I decided to make a new improved version of my DC-coupled preamp. I have made a completely new mechanical design, actually I have built 2 completely different ones, the one seen here is the one I like best.
The amplifier is built with 2 almost identical gain stages, the main difference is in the feedback loop with RIAA correction in the phono stage.
The anode voltage is regulated with a electronic voltage regulator built with a 6DR7 unequal dual triode, it works very well with stable voltage and low hum, the negative voltage is regulated with a solid state regulator. Heater voltage is rectified but not regulated, (it is not needed to eliminate hum).
The open loop bandwidth of each stage is ~3000 so especially the line stage need heavy feedback in order to reduce gain to useful values, the gain is ~4 for CD and ~16 for other high level inputs. I have been very careful to get good phase margin in order to get good transient response without ringing. The rise time is about 0.5us and the amplifier is stable with any load. The output impedance is less than 1 ohm with any gain. This amplifier sound very good and is also very quiet. The sound is noticeably better than the one I used before, see Marantz 7 type preamplifier.
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Created on ... October 6, 2002