What's the best module for heavy subwoofer use?
I use two pair of infinite baffle 15" subwoofers 8 ohm each for 4 ohm per channel.
What module would be recommended to drive them 75V rails?
What is the best powersupply type for such use, SM or toroid and capacitor?
Depending on the power rating of the sub, you can chose from here Connexelectronic the most suitable class D amplifier module. I recommend, for up to 500W to use one CxD500 amplifier module, supplied at +-75V with enough capacitance per rail (at least 4mF to reduce bus pumping) or IRS2092 Stereo Amplifier 2x200W in BTL mode, but supplied at lower voltage, just +-45V for higher power, either two CxD500 amplifier modules in BTL mode, or IRS2092 Stereo Amplifier 2x400W or 2x700W version in BTL mode depending on the required power.
To supply these modules I recommend to use SMPS. there is no reason to chose a heavy, low efficiency classic power supply, mains transformer, rectifier bridge and capacitors when a SMPS can do better job, at lower weight and cost, and higher efficiency.
An SMPS is essentially a regulated power source. You can design it to provide sufficient current for continuous full power operation if you would like. As you correctly mention, this type of power supply is lighter weight compared to a linear supply with a transformer that has a high enough power rating to provide continuous full power.
Music is dynamic in nature. The peak to average ratio (crest factor) can be 10-20dB or more. Peaks are short in duration. So you might need to briefly supply 10 times (or more) of the average power as clean dynamic power. A linear power supply is well suited to this task. The transformer power rating (in VA) can be chosen to be a fraction of the continuous full power and with a secondary voltage that is near the high end of the range that the amplifier can withstand. When high power levels are required from this system, it will only be capable of meeting them for brief periods of time (e.g. <100msec) before the rail voltages fall and available power falls with them. But this is exactly what is needed by very dynamic signals. An SMPS that is rated for full power would be overdesigned for this case. If you use an SMPS with a lower power rating, you run the risk of the user trying to attain constant full power from the system, which would draw too much current from the SMPS and it would shut down (hopefully). In contrast, the linear supply would be self limiting because under sustained full power demand the rail voltages will fall and the long term output power would be determined primarily by the transformer's power rating.
SMPS power supplies are perfect for PA use, where music signals are very compressed and the crest factor is low. A linear supply can be very attractive for uncompressed, dynamic signals typical of audiophile recordings.
P.S. I neglected to mention that an SMPS with a full bridge or used in a BTL arrangement is definitely better than a linear power supply for a subwoofer application. This is because the period of the low frequency signals is longer, and the short duration dynamic power that the linear PS described above will not be up to the task.
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