DOUK AUDIO ST-01 Pro amplifier performance tests with teardown

[@peterbaugh51]

Douk ST-01 Pro TUBE tip: If your amp came with Chinese 6k4 tubes (has a red star printed on the glass), mine caused distortion. I upgraded to much better sound for $17 with GE JAN 5654W matched pair tubes. 6k4 tubes can be replaced by 6J1, 6J2, 6J3, 6J4, 6J5, 6"1N, 64N, 65N, 5654/5654W, 5725, 6AK5, EF95, 6BA6, 403B, etc. Your tubes may be ok. Just an FYI free tip.

[@soumikdas5857]

Will glad to see you making a full tube amp

[@HillsWorkbench]

A tube power amp is an expensive beast, the output transformers alone can cost a pretty penney. Just not a good use of money, that's why you see units like this one.

[@soumikdas5857]

Still there's people like me who will do anything to get that expensive beast 🤧

[@paulperano9236]

Throwing in a couple of valves, is like adding a red racing stripe to your car - makes it go faster. yeah right

[@franzliszt3195]

Well, if I see a car with a red racing stripe, I assume its a beat, even if its parked.

[@francohic]

I read that a guy was thinking exactly the same, so he decided to remove the tubes and play a song to test the amp, and the volume output was almost minimal. Did you try to use without the tubes to check the theory? Because if you remove the racing stripe in car, you know nothing actually happens performance wise.

[@MichaelBeeny]

I think they have added a valve (a RF valve) for looks only. With all those 5532s the circuit offers more than enough gain. So, the valve can only really contribute to noise and harmonic distortion. Remove the valve and all its associated circuitry and you probably get less noise, lower THD. Less issues with impedance matching. Overall, it will sound better and cost probably 50% less. With such poor filter design your chosen speakers should be a very close 8 ohms with no impedance drops towards the 4 ohm area or it will sound dull. Using an RF valve for audio is not a good idea. Thats why audio valves like EF86 etc were made just for audio. Such valves do not really exist in the Chinese surplice market. so they throw in any cheap and unsuitable valve knowing the public just see a valve and get super excited.

[@HillsWorkbench]

Yeah, in most of these the tube doesn't add enough distortion to make for "the tube effect" so it's kind of pointless. Would be cool to have an adjustment on the tube stage so one could adjust from clean to prominent 2nd harmonic distortion.

[@montynorth3009]

He dodged the technicalities regarding the tubes, but liked their glowing effect, with possible led enhancement.

[@franzliszt3195]

And it still on $150. Amazing.

[@franzliszt3195]

Its great to sit in the dark and see glowing tudes!@@montynorth3009

[@franzliszt3195]

Now that is a great idea. I would like to have a amp I could transition in and out tube effect so I can hear what my grandparents heard before 1970.@@HillsWorkbench

[@antibrevity]

Glad to see frequency response is good at 8-ohm, but I wish we'd seen a distortion test as I never trust these toob products coming out of China. The tubes are a gimmick and probably reduce fidelity in most designs, though lately they do at least place the tubes in the signal path, for better or worse. There were some models where they just ran the heaters for show. As the average audio consumer becomes dumber and dumber, I suspect that adding tubes lets these sellers raise the price while adding no useful features. I actually like tubes, btw, but they are not some magical, lost technology that improves sound; simply looking at the tubes and warm glow makes anything *seem* to sound better ;). I like how many inputs this thing has and it appears to be a reasonable design and construction, but it could have been priced even lower without the tubes.

[@ford1546]

The problem here is that a speaker gives different ohms in different frequencies, which means that a class-D with a standard output filter coil design will never give a flat frequency response. often a speaker gives, for example, 4 ohms in low frequencies and, for example, 8 ohms in high frequencies, which means less bass and more treble.

[@jakubczajka4275]

Blah, blah, blah. Purify and Hypex have flat response despite load. I think they have figured out how to implement the output filter into a feedback loop.@@ford1546

[@francohic]

I have the same one powering my Yamaha Ns45 towers, and I love it. It sounds perfect to me, but it might get a bit hot after a few hours of normal usage. Also, I had some minor issues with bluetooth having micro-cuts

[@gamiwv]

Dear John lets face it. My everyday driver is a hamble CD7379 + "AD828" (fake I think from Aliexpress) powered by a 220 VAC / 12 VAC 4,5A transformer for halogen spot lights. I can't hear even the slightest back-ground noise and I'm very satisfied from the audio quality (not for "critical listening) in conjunction with a pair of full-range speakers (open faffle) . In contrast OVER A FEW MONTHS I'm trying to make a class-D amplifier to drive the same speakers with a descent "quality" (not screaming highs) and S/N ratio, BUT IN VAIN !

[@SheikhN-bible-syndrome]

Ive got one of those ic amps im impressed with how good they sound

[@rotaxtwin]

I wouldn't be surprised if they are not stepping the tube B+ up beyond the 24 volts of the supply rail. It would be interesting to measure it but that's my suspicion.

[@user-uc5vr8le7y]

I do think some of these tube buffer pre amps do add to the sound tbh. I think they add a warmth to the sound which i like with some music. Not to everyones taste obviously.

[@KissAnalog]

Thanks John! You didn’t do your normal THD test?

[@jerryking2418]

Great review. Use of tubes I find dubious.

[@franzliszt3195]

It is curious, is it not, that Mr. John did not do a distortion test.

[@franzliszt3195]

Anyway, if its a class D, then what are the tubes doing? Are they really amplifing the signal.

[@JohnAudioTech]

@@franzliszt3195 They are probably configured as a buffer in the signal chain.

[@simonzinc-trumpetharris852]

@@franzliszt3195 Cathode follower buffer.

[@simplyflipflops]

No harmonic distortion testing?

[@1pcfred]

John said he listened to the amp and it sounded good to him. That's good enough for me. I know the distortion of modern amplifiers is inaudible. At least when they're below clipping. What's more important is the power output before then. Generally the louder things are the more we like them. So as long as you have enough power to satisfy you then you're good. This amp is good for 30 Watts a channel. Which with efficient speakers is pretty loud. Really 1 Watt can be loud.

[@simplyflipflops]

@@1pcfred I was more interested in what second order "pleasing harmonics" the tubes add to the sound. Like in his other video on 12au7 tubes. kzbin.info/www/bejne/nqunYpeMotukodksi=eHaRYFH1H25y9VVN

[@1pcfred]

@@simplyflipflops the tubes likely are not even in the signal path. They usually aren't. They're just there for looks. John said he couldn't find a power supply for them. I've seen people pull those tubes and it has no effect on the equipment. But in this case who knows. None of that happened besides the one comment John made.

[@simplyflipflops]

@@1pcfred Bet you they are in the signal chain. These micro tubes can go down to a low voltage as per the specs sheet. Albeit not in their ideal range. :)

[@simplyflipflops]

@@1pcfred my last reply was probably cut off from me sharing a JohnAudioTech KZbin URL in my reply. Do a KZbin search for "johnaudiotech 12au7 headphone" and you might find even the venerable 12au7 can be run at 12v or lower on his previous videos. And he does an amazing job of talking about low voltage power supplies and harmonic distortion with tubes in those videos.

[@ford1546]

The problem here is that a speaker gives different ohms in different frequencies, which means that a class-D with a standard output filter coil design will never give a flat frequency response. often a speaker gives, for example, 4 ohms in low frequencies and, for example, 8 ohms in high frequencies, which means less bass and more treble. All cheap clas-d often have the same main design. Correct me if I'm wrong. This is a completely normal CHEAP class-d amplifier with a slightly fancier preamplifier. Nothing pro quality about this amplifier no.

[@lunam7249]

ohm load is not a concern in "freq responce", d amp do provide stable constant volts ac out....modern class d are extremely clean, BLIND AB TEST FOR YOURSELF, for the $$ its a great value

[@davidgriffin79]

@@lunam7249 Impedance is very much a factor in frequency response; there's a white paper over on Benchmark's website (Audio Myth - "Damping Factor Isn't Much of a Factor") which eloquently explains why the output impedance/damping factor of an amp will influence a speaker's frequency response if the speaker has varying impedance vs. frequency.

[@amitghosh6966]

Really professional looks

[@Montebruno]

thank you for this Do you know what is the amp's input impedance (resistance)? thank you

[@francohic]

If you remove the tubes and use it anyways: does the volumen or the quality get affected? I read somewhere that a random guy posted that he removed the tubes and the volume was almost mute.

[@JohnAudioTech]

Yes it needs the tubes to function.

[@haraldh.9354]

nice and funny music insert -thx

[@davidgriffin79]

1:03 Brings me back to my student days.. 2:20 Ah yes, I've got AYIMA version of this, tried using as a preamp only into an Arcam Alpha 9 power amp, not particularly impressive; tube rolling won't help it's the fundamental design of the circuit which ultimately dictates how good the sound is.

[@indridcold4210]

Don't most speakers have a rising impedance above the nominal rating with increasing frequency in the treble region? So if the tweeter in a speaker that is rated 4 ohm nominal.....the tweeter would still be having a flat response if it was near or above 8 ohms?

[@JohnAudioTech]

Yes, that would be true.

[@jakubczajka4275]

Basically, someone has installed heat sink on the wrong side of the board.

[@MichaelBeeny]

Heatsinking is always an issue on most class D stuff. I'm not sure if they simply do not understand or simply just don't care, both probably.

[@franzliszt3195]

I'm sure it was tested a lot. Unless you play the first 5 seconds of Bach's Tocatta Fuge in a continuous loop your probably OK.@@MichaelBeeny

[@publicmail2]

100+100 watts max power lying dishonest specs on website, .07%THD.

[@MichaelBeeny]

I have NEVER tested one such Class D that makes its published power spec. Often, not by a very long way.

[@publicmail2]

I'm from the FTC days when amps had to have weighted true specs.

[@franzliszt3195]

Mispronounce 'reticule.' No FFT to show distortion.

[@javanpannadi]

This is nothing But Aiyima T9 (not pro) ..once t9 pro sales got spiked against its predecessor they have given another model name and didd co name..i beleive

[@jesses1589]

I've seen a side by side teardown of both units. Internally they are indeed different in their board layouts, not quite the same. I don't think it's best to do an out of the box sound test. He should test now vs burning the unit in for a couple weeks.