I ask friends for advice.
I bought a Little Dot MKII with the 6ZH1P-EV and 6N23P Voskhod tubes that worked without distortion with the Philips SHP9500 headphones.
I sold the SHP9500 and bought the HiFiMan HE400SE which has distortions when I turn up the volume.
I tried it with the GE Jan 5654W and the distortion subsided, but it didn't go away.
Little Dot MKII did not come with the original valve kit.
It is set to EF95 (6J1).
I have tried every possible gain adjustment and the distortions at high volume continue.
Does anyone have any ideas on how to get rid of the distortions?
HiFiMan HE400SE works perfectly with Topping L30.
I appreciate the help.
Here is what I see:
Philips SHP9500
-Sensitivity 101 dB
-Impedance 32 Ohms
HIFIMAN HE400SE
-Sensitivity 91 dB
-Impedance 25 Ohms
Little-Dot MKII Power Output:
300 mW (600/300 ohm)
200 mW (120 ohm)
100 mW (32 ohm)
So really, neither of those headphones are ideal for the MKII. The MKII, being a tube amp with no output transformer (aka OTL), will put out more power into HIGHER impedance headphones, and less power into lower impedance headphones. This is the opposite of what you see in Solid-State devices, or tube amps that do have an output transformer. The reason is because low impedance requires a lot of current, while high impedance requires a lot of voltage. The MKII design can supply a lot of voltage, but not much current.
With the Philips SHP9500, you were already pushing the limit of the amp with only 100mw of power available, but at least they had a pretty high 101dB Sensitivity. Moving to the HIFIMAN HE400SE, you not only decreased impedance from 32 to 25 Ohms, but those headphones also have significantly less sensitivity at 91dB.
So, simply put, the Little-Dot MKII doesn't have enough power for those headphones. The distortion that you are hearing is almost certainly a result of clipping (soft-clipping and/or hard-clipping).
Unfortunately you don't have any great options other than to get a headphone with a higher impedance or get a different amp that is able to deliver more power into low-impedance headphones.
Compare the output of the Little-Dot MKII with the output of the Little-Dot I+ for example. The Little-Dot I+ is a hybrid tube amp that uses transistors in the output stage.
Little-Dot I+ Power Output:
150 mW (600/300 ohm)
300 mW (120 ohm)
800 mW (32 ohm)
You can see that this would give you 8 times as much power output when using your Philips SHP9500, and likely even more than that when using the HIFIMAN HE400SE. I use my Little-Dot I+ with my 24 Ohm Sony XB700 Headphones and they are a perfect match, with the Little-Dot I+ potentially putting out close to an entire watt (1000mW) at 24 ohms, and it definitely needs that power when playing bass-heavy music. I also have a Little-Dot II+, which is basically a predecessor to the MKII, also an OTL design, with similar power output to the MKII. It simply doesn't have enough power for the 24-ohm XB700 headphones, very similar to your situation with the HIFIMAN HE400SE and the MKII. I keep both amps, using the Little-Dot I+ with my low-impedance headphones, and my Little-Dot II+ with my high-impedance headphones.
