Dynaco PAS-3X

A five-tube control center from Philadelphia that took the most famous preamp circuit of its day and pulled the tone controls out of the signal path - with a trick of the knob.

Overview

The PAS-3X is Dynaco's tube stereo preamplifier, a two-channel control center built to handle both monophonic and stereophonic reproduction from any program source with any modern amplifier. It is the updated PAS-3, and the update is specific. The PAS-3X added a cathode-follower output stage and a more flexible input setup with dedicated circuits for phono, microphone, tape, and FM sources. Everything else is the Dynaco house approach: a simple circuit, negative feedback across the entire amplifier, and a parts budget that kept it within reach of ordinary listeners.

Owners who have worked inside one tend to praise the assembly quality. One long-time repairman called it the neatest wire job he had seen in forty years of servicing this equipment. Another owner reported the unit running quiet, with no hum and no noise. That second claim holds up. Dynaco's manual points to the power supply's separate rectifying system for the tube filaments, which is one of the reasons the design runs with practically no hum.

The interesting part on the bench is what Dynaco did with the tone controls. The unit uses a special volume-type control. When the knob sits at the mechanical center of its rotation, the tone control circuit is disconnected from the signal path altogether, and the preamp becomes a flat amplifier. Dynaco's own literature called this the first major change to the preamp design since it was introduced. It also claimed this made continuous controls superior to the far more expensive step-switch type, because they keep infinite resolution while removing the frequency- and phase-discriminating networks when they aren't wanted. Whether a listener can hear the difference is another matter. Dynaco's own advertising admitted as much, noting that the preamp was already remarkably good and that the improvement was made for its own sake.

Key Features

Tone Controls That Leave the Circuit

The tone control arrangement is the defining feature of the PAS-3X. Instead of a step switch, the unit uses a continuous control wired so that at mechanical center the tone-shaping networks are pulled out of the signal path and the preamp behaves as a flat amplifier. Bass and treble are adjustable independently for left and right channels. The bass control offers ±20 dB at 20 Hz and the treble ±18 dB at 20 kHz, though a 1973 catalog lists the treble figure at ±14 dB, so the range shifted slightly over the run. The tone controls are the reason many buyers chose this version over the standard PAS-3.

Cathode-Follower Output Stage

The biggest concrete change from the PAS-3 is at the output. The added cathode-follower stage gives the preamp a lower output impedance and lets it drive power amplifiers more comfortably. The manual states the design works into power amplifiers with input impedances of 100,000 ohms or higher, requiring between 0.5 and 2.5 volts input for rated output. That covers essentially all generally available tube and transistor amplifiers of the period. Dynaco noted that amplifiers with lower input impedance would need slight adaptation of the preamp, with factory consultation available to get optimum results.

Four 12AX7s and a 12X4

The tube complement is four 12AX7s (also designated ECC-83) and a single 12X4, five tubes in total. The 12AX7 is the workhorse dual-triode of the era, widely available and easy to substitute, and the all-tube signal path is a big part of why the PAS-3X remains serviceable today. The stated harmonic distortion is below the measurement limit of regular commercial-grade test equipment, which means the bench gear of the day couldn't reliably resolve it. Intermodulation distortion is quoted at 0.05% or less at 2 volts rated output, and it does not increase at lower volume settings. Dynaco emphasized that detail, since many preamps of the period degraded as the volume control came down.

Flexible Input Switching

The input section is where the "more flexible" claim earns its keep. The PAS-3X accepts three stereo low-level sources: RIAA phono, NAB tape head at 7½ ips, and a "Special" input that can serve as a second phono or microphone input. It also has four stereo high-level inputs covering FM-AM, FM-multiplex, a spare, and tape. Input sensitivity is 2.5 mV for phono, 188 mV for tape head, and 100 mV for the special input. Low-level input impedance is 50k Ω, and high-level is 250k Ω. Phono will tolerate up to 250 mV of input before overload. The manual describes the configuration as making it possible to use a microphone or a phono source through the special input circuit, and owners who wanted a third phono position could repurpose the mic input for it.

Frequency Response and Output

The auxiliary input runs 10 Hz to 40 kHz within ±0.5 dB, and that response is not affected by the position of the volume control. Output is generous at 10 volts into 100k Ω, enough swing to drive a power amplifier directly. The manual notes the preamp is intended to work into loads of 100,000 ohms or higher and that it will deliver between 0.5 and 2.5 volts for rated output, depending on the amplifier it feeds.

Compact Chassis and Optional Wood Case

The PAS-3X measures 343 mm wide, 108 mm tall, and 230 mm deep. It is a slim, shallow box that slides into a cabinet or shelf easily. Ventilation is not critical unless the unit is run continuously, since the preamp produces very little heat. For panel mounting, Dynaco suggested a supporting shelf flush with the bottom of the cutout, with a PBK bracket kit available for $2 to accommodate panel thicknesses up to 1 inch. A wooden case was sold separately. The PAT-4/PAS-3X common wooden case listed at ¥3,900 around 1970, and the DW-1 wooden case at ¥6,500 around 1976.

Historical Context

The PAS-3X was positioned as an update to the PAS-3 preamplifier, and it is generally regarded as the more desirable of the two. It was built in the United States by Dynaco of Philadelphia, and it joined a product family that included the PAS-2, PAS-3, PAT-4, ST-70, ST-35, SCA-35, FM-1, FM-3, FMX-3, and Mark III. The PAS line as a whole is often described as the most famous stereo preamplifier of its time, and the PAS-3X is the last and most refined version of that design.

Production years are not reliably documented. One source dates the PAS design to 1959, while a service database places the PAS-3X itself at 1971-72, and a catalog description suggests 1973-1976. Those dates conflict. The safest reading is that the PAS-3X was a 1970s refinement of a much older circuit. Pricing survives mainly from the Japanese market: ¥42,000 around 1970, ¥49,500 around 1973, and ¥92,000 around 1976 for the finished unit, with the kit version at ¥64,900 around 1976. No original US pricing has been located.

Dynaco marketed the PAS-3X alongside the FM-3 tuner and Stereo 120 amplifier as a complete system. An advertisement in High Fidelity (March 1968, p.99) presented the combination as representing the highest level of quality attainable with high-fidelity components, and the same campaign ran in Audio (June 1968, p.31) and HiFi-Stereo (October 1967, p.133). The PAS-3X kit was listed at $69.95 and the factory-assembled version at $109.95. Those are advertising prices, not independent verdicts, but they show where Dynaco positioned the preamp in the market: at the low end of the serious tube preamplifier field, deliberately.

Collectibility & Value

The PAS-3X is one of the more approachable vintage tube preamps to collect, partly because so many were sold and partly because the circuit is simple enough that the restoration community has kept it alive. Complete units have been listed around $500 for a boxed example, and European listings have appeared in the €650 range. Parts and accessories circulate separately. A pair of brass RCA input jack kits for the PAS-3X sells for around $29.90, so many listings under this model name are components rather than whole preamps.

Two areas need attention. First, the filament supply was updated at some point to facilitate operation, and power supply repair is a common topic in service discussions. A YouTube walkthrough on fixing the PAS power supply is one of the more frequently referenced repair resources. Second, the original tubes matter. Examples have surfaced with Telefunken smooth-plate 12AX7s still installed, and those are worth preserving rather than swapping out casually. For owners who want to go further, replacement boards exist as an aftermarket option: the PAS Purist Line Amplifier, designed by Norman Koren for the PAS-2, PAS-3, and PAS-3X. The designer's own disclaimer notes that Classic Valve Design assumes no responsibility for circuit or user damage from their use.

Documentation is well supported. The original assembly manual with full-sized fold-out inserts survives, as does an owner's manual and at least one independent test report.