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    PAS3 RIAA mod

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    tubes4hifi
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    PAS3 RIAA mod

    Post by tubes4hifi on 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.



    MexicoMike

    Posts : 28
    Join date : 2014-11-21

    Re: PAS3 RIAA mod

    Post by MexicoMike on 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.

    tubes4hifi
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    Re: PAS3 RIAA mod

    Post by tubes4hifi on 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

    alexg

    Posts : 7
    Join date : 2012-02-24

    Re: PAS3 RIAA mod

    Post by alexg on 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?

    tubes4hifi
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    Re: PAS3 RIAA mod

    Post by tubes4hifi on 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

    Posts : 4
    Join date : 2015-01-10
    Location : Sendai, Japan

    Re: PAS3 RIAA mod

    Post by lorne on 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

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: PAS3 RIAA mod

    Post by peterh on 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

    Posts : 4
    Join date : 2015-01-10
    Location : Sendai, Japan

    Re: PAS3 RIAA mod

    Post by lorne on 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

    peterh

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: PAS3 RIAA mod

    Post by peterh on 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.

    alexg

    Posts : 7
    Join date : 2012-02-24

    Re: PAS3 RIAA mod

    Post by alexg on 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

    Posts : 152
    Join date : 2011-05-09
    Age : 49
    Location : Texas

    Re: PAS3 RIAA mod

    Post by denny9167 on 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!!

    denny9167

    Posts : 152
    Join date : 2011-05-09
    Age : 49
    Location : Texas

    Re: PAS3 RIAA mod

    Post by denny9167 on Tue Aug 04, 2015 6:17 pm

    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.

    cheers

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: PAS3 RIAA mod

    Post by PeterCapo on Mon Oct 05, 2015 11:55 am

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

    denny9167

    Posts : 152
    Join date : 2011-05-09
    Age : 49
    Location : Texas

    Re: PAS3 RIAA mod

    Post by denny9167 on 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!!!

    Peter W.

    Posts : 79
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: PAS3 RIAA mod

    Post by Peter W. on Wed Aug 10, 2016 2:13 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.

    Never going to happen. First, there is far to much legacy equipment out there for FM to go away.
    Second, Many (but not all) digital OTA sources are pretty wretched. And there is no real incentive for that to change.
    Third, the cost of conversion will be far too high for the tens of thousands of small stations out there. Keep in mind that this extends all the way down to Drive In theatres - a growing trend, at least here in the East.
    Fourth, Great Aunt Esmeralda will not let this happen. Remember that she delayed Digital Television for two (2) full years even though it was (and remains) a vast improvement over analog TV - which DAB is not. If you do not grasp the analogy, I will explain if necessary.

    Now, the same people concerned over an RIAA pre-amp will also likely be hard-core FM users as well.

    Now for the 'heresy' part:

    0.8dB is fine for adherence to the RIAA curve. Closer does not necessarily mean audibly better. There are those who would argue that a tube phono-pre will be better than a SS unit. And those that would argue otherwise. But, if tubes are the basis of comparison, 0.8 dB to curve will be just fine.

    Writing entirely for myself and my fully tweaked PAS3x, and other legacy equipment, from Dynaco to Revox, tube and solid-state, I find the Revox A720 to have the best on-board phono-pre-amp, followed by the Dynaco PAT5 biFet, followed by, actually tied with, the AR Receiver (or integrated amp). I have an HK Citation 17 that is no slouch either. After all of these, comes the PAS, even equipped with mil-spec. Sylvania 5751s. The greater issue for me is that the first three listed above will also accept most (but not all) MC cartridges without an outboard amp or transformer.

    End Heresy.

    Peter Wieck
    Melrose Park, PA

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