autoformer attenuator and step up transformers

If the transformer steps up the voltage provided by the source by a factor of 4 (which is a 12 db increase in gain), it will result in the source component seeing a load impedance of 1/16th of the impedance that exists "looking forward" from the output of the transformer.

If the autoformer is set so as to step down the voltage that is applied to it to a significant degree, it would result in the impedance "looking forward" from the output of the transformer being significantly greater than the input impedance of the amplifier (or whatever component is being driven). But since what you are apparently trying to accomplish is a net overall increase in voltage, presumably the attenuator would often be set to not much lower than 1:1. For a 1:1 setting of the attenuator and a 4:1 transformer ratio the source component would see a load impedance of 1/16th of the input impedance of the amp. Which in many cases will result in a poor impedance match and consequently in degraded sonics, depending on the impedances of the specific components and on how those impedances vary as a function of frequency.

Also, while I’m not certain, it seems conceivable to me that sonics could also be adversely affected by things like ringing, phase shifts, etc.

Regards,
-- Al

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