xDuoo TA-66 OTL Tube Headphone Amplifier: A Kind of Magic

and why would you power on your amp before powering on your preamp?
Just power on the preamp first, given it is a tube preamp give it a moment, then power on the amp, problem solved.

 

Always turn the preamp on first, let it warm up, then the power amp. That's been the rule for, um, ever.

 

Yes, that's what I'm gonna do in the future if I keep the unit. I just didn't know if this is a sign of a fault or not.The humm is of bigger concern to me.

 

If the hum is a problem for you on your system then it's a problem. There's definitely some hum on the headphone out, but I only hear it on the lowest pot setting (a lot of pots have screwy behavior around there, I guess a stepped pot can too). So the amp isn't perfectly dead quiet.

 

Try different or shorter interconnects?

Sometimes placement of gear matters. You can try shifting equipment around.

 

And the reverse for power off!

Always a good reminder.

ON: upstream source > amp > plug in headphones
OFF: unplug headphones > downstream gear > source

 

Thanks for the advice guys, but I'm gonna send the amp back. In silent passages I can hear the hum from 10' feet away. A pity.

 

How does this amp do with bassier 300-ohm HPs like the Verité Closed?

Too warm? Too bottom heavy?

 

Do you unplug headphones prior to turning off OTL and transformer coupled tube gear? And do you turn on tube amp without a load?

I was told otherwise as tube amp always needs a load unlike solid state. Is that not true?

 

What are some NOS tube equivalents to replace stock tubes with?

 

I've heard that, too, but I think it's more applicable to speaker amps.

 

My unit also had a bad hum on the right channel pre-amp output. I'm sending it in for an exchange, let us see. Tried reseating the tube, checked for good grounding.

 

I don't know, it seems like a lot of people are sending them back because of noise issues. It seems like a pain to have to send it back one or more times to get a good unit.

 

Yeah, if there are no compromises, it is truly too good to be true for $250. Looks like QC is the compromise.

 

There are some compromises, and one should be aware:

• Noise. You might experience some faint hum. If you’re very sensitive to this when the music changes, you might find this unacceptable.
• High impedance only, as mentioned.
• It’s limited in moddability if that’s your thing, because the case is small.
• You might experience microphonics because you’re dealing with tubes. Also you’re playing Russian (and Chinese) roulette with the supplied tubes, but my experience is that the stock tubes are good. Really good. But you know, life’s a box of chocolates.

It’s still a wonderful amp. With regards to QC, I feel that it’s worth getting because companies need to know we will buy quality, and they need to improve on their QC via returns. With decent sales, they would need to adjust because the QC returns eat into their profits.

Anyway, I suggest buying from a trusted vendor, and providing feedback.

 

Mine is a 45-50db hum audible 10ft+ away from the powered speakers connected on the preemp out.

Also the hum doesn't attenuate with volume knob on the amp.

 

Yeah, my first unit had a really noisy gain tube. Second try was fine. I hate buying stuff on Amazon, but it really makes dealing with this easier.

I dunno, it seems like some companies just put up with it and accept it as a cost of business. At some point the message needs to get through that great products should have great QC as well, even if it costs a bit more. FiiO seems to be going through this with the FT1 as well: it's been fairly universally praised for sound quality, but the design quality is lacking and there's a number of reports of problems where the cups attach to the headband.

Do you get the same hum if you run the preamp out into another headphone amp? This really seems like a ground loop to me, especially since it's not affected by the volume knob.

 

This is true for power tube amplifiers but not for cathode-followers in this amplifier.
So do not worry abut running it without load.

What load can change is output signal from preamp outputs (better or worse - depends).

 

Just adding my experience since I got the unit recently. I had audible hum on the left channel and was very disappointed that I got a bad unit, then tried to run with other dacs and eventually found out that the hum is present when using USB out from my macbook to the DAC (charging through my monitor), using a phone or DAP doesn't cause any hum or static. I then tried the laptop unplugged from the monitor and no hum / static.