I've a set of speakers (front, center, back) rated a 6ohms. Can I safely connect them to a Sony STR-DE895 receiver? Sony manual says: Reference Power Output: (8 ohms, ...) Does this means I should only connect 8ohms speakers? Or if I want the power output stated (100W /ch) I must match the required impedance (8 ohms)?
you can connect them, but I would not recommend it. The STRs are not very tolerant of impedance changes, most especially 4 ohms, it will tend to over heat and shorten the life span of your unit. cheers
Mario, welcome. You can connect them to the Sony STR-DE895 receiver. The referenced power is for "nominal" impedance and is what Sony tested with. This is not to say that 6ohm speakers may not go really low (below 3ohms) and cause the system to go into protection mode. All speakers have varied impedance across the frequency spectrum with the lower frequencies having the lowest impedance. Those speakers (6ohm) will probably cross 4ohms during low frequency portions of programme material.
The only problem with the Sony STR-DE895 receiver is that it doesn't have the power reserves to adequately deal with 7 channels. So 6ohm speakers may push it a little... 4ohm speakers should definately not be connected.
And yes, the "rating" of the power is based on 8ohms. Lower impedance yields higher power. But, as I said, the Sony STR-DE895 series doesn't really put out 110W into 6 channels with ALL CHANNELS DRIVEN.
"All channels driven" is essential. So is "continuous". Some brands quote the power value of a short burst, like calculating the time a sprinter could run a marathon if he just kept going at the same speed.