I developed this preamp to drive various power amplfiers that I could make as single gain stage amplifiers that otherwise could not have been. The output could have a voltage swing much larger than most preamplifiers. Using an output transformer, that was custom made, allowed for maximum gain from the medium to low mu tubes this preamp was made with in mind. This preamp could be used with any 6922, 6N1P-EV, 6N6P, or 6H30P-DR. It got its name A2 preamp from its ability to drive some tubes in to A2 operation creating more power from them than normally would be the case. Another noteworthy feature was the use of a copper lined wood chassis, the sonic advantages of this are debatable. Also this amplifier uses a switch mode power supply that is not very common in tube audio. The power supply has super low impedance helping preserve transients, with very low noise.

Comments

• TerryG
• 9 November 2012 at 10:29PM
• The little lines on the knobs I planed to put brass guitar fret pieces, but never got around to it. The dark edges was burnished by saw dust when I was turning them. Hard to get them all to loo exactly alike.

• TerryG
• 9 November 2012 at 10:31PM
• Later I might post some information on the matching power amp. I came up with the idea after the preamp to use aluminum plates to mount the tubes on so that I could use the same chassis later for a different amp, with different tubes. This expands the use of the same style chassis.

• TerryG
• 9 November 2012 at 10:34PM
• I keep forgetting stuff. The preamp has some output transformers per my own design like a power amplifier for speakers. This way I didn't have to use a large capacitor, and avoid capacitor coupling. Likewise it makes for a very very simple circuit, since I am biasing the tubes with LED's there are not resistors in the circuit. The input volume control is an opto-coupler volume control that also doubles as the grid leak for the control grid.