After I had built my copy of the Dynaco PAS3 preamp, I read several books about tube amplifier technique like "Vacuum tube amplifiers" written by Valley and Wallman, and others. After reading these books I started to analyse different tube amplifier circuits and soon I started to think about building a all DC coupled preamp using techniques earlier used in analogue computer amplifiers. A good DC coupled operational amplifier built with tubes need to use a lot of tubes and that mean a lot of excessive heat generated.
Some compomise was called for, I tried to design a gain stage that should use a minimum number of tubes and the same time have high open loop gain, wide frequency range and should be stable.
I finally decided to use the circuit below as gain stage in the amp. this gain stage consist of a SRPP with 1/2 12AT7 with a 1/2 12AX7 as anode load. The SRPP is coupled directly to a grounded cathode 1/2 12AX7, the voltage on the cathode of this stage is raised by Zener diodes. After that the signal is fed to a 1/2 12AT7 as cathode follower. The DC voltage on the output is restored by a chain of Zener diodes and the DC operating point is controlled by adjusting the cathode resistor of the SRPP anode load.
The open loop gain of this gain stage is ~3000 and feedback is used to control the gain. The open loop frequency response is flat up to ~200kHz, the frequency response is basically determined by the miller capacitance of the cathode coupled 12AX7 stage.
I used two of these gain stages in the preamp, one was used as RIAA phono amp and the second as line stage. The power supply was electronically regulated both for anode voltage and for heaters. I used this pre amp until I moved to Japan in 1996.
All DC coupled gain stage, open loop gain ~3000 here shown as line amplifier with gain of ~15.
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Created on ... december 16, 2000