What kind of things decide the QUALITY of mixers and expecially turntables? What makes one 300$, another 73K and another 100$? And what features should a DJ look for when buying his first turntables?

when buying DJ turntables, look for the words technics 1200 SL.

as for mixers and CD decks, that's a matter of opinion.

personally, I think I am going to get a Pioneer DJM600.

Turntables
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The most important things to look for when buying turntables are torque and platter stability.

The first TT's I had, had incredibly wobbly platters. This is terrible for cueing up/scratching records, because your needle starts jumping all over the place. Generally, any TT under $250ish will probably have platter stability issues.

Torque is the other main priority. You need the platter to keep turning under the record when you hold it, so that when you release it it starts up straight away. If you don't have this, and the platter stops or slows down too much, you then have to give the record a shove to get it going. This can be a worry because you might not get it exactly right and waste time getting it back into sync before you bring in the mix, or worse, if you also have an unstable platter, you may make the record skip when you push it.

The final (major) consideration is wow and flutter. This is where the motor sppeds up/slows down intermittently with current changes or whatever. This throws your records out of sync.

The best TT's to do all of this and more is the Technics SL1200. If you can afford these, get them. There is nothing better.

If you can't afford these, try out the Gemini PT-2100. These are around half the price of the Technics and are very good for the price.

If you can't afford these, look for something cheaper. But make sure they are direct drive.

Mixers
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The main things to do with the quality of mixers are sound quality, build quality and fader quality.

Look for a mixer with as good sound quality as you can afford. This makes a lot of difference.

Build quality is another big issue. This varies a lot. A lot of people on this forum bang on about the Pioneer DJM500/600. These mixers are renowned for their poor quality components and general poor quality build. The circuit boards inside these are mounted horizontally which means that any liquid that gets spilt inside (i.e. beer. And it will happen.) will completely mess up your mixer. They also have relatively poor sound quality for this price. They are by no means good value and are tremendously overpriced.

Mixers by Allen & Heath on the other hand, have fantastic sound quality (only beaten perhaps by some of the Rane or Urei rotary mixers). They are expensive, but you get what you pay for. The circuit boards are all mounted vertically to avoid any spillages, and they use only the highest quality components. Their crossfaders, made by Penny & Giles, last for years and produce the highest quality sound.

If you can afford it, buy the Allen & Heath Xone 62. You will never buy another mixer.

If you can't, look at some of the higher-end Vestax mixers, the PCV-175 for example. In this range as well, look at mixers made by Ecler.

Slightly less expensive is the Gemini UMX range. These are very good quality for the price and offer good quality VCA faders that don't wear out.

If you are in the budget range, look at mixers by Numark. These are extremely good value for money and have all the qualities of a mixer that should be at least double the price.