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lorne
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MexicoMike
Roy Mottram
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    PAS3 RIAA mod

    Roy Mottram
    Roy Mottram
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    Post by Roy Mottram Wed May 01, 2013 2:02 pm

    this was posted 4 years ago but I'm re-posting here as a "sticky" to make it easier to find especially for our newer visitors

    If you have a PAS-2 or PAS-3 preamp and also use the phono section you may be interested in upgrading the RIAA equalization which (with stock parts) departs from the "ideal" equalization by + or - 2 dB. Also consider that the small value stock picofarad caps have probably drifted from their original values by as much as 50% in the past 40+ years which could now result in as much as a + or -4 dB drift from the ideal equalization curve. The steps are outlined below and the schematic shown at the bottom has red dots where the changes are to be made.

    1. Remove the 750pf caps from the selector switch on both channels.
    2. Remove the 27K resistors from the selector switch on both channels.
    3. Replace the 68pf cap on the PC-6 pc board with a 820pf silver mica cap.
    4. Parallel the 100K resistor with a 2 Meg metal film resistor
    5. Replace the 2750pf cap with a parallel combination of 2200pf and 470pf.
    6. Replace the 4.7Meg resistor with a 2 Meg metal film resistor
    7. Replace the two .1uF caps connected between teminals 1 & 3 (left channel) and 7 & 9 (right channel)
    with a larger .33 or .47 cap at 400 volts or higher.

    If you do the changes above your RIAA equalization should now be off no more than .3dB (3/10 of a dB) of the ideal equalization.


    PAS3 RIAA mod PAS3-RIAAmod
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    MexicoMike


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    Post by MexicoMike Sat Nov 22, 2014 10:56 am

    I read a Hirsch/Houk test of the Pas3 from the mid 60's and they commented that it had one of the most accurate RIAA curves available. Their test showed that the maximum error was .8 dB.
    Is it felt that the reason it is not accurate now is simply due to parts age? That makes total sense, of course but it seems like the original design was quite good.
    Roy Mottram
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    Post by Roy Mottram Sat Nov 22, 2014 2:58 pm

    these days it's rare to see a phono preamp that doesn't keep within 0.1db of the RIAA curve from 20Hz to 20KHz
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    alexg


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    Post by alexg Thu Dec 11, 2014 2:26 am

    i have a question on this circuit, if I increase the 0.1uf that connects the plate of the second triode to #9 to let us say 0.33 or 0.47uf and remove the 0.1uf going to the output, would it affect the equalization?
    Roy Mottram
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    Post by Roy Mottram Thu Dec 11, 2014 2:39 pm

    you can change the value of the output capacitor (between 7 & 9), but if you change the coupling capacitor, then you are
    changing the plate impedance to the RIAA EQ circuit, which will of course change the EQ
    lorne
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    Post by lorne Wed Jan 14, 2015 8:31 am

    Some years ago I began, and nearly finished, a PAS-3 that was intended to be ONLY an RIAA amplifier. Someday I hope to feed this "phono amp" to my Melos GK- 1+1 and see if it betters the internal one — which BTW is very good.

    And so someday — as other projects are cleared off the bench —  I intend to un-bury this endeavour and continue. I stumble and grope  around in the dark in regards to electronics, but sometimes I get it right. I hope that this project will be one of the successful ones.

    If there is any interest, I will post photos and details concerning what I have done so far. I think I have documentation, schematics and so on — somewhere. I seem to remember that I have already built the RIAA board.

    NOTES:Modifying PAS-3 Phono Section — (my mods as RIAA amp only).

    1) Remove the 750 pF caps from the selector — (I deleted the switch).

    2) Remove 27K R from selector — (I deleted the switch)

    3) Replace the RIAA 68 pF cap on PC-6 with 820 pF polystyrene or silver mica

    4) Parallel the 100K RIAA R with 2Meg (metal film)

    5) Replace 2750 pF RIAA cap  with paralleled 2200 pF and 470 pF

    6) Replace the 4.7 Meg RIAA R with 2Meg (metal film)

    7) DON'T remove  the 47 K feedback R's between the cathodes of each tube

    Low Frequency Roll-off Fix:

    1) Jumper the 0.1 output cap connected from the ...garbled ... of the second stages with a solid wire (both channels).

    2)Change the value of the 0.1 uF output cap (coupling cap?) connecting terminals 1 and 3 (left channel) and 7 and 9 (right channel) to 0.47 uF/ 400 VDC

    Considering the cost of tube phono amps these days — either kit or factory assembled — a PAS-3 that can be modified with quite ordinary parts a seems a very attractive option. I chose to make my PAS-3 a dedicated phono amp because I have preamps that I think trump it — IMHO. If the PAS-3 can be turned into a nice phono amp, the conservation of the essential core of a classic is work of love and conservation. Moreover, it may turn out to be a darn good phono preamp. I'm counting on it — someday Smile
    peterh
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    Post by peterh Wed Jan 14, 2015 11:33 am

    Using a PAS-3 to build a dedicated riaa amp is possible but a waste (imho) . Use a FM-1 or FM-3
    instead as these will be made extinct by the move to DAB.
    lorne
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    Post by lorne Wed Jan 14, 2015 3:13 pm

    peterh wrote:Using a PAS-3 to build a dedicated riaa amp  is possible but a waste (imho) . Use a FM-1 or FM-3
    instead as these will be made extinct by the move to DAB.
    You will please forgive me when I say that I do not follow your logic: don't use a PAS-3 for analog audio but instead concentrate on an instrument which is about to be made utterly obsolete and unusable.

    I'm sure you have an interesting comment. I hope to read your answer.

    I have two FM-3's. One of them that had played many tens of thousands of hours in Canada for 20 years came to Japan with me. I found a skilled technician who converted it to the Japanese bandwidth. He said that it was not an easy task, and he warned me to stay away from some sections of the unit in order to not upset the alignment.

    After it developed a very audible hum, I proceeded to make a cap board for the power supply. I had already made discrete PEC boards for it, which greatly improved the sound. Alas, this is yet another project parked in a box because I decided to rewire the whole radio and never got around to finishing it. In it's place there is a re-capped Pioneer TX; it is not as lush in tone but it delivers more detailed information.

    I am currently working on getting my old ST-70 back on line. The PAS-3 as a dedicated phono-amp is next. The FM-3 the next after that. But ... you could very possibly convince me to abandon the PAS, so please explain. Thanks — Lorne
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    Post by peterh Wed Jan 14, 2015 4:23 pm

    If you already have excess fm-3, of which one is broken, then it's a good suggestion to
    use one of them as chassies to build a dedicated riaa box. Pull the tubes, remove the
    mpx and build the riaa on a sheet that fit's the hole left by the mpx. The only external
    thing to add is rca-connectors, the powersupply may be used as-is.
    There floats schematics when 2 ecc88 is used giving excellent riaa performance. ( edison 1 is
    one example). Or to make it simple purchase a riaa board from tubes4hifi and you are done
    with some screws and solderings.
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    alexg


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    Post by alexg Tue Jan 20, 2015 3:24 am

    lorne wrote:
    Low Frequency Roll-off Fix:

    1) Jumper the 0.1 output cap connected from the ...garbled ... of the second stages with a solid wire (both channels).

    2)Change the value of the 0.1 uF output cap (coupling cap?) connecting terminals 1 and 3 (left channel) and 7 and 9 (right channel) to 0.47 uF/ 400 VDC

    This is similar to what I wanted to do, but tubes4hifi said that it will change the plate impedance if I replace the first 0.1uf from the plate and jumper the wire between 1&3 and 7&9.

    As quoted above, it seems that one would jumper the 0.1uf directly connected to the plate and increase capacitance of the output cap. Would soldering a jumper on the first 0.1uf connected to the plate change the RIAA?

    Thanks.
    denny9167
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    Post by denny9167 Sat Aug 01, 2015 12:24 am

    MexicoMike wrote:I read a Hirsch/Houk test of the Pas3 from the mid 60's and they commented that it had one of the most accurate RIAA curves available.  Their test showed that the maximum error was .8 dB.
    Is it felt that the reason it is not accurate now is simply due to parts age?  That makes total sense, of course but it seems like the original design was quite good.
    he

    You are quite right!! I ran the original through B2 Spice and came up with the same results!! The original topology is the best sound!! Ive been an audiophile and musician for over 25 years, and I Know what sounds good, and what looks good technically!!
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    Post by Guest Mon Oct 05, 2015 11:55 am

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 9:00 pm; edited 1 time in total
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    Post by denny9167 Wed Dec 09, 2015 10:30 pm

    PeterCapo wrote:Here's an exhaustive, enlightening and definitive "must read" regarding the classic PAS phono stage: http://www.audioregenesis.com/documents/PAS_Phono.pdf

    Norman Koren pretty much hit the nail on the head on the RIAA curve, so far as the PAS Phono stage is concerned, B2 Spice says it all, within 0.3 db I Think, pretty dang good!!!
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    Post by mark four Mon Nov 15, 2021 6:06 pm

    I have a pas 3 that needs a rebuild on the phono stage . The low level is fine but phono is wonky one channel doesn’t work . Could someone recommend a service. I am in Philadelphia , pa .
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    Post by carcy Fri Jan 20, 2023 2:28 pm

    Hello everybody !
    First excuse my poor english... I'm french citizen !!! I read your forum since a long time and I wanted to join you in this beautiful adventure of tube !!
    I started a dynaco pas3 preamp restoration... Many many many datas in this forum !!
    the use of the pas3 will be for me, the phono stage, coupled with a st70.
    Then i'm interested with this section PAS3 RIAA mod...
    That's why I've some questions on the PC6 board...
    First, you say to replace the 4M7 resistors with 2M R metal film... OK... but 2M2 will be ok ?
    Second... We have to replace 2 caps 0.1µF with wire... The caps are the ones shown in red on the photo below ? right ?
    Third... We have to replace 2 0.1µF caps with 0.47µF/400V caps... They are shown in green on the photo... Is it OK ? what is the best cap to use ? MKT ? MKP ???
    Third : about the 100K resistors... The one to replace with 95.3K, are shown in orange... Always OK ???
    Fourth... The 2 others 100K to parallel with 2MEG resistors are indicated in blue. True or not??? Always the same question with 2MEG resistors... Will it work with 2.2MEG resistors ? or can we replace directly these 2 100k resistors with 2.2MEG resistors ?
    Finally (not shown in my photo... Can you confirm we have to replace the 2 4.7MEG resistors with 2.2MEG ones ?
    PAS3 RIAA mod Riaa10

    Many many thanks for your help !!!
    Best regards
    Jean-Luc
    Roy Mottram
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    Post by Roy Mottram Fri Jan 20, 2023 2:55 pm

    Carcy,
    refer back to the #1 post at the top of this thread for correct instructions.
    The 2M resistors in parallel with 100K make 95.24K, using 2.2M with 100K makes 95.65K, a 0.4% difference, OK.
    Use any good quality PP type capacitor, MKP is fine
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    Post by carcy Fri Jan 20, 2023 3:03 pm

    Thank you for your answer !!!!!!!
    I'll do that... OK for the resistors in parallel !
    BUT
    I'm confused about the values of the Big Black capacitors on the PC6 board for a correct RIAA.
    There are many different values on google...

    According to you, we have to let A caps in place on the board ? WE DO NOT HAVE TO SHORT WITH WIRE OK (as lorne say above "1) Jumper the 0.1 output cap connected from the ...garbled ... of the second stages with a solid wire (both channels)."?

    We have to replace the caps named C (blue squares) with 0.47µ 400V or 630V OK ?

    Then, what is the value of the B capacitors ? 0.05µF or 0.1µF or other value ???

    PAS3 RIAA mod Capa10


    After that, all we be ok for me !
    Thanks for all !


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    Post by carcy Mon Jan 23, 2023 7:26 am

    Someone to help me please ?
    We change caps C with 0.47µ (400 or 630V), and we let caps B (or change with new ones) 0.047µ (400 or 630C) ?
    Is it right ?
    Thanks guys !

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