This is an AI answer, but perhaps others can help confirm or add to this.
CLAUDE: "Your system is well-matched on paper, and you've made thoughtful choices. Let me walk through the technical relationships:
Impedance Matching (Preamp to Amps)
Your Motherload XL has a 400Ω output impedance and recommends feeding loads ≥10kΩ. Both your amps exceed this comfortably:
- SET 45: 91kΩ input impedance (ratio of 228:1)
- Atma-Sphere Class D: typically 20-47kΩ (Class D amps generally have high input impedance)
The standard rule is a 10:1 ratio minimum (load impedance to source impedance). You're well beyond this, which means minimal signal loss and excellent frequency response—no bass rolloff or other colorations from impedance interactions.
Gain Staging (The Key Question)
This is where your preamp's adjustable gain becomes crucial. You want enough gain for the system to reach satisfying volume without the preamp's volume control being either too low (degrading resolution) or maxed out (running out of headroom).
For the SET 45 (2.3V sensitivity): This is relatively high sensitivity—it needs only 2.3V to reach full power. Your preamp can output far more than this. I'd start with the lowest gain setting (3) and increase only if you find yourself pushing the volume control past 2-3 o'clock for normal listening. SETs typically have high sensitivity to preserve their low-power nature.
For the Atma-Sphere Class D: These typically need more voltage (often 2-4V for full power, but varies by model). I'd start at gain setting 5-7 and adjust downward if you have excessive volume with minimal rotation of the volume control. Class D amps generally need a bit more drive than SETs.
The real test: At comfortable listening levels, your volume control should typically sit between 9 o'clock and 1 o'clock. Too low suggests excess gain (noise/hiss); too high suggests insufficient gain (running out of headroom).
Subwoofer Integration
Your 50Hz crossover at 24dB/octave is conservative but reasonable for horn speakers, which often have excellent bass extension. The AER BD3 in Charney's bass-reflex horns likely reaches down to 40-45Hz, so crossing at 50Hz creates some overlap—which can work if phase-aligned properly.
Consider experimenting: Try the crossover at 40Hz or even 35Hz. The steep 24dB slope means less overlap, reducing potential phase cancellation between subs and mains. Listen for where the bass sounds most coherent and "of a piece" rather than localized.
Speaker Matching
Your AER BD3s are 16Ω, and you're using the 16Ω taps on your SET. Perfect. The Atma-Sphere Class D amps are typically load-invariant (they don't care about 4/8/16Ω), so those should be fine as well.
Recommended Reading
For understanding these relationships without drowning in electrical engineering:
- "The Complete Guide to High-End Audio" by Robert Harley - Chapter on system matching explains impedance, sensitivity, and gain in accessible terms
- "Sound Reproduction" by Floyd Toole - More technical but excellent on speaker-room interaction and subwoofer integration
- Audio Technical Articles: Roger Sanders' whitepapers on impedance matching (available free online) are remarkably clear
Your system is fundamentally well-conceived. The adjustable gain on your preamp is exactly what you need to optimize different amplifiers. Trust your ears, but now you understand why certain settings will work better than others."