Good comments above. As Kal indicated, loading is the concern. And using an external splitter or y-adapter, such as Viridian suggested, will result in loading effects that are identical to those that would result from the internal modification you proposed.
To assure that no sonic degradation occurs, the parallel combination of the input impedances of the headphone amp and the main power amp should ideally be 10 or more times as great as the output impedance of the preamp, at the frequency within the audible frequency range for which the preamp's output impedance is highest. If that criterion is not satisfied there may or may not be adverse sonic consequences.
The preamp has a specified output impedance of 560 ohms, which is most likely at mid-range frequencies. I wouldn't be surprised if it increases to 2K or 3K or so at 20 Hz, due to the output coupling capacitor it probably uses. So ideally you would want the parallel combination of the two load impedances to be 20K or 30K or more, or some rolloff of the bottom octave, and possibly other frequency response irregularities, may occur.
The combined input impedance of the two loads is equal to the product (multiplication) of their individual input impedances divided by their sum. For instance, if the headphone amp has an input impedance of 50K and the power amp has an input impedance of 100K, their combined input impedance would be (50 x 100)/(50 + 100) = 33.3K.
You would also want to avoid cable lengths between the preamp and the headphone amp and power amp that are particularly long, and/or use cables that have low capacitance per unit length, to avoid possible upper treble rolloff as a consequence of the interaction of cable capacitance with the preamp's output impedance. The TOTAL of the capacitances of the cables to BOTH the headphone amp and the main power amp will affect the signals that are seen by BOTH of those components. In other words, the capacitance of the cable to the headphone amp, as well as the capacitance of the cable to the power amp, will affect the signal seen by the power amp. And vice versa. If either or both of the cable lengths are particularly long, post back with the lengths, and the cable makes and models if known, and the relevant calculations can be performed.
Regards,
-- Al
To assure that no sonic degradation occurs, the parallel combination of the input impedances of the headphone amp and the main power amp should ideally be 10 or more times as great as the output impedance of the preamp, at the frequency within the audible frequency range for which the preamp's output impedance is highest. If that criterion is not satisfied there may or may not be adverse sonic consequences.
The preamp has a specified output impedance of 560 ohms, which is most likely at mid-range frequencies. I wouldn't be surprised if it increases to 2K or 3K or so at 20 Hz, due to the output coupling capacitor it probably uses. So ideally you would want the parallel combination of the two load impedances to be 20K or 30K or more, or some rolloff of the bottom octave, and possibly other frequency response irregularities, may occur.
The combined input impedance of the two loads is equal to the product (multiplication) of their individual input impedances divided by their sum. For instance, if the headphone amp has an input impedance of 50K and the power amp has an input impedance of 100K, their combined input impedance would be (50 x 100)/(50 + 100) = 33.3K.
You would also want to avoid cable lengths between the preamp and the headphone amp and power amp that are particularly long, and/or use cables that have low capacitance per unit length, to avoid possible upper treble rolloff as a consequence of the interaction of cable capacitance with the preamp's output impedance. The TOTAL of the capacitances of the cables to BOTH the headphone amp and the main power amp will affect the signals that are seen by BOTH of those components. In other words, the capacitance of the cable to the headphone amp, as well as the capacitance of the cable to the power amp, will affect the signal seen by the power amp. And vice versa. If either or both of the cable lengths are particularly long, post back with the lengths, and the cable makes and models if known, and the relevant calculations can be performed.
Regards,
-- Al