Additional Notes
(1) About the necessity of pre amplifier:
If you enjoy a separates amplifier system, a pre amplifier plus a power amplifier, and want 'live sound' with the Douk Audio U4, in that case I think it's best to connect the U4's analog output directly to the power amplifier's input terminal.
I myself enjoy listening to music using this connection, but the only inconvenience I have found is that volume adjustment is touching up or down on the mobile phone. The number of steps for adjusting the volume is small, and it can be difficult to make subtle volume adjustments.
I think the only way to solve this problem is to use a preamplifier. Just fix the output on the mobile phone to a reasonable level, and use the preamplifier to adjust the volume to suitable level.
The following is my experience, even with my Model 7, which has been improved a lot, when I connected it between “Douk Audio U4” and the power amplifier.
Sorry but I heard clearly the sound lost some of its freshness and lively feeling compared to the direct connection. In the end, I had to make a decision to replace the coupling capacitor of the Model 7’s each stage from Arizona Capacitors to Jupiter Condenser Red Comet, fortunately which resulted in clearer sound quality with rich atmosphere.
Actually, I used Arizona Capacitor, 0.22 MFD, 0.33 MFD and 0.22 MFD, in three places of the Tone Amp section, and each time I replaced one by one, I checked the sound quality and experienced how the veil was lifted gradually, little by little.
Of course, this was a new and shocking experience for me, and I realized that there was a lot of room for improvement in my Model 7 tone amp, which I had boasted, was nearly perfect.
On the other hand, I once again confirmed how the single tube output stage of Douk Audio U4 or the Improved U4 outputs a lively and sharp rise music signal.
(2) Honda-san’s view of USB and Bluetooth connections:
The Douk Audio U4 is basically a USB DAC, so instead of connecting it to a mobile phone via Bluetooth, you can also connect it to a PC via USB and easily enjoy PC Audio. My friend, Nishikawa-san always enjoys YouTube surfing on his laptop computer and discovers new music. Well, Honda-san expressed his views on the current state of USB connections and the future of Bluetooth connections as follows:
“This is my experiences with USB connections that I have tried for many years. Surely, the sound changes depending on the USB cable, just like other cables. I've never had a good USB cable, which is specialized for audio purpose. General use USB cables such as ones that come with a printer are often natural. There is also a noticeable difference in sound quality depending on the type of PC with USB output, you are using."
"Furthermore, even on the same PC, there is a huge difference in the location of the USB output, especially if it is on the left or right side. If you are using a notebook PC, it may be better to play without an AC adapter, or to play from a USB memory rather than an HDD."
"With the recent Bluetooth 5.0 and later, the bit rate has increased, and it may be better than a poor USB connection with power supply noise and jitter. About the Bluetooth, although the data is not lossless, the noise are jitter are minimal, so I think wireless connections will eventually become advantageous."
"In near future, it may become possible to support rates that allow lossless transmission. When it comes to the wireless era, the operation will be made by only the smartphone, and the source will be only streaming. I believe there will be a lot of demand for this.”
(3) About the DAC used in the recent Douk Audio U4:
On the listing page for the Douk Audio U4 on Amazon Japan, the DAC used is listed as 1543. This is the Philips TDA1543, a very primitive DAC developed in the late 1980s that does not have any digital filters and is a Non-Oversampling DAC.
I thought this was an interesting choice for our amateurs, but recently the version has been changed to TI/Burr-Brown's PCM2704. However, despite its low price, this DAC seems to be used by many DAC enthusiasts, and its sound quality seems to be highly rated.
I myself no longer pull out my old Pioneer DV-AX10, an extremely high-quality DVD player, that had received high praise for its sound quality at that time.
(4) About the power supply of “Douk Audio U4”:
For power supply, a small 12V / 2A AC-DC switching power supply is included. However, the current capacity seems to be at near the limit, so if you replace it with one with a capacity of about 12V / 3A even with the same switching power supply, the Bluetooth connection will be more reliable and you can expect a much more stable sound quality.
As recommended by Honda-san, even a lithium-ion battery with a capacity size of 12V / 9,800 mAh, for example, will give you clearer mid-high range and more powerful sound than the accessory.
It seems that there is compatibility with not only the tube selection, but also the sound character of your audio system, so I think the selection criteria should be based on whether it matches the musical expression of each Jeff’s Place reader’s preference.
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