Needed to add some headphone outputs to my single-output Apollo solo interface.This connects with a single 1/4" TRS cable, so you can run the headphone out of your interface directly into this thing with one cable. Very convenient.I don't notice a difference in quality or output power between this and the interface, and the interface has one of the better headphone outputs I've seen for audio interfaces.Mackie hit a home run with this one and the price is unbeatable.Sounds good. Very affordable at about $40.I'm beginning to explore my engineering chops and based on research these headphone amps were a must. So far my production is clear and clean. I also enjoy controlling the volume from the amp instead of reaching for my Focusrite.Product works great for the price!I recommend the HM-4 to anyone that needs multiple headphone jacks. It’s truly amazing.This headphone amp is well worth the $40 that I paid for it. Well built, solid construction with super clear sound, with plenty of volume. And for those reviewers that say this is a mono amp, its not!! It has stereo input/outputs. I think these people who say it's mono don't realize that you need to use a TRS (tip, ring, sleeve) cable that sends a stereo signal as opposed to a regular instrument cable (TS) that only sends a mono signal. Don't hesitate to purchase this amp if you need a small, basic portable headphone amp.This product is stereo which many people seem to be having problems with I’m willing to bet they are using a ts cord or 1/4 guitar cord to bring in signal not a trs cord or balanced stereo cord. Of course it’s going to be mono and missing a channel if you use a single channel cord. Being a entry level product I’m willing to bet this is their problem. My theory is further supported when I see pictures of people summing a mono line to both channels by holding it at half plug. Technically they might as well be using a monto to stereo converter.My major gripe with the amp is it can get a lot of noise on the input if you don’t have your cord twisted the right direction allowing a proper ground. One you have it set and twisted right the amp works just fine. I have plugged 5 different sets of phones into it. From 40 dollars to 400 dollars. The amp will do what described but the product could be built better. I would rather pay 60 for it if they would put better jacks in the unit, specifically the input of i could only choose one. The pots them self i Are smooth and clean when new. Remember abuse and smoke will mess them up.NiceCould not fine anythink wrong great product for the money great sound nice and loudI have tested both behringer h400 and Mackie hm 4. From my opinion1. bulit quality of hm 4 is better than h 400.2.packaging quality of hm4 is better.3.weight of hm4 is more, which make it more stability for Maintaining it's place.(displacement will less)Now about sounds quality.Sound quality of both amp are similar,here behringer got louder volume while hm4 have balance .Sounds quality of both product is not up to the mark, actually these amp are only working as splitter.Not good for mixing and mastering.But ok for recording and hearing music.But in this price range it's ok ,both product works good. If you ask which one to buy definitely I will say Mackie hm4,And if you need louder sounds go for h400.But for headphones both product have enough power to distorted you headphones speaker.If you want to connect speaker in it definitely go for behringer h400.Mackie HM-4 HeadphoneThis headphone distribution amplifier is absolutely outstanding for my purposes in my opinion. I like to switch between three different headphones during my hobby recordings. I use two closed back headphones that are 80 ohm and 250 ohm as well as my third open back headphones that are 300 ohms. This Mackie headphone amplifier does an outstanding job driving all three headphones and I love the fact that I have independent volume knobs for each one of them.In my setup I use a focusrite Scarlett audio interface. From the Scarlett, two 1/4" outputs fr the interface connects to the Mackie HM-4 using a L+R 1/4" splitter cable that merges channels into one stereo 1/4" connector which is plugged into the HM-4. With this setup the direct monitor button on the scarlett audio interface still works.I use to closed back headphones to do recording so there's no sound leakage. And when I do mixing and mastering I use my open back headphones. Having the HM4 really is convenient so that I don't have to keep plugging in unplugging different headphones.... And obviously independent controls really helps.Awesome.I bought this Mackie amp because I have *three* superb Mackie mixers of various sizes. But this amp is definitely not the same quality as the mixers. One knob/potentiometer feels like something is loose on the PC board inside. The whole thing wobbles. If it breaks, I have three other channels I can use. The sound is ok; no scratches when moving the knobs that I have heard after several weeks.best budget 4 channel headphone amp for studio, jam rooms theatre etc, even though it's a 4 channel.. actually there is 12 headphone out, 2 monitor out and many inputs.. at this price i wasn't expecting that much quality but after testing even on full volume there is almost no distortion in the audio, reliable and sturdy, as it is black colour you can also hide them easily and make your desk tidy... thanks mackie for an affordable HA... love it.