10 March, 2018

Dynaco PAS 3 Mods

Click pic to enlarge. L: Slightly modified PAS 3.

Dynaco PAS 3, Part II
Letter from NYC (84) 2018 (10)

Revised Mar 11, 2018.

PAS 3, Part I saw the stock Dynaco PAS 3 ran for may hours. The sound then was remarkably detailed at low volume, but stressed when pushed harder. While mods were definitely on the agenda, as is my wont I decided to go against the fashion of sea change (replacing everything) and start with a minimalist approach. First, I researched on the internet, but found the documentation of sonic changes to be quite lacking, even contradictory (as opposed to electrical changes, for which most would agree on). I ordered just a few caps and resolved to do one mod at a time to hear the individual impact each has on the sound. Yes, I heard enough potential to think my efforts would be repaid. They were, amply.

Tone Control Mod Dilemma
Most people bypass the tone controls, which is not hard to do. One popularly discussed method is the Curcio Tone Control Bypass. It seems, however, this increases the DC present at the PAS 3 Output, which can be resolved by adding a 0.1 Output Cap, as done at audioregenesis. As I did not have the requisite caps, for the moment I decided to leave the tone controls in the circuit.

Now the Mods I have done this time, numbered in order and rated in efficacy from * to ***:

1. The Amazing $0.25 Non-Electrical Mod (***)
While waiting for the caps to arrive, I thought about how to improve the preamp. Staring at the innards, I saw both PC Boards (PC-5 and PC-6) were affixed to the chassis by 4 screws, bolts and washers. Since the chassis also has the transformer directly bolted on it, the idea of providing some measure of isolation against vibrations came to me. This did not come out of nowhere. Similar tweaks can be found in some modern manufacturers, but also in vintage equipment (e.g. the Marantz 7 has some isolation for the tube socket mounting plate.) As I did not have rubber washers around and as I was working on some Gotham DGS-1 cables, I slotted in small cross sections of the springy PVC jacket between each screw and the PC board, and between the chassis and transformer. The length was not enough to completely encircle some screws, particularly the ones fixing down the transformer, but that would have to do for the moment. I figure, proper rubber washers would cost you less than 25 cents. With just this done, I sat down to listen. !Oh My! The difference was unbelievable! Not only did the sound smooth out a bit, I now could play significantly louder before the sound tightened up. No, it did not completely cure this particular ill, but it went quite a way towards ameliorating it. Even if you have already done all your mods, I can still confidently recommend this easy and reversible step. Do keep in mind that the PC Boards are a little looser than before and a little more care should be exercised when swapping tubes, particularly during insertion (use gentle wiggles).

2. Output Cable Change (*1/2)
The stock hook up cables are tinned solid cores. As I usually prefer stranded cables in interconnects, I decided to first change the high level cable from the output tube to the main out. I swapped in a run of Gotham 2-conductor GAC-2, one of my reference interconnect cables (center run in pic). The copper shield was not connected at either end. Compared to the above, the result was a small (but audible) improvement in the same qualities.

3. $3 Capacitor Change (***)
I replaced the two 0.22 Output Coupling Caps (the green globs seen in the Part 1 pic) with German Audyn (Parts Express, bright red ones in pic). I wanted to also replace the two 0.02 caps that couple the two sections of the 12AX7, but that would have to wait as I don't have the parts, and the original Black Cats are not bad in my previous experiences. The Sonic Improvement was Dramatic, on par with the isolation measures above. Considering that there were only two cap changes, the degree in improvement is quite amazing! As the tone control circuits, which employ all Black Cats, are still in-circuit, this strengthens my belief that they are not as nearly as bad as they are often made out to be.

4. Line Input Cable Change (*1/2)
Emboldened, I then re-cabled with GAC-2 one of the Line Inputs ("Spare") and the same small incremental improvement was audible. Note that I only replaced the segment immediately leading out from the Input Sockets (the longest, see pic). The Selector has three gangs, and it would be a major task to replace all wires going to the PC Boards and Switches (it would be simpler to install a new selector).

5. Phono Input Cable Change (**)
I then made the same cable change to the Phono Input, though this time I used the shield, which was tied at the Input End only (not in the pic, as this was done after). The sonic difference was more substantial than cable change in the line section, resulting in a smoother sound and quieter background. However, I did notice a very subtle drop in the presence factor, but that did not worry me as the PAS 3 is super strong in this department.

  • Chez Moi As reported in Part I, the PAS 3 always had excellent transient speed and great presence. The challenge was to get it to smooth out, and it was immensely gratifying to hear it do exactly that as the mods progressed. After the Cap and Line Input Cable Change I went through the CDs I used in the last visit to R (here). I knew my job in the line section was complete when I tried out Masekela's Stimela; yes, that crescendo was perfect; the train imitations were strongly rhythmic; and the loud cries were heard without strain. After the Phono Input Cable Change I listened to cuts from Tchaikovsky's Swan Lake (Ozawa/BSO, DG), which had previously sounded strained, and all was well. Finally, it was time for the real litmus test - the two LPs I heard last time at Andy's (here) that so impressed me. Moravec's Beethoven (Connoisseur Society LP) showed considerable dynamic range, and the inner voices were clear and vital, the atmosphere charged. The PDQ Bach LP (Vanguard) was very lively; though occasionally the recording still appeared a little hot, it was quite listenable and never strident. The PAS 3 now joins my Reference System II, which has just been completely revamped (sidebar)!
  • Chez Andy As with most of my projects, I took it to Andy's for a final assessment. Kevin was also present for the excellent lunch of rack of lamb. In his system the PAS 3 sounded slightly brighter than usual but with less grain than the Citation I even when pushed hard. Throughout the afternoon, we played many records and everything sounded very good. Yes, replay of the PDQ Bach and Beethoven were just as beautiful as before, but it was the album below that deserved the limelight. Project Accomplished!
  • Radka Toneff Transformed Most amazingly, Andy played his original pressing of her mostly digitally recorded Fairy Tales (Odin, LP). Her rendition of The Moon is a Harsh Mistress needs no introduction to audiophiles, but although I have probably heard it close to a hundred times in the past two decades, I have never taken to it, or understood what the fuss was all about. This time, however, the fine modulations of her voice were more impressive and I actually managed to hear several other tracks. Most importantly, I finally heard at least some emotion and charged atmosphere, which were devoid from all the CDs and 2015 re-issue LP I have heard in other systems in HK. Incidentally, a new re-issue is just available and the April Stereophile (not online yet) has a story on it by Atkinson. Apparently, the 2015 issue was flawed technically (see this google translated link). I note too that I had also heard the original LPs in quite a few very expensive systems in HK, but never to this effect, attesting that the horn reigns supreme.
  • PAS 3 Then and Now The transformation of the Dynaco PAS 3 was utterly amazing. Indeed I had to ask the same question that I asked after I brought my Citation I back from obscurity: Why had I not appreciated it before? Likewise, there is no easy answer. Horns perhaps just show everything in a new light. For more musings on this subject, see here.
  • Thought on PAS 3 Mod As you have read, my mods are minimal, cost almost nothing and can be accomplished in little time. I believe the isolation work is very effective. Also, before you decide to embark on a wholesale mod, ponder this: as I had changed only two caps, I believe the original Black Cats are no at all as bad as they have often been made out to be. For the moment, I do not have that great a desire to do further mods. It is already an excellent preamp and can fully stand alongside the Citation I.

1 comment:

  1. Thank you again for posting the food & wine pics. The omelette and homemade sausage looked scrumptious in your last entry as well.