I am selling a wonderful Monolithic Sound Passive Stereo Preamp PA-1 with Power Supply HC-1B. This is highly acclaimed in audio circles and sounds sublime. Here is an excerpt from a national review:
The Monolithic Sound PA-1 line-stage is passive . . . and active, too. "How's that," you ask? Without giving away any secrets of this clever circuit, here's the concept:
The signal is directed in two separate paths after the input. One route is the true passive track, essentially through the volume control, then to the preamp's output. The other route goes to a simple, Class A, dual-mono, solid state gain-stage, the output of which is directed back to the 0 dB position at the top of the volume pot. The custom Alps attenuator allows the completely passive (and unbuffered) signal to be paralleled by an active signal when the volume control hits 0 dB (12 o'clock position), where there is a slight but noticeable detent in the otherwise smooth turning motion. Once engaged, the active gain continues as the volume control is rotated clockwise, to about the 5 o'clock end position of the control. At that point, the active stage is providing its maximum 6 dB of gain. This gain is available to the user, but only when you need it; otherwise, the PA-1 is operating completely passive. Also, it's worth mentioning that 6 dB of gain is much less than most other active preamps, and therefore less imposing of itself on the signal.
As I mentioned, this is a clever circuit, and best of all, very effective. Monolithic considered a number of options, and performed a ton of listening tests, before finally settling on the circuit that is now the PA-1. It came down to choosing a design that would do as little as possible to the signal, and in my opinion that was a great choice.
The chassis of the PA-1 is compact at 8.5" x 6" x 2.5", which means you can place it and the optional HC-1 power supply (more on that later) side-by-side on a single component shelf. Peering internally, the PA-1 is tidy and the parts quality is good. Looking inside inspires confidence and offers no unpleasant surprises. This also reinforces that you don't have to go overboard with designer parts to come up with a great result. There is an external power supply that connects to the PA-1 via a 5-pin DIN connector. The stock PS sounded good, but I preferred the PA-1 with the upgraded HC-1 power supply option.