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

IIRC, TA-84 was always billed as having an OTL based topology (as the TA-26) as well. It's what I came across when I was once trying to find the spiritual successor to the darkvoice.

 

Quick prelim based on those 2 phones indicates something between 75 & 150, but the Grado's curve is a bit bouncy and I'm using an 800 one from Tyll, which might be different from yours. I'll do it properly when I get back home. Meantime, you could try FR with the Caldera Closed - might be an interesting listening experience, too

 

Z out is not so hard to find out, following https://www.valvewizard.co.uk/accf.html
Z out = 1/gm

6N5P=6080=6as7 and gm is 4,2 mA/V
https://frank.pocnet.net/sheets/113/6/6N5P.pdf

That gives Zout=238 Ω

What makes me wondering is opinion about unusual clarity of this little simply amplifier.
But I can believe in it. I think I should try use this tube this way and run AKG K340 with it's Z in 440 Ω.

 

Calculating from impedance & frequency response measurements and voltage division, here are estimates from the different responses of, first, the Grados, and second, the HD800s.

In both plots, brown is the impedance vs frequency curve of the headphones, green is the FR from the Magni, orange the FR from the TA-66, and purple the calculated response from voltage division from amps of 80 (Grado graph) and 150 (HD800 graph) Ohms output impedance. For convenience the Magni's output impedance is assumed to be zero Ohms (mfr spec is "less than 0.1").





On the Grado plot, the divergence of the 80 Ohm curve and the TA-66 measurement in the bass is probably due to the capacitance-filter effect @purrin mentioned (ie the Grado's bass response change with the TA-66 is not due solely to voltage division).

The different results from the different headphones is likely down to measurement precision & accuracy. Using 'phones with more extreme impedance/frequency curves would likely give better results.

Somebody with EE expertise (that's not me) can comment on the differences between these estimates and @Krzysztof 's calculation from the tube gm. What can be said for sure, though, is that the TA-66's output impedance is much greater than 5.5 Ohms; it's really an amp for high-impedance headphones.

 

@purr1n Would you recommend this amp for mainly Grado listening or is it just not terrible with them?

IIRC Folkvangr is ~20 ohms on high gain which was about perfect with the RS1x with the Geekria smaller bowl pads. Is this a lot bassier at 50+ ohms?

 

Some ppl may like bump in mid bass when Grado is combined with TA-66, as we can see on the graph.
So we cannot say it is only for high impedance headphones.
But I wouldn't recommend this amp to anyone for headphones lower than 200 Ω without testing.
Such condition will force this TA-66 to produce higher distortion and final result can change a lot sound of well known headphones.

 

Regardless of pads, the TA-66 doesn’t do anything for me with the SR325x. The mid-bass bump is not something this headphone needs or copes well with. It’s not as awful as the Clears, but not great either.

Agreed in general, with the caveat that a negative gain setting can be useful to help keep the noise floor down. I do like the Pietus with the SR325x for this reason, and I imagine Magni Unity would be a great pairing too. Also many dongles are built around a balanced output with the single-ended out being an afterthought, so that’s another reason to consider a good single-ended amp unless you’ve done some surgery on your Grados.

 

Purr1n said ..... I would be hesitant about using the octals,6SL7, 6SN7. They use twice the heater current of the 12AX7 / 6N5P types. It's possible it could not be an issue, but I would lift the hood and see what's under it.


The 6SL7 uses 0.3 amps of heater current just like the 12AX7. The 6SN7 uses 0.6 amps of heater current. The 6N5P needs. a whopping 2.5 amps of heater current.

If you meant 6N9P/S not the 6N5P that tube also has a heater current requirement of 0.3 amps.

Edit: While I was thinking about this I remembered the 6N2P has a heater current of 0.345 amps.

 

The Clears at 55 ohms may be just below the TA-66’s impedance floor. The Elex (or Utopia) at 80 might work better.

 

I’m currently running a 6N9S (6SL7) on an adapter. I believe a 12AT7* might work but the gain will be lower.

Edit: adapter required. Don’t just plug it in because it seems to fit.

 

@darmok yeah, the Focals are kinda weak here.

I tried them with the TA-66 and they sound a bit thin—just off and not showing their capabilities fully. Switching back to the HD600, and the pairing was excellent. Now I’m curious how a HD800 sounds.


[edit: added more comments]

 

The three things I've tried on this are HD800, HD6XX, and Elex. HD800 was the best by a lot. I didn't like HD6XX much, felt it was warm on warm on warm (but in this respect my source is not helping). I thought Elex actually sounded fine, like it could work, but it was a tiny bit boomier than I wanted and I really just wanted to get back to listening on the HD800. I should try Eikon from them at some point.

But really, if someone told me this was specifically designed with the HD800 in mind I would believe them.

 

It does really well with the DT1990s (OG, not the new low-Z model) and I’m curious how it’d do with the DT880 250 or 600 ohms. But yeah, once I put the HD 800 S on, that was the pairing that stuck.

 

I pulled out the 58X to refresh my memory of how it sounded on this amp, and it’s really good. Now I’ve got the ZMF perf suede pads on mine, but I suspect it’d still be good with the stock pads. Not going to win any awards for plankton, but the micro- and macrodynamics are great.

 

@purr1n have you tried Modius E with the Cinemags on the TA-66?

 

I ordered a TA-66 specifically for preamp duties but there is way too much humm coming from the speakers (for my liking) and also if I turn on the unit after the power amp there is a not very healthy sounding "bang" coming from the speakers once the unit gets operational.
Has anybody else experienced similar issues with their TA-66 or is this behaviour normal? I'm asking because it's my first tube gear.

No issues with headphone whatsoever though.