The Modular Preamplifier, Part 10: 6AH4 Linestage

Hi!

The linestage version of the modular preamp sytsem with indirectly heated triode is finished.

It uses the 6AH4. Like all gain stages of the modular preamp it has it's own external power supply.

The circuit is exactly the same as the linestage section of the Octal preamplihier Mk2. The power supply uses the same full wave bridge with 4 6AX4 TV dampers and choke input filter. Only the heater supply is different. In the Octal preamp all heaters are supplied with DC, since it also contains a hum sensitive phono section. In a linestage it is sufficient to heat the tubes with AC. This simplifies the supply and works hum free. The heaters are just referenced to ground through two resistors across the heater line.

Here a photo which shows the inside construction of the preamp and power supply. The PSU is completed. The preamp still misses all the wiring from the inputs to the selector switch in this picture.

The linestage has 5 inputs. Four of them are regular inputs with 100kOhm input impedance for all kinds of line sources. One set of inputs goes through 1:4 step up transformers. These are reserved for low impedance sources which need some additional gain.

The two sets of output jacks are wired in parallel. Separate binding posts which connect to chassis and signal ground. A ground lift switch provides the option to either connect signal ground with chassis or have them disconnected. This allows for adjustment in case of ground loops in the system. The power supply is hooked up to the preamp trhough an umbilical with 5 pin Amphenol connectors.

The sound is the same as the linestage section of the Octal preamp Mk2. It uses the same AVC from intactaudio. Maybe a tad more refined since it has it's own dedicated power supply, which is shared with the phono section in The Octal preamp.

The next part of the modular preamp to be finished will be the linestage with the directly heated 10Y.

Best regards

Thomas