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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products

    PAS-3 upgrades



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    PAS-3 upgrades

    Post by tubes4hifi on Wed Apr 08, 2009 5:26 pm

    I thought this was old old OLD news, but I still get this question almost every week . . .

    > I have a PAS-3 I'd like to upgrade, boards,switches, volume, jacks, etc.
    > what are the sonic differences between the original pc-5 & pc-6
    > boards as opposed to your sp-6 and sp-8 boards ?
    > also do you sell the boards already built up for a nominal fee ?
    > and do any of these board still retain the tone controls ?
    > OR is it also possible to ship you my pre-amp and have
    > these upgrades done ?

    And here is the answer again for anyone interested . . . .

    see my PAS3 upgrade page and see my SP8 preamp page (links at bottom of message)

    Also as I've posted elsewhere on this or some other Dyna forum, I've owned more than a dozen preamps over the past dozen years, and although some people will disagree with me, the PAS3 sits
    near the very bottom of the list. Moving up the list in both price and quality, are all of the classic CJ preamps (PV5 thru PV12), Audio Research SP6, SP8 thru SP11, Transcendence Grounded-Grid, Tubes4HiFi SP6 or SP8, and the Glassware AIKIDO. I believe the Foreplay is overpriced and although better than the PAS-3, certainly not worth it unless you can buy a used one for half price.
    On a value basis, especially if you are looking for a preamp WITH a phono stage (rather than just a separate line stage) I know of NO other preamp kit under $1000 that matches the Tubes4HiFi SP6 or SP8.
    As for tone controls, most modern preamps do not use them as they have a detremental effect on the quality, and need an extra gain stage to make up the 20db or so of signal losses imposed.
    I do have a PAS3 upgrade that includes a premium quality line stage with tone controls.
    I can supply finished PCBs or KITs ready to install in the chassis of your choice.
    See these pages for more info . . .

    PAS3 upgrade page
    SP8 preamp page

    Last edited by tubes4hifi on Sun Mar 04, 2012 3:11 am; edited 2 times in total (Reason for editing : updated links)


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    Re: PAS-3 upgrades

    Post by Bugs on Fri Apr 17, 2009 2:01 pm

    Does the PAS 3 have an achillies tendon or is it a general design problem?



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    Re: PAS-3 upgrades

    Post by tubes4hifi on Sun Apr 19, 2009 1:43 am

    Like the ST70, the PAS2 and PAS3 preamps were designed to meet a budget price, and had alot of compromises necessary to keep the price down, however, both products were best sellers and opened up a huge market for tube hi-fi not just 50 years ago but continuing today.
    The PAS preamps have 3 main shortcomings
    1 - marginal power supply, which also is the limiting factor in the design
    of the phono and line circuits.
    2 - phono circuit has inaccurate RIAA, and very high output impedance
    3 - line amp circuit has very poor tone controls, and very high output impedance
    4 - poor quality selector switch with lots of extra wiring and equalization circuits
    not needed - All of these problems have a fairly simple fix
    1 - replace the original QUAD cap and filament supply
    2 - replace and change resistors and capacitors in phono circuit for improved RIAA
    3 - remove tone controls from the circuit
    4 - remove and replace the selector switch
    These changes will help alot and can be done for under $100.
    In my personal opinion, this will improve the sound from a 1 to a 2, on my scale of 1 to 10.
    You'll still wind up with a 50 year old preamp with poor quality RCA jacks, poor quality
    volume and balance controls, and high output impedance.

    Bob Latino

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    Re: PAS-3 upgrades

    Post by Bob Latino on Sun Apr 19, 2009 9:52 am

    I agree with Roy here. Of the Dynaco preamps and the Dynaco amps - the amps were better designed and require less effort and money to bring up to today's standards. You have to remember though that both the preamps and the amps were designed in an era when digital music was nonexistent. The main signal source at the time, vinyl records, have no where near the dynamic range of CD's, SACD etc. Records usually max out at 50 dB while a CD has a theoretical dynamic range of 96 dB. 24 bit digital sources can have a 144 dB theoretical dynamic range. In order to deal with these types of dynamic ranges you need a power supply that can respond quickly with large current swings. The original power supplys in all the Dynaco tube gear were way low in storage capacity. The original ST-70 had a 30, 20, 20, 20 Mfd quad cap with one of the 20's trying to power TWO output ransformers. There is no way that 20 mfd can has enough DC power storage to power two output transformers at higher volume levels. I can tell you a story that illustrates this. I use to play a good vinyl recording of "Also Sprach Zarathustra" back in the 1970's but everytime I tried to get any volume out of my ST-70 I would get a crackling noise on every beat of the tympani drum. Must be bad tracking I thought so I increased the tracking force on the cartridge. It didn't help. I just couldn't play that amp at volume levels that even approached a normal symphony level. One day my audio friend Paul brought over his new Audionics 60 WPC solid state amp. These amps were made in Oregon and were "state of the art" (for the late '70's) solid state amps. We put it in place of my ST-70 - WOW - this thing could play loud and now no crackling noise on the drum strikes at the same and higher volume levels that the ST-70 played at. The 32 Hz note that starts the piece that I could barely hear on the ST-70 also had more impact now. The down side of the Audionics amp was when I played some of my acoustic jazz and vocal records. The ST-70 destroyed the Audionics amp (IMHO) in the mid range and it soundstaged much better - the sound was more holographic with the ST-70 through my Infinity speakers. Frank Sinatra sounded much more real with the ST-70. The Audionics sounded flat with less depth and width to the image. There are just two realtively simple things you can do to bring the Dynaco amps up to where they are better able to deal with digital music sources.

    1. The power storage issue with the ST-70, Mark II, Mark III or Mark IV can be vastly improved by pulling the original quad cap and replacing it with an 80, 40, 30, 20 cap from Dynakitparts. The 80 mfd section will now power the output transformers instead of a 20 Mfd section. Your amp will be able to play louder with less train. Cost > $50
    2. Pull the driver board and replace it with the VTA driver board. You also now have a new and far superior bias system than the original amp. You are also not dealing now with a circuit that was designed by "bean counters" at Dynaco to save a buck and whose implementation gave all original Dynaco tube amps a slow and mellow characteristic that is not particularly accurate. Cost > $90

    For $140 you have an amp that will sound much better and an amp that will play louder with less strain.

    A PAS preamp can be improved somewhat as Roy has mentioned above. Even if you do the 4 changes he mentions, however, the general character of the amp will not improve that much. There are many Dynaco dudes that love the sound of a PAS preamp (slow, mellow but not particularly accurate) because it can gloss over other inadequacies in their music system. The other thing you should consider is that if you do alter your PAS preamp you will probably devalue the preamp should you ever decide to sell it. Totally original PAS preamps always sell for more than "improved" PAS preamps on Ebay. People are usually suspicious of the "improvements" and would gladly pay more for a preamp that has not been altered. It is usually more cost effective to just sell your unaltered PAS preamp and put the money towards a modern used preamp from Audiogon or build yourself a new preamp kit like the SP-8 kit from tubes4hifi.



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    Re: PAS-3 upgrades

    Post by fortherecord on Tue May 26, 2009 10:56 pm

    I've been switching my different preamps (Dyna PAS 3, Audible Illusions Modulus 3a, Music Reference RM 5) in and out of my system this week. So far, my favorite match with my Dynaco ST70 is the Music Reference preamp which utilized one 6922 in the line stage. I used to have a CJ PV 6 but sold it a couple years ago. With the gear I had at the time, it seemed to be too dark and opaque sounding. I borrowed a friend's CJ PV 5 once and loved the sound of that. I don't see many PV5 up for sale these days. BTW: my ST 70 with new Sound Valves board and upgraded power transformer is my favorite amp right now, even compared to my MR amps. It is much more detailed.


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    C-J preamps

    Post by tubes4hifi on Fri May 29, 2009 1:20 pm

    I've had dozens of preamps over the past twenty years, and I must say that I do like the Conrad-Johnson line, and the used classics are generally a bargain. Especially the PV5, PV6, PV8, PV10, and PV12. You don't see these for sale often as most people keep them, but I have seen and bought all of these for prices between $400-700.


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    Re: PAS-3 upgrades

    Post by myaddiction on Thu Aug 20, 2009 9:39 pm

    So how do I lower the output impedance on my PAS3?


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    Re: PAS-3 upgrades

    Post by GP49 on Fri Aug 21, 2009 11:55 am

    myaddiction wrote:So how do I lower the output impedance on my PAS3?

    Major rebuild such as adding a cathode follower. And then you would be excessively loading down the stock power supply, including the transformer.

    The main audible consequence of driving a too-low input impedance...rolled-off bass...can be ameliorated somewhat by greatly increasing the coupling capacitance from the last tube in the linestage. But that won't cure a major mismatch. Also the same problem exists for the phono stage trying to drive the TAPE OUT connected to a typical transistorized cassette deck, CD recorder, computer line input for burning CDs, or wireless headphone transmitter. Such a low-impedance load will roll off the phono section's bass response. A lot of people do that and wonder why their phono sounds bass-shy compared to CDs; the rolled-off bass will be apparent not just in any recordings that are done, but in normal playback too.

    The PAS was designed when such impedance considerations did not exist in any high-quality audio system. There were no good transistor amplifiers, and it was intended for use with tube amps.

    Tube Nube

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    Re: PAS-3 upgrades

    Post by Tube Nube on Fri Aug 21, 2009 11:15 pm

    I'm almost afraid to ask this, for fear that the answer will make me order an SP8 by noon tomorrow...but, say, Roy, how does the SP8 compare to the Transcendent Grounded Grid pre-amp.

    I know you're in a position to have an informed opinion on this, if I'm not mistaken.



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    GG v.s. VTA SP8

    Post by tubes4hifi on Sat Aug 22, 2009 2:01 am

    Hi Brenton,
    I've probably tried every tube amp and preamp available for under $1000.
    When I first heard the Grounded Grid preamp, I was pretty impressed, especially since it's available as a chinese clone kit for around $100, of course a full DIY project preamp is still gonna run over $400, and you can buy the full kit ready to go for around $500.
    I rated it an 8 or 9 on my scale of 1 to 10. At the time, I was rating my SP6 kit as a 7 or 8.
    But then I had about a dozen of my own customers write back telling me my SP6 kit sounded better.
    I had one customer who actually bought several hi-end classic preamps, including some Marantz and McIntosh gear, and Conrad-Johnson, and Audio Research. I loaned him my Grounded Grid, and my own VTA SP6, and he told me they all sounded good, but none of them had the "magic" sound he was looking for except my SP6 preamp, so he sent the GG pre back and bought my VTA SP6.
    I've made several improvements to the circuit since and upgraded several of the component parts, now selling as SP8 with regulated power supplies, and available with h-end resistors and capacitors in a "Special Edition" package. I currently have an extra SP8-SE available for sale ready to go. I also have a repeat customer selling his current SP6 at an absolute steal of a price so he can upgrade to my SP8-SE version.
    The OTHER determining factor here most people overlook is that all those other preamp kits are ONLY line stage. My SP6 and SP8 kits are both LINE stage AND PHONO stage, with integrated power supply all on one compact PCB.

    Tube Nube

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    Re: PAS-3 upgrades

    Post by Tube Nube on Mon Aug 24, 2009 11:13 am

    Hi Roy,

    Thanks for this explanation. Sounds like it would have been better value for me, had your kit been available last Christmas (please confirm for me it wasn't!), since I also got the Transcendent phono stage to go with my GG.

    I'm tempted to try your SP8, but maybe should restrain myself for the time being. It's too bad, in a way, we're all scattered so far and wide. Can't get folks together so easily. Then again, were in not for the modern marvel of the WWW, I'd have never found my way to my VTA - ST 70!

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    Re: PAS-3 upgrades

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