If its not an amp, I cannot think of anything larger than 20 inches deep, although maybe another A goner can think of one. A consideration should be height ie a Turntable or a higher end CD player may be a top loader.
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If you make sure your rack can handle 19" widths and 20" depths you should be ok. I used one sheet of finish grade plywood to build a Salamander clone. A Salamander clone allows the heights of the shelves to be adjusted as the shelves are held by nuts and washers on threaded rod. My Parasound HCA 3500 sits on the bottom. The rack is 48" wide by 24" deep. The Parasound actually sticks out the front and back a couple of inches but fits. No other components would be so large. These racks are also very strong. Do a search over at AA. You can build one for a couple of hundred dollars.