Hi Bob,

No, the 4 ohm and 8 ohm tap designations do not signify that the output impedance of the amplifier is 4 ohms or 8 ohms, and in most cases the output impedance will be significantly less than those numbers. What they signify is just that the tap is intended for use with speakers that have those impedances, at least nominally.

Keep in mind that damping factor, which for an 8 ohm tap would be defined as output impedance divided into 8 ohms, would be extremely poor (exactly 1) if the amp's output impedance on that tap were 8 ohms.

Also, at audio frequencies the power delivered to the load is not maximized when output impedance = load impedance. That is a concept that is applicable to rf frequencies, where impedance mismatches result in some fraction of the signal delivered to the load being reflected back to the source. At audio frequencies, if an 8 ohm source impedance is connected to an 8 ohm load, one-half of the power generated by the output stage of the amp would be dissipated across the output impedance, and therefore wasted.

Think of it in terms of the amplifier being an ideal voltage source (zero output impedance) in series with a resistor equal to the output impedance, with the speaker impedance being connected between the output side of that resistor and ground. So what you have is essentially a voltage divider, resulting in progressively less voltage being placed across the load as the output impedance increases.

On the other hand, though, keep in mind that highish output impedances can be advantageous with some speakers, as you've no doubt seen in Ralph's "Competing Paradigms" paper.

Best regards,

-- Al

No, the 4 ohm and 8 ohm tap designations do not signify that the output impedance of the amplifier is 4 ohms or 8 ohms, and in most cases the output impedance will be significantly less than those numbers. What they signify is just that the tap is intended for use with speakers that have those impedances, at least nominally.

Keep in mind that damping factor, which for an 8 ohm tap would be defined as output impedance divided into 8 ohms, would be extremely poor (exactly 1) if the amp's output impedance on that tap were 8 ohms.

Also, at audio frequencies the power delivered to the load is not maximized when output impedance = load impedance. That is a concept that is applicable to rf frequencies, where impedance mismatches result in some fraction of the signal delivered to the load being reflected back to the source. At audio frequencies, if an 8 ohm source impedance is connected to an 8 ohm load, one-half of the power generated by the output stage of the amp would be dissipated across the output impedance, and therefore wasted.

Think of it in terms of the amplifier being an ideal voltage source (zero output impedance) in series with a resistor equal to the output impedance, with the speaker impedance being connected between the output side of that resistor and ground. So what you have is essentially a voltage divider, resulting in progressively less voltage being placed across the load as the output impedance increases.

On the other hand, though, keep in mind that highish output impedances can be advantageous with some speakers, as you've no doubt seen in Ralph's "Competing Paradigms" paper.

Best regards,

-- Al