortions), by far sharper than other types, and has rounded corners, which makes it easier not to drag cutting area. As it is the blade is very flexible and won't be suitable for cutting the ribbons, so we had to modify the barber holder to make it work. We change the blade every 10, or so ribbons.

Best, M

 

[12afael]

thanks Marik that is good information.

 

[abbey road d enfer]

We have a fixture with .001" graduation micrometers on each side to set depth of the cut. The ribbon is sandwiched in tracing paper and for the blade we use barber shaving blade in a custom holder. The reason for that style of the blade--it is much thinner (=much less tracking distortions), by far sharper than other types, and has rounded corners, which makes it easier not to drag cutting area. As it is the blade is very flexible and won't be suitable for cutting the ribbons, so we had to modify the barber holder to make it work. We change the blade every 10, or so ribbons.

Best, M

Thanks for the explanation!

 

[Whoops]

Any way of testing the Resonant Frequency of a Ribbon like if it was a speaker?

By sending a sweep signal into the motor?

 

[abbey road d enfer]

Any way of testing the Resonant Frequency of a Ribbon like if it was a speaker?

By sending a sweep signal into the motor?

At 30Hz, the efficiency of a ribbon motor is very poor. You would not hear it, unless you drive it with dangerous power, and even then, I think you would only hear aerodynamic noises and distortion products.
You may try visualizing the excursion with a sync'd strobe light or a laser distance detector, but it's more complicated than measuring impedance.

 

[Whoops]

Hi mates, I want to share a really nice video from Royer Labs, were they show mics contruction, Ribbon testing and circuit testing.
Really interesting Video:

 

[ricardo]

This oscilloscope method is the most accurate way of determining 'resonance'

It is essentially treating the ribbon as a speaker, sending a VERY SMALL current through it and comparing current & voltage phase.

The more sophisticated speaker testers use a similar method.

 

[Whoops]

This oscilloscope method is the most accurate way of determining 'resonance'

It is essentially treating the ribbon as a speaker, sending a VERY SMALL current through it and comparing current & voltage phase.

The more sophisticated speaker testers use a similar method.

I was thinking about this method, and would like to try it out.
Could you guys help me out on how to setp this.

So if I understood correctly, I should send a small audio signal directly to the ribbon, then where I connect the Oscilloscope probes? also to both sides of the ribbon? or to the mic output after the output transformer?

thank you so much