Effects of damping factor on planar magnetics/orthodynamics ?

Hi everyone,

Lately I've been researching on electrical damping factor, and especially the effects a high impedance output could have on planar magnetic drivers. It has been well known for a long time that electrical damping can have an influence on the frequency response of drivers, as clearly exposed in the article reproduced here, which dates back to 1954 : http://www.paulspeltz.com/tomcik/index.html

Obviously this is a concept that a lot of people understands here, most likely much better than me. It is also quite well known that low damping factor is affecting the bass frequency response in regular drivers. However I have some difficulties to find some conclusive information on low damping factor with planar magnetics, given their essentially flat impedance curve. Does anyone know about studies done on the subject ?

Thanks to both of you for the answers.

@JadeEast

I did exactly that, but I didn't get much additional info, especially with "isodynamic". "Orthodynamic" basically points towards headphone boards and especially head-fi, while "planar magnetic" points towards speakers boards. I guess I'll have to go through the patent applications, and see if I can get anything from there.

@Gurubhai

Thanks for pointing that out, that's an interesting point from Yamaha. I don't know if that can be applied to other constructions (Audeze/HifiMan). The driver construction is different, isn't it ?

So far I have been able to find the following information :

1. Orthodynamic drivers are purely resistive loads, given their operation mode, different from dynamic drivers. As a consequence, they are not affected by the damping factor per se, as their impedance curve is virtually flat.

2. Given their construction (at least in the case of Audeze and HifiMan), and the lack of resistive load, it is necessary to keep a high damping factor. In other words, it is necessary to keep the drivers under control. This last point is interesting, but unfortunately I have nothing to substantiate it further, besides the fact that it's coming from K. Gilmore, on a different board.

3. Thread about Magnepans on speaker boards also point to the necessity of damping factor, to avoid driver "flapping". 

So far, it seems like electrical damping is necessary to prevent the driver from getting out of control. From the Yamaha HP-1 brochure, we can see that an impedance of up to 300 Ohms is still ok, at least with that construction. Given that the impedance of the Yamaha is 150 Ohms, I suppose we could take a damping factor of 0.5 as the minimum acceptable. That is pretty low and a far cry from the 1/8 rule of thumb (I've also seen 1/20 recommanded).

Thanks MacieKN and Steve for chiming in.

The reasoning behind K. Gilmore point of view is that the lack of mechanical load in the orthodymanic drivers, contrary to dynamic drivers, would lead to a potential lack of control. The analogy that's often used is that dynamic drivers have their own "brake" built-in, as the magnet makes resistance. The orthodynamic drivers, being built differently and without a driving magnet attached to the ribbon, would then need to be properly damped to avoid getting out of their mecanichal operation range. This is what is referred to as "flapping" on magnepans' boards.

Steve Eddy's experience is interesting because he seems to hear no difference, even though the electrical damping is close to a ratio of 0.15. I wish I had the equipment to measure this too, I'm really wondering if distortion is going up, or if that's still within the mechanical range of the drivers (for the Audeze at least, but I see no reason to have a difference with HifiMan).

MAcieKN, HifiMan models have close to zero mechanical damping, and I think it's the same for Audeze's models. Yamaha and Fostex, however, do use that kind of damping. Hence my question on the applicability of the numbers from the Yamaha brochure to the Audeze and HifiMan models (open-back orthos).

Thanks for the additional answer Eddy. I'm beginning to wonder whether the electrical damping that seems necessary with magnepans speakers might not be with our orthos. Different construction perhaps ?

Thanks JCX.

That's what the theory tell us, yes. In this case it's pretty easy to conclude that there's no real problem with a high Zout, in regular conditions. Say, something like 25/30 Ohms.

Maybe I wasn't clear, but here I'm interested in papers that would document the higher limits of orthos. The Yamaha brochure gives us a first glimpse, and Steve another one, with damping factors that definitely exceeds the limits usually accepted for headphones. 

My interest here, is to see whether I could use my HE500 connected to the balanced XLR of my DAC1. But the Zout of these is quite high (43/135/425/60 Ohms, depending on the gain), especially in the gain positions that are usable with a balanced cable (135/425 Ohms).

However, seeing the impedance Steve is using with his LCD2 makes me believe it might not be a problem. At least it is possible without blowing up the drivers. Now, without testing by myself, seeing what kind of influence it can have on the overall performance (distortion in particular) will probably be difficult.

Quote:

Originally Posted by jcx 

you want to look at the headphone's impedance vs frequency curve, any bumps, dips means there will be corresponding frequency response changes as amp output Z increases - 0 output Z means the amp completely controls the driver terminal voltage

 

orthodynamics seem to be extremely flat impedance - very little sensitivity to amp output Z

The DAC1 doesn't have balanced headphone output, you're absolutely right. I wish it had though, as it would makes things simpler for me Let's say I was contemplating the idea of using the balanced outputs.

The XLR outputs are factory calibrated to give +4 dBu, or about 1.2V RMS, which fits pretty much in the needs of the HE500 for regular operation. So unless my reasoning is flawed (I'm not excluding the possibility at all), in the worst case it should simply be unable to give enough power to the headphones. Happy to hear otherwise and learn something I don't though. EDIT: my reasoning is indeed flawed. definitely not enough power available. At least with the factory calibration settings.

Benchmark engineer (Elias Gwinn) has advised against driving headphones from the XLR, but it was before the arrival of orthos in the market. I simply assumed he was talking about the electrical damping. So you think the output wouldn't give enough current to drive the headphones. Maybe that's what he meant, and not the distortion from the lack of damping.

Quote:

Originally Posted by mikeaj 

Wait, you want to drive (somewhat insensitive, lower-impedance) headphones from a balanced line out?  I'm guessing something like a DAC1 doesn't have a balanced headphone output, or does it?

 

The problem is that a line output probably does not have the capability to handle such high current.  It works fine driving a high impedance load like a device's line input, but all bets are off into headphones.  You'd be running it way out of its (mostly) linear operating region, so really high distortion is easily a possibility.  It depends on how the line output is implemented though.