Samadhi Acoustics offers an excellent full range speaker (28hz up) rated at 94db and 4 ohms. A custom interstage transformer coupled SET 300b with 7 watts can drive these speakers with ease. The transformers are custom wound Electraprint and designed for 4 ohm loads.
There is an excellent article on 4 ohm and 8 ohm speakers on Roger Russell's McIntosh history website. Here is what it says:
Impedance of Home Systems is Changed To 4 Ohms
All systems manufactured prior to 1993 are 8 ohms and have an industry standard sensitivity rating of 1 watt into 8 ohms at a distance of 1 meter. All systems made after this date are 4 ohms and have a sensitivity rating of 2.83 volts into 4 ohms. This turns out to be 2 watts into 4 ohms, making them appear to have greater sensitivity. To convert to a 1-watt level, subtract 3 dB from the rated sensitivity. For example: the LS350 is a 4-ohm system rated at 89dB for 2.83 volts. This converts to the industry standard rating of 86dB for 1 watt at a distance of 1 meter.
Generally, a 4 ohm speaker will play louder on a direct coupled amplifier than an 8 ohm speaker simply because it draws twice the power with the lower impedance. A few companies have used this strategy for many years. The idea is that when a simple switch arrangement is used in a dealer store to compare different speakers, the louder one is supposed to be the one that gets the sales—and this does help.
Reducing the speaker impedance to 4 ohms does not improve the accuracy of the speaker, reduce distortion or make any other improvements. However, it does create problems for the customer. It means that the customer must purchase heavier speaker wire to maintain the same low losses compared to the earlier 8-ohm speakers. The total DC resistance of the wire must now be less than 0.4 ohms instead of 0.8 ohms. It also requires better connectors at the speaker and amplifier terminals to make the contact resistance even lower.
There is an excellent article on 4 ohm and 8 ohm speakers on Roger Russell's McIntosh history website. Here is what it says:
Impedance of Home Systems is Changed To 4 Ohms
All systems manufactured prior to 1993 are 8 ohms and have an industry standard sensitivity rating of 1 watt into 8 ohms at a distance of 1 meter. All systems made after this date are 4 ohms and have a sensitivity rating of 2.83 volts into 4 ohms. This turns out to be 2 watts into 4 ohms, making them appear to have greater sensitivity. To convert to a 1-watt level, subtract 3 dB from the rated sensitivity. For example: the LS350 is a 4-ohm system rated at 89dB for 2.83 volts. This converts to the industry standard rating of 86dB for 1 watt at a distance of 1 meter.
Generally, a 4 ohm speaker will play louder on a direct coupled amplifier than an 8 ohm speaker simply because it draws twice the power with the lower impedance. A few companies have used this strategy for many years. The idea is that when a simple switch arrangement is used in a dealer store to compare different speakers, the louder one is supposed to be the one that gets the sales—and this does help.
Reducing the speaker impedance to 4 ohms does not improve the accuracy of the speaker, reduce distortion or make any other improvements. However, it does create problems for the customer. It means that the customer must purchase heavier speaker wire to maintain the same low losses compared to the earlier 8-ohm speakers. The total DC resistance of the wire must now be less than 0.4 ohms instead of 0.8 ohms. It also requires better connectors at the speaker and amplifier terminals to make the contact resistance even lower.