Linearizzazione di impedenza
- Green: Impedance without linearization,
- Orange: Impedance with cross over impedance linearization but without Low frequency-impedance linearization
- Blue: low frequency impedance linearization put on Low
- Purple: low frequency impedance linearization put on Medium
- Yellow: low frequency impedance linearization put on High
The cross-over impedance linearization is in our experience useful on all tube amplifiers (some people prefer to keep it switched off - question of taste and overall system).
The Bass impedance linearization leads to higher definition and better control in the low frequencies. The fact is that only rare tube and hybrid power amplifier need it, suggests us not to stress the construction to achieve this result.
In fact, each improvement is only useful, as long as the drawback it brings, is smaller than the problem it corrects
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There is a convention to the use of the term "nominal impedance", and if the impedance over the majority of the bandwidth, specifically covering the range in spectrum where majority of the musical spectral power occurs, it's 8 ohms. A single number cannot tell all there is to tell about an impedance that varies with frequency. You must keep in mind that 'nominal impedance' is not defined in IEC. Indeed, the electronics industry was advised when the Trade Descriptions Act was introduced, that the word 'nominal' should no longer be used in specifications. That is why the IEC concept of 'rated value' is so useful. There is a very detailed definition and explanation of this term in IEC60268-2. The IEC standard (IEC60268-3) allows any "increase" above the rated value, but limits the "decrease". The standard does not allow the impedance to fall below the 80 % of the nominal value at any frequency, including DC.
Saying that there is a nominal resistance of 8 Ω, does not mean that the impedance is flat 8Ω over the whole frequency spectrum.
One critical range is at the cross over frequency, where normally there is a peak (blue curve in the drawing). Therefore we developed and implemented in our loudspeakers a switchable impedance linearization for the cross over frequency. This flattens the impedance curve in the critical range (green curve in the drawing).