Looking for personal opinions. If you were going to be driving a speaker with 4 ohm woofers and 8 ohm mid and tweeters (above 200-800Hz) would you use a 4 ohm OPT or split the difference with a 6 ohm?
I would use a 4 ohm OPT if you want maximize power delivery in the lowest octaves where you need it most. The amount of power required above 500Hz should be a quite a bit less. What is the overall efficiency of the speaker system and what is your power target?
I agree with kevinkr.
Use an OPT with a 4 Ohm tap.
But . . .
Be sure to measure the DCR of the Woofer voice coil.
You may be surprised.
(the minimum impedance at one or more frequency ranges is about the same as the DCR).
Typical examples of approximate woofer impedance:
Ported enclosure; impedance, Z = DCR at 20Hz, = DCR at the port tuning frequency, and = DCR at 200Hz to 400Hz.
Closed box enclosure; impedance, Z = DCR at 20Hz, and DCR at 200Hz.
Just my opinions
Use an OPT with a 4 Ohm tap.
But . . .
Be sure to measure the DCR of the Woofer voice coil.
You may be surprised.
(the minimum impedance at one or more frequency ranges is about the same as the DCR).
Typical examples of approximate woofer impedance:
Ported enclosure; impedance, Z = DCR at 20Hz, = DCR at the port tuning frequency, and = DCR at 200Hz to 400Hz.
Closed box enclosure; impedance, Z = DCR at 20Hz, and DCR at 200Hz.
Just my opinions
The woofer is also the driver that is perhaps most sensitive to driving source impedance depending on speaker and crossover design.
I designed my Onkens to be driven by a non-ideal quasi voltage source. (AKA GM70 SE amp with approximately 2R output impedance) Driving source impedance for most modern speaker designs is basically 0R as provided by an "ideal" voltage source (most solid state amplifiers.).
I have also found to my chagrin that untapped secondaries perform slightly better, it's that leakage inductance thing (poorer coupling) that enters the equation when you have unused parts of the secondary. 😈
I designed my Onkens to be driven by a non-ideal quasi voltage source. (AKA GM70 SE amp with approximately 2R output impedance) Driving source impedance for most modern speaker designs is basically 0R as provided by an "ideal" voltage source (most solid state amplifiers.).
I have also found to my chagrin that untapped secondaries perform slightly better, it's that leakage inductance thing (poorer coupling) that enters the equation when you have unused parts of the secondary. 😈