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 run 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.

Impressions
  • 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.
Wunderbar!

16 comments:

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

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  2. Great article. Hoping you are still around to answer a question. I opened up ky pas3x to install the rubber washers and first thing I noticed is that my two boards are connected via rivets instead of screws like yours. I added washers on the transformer but would need to drill out the rivets.

    Given that I have already damped vibrations at the transformer is it worth drilling the rivets?

    Already enjoying a sonic improvement from the two washers. It doesnt sound like an antique anymore!

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    1. Thanks for the comment! I am ALWAYS here! Even more so during this damn lockdown, and even more looking forward to any interactions!

      Well, I really can't answer your question as I did the isolation thing in one stroke. But I would venture to guess, as your own observation tends to corroborate, isolation of the transformer probably makes the major contribution.

      Drilling out the rivets sounds like a major pain and is not without risk, so I personally would not do it. But should you see some clearance between the PC Boards and chassis you may experiment on slipping some thin slivers of rubber in between. That may help a bit, but I don't know.

      In any case, it is gratifying that someone agrees with me on that tiny mod! Thx for the feedback.

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  3. Yeah that rivets are pretty tight. Not much room to wiggle anything in between. Not sure I want to drill them out and ruin the "stock" quality of it. Guess I'll move on to replacing caps and wires.

    To me it sounded less compressed after isolating the transformer. But also a little stretched like the same amount of sound was filling a bigger space. Not sure if that makes sense! Hoping the caps will restore "presence".

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    1. ya, those 2 caps make a big difference. Enjoy this bargain of bargains!

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    2. Also, I didn't do it, but isolating the selenium rectifier may have some benefits. If you do it, give us a feedback.

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  4. AnonymousJuly 07, 2020

    I did another round of changes this weekend. First I used rubber washers on the long screw that holds the rectifier to the chassis. Given how painful it was to thread all that stuff together (without desoldering everything) I want to say it made an improvement but honestly I couldn't prove it in a blind test. I'd give it an imaginary star because it made me feel better that I'd done everything I could.

    Second I replaced the two .22uF caps with Jentzen caps and wow! I don't think I have ever really heard what NHT C3's before. Soundstage opened up, clarity improved, I think you said this (or maybe I read it elsewhere) it has the warmth of a tube amp with the precision of solid state. Before these mods I was disappointed in the Pas. Stock to me sounded compressed and grainy with limited frequency range. Worst of both worlds. Maybe my caps were just in bad shape.

    One day I may replace the power supply with a modern board. For now I'll be moving on to improving the ST-70 because I'm pretty happy with the Pas3x with your simple mods. Thanks for this article!

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    1. And I am so glad you came back to tell us! The ST-70 is a sweet amp. A lot of people mod it but I think it sounds pretty good stock (I particularly love its smooth treble). It will never be everything to everyone (what amp is). Personally I'd look for some isolation, though effect will be less than with the preamp. If the input board is screwed on, this can be done. Perhaps u can do this to the choke inside the chassis too. I'd leave the power and output transformers alone. Personally at most I'd change a cap or two rather than rebuild using any of the aftermarket boards out there. Do report back your AT70 efforts.

      Is your ST70 with the earlier transformers? They are SIGNIFICANTLY bettersounding than the later Japanese ones. I have had both.

      Oh, while we are here, the Dyna EL84 amps are FABULOUS and even better sounding because EL84 is simply an even better output tube (if your speakers are sensitive enough) and one of my favs. Even the more common SCA35 is a fabulous integrated!

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  5. AnonymousJuly 08, 2020

    Yeah it has the A470 transformers. Just curious, why do you say to leave the output transformers alone? My first plan was to isolate those. Mine has a common problem of a "gong" sound when I turn it on due to transformers shaking when first powered up. I'm pretty sure they continue to vibrate once stable and generate some audible hum.

    I do think it sounds good stock. I don't want to modify more than necessary. To start I was going to add or replace the cap across power leads to reduce the snap at power-on and isolate the transformers.

    For the st-70 and pas3x I mostly just want them to be welcome to use instead of feeling like antiques.

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    1. I said that because the transformer laminations are not always held together tight and I was afraid if not done properly it could lead to worse sound. I could be wrong about this.

      The "gong" sound is almost certainly from the power trans and could be related to the capacitance issue. Many older power trans develop such problems, particularly at turn on. I don't know how much the vibrations will affect the trannies. My thinking is the trannies are heavy and may not be so easily damped. If you think it can be easily done then it is worth a try. But I think you need sturdier rubber washers (but not too hard). Whatever you do, report back as I am very curious. You don't know how much your feedback on the PAS means to me!!!

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    2. AnonymousJuly 09, 2020

      Looks like you are right, in this article https://audiomods.datsunzgarage.us/st70/ the guy only isolates the main transformer.

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    3. Boy, thanks again for this interesting link! I am glad there are others doing these isolation work!

      BTW, don't worry about the 7199. Used ones are often seen on Ebay, and there there are 6U8/7199 adaptors which one can use!

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    4. AnonymousJuly 10, 2020

      Managed to get some washers under the main transformer sandwiching the chassis. Since I wanted everything cold I didnt listen right before the change so it's hard to say about any sonic improvement. I believe it improved clarity but probably wouldnt stand up to blind test.

      Also when I was working on it I noticed that the nuts were loose on top and below the chassis already. In adding the washers I tightened everything. Hard to say which change helped.

      Bolts on the board were too short to add washers so that change will have to wait.

      Bong sound still therebut happens when turning off now. Go figure. Maybe adding the cp will help.

      Overall i would do it again but verdict is out on benefits.

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    5. Thx for reporting back!

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  6. doctorjohn, really appreciate this post as well as part one as I have been contemplating buying an old Dynaco PAS and slowly restoring and upgrading it. Unfortunately it seems many others have had the same thought since the price for even a used one in a bad shape is very high now. With that in mind, do you feel it is still worth going the Dynaco path or buying one of the many preamp kits out there (for example Elekit TU-8500, Tube4hifi SP10, etc.). I know this is an open-ended question and without building and comparing these preamps in A/B tests it's not possible to tell which would do better, but my question is more in regards to the Dynaco PAS-3 sound quality and the possibilities for incremental component improvements. Appreciate any thoughts you may have on this, and have a great day.

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    1. Ah, sorry for late reply. I am not sure how to answer that. I'd say just be patient and wait for a cheaper unit to rebuild. Sonically I'd say a good PAS shall edge out the Elekit. Don't know about the Tube4hifi. Take your time is the key.

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