To help see how these resistors are an extraneous load, imagine that the tubes have been removed from the circuit.

     This last move brings to our beginning: the SRPP circuit. Yes, we arrived, but what did we lose along the way? The compound circuit offered the lowest output impedance and the greatest immunity for the grounded-cathode amplifier portion of the circuit from perturbations at the output. The bootstrapped-compound circuit offered the highest gain and a fairly low output impedance and a much improved PSRR figure. (Definitely a sleeper circuit.) How do we regain some of these better performance aspects that we lost on the path to the SRPP? 

    Notice that the current draw for the resulting circuit is 10-mA and that the two resistors do not need the tubes to complete their current path from B+ to ground, which also implies that the two tubes do not need the two resistors to complete their current path as well. So the next step is to eliminate the two resistors. Now, the circuit draws only 10-mA and enjoys a slightly higher gain.

The SA-1 makes a great addition to any audio test bench. It can also be a complete preamp solution for audiophile "purists". Consider the advantages of using a Passive Preamp like the SA-1 between your CD/DVD player(s) and your power amplifier(s): The SA-1 adds no distortion to your signal path while performing the major task of most preamps - signal level control. (Active preamps add in at least some distortion.) It uses a precision 24 position stepped attenuator for low noise and excellent channel-to-channel signal level tracking. And it costs a fraction of what an active preamp costs.

Arn Roatcap, Inc.
1248 Valerian Court #4    Sunnyvale, CA 94086
Phone: (408) 737-3920  E-mail: info@goldpt.com

http://www.goldpt.com

            Phone: (408) 737-3920   E-mail: info@goldpt.com

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