Two pairs of speakers on one amplifier: what the parallel load really does
Two 8 Ω pairs in parallel are 4 Ω on paper and 2.2 Ω at the dip. The Fosi V3 is rated for that; a receiver rated 6 Ω is out of spec by nearly a factor of three.
Signal Chain Compatibility Matrix
Select gear and parameters for live gain, impedance and SPL calculations.
Input Equipment Setup
PASS: Optimal Signal Chain
Requires minimum +60dB of clean pre-amp gain. Most budget entry interfaces max out at +50dB to +56dB and introduce hiss at max pot. An inline preamp booster (Cloudlifter CL-1 / FetHead) is strongly recommended.
The interface preamp provides sufficient headroom.
$538
$405
-25%Tonal & acoustic characteristics: Flat, wide-range frequency response for clean music and speech. Smooth, warm acoustic character with air-suspension shock isolation.
Technical advisory: With +69dB max gain, the Gen 4 Solo can drive demanding dynamics like the Shure SM7B natively without requiring an external inline booster.
Physical Interconnect Topology
Shure SM7B
Direct Cable Connection (No Intermediate Box)
1x Shielded Balanced XLR Cable (3-Pin XLR Male to XLR Female)
Focusrite Scarlett Solo (Gen 4)
Host: Apple MacBook Pro (M1/M2/M3/M4)
Only if the amplifier is rated for half the impedance — which the Fosi V3 is and most receivers are not.
Two pairs of 8 Ω speakers wired in parallel present 4 Ω. That is the number on the box; the number that decides the outcome is the dip. The RP-600M II's 4.4 Ω minimum becomes 2.2 Ω. The Fosi V3 is rated down to 2 Ω, so it survives. The Denon AVR-S570BT, rated 6 Ω minimum, is out of spec by nearly a factor of three; the Yamaha RX-V4A, rated 4 Ω, still is.
Current is the whole story. At 45 W the amplifier pushes 3.2 A into 4.4 Ω and 4.5 A into 2.2 Ω for the same voltage swing. A minimum impedance rating is a current limit written in the units of impedance — that is why it does not scale with the watt figure on the front panel.
The surprise: the level cost is zero here. The Fosi delivers 45 W into 8 Ω and 90 W into 4 Ω, so each pair still receives 45 W. That doubling is exactly what a receiver with a shared power supply cannot do, and it is why the A+B switch behaves at low volume and trips protection at a party.
If your amplifier is not rated for it: run the pairs one at a time, or use a switch box with series resistors and accept the level loss it costs.