sanity check on cable capacitance and passive ribbon mic

[klem]

I am building a cable for FM's @Marik AL959 stereo ribbon mic. I was advised that cable lengths over 3 meters could have an impact on the sound of passive ribbons (I planned to make a 10 meter cable).

I used the handy calculator at Calculation of cable length, cable capacitance C, and cutoff frequency cable attenuation treble loss damping microphone mic cable capacity wires XLR capacity - sengpielaudio Sengpiel Berlin and looked at 2 cables (both are quad, but to be used 1:1 wirein as it's for the stereo mic)

Plugging in the data below for either a Gotham or Canare, I get a value wayyyy above human hearing, and that value is cutoff frequency that's only 3dB of attenuation. So, what's the potential issue here? Or maybe there's something outside of this for what would impact the sound with a (properly shielded/terminated) cable run >3 meters?

https://www.gothamcable.com/en/gothamcables/starquad/11001gac4155 pF/m

https://www.canare.co.jp/en/products/cables/index.php?tid=4_001150 pF/m

AL959 Output Impedance: 250 Ohm

10 meter cable

Canare: 424413 Hz

Gotham: 1157490 Hz

 

[ccaudle]

Don't forget about the conductor to shield capacitance, the series combination of conductor to shield capacitance is effectively in parallel with the conductor to conductor capacitance, so roughly double what you used. That still only brings the -3dB frequency down by a factor of 2

 

[emrr]

Look at 110Ω digital audio cable, see if you can find a 2 channel suitable type. Capacitance is much lower. Now, I've tried that, Grimm TPR, and Canare in both star quad and standard, and can't say I hear any differences really.

 

[klem]

@emrr, were those 30' or more and used with passive ribbons?

@ccaudle, sounds like there are parts of the equation i'm quite literally missing. that said, it also sounds like the specs are good enough that some GAC 4/1 would be fine for my application. if you are aware of a way to do a more comprehensive calculation on total capacitance loss than what i initially did, that would be great to learn. thanks!

 

[ccaudle]

The model is pretty simple, capacitance affects any conductors in proximity, so there is capacitance between the two wires in a pair, and from each wire to the shield. In a four wire bundle there would be capacitance from one wire to all three other wires, but I didn't draw that out because it doesn't really add clarity to the basic idea.




The effective capacitance between the wires is in parallel combination with the effective capacitance formed by the series combination of wire 1 to shield and wire 2 to shield. Two equal capacitors in series have an effective capacitance of half of the individual capacitance values.

So for that Gotham wire the data sheet lists conductor to conductor as 55nF/km, so 55pF/m.
The conductor to shield is listed as 103pF/m; two of those in series would be just a hair over 51pF/m in parallel with the 55pF/m directly from conductor to conductor, for a total effective capacitance of a little over 100pF/m between the two signal wires.

 

[emrr]

@emrr, were those 30' or more and used with passive ribbons?

yes, 30 and 40 foot runs, plus wall panel to patch bay (110Ω there), plus another 6-10' to preamps.

 

[klem]

thanks for explanation @ccaudle !
i may be misunderstanding though, as i'm thinking for the GAC 4/1 total capacitance would be
(55/4)+(103/4) = 39.5pF/m (?)

 

[ccaudle]

(55/4)+(103/4)

Why divide by 4? You need to draw out the conductor diagram showing the capacitances, I don't follow where you came up with that.

 

[Marik]

In a ribbon microphone the load through the transformer comes back to the ribbon and effectively controls its motional impedance, restricting its movement. Cable capacitance, resistance, and inductance make this load frequency dependent. To more accurately calculate it you need to take the equivalent schematic of the cable and calculate it as a LCR (including loading impedance) filter.

Best, M

 

[klem]

well, @Marik just threw a wrench in my already poor understanding, but @ccaudle, my logic was
(conductor to conductor capac. / total # of conductors) + (conductor to shield / total # of conductors), based on the series explanation you gave. like i said, i'm guessing i misinterpreted it, but would like to get it straight! @Marik s info may be hopelessly out of reach with my present capacity, pun semi-intended.

 

[ccaudle]

, my logic was
(conductor to conductor capac. / total # of conductors)

Draw out a picture like I did in post #5 and you can see which capacitances are in parallel and which are in series. To have a capacitance divided by four you would need four in series. You could think of the capacitances between conductor 1 to 3, 3 to 4, and 4 to 2 as being in series, and that combination being in parallel to the direct 1 to 2 capacitance (using the numbering scheme that 1 and 2 are the conductors in one signal pair, and 3 and 4 are the second signal pair in your stereo application), but there is always going to be a direct path from 1 to 2, that is never in series with another path and so will always appear directly. In other words you can never have less than the direct capacitance between the two conductors in the signal pair, it will always be that plus some additional, so it should have jumped out immediately that something was wrong in your calculation when you came up with a total capacitance less than the starting minimum.

 

[k brown]

Mogami's AES/EBU cable is 110 ohm; and a very fine audio cable in general.

 

[kags]

Mogami's AES/EBU cable is 110 ohm; and a very fine audio cable in general.

I would add Belden’s AES cable and also Gepco X-Band. I recently used a spool of Sommer mic cable that was very nice to work with and had great specs. Also priced at lower end- probably around $0.40 per foot for a full roll.

 

[ccaudle]

put a 250 ohm resistor across pins 2&3 to terminate properly

That is excessive termination, not proper termination. You should not terminate a microphone with a load equal to its specified source impedance, that will drop the level by 6dB. Microphones are designed for pre-amps with input impedance in the 1k to 3k range.

 

[emrr]

Nonsense. You always terminate a transformer output at its impedance.

Sorry, you are wrong.

A Western Electric mic? Maybe; one of the few companies designing for matching conditions…80 years ago.

All the old classic transformer to tube grid preamps are “much higher than” bridging Z in the mid-band, and the vast majority of the pass band, and an infinitesimally small % used any sort of loading resistance. Only remote multichannel preamps with passive mic level mixers and single channel amps loaded the mics at specified source Z. Look at all the old schematics and manuals you want, you won’t find load resistors for mics.

 

[emrr]

Response to now deleted previous post————

Oh, I definitely have, not optimal. I definitely don’t want matching loading loss on a very long line with a low output mic.

A matching load on a dynamic mic creates a LPF.

Mics are designed for bridging inputs.

Have done a lot of broadcast with 1000 foot+ runs, mic, line, etc. You put an amp closer to drive the long run if you can. There’s a lot of them on the market powered by 48V, very popular, most are 3k-7k, some are 20k input Z.

 

[Marik]

Since its transformer out, use a good low impedance twisted pair cable. and on the mic pre plug side, put a 250 ohm resistor across pins 2&3 to terminate properly.

In order to smooth out resonances and get a flat response the ribbon must be loaded both acoustically and electronically. It is important to note both of those have similar effect. In a well designed ribbon microphone acoustical load is calculated so that electronic one is about 10 times of the ribbons impedance (after translated through transformer ratio).

Say, the ribbon impedance is 0.15 Ω, so with a 1:36 transformer it is about (considering the losses) 200 Ω output impedance. Say, with a typical 1,5kΩ-3kΩ input impedance of modern pres the ribbon sees a good and comfortable x10. If we load the transformer with 250 Ω then in parallel with (say) 3kΩ it's already some 230 Ω. Then the ribbon sees it as 230/36^2=0.18 Ω and its motion becomes quite restricted severely affecting sonics and performance.

For long runs of cables we recommend only active ribbons.

Best, M

 

[emrr]

Its sounds like you doing a bunch of math.
I heard it both ways, and yes, I'll keep on using a modded cable for the ribbon mic that has a termination resistor in it.

It SOUNDS like he runs a well regarded microphone company making ribbon mics….which he does!

You’re on your own with that approach, glad it works for you.

 

[rock soderstrom]

In order to smooth out resonances and get a flat response the ribbon must be loaded both acoustically and electronically. It is important to note both of those have similar effect. In a well designed ribbon microphone acoustical load is calculated so that electronic one is about 10 times of the ribbons impedance (after translated through transformer ratio).

Say, the ribbon impedance is 0.15 Ω, so with a 1:36 transformer it is about (considering the losses) 200 Ω output impedance. Say, with a typical 1,5kΩ-3kΩ input impedance of modern pres the ribbon sees a good and comfortable x10. If we load the transformer with 250 Ω then in parallel with (say) 3kΩ it's already some 230 Ω. Then the ribbon sees it as 230/36^2=0.18 Ω and its motion becomes quite restricted severely affecting sonics and performance.

For long runs of cables we recommend only active ribbons.

Best, M

HQ post. Thanks for good info!

 

[ccaudle]

I heard it both way

When you hear contradictory advice you need some way to evaluate which is correct (and when, because it could be the seemingly contradictory advice applies to different situations).

Engineering principles and math is how you evaluate answers to technical questions.

Of course empirical evidence is valuable as well. Since you have not contributed any engineering analysis to the discussion, do you have any measurements which would support your suggestion of heavily loading the output of any transformer coupled microphone?