[Arjestin]

It is generally considered a bad idea to plug planar magnetic headphones to OTL tube amps. Why?
Can anyone link a paper on the subject?

 

[Rhamnetin]

The main reason is OTL tube amps not meeting the power and current requirements for many, as planars tend to be low impedance while OTL tube amps are meant for high impedance loads. Don't have an article on the subject but you can always look at the impedance and sensitivity ratings for the headphone to get an idea of what it needs.

 

[ProtegeManiac]

It is generally considered a bad idea to plug planar magnetic headphones to OTL tube amps. Why?
Can anyone link a paper on the subject?

Low nominal impedance and usually lower sensitivity of planars don't work with how OTL amps work, which is deliver their peak output performance into high impedance loads and also have high output impedance (this is a lot less of a problem than the first).

For example, take planars with 35ohm, 93dB/1mW sensitivity or 70ohm, 91dB/1mW sensitivity, they're going to need roughly around 512mW and 1000mW (note that you will not literally be using all that power all the time doing 120dB, but without reserve power your headphones will sound somewhat lazy). OTL amps like the WooAudio WA3 produces 150mW at 32ohm and 270mW at 120ohms, and that's already a lot for an OTL amp. By contrast, another $100 and you can get the WA6, which, despite being biased for 32ohm power delivery, actually still delivers more power into 300ohms than the WA3.

Output impedance is also a problem. The Valhalla2's 40ohms is very low for an OTL amp. As much as planars don't require very high damping factor (which drops when the impedance of the headphone isn't high enough vs the amp's output impedance, though dropping it from optimal range does not automatically mean you've dropped it enough to be a problem), having an amp with damping factor higher than the nominal impedance of the planar can still introduce some EQ effect to the sound.

So in reality it's not so much that all planars can't work with all OTL amps. It's just that most planars if not all have specs that aren't conducive for use with an amp designed to drive high impedance, higher sensitivity headphones on the cheap (which is why the Darkvoice DV336se is often recommended, but not necessarily the WA3).

 

[Monsterzero]

I will add to this topic that the La Figaro 339 is exceptional with a few planars like the HE500 and LCD-2.

 

[Arjestin]

It's just that most planars if not all have specs that aren't conducive for use with an amp designed to drive high impedance, higher sensitivity headphones on the cheap (which is why the Darkvoice DV336se is often recommended, but not necessarily the WA3).

The OTL amp I use is indeed the popular DarkVoice 336SE (Winged "C" 6AS7G + Tung-Sol 6SN7GTB tubes). Works great with my HD600, which is a 300Ω 97dB/1mW dynamic headphone.

Some planar magnetic headphone specs:
Fostex T60RP is 50Ω 92dB/1mW.
OPPO PM-1 is 32Ω 102dB/1mW.

Some dynamic headphone specs:
Massdrop Fostex TH-X00 is 25Ω 94dB/1mW.
Audio-Technica ATH-AD2000X is 40Ω 103dB/1mW.

DarkVoice 336SE output impedance is 32-600Ω.

Based on these specs, how can one tell if a headphone matches the amp? All of these headphones have a relatively low impedance. If OTL amps "deliver their peak output performance into high impedance loads", will these headphones be under-powered and "sound somewhat lazy"?

 

[ProtegeManiac]

DarkVoice 336SE output impedance is 32-600Ω.

That doesn't tell you how well it can drive all 32ohm loads. It just tells you that the amp will not be unstable to as low as 32ohm nominal loads, same way speaker amps will state if they can handle 4ohm speakers or not.

Some 32ohm loads can work quite well, if subjectively. Grados have very high sensitivity so the lower output doesn't matter. OTL amps have a tendency to boost the low end on Grados and while from a technical standpoint that doesn't seem automatically that bad considering how the SR and some PS series roll off earlier than the RS and GS lines, the boost can be too much that instead of getting a low end tha just has a deeper "thump!" you get "thwuuuUUUuuump."

Based on these specs, how can one tell if a headphone matches the amp? All of these headphones have a relatively low impedance. If OTL amps "deliver their peak output performance into high impedance loads", will these headphones be under-powered and "sound somewhat lazy"?

They'll likely be since you're not getting as much clean power to them. Part of what makes Grados sound so exciting isn't just their response but has a lot to do with their low impedance and high sensitivity, so most amp circuits - even CMOYs - don't struggle driving them. Even if you're not getting obviously bad distortion, having an absolutely severe disparity in how a Grado sounds dynamic vs another system is usually due to lower power getting to a low sensitivity load.

In any case you have to look at the power rating of the amp at the nominal impedance of the headphone. To guesstimate how much power is optimal if you really want to maximize the headphone, look at the sensitivity and add 3dB until you get to 120dB (or at least 115dB), with every 3dB requiring the power to double.

Also, note that in the case of the LaFigaro 339 mentioned above, it's a dual mono OTL amp, practically making it two OTL amps working as one - kind of like the equivalent Little Dot except that one runs in balanced drive (or how you can use three Beta22 boards to run active ground or four to run differential drive). Power delivery is still the same but of course you get more power at low impedance loads, just that it also pours in a lot of (unnecessary) power into 300ohm to 600ohm loads.

Output impedance is still high but if you subjectively prefer doing that rather than just use an EQ well it's not like people are going to actually stop you from doing things that way.

 

[buke9]

Not all OTL’s are the same as mentioned with the LaFigaro and another is the MicroZotl 2 by Linear Tube Audio just is fantastic with planars but a bit pricey but is really good.

 

[dgozalie]

i am currently using yamaha orthodynamic hp1 and hp3.. hp1 play very well with my bottlehead + speedball...
my yamaha is 150ohm
i guess audeze lcd4 will sings with crack or any high impedence tube ampli..

 

[LoryWiv]

Low nominal impedance and usually lower sensitivity of planars don't work with how OTL amps work, which is deliver their peak output performance into high impedance loads and also have high output impedance (this is a lot less of a problem than the first).

For example, take planars with 35ohm, 93dB/1mW sensitivity or 70ohm, 91dB/1mW sensitivity, they're going to need roughly around 512mW and 1000mW (note that you will not literally be using all that power all the time doing 120dB, but without reserve power your headphones will sound somewhat lazy). OTL amps like the WooAudio WA3 produces 150mW at 32ohm and 270mW at 120ohms, and that's already a lot for an OTL amp. By contrast, another $100 and you can get the WA6, which, despite being biased for 32ohm power delivery, actually still delivers more power into 300ohms than the WA3.

Output impedance is also a problem. The Valhalla2's 40ohms is very low for an OTL amp. As much as planars don't require very high damping factor (which drops when the impedance of the headphone isn't high enough vs the amp's output impedance, though dropping it from optimal range does not automatically mean you've dropped it enough to be a problem), having an amp with damping factor higher than the nominal impedance of the planar can still introduce some EQ effect to the sound.

So in reality it's not so much that all planars can't work with all OTL amps. It's just that most planars if not all have specs that aren't conducive for use with an amp designed to drive high impedance, higher sensitivity headphones on the cheap (which is why the Darkvoice DV336se is often recommended, but not necessarily the WA3).

Is there a formula / equation by which the above calculation is done. I own a Feliks-Audio Elise which I believe has output impedance of ~ 50 ohm and max. power output of ~ 200mW. Could Elise power LCD-2 (70 ohm, 101 db sensitivity) or HiFiMan HE1000V2 (35 ohm 90 db). I think the Dan Clark Aeon 2 Noire (13 ohm, 91 dB) is a no go even with out knowing how to calculate, correct?

I love my OTL and high impedance dynamic (ZMF Auteur) with it, but interested in trying the planar "flavor".

Similarly, does the above calculation apply to non-planars (for example, also intriguing new HP, Focal Clear Mg, with impedance 55ohm, sensitivity 105 dB)

Insight / recommendations appreciated!

 

[leftside]

When I had @A2029 design the OTL amp for my 200 ohm LCD4, we tried to lower the output impedance of the OTL amp as much as possible. We did that by having 6 output tubes and a totally ridiculous separate power supply. I think this brought the output impedance of the amp down to 15 ohms, but this is not a "normal" OTL amp.

Typically, you'll want the impedance of the headphones to be greater than the output impedance of the amp. As OTL amps are usually high output impedance, you'll want fairly high impedance headphones. "Power of 8" is what I've seen mentioned before - i.e. 8 * the output impedance of the amp.