Review: Sybasonic S21A Hardware EQ Headphone Amplifier

Firstly I would like to thank Peter at Sybasonic for sending me this sample for review, as always I will try and write an honest review.

Gear Used:
Dell XPS 15 > Syabsonic D31A > Line out > S21A > German Maestro GMP 400/In-Earz IE-P350

Tech Specs:
Power USB Micro-B 5V DC
Input /Output   3.5mm Headphone Out
                                3.5mm Line Out
                                3.5mm Line in
                                Line in for mobile device
Tone Control       Treble
                                         - Control Range: 3KHz-20KHz
                                         - Trim Range: -12dB to +12dB
                                         - Center Freq: 18KHz
                                         - Control Range: 200Hz-4KHz
                                         - Trim Range: -10dB to +10dB
                                         - Center Freq: 1KHz
                                         - Control Range: 20-250Hz
                                         - Trim Range: -14dB to +14dB
                                         - Center Freq: 0Hz 
Stereo Crosstalk 90dB
Signal to Noise Ratio (SNR) 112dB (A-weighting)
Total Harmonic Distortion (THD) 0.003%(A-weighting)
Headphone Impedence 16 - 600ohm 
Max. Headphone Output power:  around 200mW / 16ohm ,120mW / 32ohm at 1KHz
MSRP $188

Packaging, Accessories and Build Quality
The packaging is the same as other Sybasonic products, a brown card box with a white sleeve, this sleeve contains all the technical information and also a picture of the product. Inside the box the S21A is held neatly in place with foam inserts, this helps protect the device from damage during shipping, underneath this you will find the accessories. A well thought out, fully functional packaging, not the most luxurious but it does the job very well.

Accessories included are good, it comes with a micro USB cable to power it, a 3.5mm-3.5mm cable, and also a 3.5mm to RCA cable. It doesn't really need any accessories, so the included are the essential to get it up and running.

Build quality is similar to other Sybasonic products, a very sturdy metal case with no flex, held together with hex screws. A very strong piece of kit, with LED's to show you it's on and one to show it's active. The knobs are all smooth and centre with a click, the push buttons also feel sturdy and are easy to use. All headphone sockets are high quality and everything looks like it will hold up well over time.

Easy to use, this device is powered by a micro USB socket, and you simply plug your device in to this via the line in (front or rear) and plug your headphones in to the headphone out, or connect the line out to your other audio equipment and fiddle about with the options, the volume knob is the power switch.

Just to note this device is very well designed, it uses separate op-amps for each channel to minimize crosstalk, and uses high quality components and power filtering, for low noise, it is not your run of the mill EQ.

Sound and Functions:
This little device has a lot of functions, and can be integrated in to most audio systems. It has a tone defeat button, which means it will bypass the EQ when active, it also has a Loudness button which basically boosts the sound and certain frequencies more than others.

This is a great headphone amp, and with everything disabled it is flat as you would expect, it is also fairly powerful and can drive most headphones with relative ease, even my German Maestro GMP 400 which are quite hard to drive.

Now let's get to the main feature of this, the EQ knobs.
Balanced does what it says, you can shift it from right to left, and it works well as expected.

Bass: the bass knob has a range of 20-250hz, and it seems to be accurate, however the mid-bass seems to be the most noticeable when boosted. The bass boost doesn't bleed in to the mids, and is very controlled, it never becomes sloppy or bloated. With the GMP 400 this is a great feature as they are slight thin sounding, so have a little boost down low really helps them sound a little more natural. The EQ is also fairly subtle so you can increase it gradually and not notice a huge difference, which is great for fine tuning the sound of a headphone.

Mids: Again the EQ is very subtle, and this would help those V sound signature headphones a lot, it really brings the vocals and guitar to the center stage, it really does only affect the mids and in a very good way. I found it to have quite a positive impact on the sound if used carefully.

Highs: This one is a good one to have if you are treble sensitive, I found it to greatly decrease listening fatigue without taking away the detail. It will just take the edge off the highs and dull them a little, if used excessively it will make the sound very dull, but again moderately used it can be a great aid in reducing listening fatigue.

Conclusion, I found this device to be great fun to play around with, I have never been a fan of software EQ, I prefer to use headphones and amps to get the sound I want. But I do really like this device, it is a little different and also very effective. You can use as little or as much as you like, and the changes are subtle so if you adjust it a little you won't get a huge increment, it means you can fine tune your headphones to your liking, you don't have to settle for a bass boost switch, you can fine tune it as much as you like. I found myself using this with the German Maestro GMP 400 a lot, giving it a slight boost in the lows and a tiny bit less treble which made them a lot more enjoyable to listen to.

A great product, that is well built and does exactly what it says very well. Recommended to those who would like to fine tune the sound of their headphones or speaker system.

Sound Perfection Rating: 10/10 (a definite improvement, a more enjoyable listening experience)


  1. It's beyond sturdy, it's built like a rock. The pots are silky-smooth to turn. The stereo balance is perfectly maintained even at low levels. This is actually a rather high-end product. I like mine a lot. I wonder why they didn't include one of their inexpensive but good DACs? Surely there'd be room. I've used it with the Breeze Audio DAC with the sexy case, and their own SybaSon24/96 DAC with great results.

  2. Wow wow wow this is a fantastic product. I've tried tons of dac's etc. but this baby was the key to the sound I have been looking for. My search is over! I can literally tune my $400 high end earbuds to perfection, I am in sonic bliss! I turn up the mids and bass 1/4 past half (roughly). THANK YOU SYBASONIC I can't believe this guy was all I needed I'm blown away. Now if I could only find a place where I can buy the S21C! If anyone knows where I could get one please let me know. Thanks!!!

  3. is it battery powered or does it need the USB to run?


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