Dynaco PAS-3 (1963–1980s)
A four-tube preamp from Philadelphia that sold by the tens of thousands, sounded warm, and still gets modified - sometimes to the regret of the person holding the soldering iron.
Overview
The PAS-3 is a tube stereo preamplifier built by Dynaco of Philadelphia. It was introduced in 1963 and produced into the 1980s. It's the preamp most people picture when they picture a Dynaco system: a slim silver faceplate, a row of source positions that reads like a 1960s hi-fi glossary, and a circuit simple enough that owners have been rebuilding and reworking it for six decades. It sold as a kit and as a factory-assembled unit. It was designed from the start to sit above the ST-70 power amplifier.
The measured numbers are respectable for the period. The auxiliary input runs 10 Hz to 40 kHz within ±0.5 dB, harmonic distortion sits below the measurement limit, and cross modulation distortion is quoted at 0.05% or less at 2 V rated output. Signal-to-noise is listed at 85 dB for aux and 74 dB for phono. The phono stage is the noisier of the two, which is normal for a tube phono section, but worth knowing before you plug in a low-output cartridge. Output capability is generous: 10 V into 100k Ω, enough swing to drive a power amp directly without a second gain stage in between.
What owners actually talk about is the sound. The consensus is a warm, slightly lush character with a fat low-mid presence that flatters the ST-70 rather than fighting it. It does not chase neutrality, and it doesn't pretend to. Some listeners call that musical. Others, working with the wrong speakers or the wrong cartridge, call it bright. The PAS-3 rewards matching more than most preamps of its class.
It is not a transparent modern line stage with tubes bolted on. It has a phono section built around the moving-magnet cartridges of the mid-1960s, no RIAA equalization switch and no subsonic filter. Switching between line sources can produce bleed or interaction if cables aren't well shielded. Owners report it, and the source documentation doesn't publish a crosstalk figure for the line inputs to argue with them.
Key Features
Four 12AX7s and a 12X4
The signal path is all tube: four 12AX7 dual-triodes for amplification and a single 12X4, which the documentation suggests handles rectification or buffering. The exact function isn't spelled out in the sources that survive. There are no solid-state devices in the audio path. The 12AX7 is one of the most widely available and swappable tubes ever made, and rolling different brands through a PAS-3 is a long-standing pastime. Dynaco's original circuit was voiced for standard-spec tubes, so exotic replacements change the sound more than they improve it. Owners who like to tune by ear don't seem to mind.
Tone Controls That Leave the Circuit at Center
The PAS-3's cleverest bit of engineering is the tone control arrangement. A special volume potentiometer is used, and when the knob sits at the mechanical midpoint, the tone control network is physically detached from the signal path. The preamp becomes a flat amplifier. That's a real difference from designs where "bypass" still leaves capacitors and resistors hanging off the signal. Bass and treble adjust left and right channels independently, which is useful for correcting room asymmetry and easy to get wrong if you nudge one side and forget. The ranges are wide: bass ±20 dB at 20 Hz, treble ±18 dB at 20 kHz, though a 1973 catalog lists the treble figure at ±14 dB. Treat the treble range as somewhere in that neighborhood rather than a fixed number.
Inputs for Everything You Owned in 1965
The input list is long: Phono, Tape head, Special (used as Phono2), Mic (usable as Phono3), Tape head AM-FM, FM-multi, Tape, and Spear. The Special input was commonly pressed into service for a reel-to-reel deck. The Mic input, with its low-level gain, could be repurposed as a second or third phono stage, handy for collectors running both mono and stereo turntables. Input impedance is 50k Ω for low-level sources and 250k Ω for high-level. Phono sensitivity is 2.5 mV with a maximum allowable input of 250 mV. That 2.5 mV figure tells you this phono stage wants a moving magnet cartridge, and it will struggle with anything quieter.
Global Negative Feedback
Dynaco reduced distortion by applying negative feedback across the entire circuit rather than in local stages. Measured performance holds up well for a tube design of the era, and the distortion figures are low enough that the company simply called them below the measurement limit. Some owners argue the global feedback costs the preamp a bit of the "liveness" that single-ended designs deliver. That's a matter of taste. The flip side is stability: the PAS-3 drives long interconnects and a range of power amplifiers without oscillating or loading down.
Silver Faceplate, Sold Separately in Wood
The PAS-3 is the second iteration of the PAS line. The PAS-2 came first, with a gold faceplate and brown knobs, and the PAS-3 updated the cosmetics to a silver faceplate with matching knobs. It's a small thing, but it's the easiest way to tell the two apart across a crowded table at a swap meet. A wooden case was available separately, and the chassis measures 343 mm wide by 108 mm tall by 230 mm deep. Weight is listed at 4.8 kg in a 1973 catalog, with 5 kg and 4 kg appearing in 1974 and 1976 catalogs respectively. Call it roughly 4 to 5 kg depending on the year and whether the wood case is attached.
Historical Context
Dynaco built its reputation on kit electronics that delivered near-commercial performance at do-it-yourself prices, and the PAS-3 was central to that strategy. Alongside the FM-3 tuner, it formed a complete system that could be assembled for under $200. The PAS-3 was the companion preamplifier to the ST-70, and together the two became one of the most popular tube amplifier combinations in hi-fi history, a pairing that shows up in period coverage of the era. High Fidelity (April 1966, p.31) ran Dynaco's own "Dynaco Is Best" advertising alongside editorial selections from the Modern Hi-Fi & Stereo Guide, which picked a Dyna system built around the PAS-3 as its "Maximum Fi" recommendation. The PAS-3 refined the formula laid down by the PAS-2 with improved layout, better shielding, and more input options. Production continued into the 1980s.
Collectibility & Value
The PAS-3 is common enough that nobody needs to pay a premium for a project, and clean examples still hold real value. Sold listings have landed around $495, with very good examples, a 1969 model among them, asking $699. Unrestored units trade closer to $375, which is roughly half of what a $750 asking price represents once you account for the work ahead. A single Facebook group report of a $1,825 sale exists, but that likely reflects a fully restored or unusually configured unit rather than the going rate. A Canadian listing for a clean working example came in at CA$645.
The known failure points are predictable for a tube preamp of this age. Static during volume adjustment is common, and the molded paper and electrolytic capacitors in the power supply degrade with time, often badly. Restoring a PAS-3 frequently means installing a new power supply board, and complete recap kits are widely available. Filter capacitor replacement and volume control modifications are documented in enough detail online that a first-time restorer can follow along. Owners who performed the Van Alstine modification decades ago have publicly regretted it, since the mod is not easily reversed. If a PAS-3 is original, think hard before cutting into it.
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