I rebuilt two Dynaco PAS preamps with AVA rebuild kits: first, a PAS-2 that I further modified after getting it back from AVA’s free check up. The second time around I rebuilt a PAS-3, with boards supplied by AVA and my own parts, the same that went in the finalized version of the PAS-2. Stereophile reviewed the AVA-rebuild of the Dynaco PAS preamp back in 1988. John Atkinson was mildly positive in his review and recommended the product with a C rating. From the photograph and description, it appears that Frank Van Alstine is still selling the same kit. Maybe a few caps and resistors have changed, but I’d bet that’s about all.
I listened to the AVA PAS-2 after it returned from the free check up. It sounded at least as good as one might expect for the money invested – and here I’m not considering the value for my labor of some tens of hours. But still, it wasn’t quite up to the task of doing duty in a modern hi-end system. Here’s the thing: if one were to rebuild the preamp exactly to the specifications of AVA and stop there, they would be missing out on some seriously unrealized potential. Further mods can bring this little retro unit up to a level that will compete with the best of today’s preamps, making it worthy of at least an A- Stereophile rating. It’s fair to admit the caveats: as the gain is not large, and the output impedance is fairly high it should drive only a single amp with high input impedance (>=67k ohm) via interconnects that are not too long or high in impedance. But these limitations will not be problematic to most.
The wonderful thing about the AVA-PAS preamp is its simplicity. Simplicity is a virtue in itself in minimal corruption of the audio signal, and offers the secondary benefit in that one can afford to install exotic audiophile parts, as they are few in number. Another good thing is that compared to other components, the PAS preamp is really easy to work on. Once you cut out a hole under the power supply section (described below), everything is readily accessible. Below are the mods I did after first building the kit to AVA specs. Most of my parts came from the Parts Connexion in Canada; I also got some from Sonic Craft, eBay and Triode Electronics.
1. Film capacitors. Maybe 70% of the sonic upgrade ultimately realized was from the simple act of replacing the Mylar (aka polyester) coupling capacitors. When you check out the AVA website, you will see that the company’s new products contain these horrid capacitors. That’s really sad, because I have every reason to believe that the circuits are well designed. Most high-end tube amp manufacturers of today use metalized polypropylene caps or better, and for good reason. Mylar caps suck the life out of the music, giving a slow, muddy, closed-in, spatially unfocused sound with weak bass, and dramatically rolled-off highs. I replaced the 6.8 uF Mylar caps with Auricap metalized polypropylene; replaced the 0.33 and 3.3 uF Mylar caps with Mundorf oil/silver; and the 0.022 and 0.047 uF caps with Sonicap Platinum (Teflon film/ tin foil). If you are interested, there are plenty of online reviews of these products. I can say, conservatively, that the sonic upgrade was astounding. If you are disinclined to go all out with the many changes suggested below, at least dump your $0.50 Mylar caps in favor or something better, such as those listed above. The replacement caps will be bigger, but you can get them in there, in some cases by installing them upright.
2. In order to tweak the power supply section, you’ll need to be able to get at it. Drill a hole in the chassis under the power supply board big enough for your nibbler, then nibble out a rectangular hole big enough to get to all the parts on the underside of the PC board. Before you start, loosen the screws that attach the circuit boards, so that the vibrations of drilling and nibbling won’t be transmitted to the boards and their parts. It will take you a while to do the nibbling, so it’s a good project for a rainy weekend.
3. Resistors. The kit provides mil-spec, general purpose Dale resistors. These are decent, but you can definitely do better. In the power supply section I installed Mills 2W wirewound resistors. These are remarkably uncolored and are the first choice for higher wattage applications. Don’t bother replacing the resistor supporting the Power On LED; it won’t make any difference. In the signal path of the line stage and phono sections, I went with Vishay S102/Texas Component TX2352. These bulk metal foil resistors are dear in price, but you’ll only need 7-8 of them per channel, and they are transparent in a way that is almost unimaginable. In areas of lesser importance, I used PRP resistors that are still a big upgrade from the dry sounding Dales. If you don’t want to spring for the expensive Vishays, PRPs everywhere is still and excellent choice.
4. Wire part 1. Wire colors the sound, just like passive parts. Cutting out the underside of the PS board means your AVA kit wires are a lot longer than they need to be. So replace them with shorter lengths of high-end wire. I used 20 and 24 AWG “silverfuse” hookup wire sourced from LAT International. It’s wonderful stuff and affordable at a less than $2 per foot. I used 20 AWG for power supply wiring (and this is not too wimpy as the preamp only consumes ¼ amp, but you can do a double run if you want) and 24 AWG for signal wiring. The kit-supplied wire is good enough for the filament supply - both phono and linestage - and the off/on LED; it’s pointless to replace this.
5. Wire part 2. Speaking of wiring, do you really need the ungrounded AC receptables on the back panel? These went out with bell-bottom pants. Yank them all out and this will enable you to shorten and simplify the AC wiring. Their absence will leave holes in the back that will improve ventilation, which I feel is otherwise inadequate. After-market power cords sound vastly better than the hardware store lamp cord provided with the kit. That’s because a good power cord acts as a low pass filter, rejecting the high frequency noise on the AC mains. So again, get out your nibbler and cut out a slot adjacent to the transformer for a male IEC connector. It shouldn’t be hard figuring out how to wire it. The middle ground tap of the connector is a bonus that can be used in a way I describe later. Then plug in a power cord worthy of this component that you are steadily improving. I like the LAT International AC-2.
6. Electrolytic capacitors. The caps provided with the kit are general-purpose industrial types that are okay, but again, you can do better. Don’t bother replacing the caps that support the tube filaments, because you won’t hear a difference. But the others that filter the B+ supply voltage (6 of them in all on the PS board) should be upgraded. If you never use the phono section the number drops to 4. I got very good results with Mundorf Mlytic types. Make sure to equal or exceed both voltage and capacitance ratings of the originals. Adding more capacitance will lower the noise floor and improve the dynamics and bass. And it’s not just about quantity; some caps filter better than others. Also, consider Jensen 4-pole electrolytics. The replacements will likely be bigger, but there is room for them in the box.
7. Diodes. I’ve replaced diodes in other components with high-speed FREDs, with excellent results, but in this component there is nothing to be gained in diode replacement.
8. Mica capacitors. The silver mica capacitors in the phono section are just middling. Upgrade to Rel-cap polystyrene film/foil types for a sweeter more articulated sound on LP playback.
9. Switches. The kit runs input signal wire from the tape loop switch to the balance control, back to the volume control, back in the opposite direction to the blend switch, then to the line stage inputs. This is needlessly complicated. Here’s how I simplified things. First, nix the blend switch. If you are like me, 99.9% of the time you leave it on the full stereo setting, anyway, so you won’t miss it. In its place I installed a mono ladder stepped attenuator for the right channel, that I built myself with PRP resistors and a 24-pole TKD selector switch from eBay. (With a quick web search you can find instructions for building these; or you can get a pre-assembled one). Where the stereo volume control is, you will, of course, install an identical mono ladder stepped attenuator for the left channel. If you hold out for a good price for the switches on eBay, you can build the pair of attenuators for around $50. You will find that compared to the metalized plastic volume controls, they seriously open things up and are thus worth every bit of cost and assembly time. Two mono attenuators with a 2.8 dB taper that I used, give reasonable control over balance, so you can disconnect the balance control without missing it. Now, the signal runs from the selector switch, to the tape loop switch, to an attenuator, to the line board. Your wire runs will be much shorter, which means better sound. You could even go one better and bypass the tape loop switch in the likelihood that you gave your tape deck to the Goodwill years ago. One more thing: the schematic shows a 47 k resistor in front of the volume control. Since you are using 100 k pots or attenuators, you already have a large input impedance. So remove the 47 k resistor and you’ll get a little more output and transparency, with no adverse effect.
10. Transformer. The original tranny in my PAS-2 seemed to be cooked and the wiring insulation was so brittle it broke off when touched. So I took, through eBay, a replacement from Triode Electronics in Chicago, the same company that built the original trannies. But they claim the latest models user thinner enamel on the wire, allowing fatter diameter conductors, and thus lower DC resistance. I can’t verify by comparative listening, however, that the replacement sounds better than the original. I once saw a toroidal transformer on eBay, marketed for use in PAS preamps, but by the time I made up my mind to act the listing disappeared. It should be worth the try if it reappears.
11. Ventillation. The cooked tranny and boards in my original units are evidence that the preamp hasn’t sufficient ventilation. About 25W is not a lot of heat, but if left on for long periods in a box that can’t breath, things inside can still get pretty hot. So I took a pencil and ruler and drew a diamond shaped pattern of grid lines on the lid, and where they intersected I drilled holes with a drill press. I don’t think that the spacing and size of the holes or arrangement are very important. What counts is there are sufficient openings for cross-ventilation.
12. Tube anchors. Mapleshade makes thick-walled brass cylinders that clamp onto tubes. They add substantial mass, which reduces microphonics, and the high thermal conductivity of brass improves heat transfer. Additionally, they have a wire connection that can be run to earth ground to drain off excess charge. This is where the ground tap of the male IEC connector comes in. The tube anchors should give tighter bass, better-defined images and more detail. I recall that they cost $18 each, and for me, it’s a must-have upgrade.
13. Tubes. The original Telefunkens are very good. JAN-Phillips 5751s work very well in the line stage, but are not recommended for the phono section where high gain might be required. This preamp is not fussy about tubes, and even those of low pedigree manage to sound good. I believe this is because the plate voltage is considerably under the original RCA specification for the 12AX7 tube.
13. Miscellaneous. I painted on a thick layer of Soundcoat to the underside of the lid and bottom plate to control resonances. Less messy stick-on Soundcoat is available. There are companies on the web that will clean up the faceplate or who will sell you a new one. I took the latter and got new knobs in the deal. It looks much better. The feet could be upgraded to various vibration dampening types. Adding brass weights to the lid, such as the heavy hats sold by Mapleshade, will yield the same sonic benefit as all vibration control measures: a lower noise floor, tighter bass, and better defined images.
This preamp allows almost unlimited upgrade potential. The simple, easily accessible layout makes that possible, and allows you to listen to each upgrade right after it’s made to evaluate the change in performance. After you have implemented many of these changes, you won’t want to let go of the preamp, because of the natural tendency to bond with something you’ve put so much effort into, and most of all, because it sounds so amazing!
I actually have two of these preamps, both PAS-2s, which are just the same as the vintage PAS-3, but with older, brass faceplate, which I actually like more. Got one that was restored on e-bay for 179, another that was unrestored on e-bay for 65. ( a deal!). The unrestored one has Telefunken tubes, which was a surprise to me, as they weren't mentioned. This preamp is simple, lively, and fun. Excellent sound for the $, excellent introduction to tube pre-amps. Many upgrades are possible, it is easy to work on if you have some soldering skill. Lots of information on the web about these preamps. Matches up well with either a solid state or tube amp, in my experience. I am running my unrestored one with an Adcom GFA-535II, and the restored one is operating with a Dynaco ST-70, that was restored. The midrange, and presence and liveliness is hard to define, but it is much superior to other solid state preamps to which I have compared it (NAD 1020, Hafler DH-101). It can be finicky, but it's old! I'm not sure how the more radical reconstructions (e.g. Van Alstine) effect the sound. I'm also not sure how taking out the tone controls effects the sound, but it is a very simple procedure (On Joe Curcio's web site).
What I purchased was actually a PAS2, which has the same circuit as the PAS3 but is a lot cuter looking if you're into art deco et al. I probably overpaid for it but it's hard to find one with a really clean faceplate, which this had.
While I knew that David Hafler's PAS design was highly respected in its day, what really convinced me to buy one was a remark by Steve Melkisethian in his Angela Instruments catalog some years back -- that he'd rather listen to a well-serviced PAS than any solid state pre extant! After listening I understand his enthusiasm. The PAS has a distinct personality and despite some rough spots, it is a classy sounding pre that is lively and musical, and its limitations should no more disqualify it than those of a Quad ELS, LS3/5a or triode power amp. While the PAS is a favorite with modifiers, I would hesitate to mess with David Hafler's design -- instead I would focus on upgrading parts and bringing everything to perfect spec -- I'd hate to lose the "PAS sound", as I experience it.
I had a lot of trouble getting my PAS up and running. At one point, the dam burst on the volume control and I was treated to instant full blast at 11PM. Anyone buying one of these things should be or have access to someone who can work on it. When things were finally straightened out I played some old lps and CDs, still angry at all the hassle I had put up with, and prepared to dislike it. Listening quickly won me over. The PAS, to my surprise, sounded quite dynamic, with lots of detail, three-dimensionality and atmosphere. Imaging was, frankly, as good as I've heard. There was warmth and liquidity and sheen, yes, but this was no old biddy designed to tame the music's savage breast; instead it sounded lively, fast and funky through the mids and treble. The bass was a little subdued, but, mine being an unmodded unit, I availed myself of the liberty of goosing the bass and treble knobs just a tad and really liked the results. It had the clarity of a modern set together with a certain life to it that solid state often misses.
I suspect that even after stuffing it with the best available tubes and caps the PAS would lack the sonic refinement of my new/used c-j PV9, but that's just a conjecture. The PAS is so agreeable to listen to -- if only it had a remote! If you've never owned tubes I suggest you take one for a test drive today!
I have Magggies driven By 2 Velemen monoblocks. When I first hooked wasn'tbad but then tubes warmed up after a while and what an amazing thing i love the sound mids and highs are clear as anything...I cant wait to replace the caps and clean it up can't even imagine what it sounds like then
a Audio Enthusiast
Date Reviewed: September 4, 2001
What can I say? It's a PAS, and it's mine ! Everything that's said in the other reviews is true as far as I'm concerned, so I won't waste your time repeating others. I recently bought the unit and plan on totally revamping it. All capacitors Philips HQ polypropylenes, Bourns pots, 1% metal film resistors, stab. heater supply, ond so on. But no way I'm replacing the tube rectifier!!! It's a great asset to the unit's sound. It does got hot however, almost boiling the high tension capacitor, but that will be replaced by a couple of 100uF's anyway.
All this should bring it up to five stars (I hope!).
Tip: if you suffer right-channel hum on a factory-wired unit (especially when using a high-gain power amp), this is caused by 2 mains wires running directly underneath the R tube socket of the line stage PCB. Yes they're twisted but still they're the cause of this. Just bend them so they run along the edge of the PCB.