Volume change by assigning the THRU socket
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Anyone who uses the BluBOX in combination with the AMP1 may have already noticed that it makes a sound difference whether a loudspeaker is connected to the THRU socket of the BluBOX or not. (In contrast to traditional tube amps, the AMP1 can be operated without a load (speakers, reactive load, etc.) connected.) In fact, several things potentially happen here that can cause a sound difference at the outputs of the BluBOX:
Loudspeaker (or reactive load) vs. no loudspeaker (or pure resistance)
The impedance curve of a loudspeaker or a reactive load changes the frequency response of the output stage, depending on the negative feedback design of the output stage used. This effect is used to create more punch and bite, e.g. for heavy sounds. If no loudspeaker or only a resistor is connected, the frequency response remains unaffected, so it sounds rather "flat" (which is not necessarily a bad thing!).
The AMP1 sounds different depending on whether a loudspeaker is connected to the THRU socket of the BluBOX or not. Incidentally, this changed frequency curve can also manifest itself in a measurable difference in volume: if a sound has more mids, it automatically becomes louder.
Compensation of the frequency curve differences through the BluBOX
The BluBOX compensates for the phenomenon described above by adapting the frequency curve, depending on whether the THRU socket is occupied or not. As a result, the tonal difference at the outputs of the BluBOX (BALANCED MIC OUT and LINE OUT) between the two scenarios - "loudspeaker connected" and "no loudspeaker connected" - is slightly smaller.
PLEASE NOTE: if either only a dummy plug or only a cable without connected loudspeakers or reactive load is connected to the THRU socket of the BluBOX, the frequency curve compensation of the BluBOX is activated. The frequency response of the amplifier remains unaffected by the lack of load. The BluBOX compensates for a change in the frequency curve that did not take place at all.
Last but not least, the AMP1 itself is a bit louder when no load is connected.