Modified MINI USB DAC

I have been listening to the MINI USB DAC for a few days now, and during this time I have paid close attention to its sound characteristics.

Why go to "Mo"?

I don't support people tinkering with their devices. If you have the idea that "tweaking it will make it better" for everything, I think you have obsessive-compulsive disorder. If you buy a device you don't like and then tinker with it, isn't that just looking for trouble? Are you playing with the equipment, or are you being played by it?

Having said that, online sales cannot be heard before buying. If I have a choice , I would rather be a user of equipment than a "repairer" of equipment, although I also think DIY is a rare pleasure. The primary purpose of buying equipment is to enjoy the functions it brings. If you have to go through certain processing to make the equipment in your hand meet your needs, then let's modify the machine!

1. Set a standard.

As a commercially produced device, I am satisfied with the quality of MUD.

No equipment can be suitable for all ears. I mentioned earlier that the mid-high frequency of MUD is too prominent, the mid-low frequency is powerful but too tight, and the vocal performance is burred and dry, which is unbearable to listen to. Of course, this is just a reflection of my own ear curve. Fortunately, this is a minor problem.

Next, I set a standard for myself: 10%. In other words, in the current state, I think I can only accept 10% of the machine cost to be spent on MUD. Why not spend more resources and choose a more suitable one when buying?

If you don't set a clear standard for yourself, you may waste a lot of time.

There are so many hobbies in life, and there are many other things waiting for you. Remember, the biggest cost is time!

(These days, I have been listening to MUD on my computer)

What is "Nai Ting"? We can know it from an example.

When you are still using the computer's built-in sound card as the sound source, you turn on the speakers to listen to music and surf the Internet. Gradually, you will feel that the music is a bit noisy, so you turn down the volume or turn off the power directly . In fact, the volume is not loud at this time. This is a manifestation of intolerance.

When you are writing while listening to music, you may not hear clearly what is being sung, but just know that the music is pleasant, which is called pleasant to listen to. It is better to prefer narrow frequency response and loud noise. Music that is not pleasant to listen to is very annoying, which is completely contradictory to appreciating music and relaxing.

3. How to “Mo”?

Finding the problem is the key.

There is one thing you should pay attention to when modifying a machine: you should make adjustments based on the basic framework of the equipment itself, rather than "changing its appearance". Some people who are good at modifying machines have good skills and can make earth-shaking changes, but this can only be regarded as modification. Unless you make a major change to your DAC, the DAC chip still plays a decisive role, and any adjustment you make cannot turn the DAC into another chip. To be honest, the embryo of 1793 is still good, the key is how to make 1793 play more suitable for your own taste.

By comparing the built-in headphone amplifier with the external headphone amplifier, it was found that the problem came from the DAC output signal.

Looking through the 1793 datasheet, the LPF circuit is basically the same as the machine, and the change in the circuit value is meaningless. The resistors and capacitors used are not too bad or unreasonable, as long as they are properly matched, they can produce good sound. Besides, the parts of MUD are already quite good. The LPF uses OPA2134, which is a recent star op amp, but I have never heard of it before, so I think the action should start here.

Replacing the op amp is a common method.

(Replaced the op amp)

Replacing high-end resistors and capacitors can be called "tonics", and replacing the op amp actually changes the form of the circuit. It does not necessarily mean that the more expensive one is, the better it sounds.

I don't have many parts on hand, including ceramic sealed AD827, ceramic sealed OPA2604, LT1057, DaS ceramic sealed 5532 and LF356N. Based on the current situation, I will first replace 2134 with 2604. 2604 has always been said to have a slightly dark high frequency, but through the external headphone amplifier, it is very suitable for my taste. Then I connected the headphone amplifier of MUD, and then something unexpected happened.

Because the headphone amplifier also uses 2604, the two 2604s work together, the low frequency is mixed, and the low frequency performance of my headphones is a bit soft. Should I also replace the op amp of the headphone amplifier? I have experimented with this headphone amplifier, and 2604 is the best match for it. I don't want to make any more changes.

So I put other op amps in the LPF position one by one: 827 is really unbearable to listen to, the high and low frequencies are too prominent and the human voice is tasteless, 5532 seems more muddy here, 1057 has full mid-low frequencies but the treble is still bright, and the confident 356 is brighter than the 1057 in terms of high frequencies, but the mid-low frequencies are quite comfortable. After trying this, is there no one that can be used? I soon had a clear answer in my mind: 12 yuan.

(Some of the op amps compared in this test, the one on the upper right is the original OPA2134 in dark letters)

4. The story of 12 yuan

The audio output of DAC must be good, because it is the main sound source, in addition to the built-in headphone amplifier, it is also necessary to equip a headphone amplifier and a power amplifier. After using the above op amps, I thought of another op amp. As long as it is used, it can fully meet my requirements, and it is quite cheap!

Once when I was out, I turned into a counter and took out 12 yuan to buy two LM318s, a single-to-double-chip board, two capacitors and six resistors. LM318 is a very common single op amp, so old that it is hard to see any circuit using it now. But I have a very deep impression of its sound. As an enthusiast player, I always go for famous op amps. Can it work miracles on MUD? The answer is yes!

(LM318 is already in the machine for warm-up, LF356 is a single op amp used for comparison test)

(LF356 was originally favored, but after a round of competition, LM318 won)

Remove the two small decoupling capacitors next to the original LPF op amp, and install the two LM318s on the small conversion board. Since the leads are extended, solder two newly purchased 0.47u multilayer capacitors on the small board for decoupling. Power on, measure the DC potential at the output end, and turn on the signal to warm up the machine for a while after it is normal.

After turning on the sound, the mid-high frequency is a little more restrained than 2134, and the saliva sound of the human voice no longer makes people shudder from time to time. The mid-low frequency is very full and elastic, and the human voice that was originally the most unsatisfactory has become smooth and magnetic. Plug in the headphones and turn on the built-in headphone amplifier, and you will have the same feeling, but the mid-low frequency is not warm enough. Basically, the adjustment of DAC LPF meets my requirements. Listen to it for dozens of minutes in a row - very durable!

(The final solution for the DAC part)

That's it? Aren't there still a few resistors left? Yes, next we make a small change to the headphone amplifier circuit.

Four 33 ohm DALE resistors are connected in parallel to the original 33 ohm resistors of the four tubes to increase the static bias current. (Theoretically, the current is twice as large when the resistor is twice as small) The original static current is 6 mA, and this current cannot be doubled due to the limitation of the bias circuit itself, so it is increased to 8.5 mA. It seems that in this bias state, the plan to replace the existing tubes in hand is suspended for the time being.

As mentioned when I first modified it, I short-circuited the output resistor of 33 ohms. Now I have connected a 200 ohm adjustable resistor in series, and adjusted it to 150, 100, 50, and 25 respectively, to test which resistance value is more suitable for my headphones. Finally, I decided to replace it with a 51 ohm resistor based on my ears. The mid-low frequency is the most comfortable at this time, and I am already intoxicated by the music. This is the end of my "internal massage" on MUD. I spent an extra 12 yuan for my ears, which is worth it!

(Modify output resistance (DALE))

(Emitter resistor)