Review of AIYIMA T2 6K4 Tube Preamplifier Can this Solid Little Asian Beauty Sing?

 March 20, 2024

#477

Gentle reader,

This video takes over from the previous one here: The Robb Collections: UPDATE Inexpensive Chinese-Made TUBE BUFFER amplifiers. A Review of One using 6J1 Tubes Upgraded to GE JAN 5654W Tubes

The Chinese people have invented many things and were far ahead of much of the Earth's peoples in many ways for many centuries. Today, they are known as the world's factory and have an infrastructure that is the world's envy.

Sure, there are a lot of cheap items that come out of China and land in dollar stores all around the US. But, they are an industrious people and have created items that are designed to attract the West's collective eyes.

My interest in Tube electronics was mainly in the 1970's, starting with Dad's old Hallicrafters multi-band radio. It predated FM, so all I could bring in was AM radio. Later, my little brother found a Bogen Tube integrated amplifier.

I have found a tube radio or two over the years in thrift stores. None of them, including this find from a year or two ago, did anything more than hum or hiss. 

That all changed when one of my readers, who became a real friend, came up with the idea of us three (including another reader / friend) should all build a DIY tube buffer kit. He provided the kits and PCB soldering kit. I found the needed 12 volt AC transformers and knobs. 

This is an image from an eBay seller's auction page. He supplied the transformer and case (which is clear plastic, that one still has the protective paper on it) and knob.

Well, none of us got around to actually building one. I bought a couple completed ones via eBay. The link below is to an article I wrote about finding a major flaw in the ten-dollars-or-so kits. At least when using the massive AC transformers I found: 60 Hertz hum. The Robb Collections: UPDATE Inexpensive Chinese-Made TUBE BUFFER amplifiers. A Review of One using 6J1 Tubes Upgraded to GE JAN 5654W Tubes

Which leads me to this article in which I gave up on cheap Chinese tube buffer kits and bought a pair of REAL tube preamplifiers with solid engineering behind them and real electronics and DC (not AC!) transformers.

This is an image of the inside of the T2 Pro. The standard T2 can also be had in silver. I was looking for a photograph of the inside of the unit. This the closest I could find. A whole lot more "guts" than the ten dollar plastic cased tube buffer. But, these do offer tone controls and Bluetooth input and it's associated electronics.

Some of the images above are from the Amazon page, others were found by doing an image search for the unit. from which one can purchase the AIYIMA T2 6K4 Tube Preamplifier for (at least in my case) less than sixty bucks US. I ordered them on Monday and Tuesday morning, early, the box was on my front porch. Inside were these two boxes.

While "un-boxing" is usually reserved for YouTube videos, I am old school and prefer still images.

While there is an "op-amp" inside, I don't think any digital effects are happening inside. But, I could be wrong. No transmitting either, only receiving. Bluetooth, that is.

Another thing the Chinese are really good at is packing their wares really well.

I'm not sure what I am supposed to do with the stickers.

6K4 tubes are taller than the 6J1 tubes I have been fiddling with previously. Are they "better"? I don't know. Let me know in the comments area below.

The metal case is very well made and the whole unit is heavy for it's size.

Volume control is nicely detented and all knobs are solid metal. All the controls seem to be of good quality. 

I was hoping to get the case off to see what the internals look like. Sadly, more than just the two side screws hold it on. I did not want to break something, so the "guts" will have to remain a mystery.

There could be screws hidden beneath the rubber feet which seem to be made of Sorbothane.

Comparing the two units is like comparing "cheap Chinese crap" and well made precision equipment. 

The fact that the DIY kits and preassembled 6J1 tube buffers are being sold for less than fifteen bucks (ten when I bought the first built pair) after being manufactured, shipped from China, bought from the warehouse should be a clear indicator that they are cheap Chinese crap and not to be trusted. 

I have two stereo systems, plus a nice home theater system, in the house. The listening room uses 1990's Panor-era Dynaco equipment and the stack of vintage Yamaha 500U equipment with an OPPO universal player taking the place of a finicky Yamaha 500U CD player. 

Right now, I am listening to a SACD of Herbert von Karajan conducting the Berlin Philharmonic performing Beethoven's 9th symphony way back in 1963 on the DG label through the AIYIMA T2 6K4 and my outstanding Audio-Technica AIR ATH-AD700 headphones. And, let me tell you, it sound's FANTASTIC!

Here are some more images of this amazing little gem in the listening room system. In order of my trying one of them out first, then moving it down to it's final destination, sitting atop the Dynaco PAT-6 preamp/tuner where it is safe from being bumped.

The system above is the culmination of almost fifty years buying and trying literally hundreds of components and speakers. This AIYIMA T2 is the first change I have made to it in many years. 

NOTE: The unit I added to the Yamaha stack (next to my PC) seemed to have failed the next day! The left channel was weak, then gone altogether. I tried swapping tubes (again) and found it was still limited to the left Channel. Removing the unit and using double-female RCA connectors resulted in both channels working.  Similar to below, but mine are Monster Cable units.

Amazon quickly sent a replacement T2 preamp, and took the back one back with free shipping. The new one did the same thing! I thought, "Wait a minute, what if I unplug the transformer rather than using the unit's on/off/input selector switch? Let's see if that helps." THAT did the trick! 

So, in both systems (both rooms) I remove the transformer from the AC source and plug it back in each time I turn on the system. Both channels come on every time. Yay!

Below are some information I have gleaned from the Amazon page as well as AIYIMA's own web site:

"AIYIMA T2 6K4 Tube Preamplifier Bluetooth 5.0 with Treble, Midrange & Bass Controls, Hi-fi Audio Preamp for Home Audio Amplifier System with DC12V Power Adapter.

Item Dimensions LxWxH         4.69 x 3.74 x 1.22 inches

Voltage                 12 Volts

Manufacturer                 YIMA TECH

Mounting Type         Surface Mount

Brand                 AIYIMA

This Bluetooth 5.0 equipped Vacuum Tube Preamplifier has Bass/Medium/Treble controls, has really sound good quality. (sic)

It lets you enjoy sweet and warm sound quality associated with Tube equipment.

1 pair Stereo RCA inputs and Bluetooth Audio input (includes antenna) with manual switch mode. 

2 pairs Stereo RCA outputs can connect to two amplifier devices.

Equipped with DC12V/1A power supply. 

Can be upgraded using the 6K4 tubes design for easy replacement. Designed so the tubes can be replaced to 6J1, 6J2, GE5654, 6J3, 6J4, 6J5, 6*1N, 6AK5, etc.

Build quality is very solid, the unit is surprisingly heavy.

The power circuit is completely separate from the audio circuit, minimizing interference for a clear sound. 

This AIYIMA T2 is a preamplifier use for Audio Component Power Amplifiers.

And/or it can be connected to a pair of powered speakers and a rear amplifier.

Equipped with warm sounding 6K4 Vacuum tubes. These tubes can be replaced with: 6J1, 6J2, GE5654, 6J3, 6J4, 6J5, 6*1N, 6AK5, etc.

Because the design has current passing through the tubes, the tube must be fully inserted during use!

Both Bluetooth and RCA audio input modes are selectable.

Use Bluetooth 5.0 Version, it can offer high bit-rate transfer and supports AAC, SBC, APTX, APTX-LL, APTX-HD. 

And the RCA input (red/white) is compatible with most audio devices, such as computers, tablets, laptops, phones, TV, CD player, etc.

Dual Channel RCA output.

This upgraded T2 tube preamplifier can use for most home audio amplifier (such as A07), or connect your active speakers directly. It provides you with clean and warm sound!

Dual Channel simultaneous output. This T2 tube Preamp is special, as it can directly output connected to 2 Amplifier devices.

Nice buttons design and knobs layout with: Treble, Medium, and Bass Control, it can achieve different sound style. This upgrade T2 is a perfect Tube preamp for your home Audio system. Use it to achieve purer music experience.

Use the NE5532 Dual Op Amp Chip. The op amp chip can be replaced to double op amp chip, such as OPA2604, OPA2314, LM4562, JRC5532, etc.

Note: If there is no soldering technology, it is not recommended to replace the op amp chip. Poor soldering will cause noise, which will damage the sound quality!

With DC 12V Power Adapter. This T2 Tube Preamplifier can not be used in cars, turntables, also it cannot be used directly for headphones or passive speakers. Please know it.

Note: To ensure sound quality, please use a good audio cable. 

Thanks!

Specifications:

Tube Model                 6K4(Can upgrade Replaceable)

Chipset                 6K4+QCC3034+NE5532

Bluetooth Version 5.0supports AAC, SBC, APTX, APTX-LL, APTX-HD

Operational amplifier chip         NE5532*2

Distortion         =0.15%

Frequency response range         20hz-20khz (±1db)

Signal-to-noise ratio         =100db

Volume adjustment range         ±6db

Audio Input mode         Bluetooth or RCA

Audio Output mode         RCA

Input voltage                 DC12V

Product information:

Item Dimensions LxWxH         4.69 x 3.74 x 1.22 inches

Voltage         12 Volts

Manufacturer                 YIMA TECH

Mounting Type        Surface Mount

Brand       AIYIMA

Number of Channels         2

Product Dimensions         4.69 x 3.74 x 1.22 inches

Item Weight         1.15 pounds

ASIN         B0BCKCDQR7

Item model number         AIYIMA T2

Date First Available         August 30, 2022

Country of Origin         China"

It does come with a warranty, which is good. I downloaded the manual, such as it is. At this point, both are working perfectly. Whether they in fact adds "sweet and warm sound quality associated with Tube equipment", I don't know. It does NOT add anything negative, that is for sure!

Thank you for taking the time to read this rather long article. I tried to get all of the specifications columns aligned vertically with the Tab key, but, every time I rechecked in Preview mode, they were still misaligned. OCD sucks sometimes. I mainly use mine for good, not evil.

Feel free to comment below or on Facebook.

Scott Robb

March 20, 2024

#477

UPDATE Inexpensive Chinese-Made TUBE BUFFER amplifiers. A Review of One using 6J1 Tubes Upgraded to GE JAN 5654W Tubes

 March 18, 2024

#476

UPDATE!

Gentle reader,

It has been a VERY long time since I have written anything about the listening room stereo. I have (after more than forty years) been very happy with the system and thus stopped trying to improve it.

Over the years of which I have been writing this blog, several of my readers have reached out to me via Facebook and we have become Facebook friends. Of them one lives within reasonable driving distance and we have gotten together a couple times. Another lives in the same town my Aunt does in Arizona. So, someday we will actually meet, when we finally go out there for vacation. We talk frequently by phone and both are dear to me.  In the mean time, these two and I have come to call ourselves The Three Amigos. Which is the title of a comedy film starring Chevy Chase, Steve Martin and Martin Short.

One of them, who has a Dynaco Museum in his home, last year came up with the idea of all three of us building a small Chinese-made tube buffer amplifier. 

The one in Arizona had these guitar pick holders custom made for the three made up for all of us and surprised the other two with them. He is a very giving person.

This is what the kit is comprised of and the parts list for it. 

This is the schematic.

This image is from an eBay seller called Yunsirui. What was NOT included with our kits was the clear plastic housing (shown with protective paper covering), switch knob or 12 VAC "wall wart" power transformer.

Sadly, none of us got around to actually building the kits! No excuses. I did provide four 12 VAC transformers and knobs (two for me, more on that later) to all of us. The transformers are for landline telephones. See below.

Note the 4.17 AMP (50VA) specification. Talk about potential GAIN!

The transformers are large and heavy, which makes sense since they have lots of potential power.

I went ahead and bought two prebuilt ones via eBay, just in case I failed at building them. The first time any of you saw the first one was here:

The Robb Collections: UPDATE on Review of SKW Cables "Audiophile" at Bargain Prices? Amazon Exclusive SKW Cables

Never one to leave well enough alone, I bought two "Audiophile Grade" 6J1 tubes which are shown below. I do NOT have golden ears and could not tell any difference between the factory supplied tubes and these.

For many months I have enjoyed this little tube buffer in my "home office" with a vintage all-Yamaha stereo stack and an OPPO universal player. No room to use a turntable with it. The stack sits inches from me. Initially, I had the buffer atop the OPPO. I used QUAKEHOLD reusable adhesive putty to hold it in place since the unit has no feet and the pull of the cables would surely snatch it off the top.

I have had zero problems with the little unit and since I have no idea what "Warmth" means as far as the sound of music, I can only say that it does sound a bit nicer than before. Plus, the gain the transformer provides can really crank out some volume. My neighbors, who put up with my occasionally loud music would draw the line should I decide to see JUST how much louder it could get.

I finally decided to assemble the second eBay-purchased board in the case, etcetera. Prior to doing so, I was looking at other similar (all likely Chinese-made) tube buffers on offer out there.  

To my dismay, the second buffer I already had, produced barely any volume from the right channel! I looked at the board and it's components and it was easy to see that the volume control had been replaced. Since I'd bought it from a Chinese vendor on eBay a while back, I set out to find a replacement from a USA vendor. And did so for just over $13.00 with free shipping.

Being curious, because some vendors offered "upgraded" tubes for additional money, I decided to see what are considered the "best" seven-pin tubes. Here are two sites I looked on to learn more:

electron Tube Data sheets - 6 and 7-Pin Vacuum Tubes 5654W Upgrade – 6AK5 6J1 6J1P EF95 6k4 – Fosi Audio

The latter stated that vintage General Electric JAN 5654W Tubes were the best, but due to demand, are getting harder to find and thus, pricy. I found a pair on Amazon for way less than those being hawked on eBay.

According to the labels, they were made in July, 1987. Which qualifies as vintage by any metric.

The seller wrote:

• Built to the TOUGHEST USA standards for Amplifier.
• JAN 5654W Made by General Electric USA, Higher Durability and Life, Heavy Duty Cycle Military Specification Grade (JAN) , Considered one of the best of the 5654 family of tubes for audio use. Large ring, enhanced mica, green powder word + matte word mark.
• The sound is full, soft, delicate and poisonous, (?) larger soundstage, Treble and bass is more noticeable. Can hear some details that did not hear before while more complex music.
• JAN 5654W Can Perfect Upgrade For Little Dot / Little Bear Amplifier, Directly replace 7-Pin 6J1 / 6Ж1 / 6J1P / 6J1P-EB / 6Zh1P-EV / 6AK5 / 6AK5W and EF95 tubes. It can also replace 6J2/6J3/6J4/6J5 and ordinary 5654. It can used for WE-403A/403B/EF95 Matched Vacuum Tubes replacement.
• Platinum Grade Pair JAN 5654W tubes show you the difference in natural voice reproduction and simple musical. Match it plate current and mutual conductance. Also pass a specialized life test, aging test and audio sound test.5654W tubes are NOS, It's new stock tubes.

In searching though one of my parts drawers, I found some small rubber feet I had saved "just in case" and two of the knobs shown (above) which are splined inside like the On/Off Volume switch is outside. The knobs I had found in the past for these were from Parts Express, and use set screws to secure them.

I tested the new unit on the stereo here beside me and it sounded fine. Next, I got started on the task of pulling the heavy stereo in the listening room away from it's location so that I could get to the back of it.

A 10mm wide-angle-lens shot of the listening room. The system is on a large and heavy glass and steel stand made for a very large HDTV and for many components. I modified it to have casters at each corner.

Since the PAT-6 only has one Tape input, I added a SONY audio/video switch for the TDK CD Recorder unit's output I have in the system. I used the QUAKEHOLD to secure it to the top of the PAT-6.

Here is an overhead view. Note the white power cord snaking away. I only had to unplug the RCA cables from the OUTPUT terminals of the PAT-6 and plug them into the (right pair) RCA outputs of the buffer. I added a good pair of RCA cables to bring the PAT'6's output to the input of the buffer.

Upon turning on the buffer, there was a low-level (and deep) hum. It seemed to me lower than 60 Hertz. "Hmm," I thought, "it didn't do that in the other room. But then, the speakers there only have eight inch woofers. These are twelve inches."

I was referring to the black Cerwin-Vega! LS-12 speakers. Which are the last models made in the United States. The smaller ones are vintage Dynaco A-25XL which have ten inch woofers. I switched to those and the hum was barely audible. Which is why I think it is not AC interference I am hearing.

Playing music, the hum is not audible and it only starts to get louder if the buffer's volume is past 50%. Nonetheless, I set out to eliminate the noise. 

"Okay," I thought, "I'll run a ground wire from the transformer."

Prior to that, I marked one terminal insulator and swapped the two AC terminals. It made no difference. 

The hum was still there! I decided to shield the white power cord. I cut some lengths of good quality aluminum tape and carefully wrapped the cord from the bundle all the way to the right-angle plug in three layers. I then fished the wires through  heat shrink tubing and shrunk it with a propane torch.

The hum seems quieter to me, but that may be wishful thinking. Again, as long as I keep the buffer's volume control below fifty percent, the hum is barely audible and not at all when the music is playing.

UPDATE: My friend in Arizona called and we talked about the issue. He concluded that maybe one of the capacitors is failing in the buffer's circuitry. My next step is to substitute that one back in this room to see if there is a hum, if not, then the one sitting on my PC into the listening room system to see if there is a hum...

In the interim, I have loaded these photos of post-cord-shielding.

I moved the unit away from the SONY switch (and wherever the preamp's transformer sits inside the cabinet) just in case that is causing the hum.

While on the subject, let me find a photo of the inside of the PAT-6 preamp-tuner...

As you can see, the transformer sits smack in the middle of the right side of the unit.

UPDATE TWO: I swapped into the listening room system the unit from the Yamaha stack (below, NOTE: I have since moved the OPPO to the top. I don't know what I was thinking.) and there WAS a hum from this unit, but not nearly as loud. Then, I swapped the transformers and there was no difference.

I first placed each unit on top of the dbx box, which of course, has a transformer inside it. No difference in the hum. Then, I gently placed each on top of the turntable and there was still a faint hum from the office's buffer and the same hum from the listening room unit.

Conclusion: Both the unit and it's transformer are hooked into the Yamaha stack. There is NO hum!* The speakers attached in here are two KLIPSCH KG-4s which have an eight inch woofer and twelve inch passive radiator on the rear. And two of the latest BOSE 301 speakers which also have eight inch woofers. Therefore, neither will go deep enough to reproduce the hum, but will still receive it. *See third update below.

To conclude-conclude, I have purchased yet another buffer board from a different US seller. I will keep you posted.

Third UPDATE March 18, 2024: I received the replacement board (identical to all the other ones I have bought) from a different US seller of these completed boards via eBay. 

It is the one in front. Note, the rubber feet, while the one in the rear is secured with Quake Hold reusable adhesive.

I tried it initially in the Listening room system and the hum was still there! But, only audible on the largest speakers. ARRGH! Here, it is using the power supply with the unshielded cord.

Here, it is powered by the modified shielded cable. I tested them both, since in the Yamaha system it is placed in the Accessory RCA in and out terminals, using my Audio-Technica AIR ATH-AD700 headphones. They are THE best headphones I have ever tried and I have enjoyed them for many years.

They are super comfortable because they rest ON your head rather than clamped to your ears by tension. There are later models now for sale in the USA.

Here is a link to Audio- web page about them: ATH-AD700 | Audio-Technica should you like to get some. Specs on that page note frequency response of 5-30,000Hz.

Prior to bringing the new unit back into this room, I tried a test tone CD played through my  Cerwin-Vega! LS-12 speakers. They have the highest efficiency and lowest reach of the many speakers I own now. Their frequency response is 28-20,000 Hz and efficiency is 98dB, one watt at one meter.

The hum was there as soon as I turned the volume control up from zero. Through my Dynaco A25XL speakers it was barely audible.

I put a test tone CD in starting at 10Hz, nada then 20, zip 30 was audible, but too deep as was 40 and 50. 60Hz, as you are now thinking was identical!

Anyway, regardless of which system these units are installed in, at least in my home, there is an interfering 60 Hertz hum, whether the speakers can reproduce it or not.

I looked on eBay for better (i.e., more expensive) tube buffers but the lowest prices were near $200. On to Amazon I went, I chose two of these: 

【Upgrade】AIYIMA T2 6K4 Tube Preamplifier Bluetooth 5.0 with Treble & Bass Control HiFi Audio Preamp for Home Audio Amplifier System with DC12V Power Adapter

They were offering me a 10% discount. They use 12 volt DC power supplies and have full metal cases which I hope should prevent this hum. They should be here tomorrow, so I'm keeping my fingers crossed that they will work, sound good and be interference free.

Thank you for taking the time to read this article and commenting if you have ideas what might be causing the hum.

Scott Robb

March 18, 2024 Updated

#476