The venerable Dynaco PAS is perhaps the most famous stereo preamplifier of all time, Much like the Dynaco ST-70, it introduced the magical sound of tubes to thousands of audiophiles throughout the past decades. The PAS is still a popular model, both in stock form or as a platform to be tweaked and modified. In this article, I will touch on a general outline of the circuit, the sonics strengths and weaknesses, some of the more popular modifications, and an idea of what to look for when rolling tubes.
The Circuit:
So what is a Dynaco PAS? There are three iterations. The PAS-2, the earliest stereo version, with circuitry taken from the PAM-1 mono unit and a gold faceplate with brown knobs. The PAS-3, with a more modern silver faceplate with matching knobs. And finally, the PAS-3X with improved tone controls. This is a full-function preamplifier with tone controls and inputs for phono, reel-to-reel, tuner, and others. Gain, of course, is higher than modern preamplifiers since this was the days before high output CD players.
All PAS preamplifiers share the same circuit: For the phono stage, one 12AX7 per channel, each section in series, using loop negative feedback connected to a capactior and resistor network to provide the RIAA equalization. For the linestage, a 12AX7 per channel, each section in series, utilizing loop feedback to lower distortion and provide a low enough output impedance to drive cables into a high impedance input of an amplifier. Power supply rectification is handled by a 12X4 tube. Filaments for the 12AX7s come from a voltage-doubler circuit, which - as the namesake suggests - doubles the supplied AC voltage from the transformer and smooths it out to DC. Having DC on the filaments also helps to reduce hum.
Stock Sound:
My own experience with this unit started back in 1990, after the electrical failure of my first preamplifier, a solid-state SAE Mark XXX that I was using with my first ever amplifier, a Dynaco 70. Compared to the SAE, the Dynaco PAS-3X had a more organic sound that was imbued with a mellowness that is often attributed to the term "tubiness." Such a strong sonic fingerprint can mask detail and transparency, but the PAS never suffered from being unmusical. Since that initial PAS, I've owned many other preamplifiers, each with different strengths and weaknesses, but none having quite the same magic as old vintage tubes. Some of that is due to nostalgia, and the urge for musical enjoyment over technical perfection. No, the Dynaco PAS is not a perfect preamp, but it sure is a fun one.
In the bass department, a stock PAS suffers from a little sloppiness and lack of control - usually a sign of aged power supply capacitors - but with a pleasant warmth reminiscent of a good tubed radio or an underdamped woofer. Rhythm, which carries the beat and timing of a song, suffers compared to some of the better tubed and solid-state units.
The midrange is the Dynaco PAS strongest point. There is a golden coloration throughout the music, which, though definitenly not neutral, is a pleasant addition for digital sources or hotly mastered albums. This same effect shortens soundstage depth and width, diminishes transparency, and hides inner detail. However, the listener may not care since the sheer musicality outweighs the apparent weaknesses.
Of course the treble follows the same character of the midrange: smooth and pleasantly colored. Ultimate extension is limited, giving a rolled-off effect that may work to the benefit of the user, depending on source and material being played. However, with some vintage speakers, the end result may lead to a rather dead sounding system.
The stock Telefunken smooth-plate 12AX7s are perhaps some of the better tubes for a stock Dynaco PAS. The top end 'hotness' compliments the circuit limitations better than say, a Mullard long-plate, but still, the Dynaco PAS is remarkably tolerant of many different types of tubes from the lowliest Chinese 12AX7B to an expensive Amperex.
Modifications:
The number of changes that can be done to a Dynaco PAS are practically innumberable. There are simple improvemnts on the stock circuit to full-bore modifications that use nothing but the chassis and power transformer. Personally I prefer to keep the general character of the PAS intact, but improve the weaknesses to the point that the preamplifier can be used as part of a high-end system. Some of these mods are simple to perform but many will require experience working with high voltage electronics. Be warned! If you don't know how to solder or understand electronic safety, please have the work done by someone who does.
Bypassing the tone controls [link] is an easy modification with a great improvement on transparency and detail. Since the signal path is shortened and the effect of various capacitors are removed from the circuit, the overall quality take a jump for the better. Bypassing the loudness switch is also a good idea since these older slider switches can be troublesome.
Signal capacitor replacement is another simple change that benefits detail. treble extension, and transparency. The stock green paper capacitors are hardly high-end fodder. As to what capacitor to use, that is up to the individual user. However, space on the PCB is limited, so gigantic oil caps in metal cases may lead to troublesome short circuits. For the budget-minded, any metallized polyproplene - Orange Drops, Panasonics, Russian military - will be an improvement. Auricap, Obligatto, and Jantzen are well-priced improvements over the cheap units. After that, the sky is the limit with some of the better Teflon, film and foil, and oil capacitors beckoning the well-heeled audiophile. All have a slightly different flavor that will work better to varying degrees depending on other system considerations: type of amplifier, speakers, room, and signal source.
The next improvement can be a little more problematic. Since the power supply capacitors - for both the main and the filament - are aged beyond any margin of safety, this should be a no-brainer. However it will be noted that this step does heavily change the character of the Dynaco PAS. Bass extension especially improves, but some of the golden glow is lost since the replacement electrolytics discharge faster and provide better regulation than the vintage pieces. Replacing the stock selenium rectifiers with moderm diodes is a necessity. If one doesn't want to source the can cap and two filament electrolytics, there are PCBs available - notably SDS and Curcio - that can be used. These boards also give the option of using solid-state diodes instead of tube rectification. Personally I prefer the 12X4 rectifier, but some may prefer the solid-state replacement. The end result is a more modern sounding preamplifier that can run with some of the entry-level pieces from Conrad-Johnson, Audio Research, etc.
The next modification isn't particularly difficult but requires some mechanical work to do it. Replacing the stock carbon volume control with a Noble, Alps, or even a Goldpoint will lead to better channel tracking and improved transparency.
The final modification is one that won't necessarily change the sonics, but will stop the user from pulling out their hair in frustration. You see the stock RCA jacks are cruddy little ceramic units that don't work particularly well with modern cables. Various vendors offer replacements that bolt into the stock location. This will require much wire tagging and soldering to accomplish, but the end result will be worth it.
End Result:
A fully modified Dynaco PAS loses some of the coloration, much of the excessive bass warmth, and the rolled-off treble. Transparency and detail is much improved and is now a good match with many more amplifiers and speakers. The phono and line sections, though still warm and full-bodied, now reveal more subtle differences between phono cartridges and DACs. Though not quite top tier material, such a PAS can easily serve as the heart of a moderately priced or even a lower-priced high-end stereo.
Simple modifications after this are of a minor nature: better signal wire, source selector switch, tube rolling, and various damping and vibration control products.
Of course major changes are still possible - whole new PCBs and circuits - but be warned that these updates will drastically change the character of the preamplifier, making it sound much more modern but losing that vintage magic. However, the amount of money one could throw into such a project may be better spent on buying a used but newer preamplifier from Quicksilver, Conrad-Johnson, Audible Illusions, or whatever your budget can afford. After all, the Dynaco PAS is a fifty-year old unit that can now have several mechanical issues: switches, AC cord, and RCA and AC jacks that will need to be addressed if long-term service is of importance. As is, the Dynaco PAS is much like a old car, requiring some work to run its best.
