Fosi Audio SK01 Headphone Amplifier Equalizer Preamp with Bass Midrange Treble Tone Control and Loudness Compensation Portable Class A Analog Amp for Electric Guitar Instrument with 3.5mm 6.35mm Jacks

Fosi SK-01 is an extremely well-constructed audio "enhancer" that has become almost indespensible in my different audio systems. Solid metal with some heft, the knobs fit tightly and move smoothly. A very slight adjustment is noticed almost immediately in the soundscape with the tone controls, the volume knob is slightly larger and provides easy access. I like the sound quality that the unit outputs and have used the SK-01 with various class D amps with only volume control - this unit is perfect for adding EQ adjustments. The tone defeat will bypass any EQ and play neutral, another feature that is really appreciated for my usage. The loudness button will bump certain frequencies and is a nice feature when playing at lower volumes. I have no complaints of this product and commend Fosi Audio for designing and bringing this device to the audio market. The 2500MaH battery will last me for several days if needed and the unit can run constant on USB 5V power. Also can work very nicely with phone/tablet/sound card as source and a usb "C" adapter with 3.5mm out will work perfectly as input source to the SK-01. There is a 3.5 headphone out, and also a 6.25" - I haven't used the SK-01 much with headphones but it has that option covered.. Great all-around device, I purchased another unit after 3 months of heavy use and I will probably always have one of these units on hand to modify an analog signal.

All I wanted was to bring out the subtle sounds in the accompanying music mix without mushing up the vocalist's track. I use Moondrop Blessing in-ear-monitors (earbuds) ...https://www.amazon.com/gp/product/B0C1ZBTD2Vand a conventional CD player (KLIM Discover)...https://www.amazon.com/dp/B0BRXH9L84with an upgrade cable (GUCraftmans cable with two pin connectors to match the Moondrop Blessing earbuds)...https://www.amazon.com/gp/product/B08N6211FFI didn't want a DAC (Digital-to-Analog Converter) or a combo DAC & Amplifier because the DAC/Amplifiers require RCA inputs or OPTICAL inputs. My CD player has only one type of output: 3.5mm (for earphones).Instead, I just wanted a headphone amplifier that would sweeten the sound of my CDs. I wanted to use my CD's player's headphone output jack (standard 3.2mm headphone jack) to connect the CD player to the Headphone Amplifier's 3.2mm input. I would then plug in my earphones to the output headphone jack on the amplifier.This amplifier is special because you can adjust the sound by manually altering the tones for bass, mids, and treble. Or, by pressing a simple button, you can bypass all tone adjustments and just let the amplifier do all the work of adjusting the volume of the track. Even without the tonal adjustments disabled, music still sounds better than without the amplifier.There are two more adjustments that can be enabled or left alone. The manufacturer describes them as follows: "Diverse Capabilities: Instantly restore original sound with the one-click tone-defeating button (I explain that one above); Immerse yourself in music during the quiet hours using the loudness function; Tailor the gain level to accommodate headphones with various impedances through the gain level switch."The loudness function is a stumper for me. I hear no difference when I press the button. I am NOT very responsible when it comes to the volume streaming into my head through the earbuds. I like it loud. I imagine the loudness function is used to keep all sounds under a certain decibel. Or, enabling the function might make the quieter sounds louder, relative to surrounding sounds. It might a "limiter" of some sort.Your own headphones or speakers or in-ear-monitors should have their respective impedance ranges documented. But whether I set the amplifier gain control to 0 or to -10, I don't know which I should choose. Depending on the content on a particular CD, I choose either setting to determine which setting sounds best.The exact title of this amplifier is "Fosi Audio SK01 Headphone Amplifier Equalizer Preamp with Bass Midrange Treble Tone Control and Loudness Compensation Portable Class A Analog Amp for Electric Guitar Instrument with 3.5mm 6.35mm Jacks," but don't let the title throw you. Yes, an electric guitar player can use this amplifier to monitor his guitar-playing, but it can be used for just about anything that you would want to make louder and sweeter...like your collection of CDs!The Fosi Audio SK01: https://www.amazon.com/gp/product/B0CF1WL1V2

Solid build, dials feel good.The only pre-amp that I found that included mid tone adjustment.Good battery life.Loudness control is a nice option -- not too common with preamps.Size is good, fits in your hand. Can be mobile, not the smallest profile, but can take it with you.Descent power output, works well with my Sennheiser HD 600 and Hifiman Sundara headphones.Good price to feature ratio.Overall very pleased with my purchase of this amp.

This seems to be something of a niche product but can probably be useful in many different situations. Im using it to support my Monsoon Planar Media 9 speakers. These speakers are excellent in sound, but havent been made in years. A weak point of the speaker system is the "puck", or wired remote that has the headphone jack and volume control that often causes problems. Since I know I cant replace them, Im trying to keep them problem free for as long as possible. So, Im using the Fosi SK01 in the place of the factory remote. An added plus is the additional features of the tone controls and gain and loudness functions. As it is working. I can leave the puck alone, and power the speaker system with a separate power strip.Anyway, the Fosi SK01 itself seems well built. It has a nice weight to it and the functions seem to work well. One thing to note though is that when headphones are connected at the jack, the speakers and the headphones will both play. The jack doesnt cancel out the signal to the rest of the system. This works well for me, and I simply leave the speakers powered off when I use the headphones. And I can just leave the headphones plugged in all the time. If I could change anything about the Fosi SK01, it would be the addition of some knurling on the knobs as they are smooth and the resistance of the pots makes them seem kinda slippery when turning them.I recommend the Fosi SK01 for anyone who needs an affordable solution.