If your amp inverts and your pre doesn't just reverse the lead as E. suggests above, red to black etc. This leaves the whole question of recordings which are made with reversed polarity ; my DAC has a reversal switch and occasionally it makes a considerable difference. Most of the time it does not.
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My concern is with the proper setting of the powered subwoofers that are part of my speaker system. The speaker manufacturer's instructions state that toggle switches on the subwoofer modules should be positioned one way if the amplifier inverts and the other way if the amplifier does not. But the speaker manufacturer's manual does not tell me how to tell one type of amplifier from the other.
In the following thread you'll find several suggested methods for setting the phasing of subs relative to mains, which is what it sounds like you are trying to do:
Keep in mind that both possible settings are probably compromises to some degree, and are influenced by the physical positioning of the subs relative to the mains.
Thanks for the information on setting up subs. In my case the actual setting up is easier. The main modules (tweeters and mid-subs) bolt on to the subwoofer modules, so the problem of positioning subs is taken care of. And the manufacturer recommends values for most of the various subwoofer settings. The end user (me) only has to choose two settings: the volume of the subs, set by ear to complement the volume of the main modules (no problem for me), and the matter of deciding whether the main amplifier is inverting or non-inverting. The phase settings on the subwoofers have only two values: 180 (for non-inverting amps) and 0 (for inverting amps). So my question is most immediately about amplifiers rather than about subwoofers.
Any of the methods indicated in the thread I linked to, or that Stan referred to, will allow you to determine the correct setting of the 0/180 switch, without first determining if the amplifier inverts.
Alternatively, you could contact the amplifier manufacturer and ask them. And sometimes their literature or website pages will indicate inverting or non-inverting. If nothing is indicated, as Elizabeth said the chances are it is non-inverting.
If you have the wrong setting, the symptom will be very weak reproduction of bass frequencies that are in the region of the crossover frequency. Nothing will be harmed, though, if that is not clear.
However, unless the sub's literature indicates it is ok to do so, it might be best to avoid changing the setting of the 0/180 switch while power is on, to prevent the possibility of transient thumps that may occur as the switch is being changed.
Several questions relate to this subject, beginning with: can you hear a difference?
To further explain Elizabeth's comments, an odd number of gain stages will invert the phase, an even number will retain the input phase. And as Stanwal states, an inverting preamp/line stage can be corrected by reversing one end of your speaker leads. But that will change all signals feeding the preamp as well. If you have a separate phono stage and it inverts and you also have other sources, that should be corrected by reversing the hot and ground connections on the cartridge.
For your subwoofers, you have two choices. Contact the manufacturer of your amp and ask if it inverts. Or test by ear as Al recommended. If you are not familiar with phasing, some test CDs (Stereophile's for one) provide a repeated track recorded in phase and out of phase. Listening to something like that without your subwoofers active may help to identify what to listen for.
I had and HAVE a pre with absolute phase switch.
Some material it can make a difference, but for R+R, which has had the phase data beaten out of it, it makes no difference. Multi miked anything is almost alway a mess, as well.
The most difference I have noted is on Solo instruments and than it is subtle and clearly preference driven.