It looks like the output impedance of the B77 is nominally 390 ohms, although if I’m interpreting the datasheet correctly it may rise to as much as 1.5K if the output level is adjusted downward (which you presumably would not be doing). While those numbers most likely correspond to mid-range frequencies, given that the B77 is a solid state design chances are they do not rise greatly at other frequencies. (Tube-based output stages commonly have coupling capacitors at their outputs that in many cases cause the output impedance to rise substantially at deep bass frequencies, although there are some solid state designs for which that is also the case).
137K divided by the factor of 16 that I mentioned would result in the B77 seeing a load impedance of 8.6K, which seems reasonably comfortable in relation to those numbers.
If I recall correctly, though, some autoformer-based passive attenuators can provide an output voltage that is up to about 6 db greater than (i.e., twice as much as) their input voltage. If that applies to your unit, and if you were to set the volume control at the top of its range, the 8.6K number would be reduced to one-fourth of that amount (2.15K), which may be too low to be optimal in relation to a 390 ohm output impedance.
Regarding the reason Dave suggested putting the transformer after the autoformer, I don’t really know. But a guess would be that he might be anticipating that the sonics provided by the autoformer would be compromised if it is asked to handle voltages as high as 6 volts or so (corresponding to 4 times the 1.55V max output of the B77, 1.55V being roughly in the neighborhood of the max voltage of many unbalanced line-level outputs).
Regards,
-- Al
137K divided by the factor of 16 that I mentioned would result in the B77 seeing a load impedance of 8.6K, which seems reasonably comfortable in relation to those numbers.
If I recall correctly, though, some autoformer-based passive attenuators can provide an output voltage that is up to about 6 db greater than (i.e., twice as much as) their input voltage. If that applies to your unit, and if you were to set the volume control at the top of its range, the 8.6K number would be reduced to one-fourth of that amount (2.15K), which may be too low to be optimal in relation to a 390 ohm output impedance.
Regarding the reason Dave suggested putting the transformer after the autoformer, I don’t really know. But a guess would be that he might be anticipating that the sonics provided by the autoformer would be compromised if it is asked to handle voltages as high as 6 volts or so (corresponding to 4 times the 1.55V max output of the B77, 1.55V being roughly in the neighborhood of the max voltage of many unbalanced line-level outputs).
Regards,
-- Al