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[DIY] Chinese Single-Ended Tube Preamp (6J4, 6P6P, 6Z5P)

In January this year, I finally decided to change out my home stereo setup and experience my music differently. In doing so, I sold my NAD C162 Stereo Preamplifier. That piece of kit was a full featured solid state preamplifier and a component I have come to adore. It did pretty much everything I asked of it and included a tape monitor function built in (great for digitizing records). I have stuck with the slight forward mid-range and darker side of things for awhile and figured it was time to pursue a more neutral and where possible, some sense of holographic scale in my music. I recently put together my first Class D ICEpower Amplifier so that takes care of the ‘neutrality’ side of things. I still needed a preamplifier of course and decided on a tube based component for that ‘holographic’ sound.

Tube based audio components are not exactly ‘cheap’ to start with hence I did take a little time researching and scouring for options. There are some DIY kits available of course but for some reason, those kits turned up as more costly options to some Chi-Fi (Chinese HiFi) gear. I know some folks who are bias to Chi-Fi due to questionable build quality and dodgy components used in the build. I think those are all valid. Some would argue that the quality of Chinese made audio components have improved significantly in the last 10 years and there are many good examples available at very competitive prices in just about everything category that will give any American, European of Japanese manufacturer a good run for their penny. Brands like Denafrips, SMSL and Topping are 3 immediate examples that come to mind in the DAC and Headphone Amplifier space. I would argue that for the very same reasons, audio enthusiast today have access to higher quality components from respected manufacturers at very competitive pricing due to Chi-Fi. Like it or not, High Fidelity sound is no longer limited to snobbery circles thanks to Chinese HiFi. Now, that cannot be a bad thing at all.

My search finally landed me on AliExpress. any would know AE is a platform Chinese companies of all sorts sell their wares to a Global market. This also happens to be a popular Chinese e-commerce site for audio enthusiast on a budget. From the odd bits and ends like machined aluminum potentiometer knobs or 1% tolerance metal film resistors by the truck load, AE is a great source of cheap parts for tinkering if you do not need them at your doorstep overnight. It takes a couple of weeks typically as sellers ship them across the world in slow boats. On AE, I found my cheap Chinese Tube Preamplifier. The neatly hand soldered piece of kit cost just under $200 SGD shipped to my door. Till date, I have not found another component that offered the same level of build quality and generous number of line inputs on the rear panel. Sure, there are smallish desktop ‘tube’ preamps available from other Chinese manufacturers like the FX-Audio Tube-01 but those are not quite the same thing and typically quite limited in functionality.

The Chinese Single-Ended Tube Preamplifier on stock tubes (almost)

For my own sanity, I am just going to refer to the Chinese Single-Ended Tube Preamplifier (6J4, 6P6P, 6Z5P) as the ‘CHN SET’ from here on. I did some digging online for reviews of the CHN SET. There was 1 YouTube short sound clip which was not what I would describe as ‘HiRes’. There were a few bad reviews on Amazon.com from grumpy buyers who seemed to dislike the unit. For $200 SGD, I decided to pull the trigger and ordered the CHN SET from a seller ‘HIFI EXQUIS’ on AliExpress. I figured it could suck for 2 full tanks of my 2018 Subaru Forester or I could be laughing my head off for picking up one of the better deals out there. There are options for a full kit with or without a Bluetooth board. I decided to opt for a fully assembled unit without Bluetooth. If I was going full old school and point-to-point soldered piece of kit, no thank you for a PCB in there just for Bluetooth. I did request for a custom 240V Output Transformer (OPT) as part of the order. The local voltages can run pretty hot and I seen them averaging at 250V in my apartment. Running a 220V OPT with tubes on a 250V power line is not a good thing at all. 238V would be fine with a +/-5% tolerance. Lijun the Store Manager at HIFI EXQUIS obliged and it probably took something like 2 weeks and the CHN SET arrived.

Hook up was pretty straight forward. I just had to install the included factory stock tubes. I had ordered some NOS tube off eBay but those took forever to arrive due to supply chain issues earlier this year. That allowed me more time listening to the stock tubes. The CHN SET had pretty high Gain of 8x typical of tube based equipment and I was running into issues early on. Where 7 o’clock on the volume dial was mute, I could not get it past 8 o’clock on normal listening volume. At 9 o’clock, my neighbors would be calling on the cops. Clearly, I had very limited ability as-is to fine tune listening volumes. I spoke with Lijun and there were a couple of options. I could either replace the 100K potentiometer with a 200K, testing my soldering skills in that tight space under the chassis. Or I could solder on a couple of resistors in parallel to the feedback resistor which had a 24k resistance value. I went with the resistor route as it was less of a test and those things are dirt cheap. I ordered a few 1/2W 1% metal film resistors off, you guessed it – AliExpress.

1/2W 1% tolerance metal film resistors

When the resistors arrived, I started with 10k value in parallel to the 24k feedback resistor. Assuming everything was as stated in the spec sheet of the CHN SET, that would drop the feedback resistance of the CHN SET to approximately 7k, about 29% of the original 24k and lowering Gain to about 2.4x – rather close to Gain of 3x, down from 8x. I listened to the CHN SET and I of course now had more ‘play’ on the volume knob. Instead of 7 o’clock, I was able to get the volume knob at 9 o’clock for normal listening levels. Somewhere closer to 11 o’clock was pretty much I could push it without any permanent hearing loss. These 2 positions were similar to my NAD C162 I had before. Since the CHN SET used a logarithmic potentiometer, I decided to try reducing the feedback resistance even lower so I could have the volume knob at 12 o’clock for louder listening volume. Ideally, 9 o’clock at about 74dB and 3 o’clock for 84dB. That should given me about 79-80dB at about 12 o’clock.

Installed 1/2W 1% tolerance metal film resistors in parallel

I removed the 10k resistors I had installed in parallel previously and replaced them with 6.3k resistors instead. This dropped the feedback resistance value to approximately 5k, 21% of original values and landing on approximate Gain of 1.67x. I listened again and while I did not do any measurements with my digital SPL meter, I was able to dial up the volume knob to 12 o’clock and get to what I would describe as ‘comfortable’ listening levels. It was a little louder then normal but had enough to stir the soul and not reach the point of risking the cops at my door. I did have some 3.5k resistors ready to go which would have lowered the feedback resistance to just 3k, 13% of original and lowered Gain of 1.02x. That would literally be Unity Gain on the CHN SET but I never got to that as I was happy with how the CHN SET sound after the second adjustment and no, there was no detrimental impact to the frequency response based on what I heard. I did contemplate pushing for Unity Gain but decided against have a third go at it.

So how does the CHN SET sound? Out of the box and with the Gain reduced, I was finally able to do some proper listening to how the CHN SET sounded. The simplest way to describe the sound signature is the CHN SET has a good amount of air and sparkle. On the upper mid-range, I noticed some grain when listening to female vocalist. That is not exactly a bad thing as I found the CHN SET allowed for a very resolving top end. The lower mid-range was clean, less musical compared to the NAD C162 while having better control and definition on sub bass frequencies. Paired with the Class D ICEpower Amplifier, the CHN SET exhibited a holographic and airy soundstage. There was nothing ‘tubey and warm’ about it but what I would describe as clean and spacious sound. Coming from a solid state meant having to accustom myself to Class D amplification of bass notes. The CHN SET did not detracted from how fast and tight bass notes sounded on good Class D setups.

Stock ShuGuang 6P6P black glass tubes with NOS Siemens EF94

I did swap out the tubes once I had what I ordered all together. I replaced the Chinese 6J4 with a couple of NOS German made Siemens EF94 input (signal) tubes and I have to say these made the biggest sonic difference. For 7 silly British pounds a piece, I think these sound really good and quite frankly have never experience such a significant sonic improvement for so little change. It took away some of the upper mid-range grain, rubbed on a little more clarity and soul into the mid-range. The next tube I replaced was the Chinese 6Z5P with a single NOS UK made Brimar 6X5GT rectifier tube. Again, the swap led to a noticeable improvement in sound. The overall presentation had better image separation and I found the bass notes to be more defined. Any initial hmm-hahs about the sub bass notes on the stock tubes was addressed. The last that I swapped out was the pair of ShuGuang 6P6P power tubes with current production Russian made Tung-Sol 6V6GT. These tube are pretty well reviewed but quite frankly, I felt the CHN SET sounded better with the original ShuGuang. I still question why there is a need for large output (power) tubes such as the 6P6P (6V6GT equivalents) in a tube preamplifier to begin with. In hindsight, the 6V6GT is a common tube in guitar amplifiers and for most part, they were reviewed for that use case. If I had to do it over again, I would skip the Tung-Sols but still went with the Brimar and Siemens swaps. The Siemens are still my pick for absolute value for money swaps and highly recommended as the first upgrades to do if any at all.

NOS Siemens EF84 + current production Tung-Sol 6V6GT

Would I recommend the $200 SGD CHN SET? Well, it depends. It depends of what you are pairing it with. The CHN SET preamp has an output impedance of 60kOhms by default through the 6P6P tubes. Most solid state power amplifiers commonly have an input impedance range of 50kOhm to 100kOhms. Ideally, you would want a ratio of 1:10 from output impedance : input impedance. So if your preamplifier had an output of 5kOhms, pairing to a power amplifier with a typical input impedance of 50kOhms should work just fine. Taking the principle of 1:10, you could see where and output impedance of 60kOhms would land. Some even suggest a 1:20 output to input impedance match for best system synergy. My Class D ICEpower amplifier is specified to have an input impedance swing of 8-10kOhms hence the match is not what most audiophiles would call ideal. Granted, some may not even hear the difference if they did not have a reference point to start from but to the trained ear and in fairly resolving setups, folks have claimed to notice dropouts between the 1-5kHz range. I have to say, during my listening tests, I did not notice such dropouts. Your mileage may vary depending on the rest of your system setup. Otherwise, $200 SGD on AliExpress for a Single-End Tube Preamplifier is still silly cheap money to give it a go. Since everything is point-to-point soldered without any PCB, it has very good modification and upgrade potential. The same Chinese Single-Ended Tube Preamplifier appears to be sold by a few resellers and under the Nobsound and Douk Audio brands. I can share that my experience with HIFI EXQUIS on AliExpress was a good one and would therefore, happily recommend them.

NOS Brimar UK 6X5GT rectifier tube on left, pair of current production Russian made Tung-Sol 6V6GT in the background and a pair of NOS German made Siemens EF94 to the right.

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