I bought 2 pioneer studio monitors (sdj-80X) but I don't know if i need an audio interface to get the best sound. I was told that I could connect it with just a connector from jack to mini jack. Is that a good idea or should I use an audio interface?
Several points are to be considered :
PC output vs dedicated Audio Interface output
A PC will output consumer line level. A dedicated interface will output either consumer line level or professional line level, or both. The quality of a dedicated interface will probably be better than the default sound chipset of a consumer PC. The difference might not be easy to hear depending on what you're listening to and your ear training (besides the quality of the loudspeaker).
Jack 3.5 vs XLR vs RCA
Your monitors have balanced (XLR or TRS/Jack) and unbalanced (RCA) inputs. The output of a PC will be unbalanced, so you should use a stereo jack 3.5 to two RCA's in this context.
If you use an external sound card, it might have balanced outputs. Balanced outputs offer a better protection against induced interferences and allow for longer cables between the output and the monitors.
Jack 3.5 are not locked in the connector, XLR's are. If you often move/install your setup, the female jack 3.5 from your computer might eventually get loose and you might have connection issues.
In conclusion, I would suggest that if you are satisfied with the sound quality of your PC output, you might give it a try for a while, and upgrade if you feel it necessary.
It is worth using an audio interface if you want to maximize sound quality, however you can also get by just fine with your built in sound card for a while if you want to save for a better interface.
The #1 difference between a professional audio interface and a built in consumer sound card is the quality of the DAC or Digital to Analog Converter. Computers work with digital audio, but you can't play sound files made of ones and zeros through a speaker. You need circuits that can take that series of ones and zeroes and make it in to an actual waveform that can drive a speaker. This is the purpose of a DAC and the better quality the DAC, the smoother and cleaner the generated analog audio signal will be.
Professional audio interfaces also have a few additional advantages. They tend to use pro level line outputs (which provide more signal, thus increasing the signal to noise ratio, which means cleaner audio) or even balanced outputs (which uses some wiring fanciness to greatly reduce picking up noise on the audio cables.)
Additionally, they use a different audio path called ASIO on Windows boxes. This allows for drastically streamlined drivers which are optimized to pass sound to the speakers quicker, reducing the latency between the time the computer starts trying to play a sound and the time it is actually produced at the speakers. This doesn't matter in every case, if you are just listening to music, it won't matter a bit, but if you are trying to record anything while you listen, you will need to correct for latency because what you are recording will lag behind what you are hearing due to the delay.
The inputs on an audio interface are also considerably better quality than consumer sound cards and often support more professional features like XLR connections and phantom power. They also have better analog to digital converters which are the opposite of the DACs and take the analog input and produce a stream of 1s and 0s that represent it. They often not only produce cleaner audio, but also can work at higher bit depths and sampling rates than consumer sound cards, which results (to a point) in better quality audio as well.
Most of these advantages are relatively minor unless you are trying to do recording or are an experienced sound guy that has trained to notice such things, but they do make an audio interface a very nice item to have, but I wouldn't classify it as critical to your needs.
I would recomend getting an audio interface. There is a sound quality difference between normal mini jack vs. XLR. If you are just using your mini jack there will be some latency; Latency is the time it takes for your computer to process information. Audio interfaces have other fantastic uses as well like connecting mic set ups that can be recorded straight to the computer without using external recorders. It helps me for alot of music things that I do or if my external recorder's battery is dead.
If you are serious about Music (who said the word Audiophile?) you want ALWAYS the best. Having said that Still now (2015) NO internal sound card (soldered in the mobo)is even remotely comparable with a dedicated one.
Sound cards HAVE TO BE external...Doesn't matter if connected via USB, Firewaire, Thunderbolt, optical, coax, ecc...ecc...EXTERNAL, remember. Lucky days now beacause (finally) consumer grade producers like Creative, Asus are coming up with "decent", sometimes exquisite (Asus Essence Muse) Sound Cards/Dacs....
Anyway a proper Hi-Fi D/A converter or a PRO Sound card is always preferred..
Prices vary from 50 something (dollars, pounds, euros) to thousands... At 200-300 price tag you begin to find proper PROsumer Cards or converters.
Another issue are STUDIO MONITORS. They are not Speakers...Well, they are, but they sound very very differently from Lounge Hi-Fi ones...
You can't change amplifiers, pres, put one with tubes, another with fets, separate,bimonos, integrated, etc... You HAVE to deal with the ones inside the box and take them as they are....The most of them pur out serious HISS, BUZZ and other noises.... something is absolutely normal/common in Studios where music is Produced, less in your Living Room or Bedroom....
Amplifiers built in the Active monitors are in NO WAY comparable to HI-FI amplifiers! They dont have to be! Monitors dont have to sound good, they have to sound TRUE!!
Today I listened to a pair of Tannoy Studio Monitors (biamped, active) and they are Night and day from the "Audiophile" line Tannoy makes (passive).
But anyway they are Warm, decent sounding for a Monitor....Well, as monitors I would never ever buy them, but for listening they are better than most speaker in the same price range and above, sometimes far above...
On their side there were a pair of Adams (higly respected in the PRO world) and they sounded rubbish in comparison, costing twice as much. But I guess they are better for mixing than Tsnnoys.
Hope to have cleared things up a bit...
The question here is really about what media you will be playing back and if an interface will benefit you.
For what its worth computer makers have stepped there game up and now internal sound cards (the ones that drive that mini jack on your comp) are much better quality and can handle much higher resolutions that they used to be able to. In the past the only way to get higher quality, was to use an external interface, and the quality jump was big. If you are only every playing back 44.1K audio (CD quality, stuff from itunes etc.) then there is really no reason IMO to get an interface. If you are recording stuff at different/higher rates, listening to lossless files/high quality audio, yeah an interface will add to it.
I powered my monitors from my mini jack for about year and was very satisfied with the quality. I stepped up to an interface when I got serious about things but when it came to general listening the interface was only as good as the files I played through it.