Speakers & Loads

An amplifier is not a perfect voltage source. It has an output impedance (Zout) that forms a voltage divider with the speaker impedance (Zload). The voltage actually delivered to the speaker is:

Solid-state: Zout < 0.1Ω typical. The voltage divider is negligible — the speaker sees essentially the full voltage regardless of its impedance. The amp behaves as a near-ideal voltage source.

Tubes (through transformer): Zout = 1–8Ω typical. The voltage divider is no longer negligible. If speaker impedance varies with frequency (and it always does), the delivered voltage varies too. The amp behaves partly as a current source.

This fundamental difference explains why the same speaker sounds different when driven by a transistor amp versus a tube amp. Speaker impedance isnever constant — it varies dramatically with frequency, with resonance peaks and inductive rise. The impedance curve matters far more than the nominal rating.

The damping factor (DF) quantifies how well the amplifier controls the speaker. When the signal stops, the cone keeps moving due to inertia. An amp with high DF effectively short-circuits the speaker's back-EMF, braking the cone quickly.

Solid-state: DF > 100 (often 200–500). Overdamped. Tight, controlled, sometimes "dry" bass.

PP tubes with NFB: DF = 5–20. Critically damped to slightly underdamped. Full, natural bass.

SET tubes (no NFB): DF = 2–5. Underdamped. Warm, "loose" bass. Many prefer this character with high-efficiency speakers.

Neither is "correct" — it is a design choice. Negative feedback (NFB) increases DF by reducing Zout.

An "8Ω" speaker is not an 8Ω resistor. The nominal value (4Ω, 8Ω, 16Ω) represents approximately the minimum of the impedance curve, not the average. Real impedance varies dramatically with frequency:

Resonance peak (Fs): At the speaker's resonance frequency (typically 30–100Hz for a woofer), impedance can rise to 20–50Ω or more.

Minimum: Just above Fs, impedance drops back toward the nominal value. This is the DC resistance of the voice coil (Re) plus a small margin.

HF rise: Above 1–2kHz, voice coil inductance causes impedance to rise. At 10kHz, an "8Ω" speaker may present 20–30Ω.

With a solid-state amp (Zout ≈ 0), these variations don't affect delivered voltage. With a tube amp (Zout > 1Ω), the frequency response is directly modulated by the impedance curve. A Zobel network (R + C in parallel with the speaker) can compensate for inductive rise.

Passive crossovers (series/parallel types) are designed assuming a 0Ω source impedance. With a tube amp, the higher Zout alters crossover behavior:

Reduced slopes: A 2nd-order filter (12dB/octave) may behave like a 1st-order (6dB/oct) when Zout is significant. The amp's Zout adds in series with the filter components.

1st order (6dB/oct): Most tube-friendly. A capacitor for the high-pass, an inductor for the low-pass. Little sensitivity to Zout.

Higher orders: Designed for Zout ≈ 0, they may misbehave with tubes. The solution: bi-amping or active crossovers before the power amp.

Speaker sensitivity specifies the SPL produced by 1W of input at 1m distance. It is the single most important parameter for tube amp matching:

SET (2–8W): 93dB+ sensitivity required, 95dB+ ideal. Full-range drivers (Lowther, Fostex, Tang Band), horn-loaded (Klipsch, Altec).

PP (15–40W): 90dB+ sensitivity recommended. Efficient multi-way (Klipsch Heresy, DeVore), vintage (JBL, Tannoy Dual Concentric).

Avoid: Modern low-sensitivity designs (<86dB) built for 100W+ solid-state amps. They require power that most tube amps cannot deliver.

Output transformers typically offer taps for 4Ω, 8Ω, and 16Ω. The basic rule: use the tap closest to the speaker's nominal impedance.

The turns ratio (N): The impedance seen by the tubes at the primary is Zp = N² × Zload. Each secondary tap adjusts the ratio to maintain the correct primary load.

Intentional mismatch: Some builders use a 4Ω speaker on the 8Ω tap to double the reflected primary impedance. This reduces maximum power but can favorably alter the distortion character. It is common in guitar amps.

Multi-driver systems: Parallel wiring halves impedance (2×8Ω parallel = 4Ω), series doubles it (2×8Ω series = 16Ω). Choose the combination matching an available tap.

Measuring Zout (two-resistor method):

Measuring speaker impedance curve: Use a sweep generator (20Hz–20kHz) and a voltmeter. Place a known series resistor (R_series, e.g. 100Ω) between the generator and speaker. Measure the voltage across the speaker (V_spk) and total voltage (V_gen) at each frequency. Speaker impedance is Z = R_series × V_spk / (V_gen − V_spk).

Interpreting results: A good amp/speaker match shows an impedance curve without extreme dips (<3Ω) and sufficient sensitivity for the available wattage. If the resonance peak is very high (>5× nominal), a Zobel network may be beneficial.