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

One of the nice things about this amp is that the synergies are dead simple if you have some idea of what your tastes are. The headphone spec rules out a lot of things. The impedance curve will tell you whether the headphone's FR is likely to change in a way that would be good or bad for you. There's a couple other things like the LF extension that can be important, but they show up on atomicbob's measurements and many people's subjective impressions, so there's advance warning.

I was lucky that every* headphone I wanted to try was measured by Tyll back in the day, complete with impedance curves (he really did a lot for us, didn't he?), and in yet another victory for the laws of physics everything basically sounded consistent with that. All the sources I've tried do what they usually do, so that part's simple too. Then all that's left is integrating your own tastes. At no point did I ever get an unintuitive or weird result (e.g. HD800 working well with Magni Piety despite those both having lots of traits that make the pairing seem to me like it should be a terrible idea).

That's not to say that everything about this amp shows up clearly on measurements or that it lacks notable sonic traits, but those traits seem to predictably manifest on any capable headphone the amp can power well. It should be pretty easy to know what this amp is likely to work with if you just take a moment to think. If anything, I'd think the specialized nature of the amp should make it easier to resist hype.

On that subject, HD800 is my favorite of what I've tried, but I think ZMF Eikon works very well too.

* OK, he didn't measure Elex, but he did measure Elear. The way I understand those headphones are related, I figure you can still make some inferences.

 

Actually, I am using the 3...and separately, I agree Custom Cans mod for the 6xx as deserving nomination for an award.

 

I've run 7 tubes in it....Stock Shuguang, Tung Sol 6080, RCA 6080 black plate and grey plate, Sovtek 6AS7G "Winged C", RCA 6AS7G (black base) and a Thompson 6080. The best bang for the buck at $9 was the 6080, the best sounding, by far, is the Thompson. The RCA 6AS7G is very good but the Thompson is better. It's a matter of detail, gain and treble grain on the majority (but not all) of the headphones I have for tubes....which I hate to admit is probably too many. I didn't care for the Sovtek and the Tung Sol was probably 50% better than the RCA 6080 but was a noise machine so I may try to find another one. No unobtanium here, I refuse to pay more for a tube than I do the amp.

I find myself wondering with all of this talk about the HD800.....would this thing tame the whackadoodle treble on the HD700? They're....um.....cheap....now. I don't know if the impedance on those being 150 would mesh well with the output impedance on the amp that you guys measured.

 

150 ohms impedance isn't a dealbreaker, those still shouldn't need a lot of power. However, the main virtue of this amp is not that it fixes screwy frequency responses. The measurement posted a few pages back has the bass up like 1.5db around the 122Hz cursor and the 5-6k peak down about 1db, versus a near-zero Zout amp. It's a noticeable effect and you will hear it, but it's not totally remaking the headphone. You should basically like the headphone to begin with.

If you dig for Tyll's HD700 chart, you see that the impedance curve has a narrower bass bump but kinda the same shape, and it's a lower impedance, so you could eyeball the effect as being stronger (somewhere between what happens on the HD800 and the Grado). But HD700's treble is much, much screwier than HD800's to begin with. The worst peak is way higher, maybe 3-5db higher. You're not going to erase that with just this amp. And we're talking 3-5db higher THAN AN HD800, and with the dip before the peak being a lot deeper too. The output impedance alone is not going to fully compensate for that big a peak.

Even if I thought this were a good idea, the used price on the HD700 doesn't actually look that low. You can get a better headphone for cheaper, surely.

 

Fair enough. That all technically makes sense. I had seen an HD700 for under $200 that needed some TLC but I don’t need another project THAT much.

 

If money is burning in your pocket, might I suggest putting that toward a different DAC/source? I think you mentioned you have several compatible headphones so adding an HD700 might not make your experience more meaningful. The amp can scale with better sources.

 

Thanks for the suggestion and I get where you are coming from. As far as DACs are concerned I have like 20 of them ranging from $13 to $1,000. I was more curious to see if I could "fix" the HD700 than anything after seeing a bunch of varying reviews, but I probably don't need another HP project seeing as I just finished 2 Beyerdynamic reclamation projects. I don't really "need" to improve anything, this is more out of a desire to fiddle than anything else.

 

Probably should have played Amazon lottery and get the amp in a couple of days rather than relying on Apos. Ordered on the 30th and it looks like the amp still hasn’t even begun to travel. I didn’t realize Apos has no US warehouse and relies on China to drop ship their stuff - I now expect to either not receive the amp at all or get a really banged up one because god knows how some Chinese shipping company will handle the amp.

I wouldn’t order from Apos going forward given the experience. They should have said we have 0 stock in the US and will drop ship from China.

 

I procured my TA-66 from Apos. Confirmation of order came from Apos. Subsequent notifications came from ztn in email which apparently is the notification service for GoFo Express. It took 11 days from order to delivery and the shipping box was in very good shape. I should also note that for 8 days there were no notifications or updates. Then suddenly a few began to trickle through. Shipping was from China.

 

I had a good experience with my Apos order. Took a little while, but came in great shape. China is far away from me!

 

I bought mine from the company that handles Apos and a few other Chinese product importers returns and warranty shipping in SoCal as an open box....apparently it was returned with a bad Tube. And thus, I got $75 off, 3 day shipping and an as new warranty. I hope it doesn't break because they said the repair process is...unpleasant......due to shipping to the Motherland. I suggest not losing the box. Just in case.

 

Finally got the amp a few days ago. Nothing is damaged but I think the packaging could use either better foam or double box. Still burning in at this point and have only listened casually. Some omission in the treble frequencies is a bit concerning, leading to a slightly muffled sound. I may try a different output tube to see if I can bring back some clarity if things don’t Improve over the next few weeks. Also, I think calling this a ZDS jr may be going a bit too far. In my brief listening I didn’t hear any ZDS mid and soundstage magic in TA66. Not bad for sub $1k category.

Edit:
System: Shanling ET3 -> Emm Labs Dac2 -> TA66 -> NDH30/650.

In terms of absolute sound quality TA66 is very good and pairs well with neutral or neutral-bright dacs. TA66 may be too warm to pair with playback designs dacs. In terms of scaling, TA66 did not sound like it was holding back emm labs dac2 in most areas, except for deep bass and ambience and ability to render an expansive space. I think going up to DAC2x, XDS1 or DA2 may leave too much of those dacs’ performance on the table.

I was able to place the amp next to my monitors, bunch of external HD and other equipment and the amp has been dead silent. Kudos to the designer for getting RF interference under control.

For those with ZDS and TA66, what has been your experience? Is TA66 just a well designed OTL SET amp or ZDS jr?

 

It took my TA-66 6n5pj about 30 hrs to settle. I wonder how much variance exists between TA-66 units and also the stock tubes.

 

I can't say anything about that specific comparison, but depending on what one means by "magic" the TA-66 may not really sound like that. I haven't heard too many tube amps, but I think Magni Piety may actually have a more overt display of that kind of sound...

 

After approximately 30 hours of burn-in, I have developed a much greater appreciation for the TA66. This improvement is significant enough that I am now actively comparing it to the ZDS, which is a big W for the TA66. ZDS still has a vastly larger soundstage, higher resolution and euphoric mid range.

@atomicbob Do you know if 6AS7G Svetlana would work as the output? Also, is 6N2 the same as 12AX7? I am trying to buy a couple replacement sets as I intent to keep the amp on 24/7 for PC use.

 

6N2 has a different pin out than a 12AX7 and less gain. The 6N2 tubes I have (Voskhod) are better sounding to me than the new production 12AX7 tubes like the Tung Sol reissues. Any 6AS7/6080 will work.

 

Any reputable supplier of Voskhod 6N2? Thanks.

 

The ZDS will have a higher level of overall performance and engagement, but it is surprising how close the TA-66 can come in satisfaction.

Svetlana 6AS7g / 6H13C should be fine.

As @ecline56 mentions, 6n2 has a different pinout with different parameters and common tubes such as 12AX7 should NOT be substituted. A 6AX7 might work, but those are hard to find and probably have different gm.

 

You'd need adapters for 12AX7 and don't go there unless you know what you're diving into. But as @ecline56 mentioned, the Voshkod 6N2P is good. @chesebert was this not the stock tube you got?

There's a higher spec 6N2P-EV version. It might be a little better (I can't tell because it takes some time to swap tubes) and the EV versions shouldn't cost you that much more.

I have tried an adapter for use of 6SL7, and this works fine. I'm currently trying a Psvane 6SL7 and there seems to be more clarity, but not necessarily better. I've also tried MELZ 6SL7 equivalent and found it microphonic. I've read that old SLs can be like that.

Adapters are available online under the Jemosa brand. Again, be conscious of your risks.

edit: emphasizing 6SL7 with adapters, not 6SN7.

 

Maybe here https://www.thetubestore.com/russian-6n2p-eb-6ax7