SRPP circuit can be optimized for that load impedance, as it is fixed. But if the external load is a power amplifier, then the load impedance might be as low as 2k and as high as 1-meg.
    Of course, if you only plan on using the line amplifier to drive one amplifier and only one amplifier, then you can certainly optimize the circuit for this amplifier. And equally obvious, no manufacturer of a line stage could optimize the SRPP for all amplifiers and shouldn't use the circuit for this application. Wait a minute, doesn't such and such make a line amplifier that uses an SRPP output stage and it sounds great? Sure, but does it sound great working into 2k or 1-meg? In the same vein, if the load is highly reactive, for example electrostatic headphones or twenty feet of patch cord, do not use a generic SRPP, as the circuit relies on a purely resistive load for proper functioning (in previous Tube CAD Journal articles, techniques for driving reactive loads are explored).

     In answering an email, I mentioned that the first tube-based phono preamp I had built was in fact completely SRPP from beginning to end. As a result of this admission, several readers asked to see the schematic. Apparently, the demand for tube phono preamp circuit information is endless. So while we are on the topic of the SRPP, below is the preamp I designed in 1982.

                      Phono preamplifier

              Line stage and power supply

     The preamp realized about 40 dB of gain in the phono section; 26 dB, in the line stage. This preamp sounded quite good, quiet and airy, sounding best with Jazz and vocals, and worked for years with no problems until it was cannibalized for parts (mistake number one that home constructors make). The power supply used a voltage doubler on an isolation transformer and caused many a wrinkled brow on my friends, but it proved amazingly reliable. (Click on the schematics to see them enlarged.)

     In sum, the SRPP circuit has its uses, but those uses are limited. Where money and space allow, better more complicated circuits can be used to better effect than the SRPP; but where dirty and cheap are needed and where the load impedance is fixed and purely resistive, the SRPP is a good choice.

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