Guy's Phono Amplifier
Posted: May 17th, 2013, 1:26 pm
I’m building a phono amplifier for my stereo. It will be set up for a moving-magnet (MM) cartridge, and can be used with a moving-coil (MC) cartridge through a step-up transformer. I am using tubes in the analog path because they sound better in general than solid-state components to me, and they are fun to work with. I am also using a collection of solid-state components for current control and an output servo, so we are looking at a hybrid amplifier. The power supply is also a hybrid of tube and solid-state devices.
Design Goals:
Approach:
I prefer using passive circuits, not feedback, to achieve RIAA correction. The passive inverse RIAA circuit could be all-in-one, but splitting it between the first and second stages makes it easier to implement and adjust: in fact, my first and second stages have identical topology. The second half of the inverse-RIAA filter has a second zero at 50 kHz, as shown in Figure 1:
Figure 1: Phono Amplifier Block Diagram
I want a lot of transconductance in the first stage to minimize noise, so I’ll use two Russian 6S45Ps, one for each channel, biased to 30 mA each. I’ll use two 6AM4s, again one for each channel, biased to 9 mA for more gain in the second stage. The line driver (third stage) is a 12SN7 with a pair of servo drivers in the cathode circuits. The line driver is biased to 8½ mA per triode.
Because I have three schematics to load, I'll add another post to continue my discussion.
Design Goals:
- low noise,
tight RIAA conformance,
low distortion,
freedom from overload and blocking,
capability to drive 10 kOhms of cables and load, and
all-tube amplification.
Approach:
I prefer using passive circuits, not feedback, to achieve RIAA correction. The passive inverse RIAA circuit could be all-in-one, but splitting it between the first and second stages makes it easier to implement and adjust: in fact, my first and second stages have identical topology. The second half of the inverse-RIAA filter has a second zero at 50 kHz, as shown in Figure 1:
Figure 1: Phono Amplifier Block Diagram
I want a lot of transconductance in the first stage to minimize noise, so I’ll use two Russian 6S45Ps, one for each channel, biased to 30 mA each. I’ll use two 6AM4s, again one for each channel, biased to 9 mA for more gain in the second stage. The line driver (third stage) is a 12SN7 with a pair of servo drivers in the cathode circuits. The line driver is biased to 8½ mA per triode.
Because I have three schematics to load, I'll add another post to continue my discussion.