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Benji90 22nd July 2021 04:16 PM

Tube amp for power hungry planar magnetics
Hello everybody,
Hope you are doing well!
I have a pair of audezee lcd3 headphones and Iím looking to build a tube amp for them. It can be both with an output transformer as well as Otl. Iím looking for some thing high quality with a great sound. The headphones have an impendance of around 100 Ohm and as all planar magnetics are relatively power hungry. Any suggestions or direct experiences? Iíve searched the net but so far didnít find anything that has convinced me 100%.

Thank you very much!

PFL200 23rd July 2021 10:30 AM

I like this one a lot but I am probably biased (it is my build; I used to be a member of diyAudio before): "Aikido" headphones amp with ECC40 and E182CC

With 100 Ohm headphones the value of the output capacitors ofcourse has to be higher.

Benji90 23rd July 2021 06:29 PM

Thanks for your reply,
Doesn't such a big output electrolytic capacitor affect negatively the sound quality?

PFL200 23rd July 2021 06:56 PM

I am not sure what you mean.

The output capacitors in my amplifier are not electrolytics. Getting more capacitance without using electrolytics may not be so practical, but it is doable. Besides that: Nowadays there are very good electrolytics so I doubt if this would present a real problem. I did not use them myself because I did not have to (but I confess that a little bit of "Mojo" was involved also).

xrk971 24th July 2021 10:42 PM

Electrolytics are used as output couplers on some of the finest sounding Class A amps. Don’t have an irrational fear of them unless you have heard them and can hear a difference.

dreamth 25th July 2021 01:07 AM

I can't remember for which ones i had to calculate the power once, hifiman or audeze but one of them needed 16 times more power than a regular 32 ohms dynamic headphones...i'd say that an output transformer might be the right way for those.

penmarker 26th July 2021 05:49 PM

The common understanding would be class A solid states would be a better pairing with planars compared to tube amps. Tube amps are able to deliver voltage required by planars easily but due to the lower sensitivity (power hungry), planars will pull more current. This is where tube amps lag behind class A solid states.

Yes output transformers are a way to go for voltage to current conversion.

Or build a PassDIY Whammy headphone amp :)

xrk971 26th July 2021 08:17 PM

The worst offender for power hungry headphones is HiFiman HE-6. 32ohm, 82dB/mW sensitivity so they require up to 3W. It’s about the only headphone that requires such extreme power. A headphone amp with an OPA541 output stage should handle it well. I am working with Jhofland and we developed a headphone amp with a SET tube buffer stage which then drives the solid state OPA451 to make about 5W into 32ohms. We have two versions, one is 2W and the other 5W. They look very similar as prototypes.

Benji90 26th July 2021 08:48 PM

I’ve always thaught that big electrolytics in the signal Line would have a negative impatto in the sound quality. 100 ohm is such an impendance that would require a big electrolytic and at the same time a relatevely high current amo inr si that maybe an otl is not the best way. Can you confirm that?
With transformers the Problem is to find the right one with the right impendance matchings. Not many produceva have specific ones for headphones.
I know a solid state class a amp is probably the most rational choice but i wanted to explore the tube option

xrk971 26th July 2021 11:28 PM

A 2200uF or two 1000uF in parallel electrolytic cap will take care of most headphones. You can get higher quality “audio grade” ones. Brands like Nichicon UKZ “Fine Gold”
or “Muses”, Panasonic SEPF polymer is one of my favorites for low ESR, Panasonic FR although not an audio cap sounds great, and Elna Silmic II are also superb sounding. Add a 2.2uF MKP film cap as bypass if you want some extra resolution.

Here is how you calculate value and bass extension.

Let’s assume worst case is 32ohm headphone impedance. Let’s say you want 10Hz before any bass fall off. Let’s take it two octaves lower or 2.5Hz for the filter -3dB point.

The corner frequency for 1st order high pass RC filter is = 1/(2pi x R x C). Solve for C and we get C =1/(2pi x 2.5Hz x 32ohms) = ~2000uF.

So a 2200uF cap will give bass response flat down to 10Hz with 32ohm headphones.

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