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    Dynaco PAS2 too quiet with distorted sound

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    gdlen1

    Posts : 17
    Join date : 2012-03-18

    Dynaco PAS2 too quiet with distorted sound

    Post by gdlen1 on Fri Sep 07, 2012 6:13 am

    Hi everyone, my first post here. I just finished adding a zmod line stage to my PAS2 preamp, I'm 99% sure I have soldered all the parts in their proper place,as the instructions were relatively easy to understand, here's where it get interesting, when I connected it and turned it on the sound was very quiet and when I turned it up audible distortion could be heard in the music and as it gets louder the distortion increases, now i'm not sure why I tried this next bit but when I take the tubes out of the phono stage ( which is still the stock one) the sound goes back to a normal volume with no distortion, although a loud buzz can be heard when there is no music playing (possibly a ground loop). when I put the tubes back in the phono stage the line stage goes back to low volume with distortion. I have the zmod phono stage as well but I was intending to do the line stage first and live with for a while to see how it sounded. The tubes in the line stage are webcor (mullard) 12au7a I bought off ebay, unfortunately I dont have any other 12au7's to try although I still have the original dynaco 12ax7's from the stock line stage, if anyone can help it would be greatly appreciated, this is one of my first hifi mods, I have already done the regulated power supply for the PAS2 and the preamp was working fine before I put the new line stage in, and I would like to find out what I have done wrong Sad

    gdlen1

    Posts : 17
    Join date : 2012-03-18

    Re: Dynaco PAS2 too quiet with distorted sound

    Post by gdlen1 on Sun Sep 09, 2012 4:42 am

    anyone have any ideas about this?, would really appreciate the help, thanks

    gdlen1

    Posts : 17
    Join date : 2012-03-18

    Re: Dynaco PAS2 too quiet with distorted sound

    Post by gdlen1 on Tue Sep 11, 2012 7:16 pm

    I think I may have solved the problem, r23 (100k ohm) on the regulated power supply is burned out and the 280vdc is not getting through to the line stage, not sure what happened there but will replace and measure voltage. anyone know of any reason this might happen? I can't see any other burned out components on the board and all other voltages are fine and reading correctly.

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: Dynaco PAS2 too quiet with distorted sound

    Post by tubes4hifi on Wed Sep 12, 2012 1:58 pm

    R23 should never burn out, it has less than 5 microamps going thru it.
    Replace it and measure the voltage drop across R3 (the 100 ohm on the output) and get back with me, that should be about 1v.
    Maybe you've got Q1 or U1 installed wrong?

    gdlen1

    Posts : 17
    Join date : 2012-03-18

    Re: Dynaco PAS2 too quiet with distorted sound

    Post by gdlen1 on Thu Sep 13, 2012 5:15 am

    hi, I have replaced the resistor and it measures just under 1v and I am now getting the approx 280vdc to the line stage, nothing else has blown , all is working, but I am getting a very audible ground loop happening. just to clarify, there are two ground wires from the output of the alps volume control connected to the input grounds for each channel on the pc-5 line stage, the output ground wires for each channel are connected to the common ground on the original i/o board ( not sure if this is correct) and the ground from resistor R23 on both channels are connected to the mounting screws near the resistors ( again not sure if this correct but not sure where else to connect them) in the last picture of on the "Dynaco PAS-3Z preamp modification" webpage it doesn't look like there are any of the resistors still there on the original I/O board, I have all of the ones that were there still connected, could this be the cause of the hum? I will try a few things and see what happens. Thanks for your help.

    tubes4hifi
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    Re: Dynaco PAS2 too quiet with distorted sound

    Post by tubes4hifi on Thu Sep 13, 2012 12:57 pm

    that all sounds correct. Is the hum there all the time or only when using phono?
    From your description, it seems one or both boards are using too much current.
    You checked R3 so the line stage current seems OK.
    Now check R4 to see how much current you have going to the phono board (V/R=I)
    I think the value of R4 was listed at 10K which may be too high, as the phono board uses 5ma, so 10K would drop 50v.
    A 4.7K would drop around 23.5v.
    Also maybe you still are using an old pilot lamp instead of an LED? A pilot lamp uses 120ma, almost as much as a tube does.
    An LED would only use about 5-10ma giving you more filament voltage and current.
    Do you have the ground wire on the power supply connected to chassis ground?
    Also you can refer to photos of the power supply installed into a PAS3 (shows two different PAS3, one stock, one with Zmods)
    www.tubes4hifi.com/PAS-REG.htm

    gdlen1

    Posts : 17
    Join date : 2012-03-18

    Re: Dynaco PAS2 too quiet with distorted sound

    Post by gdlen1 on Thu Sep 13, 2012 4:41 pm

    hi, I am getting approx 228vdc to the phono stage and a voltage drop of 27v across R4. I do have the original pilot light running and also the the house ground connected to the chassis. so far though, sounds pretty good, certainly better bass and impact than the original ( which sounded pretty transparent in it's own right) I'm going to disconnect the house ground from the chassis to see if the hum goes away, but I would prefer to find another solution for safety reasons, the hum is there on all inputs all the time and does not change volume, I Have not played any vinyl recently but when switching to the phono input there is slightly more LF hum

    tubes4hifi
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    Re: Dynaco PAS2 too quiet with distorted sound

    Post by tubes4hifi on Thu Sep 13, 2012 8:33 pm

    you will always have more hum and/or noise with phono input since it has another 40db gain over the 10-15db gain of the line stage.
    Let me know if your AC house ground makes a difference, that CAN cause a ground loop.
    I like to eliminate alot of possibilities for that by plugging ALL Hi-Fi components into one power strip, rather than individual components plugged into several different outlets. Also if you don't have a 3 wire plug you can try reversing the direction of the 2 prong plug.
    228v to the phono is perfectly fine, it will run fine from around 220vdc up to 270vdc.
    Also try temporarily disconnecting the pilot lamp (either take it out or remove one wire) to see if that makes any difference.
    Make sure all your wiring is short and not running all over the place (spaghetti !!) and lays close to the chassis.
    Any photos? You can send to me here: info@tubes4hifi.com

    gdlen1

    Posts : 17
    Join date : 2012-03-18

    Re: Dynaco PAS2 too quiet with distorted sound

    Post by gdlen1 on Fri Sep 14, 2012 5:57 am

    Unfortunately disconnecting the house ground made no difference, all of my hifi components are grounded with a 3 prong plug and all are connected with RCA plugs. I tried connecting the preamp into it's own wall socket with no difference and then plugging my cd source into the same wall socket, again with no difference, with all the complicated stuff behind me, I am so close to making this work, it's probably something quite simple.

    tubes4hifi
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    Re: Dynaco PAS2 too quiet with distorted sound

    Post by tubes4hifi on Fri Sep 14, 2012 11:28 am

    I'm out of ideas . . . what is the voltage on the cathode side (stripe) of D1 on the power supply? Should be greater than 300vdc.

    gdlen1

    Posts : 17
    Join date : 2012-03-18

    Re: Dynaco PAS2 too quiet with distorted sound

    Post by gdlen1 on Fri Sep 14, 2012 5:58 pm

    sorry, I am not sure how to measure that, when I measure across the diode I get approx 25v, I'm assuming this the voltage drop and not what you are after

    tubes4hifi
    Admin

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    Re: Dynaco PAS2 too quiet with distorted sound

    Post by tubes4hifi on Fri Sep 14, 2012 6:13 pm

    OK, that's fine, so I don't know what is causing the hum problem unless it's just a ground loop or long spaghetti wiring,
    any photos you can send? if you can't figure it out let me know via PM or email me here: info@tubes4hifi.com

    gdlen1

    Posts : 17
    Join date : 2012-03-18

    Re: Dynaco PAS2 too quiet with distorted sound

    Post by gdlen1 on Sat Sep 15, 2012 3:42 am

    hi, sent photos of top and bottom to your email, hopefully it's something really obvious

    gdlen1

    Posts : 17
    Join date : 2012-03-18

    Re: Dynaco PAS2 too quiet with distorted sound

    Post by gdlen1 on Sat Sep 15, 2012 4:34 pm

    problem solved, thanks to tubes4hifi, got rid of two ground loops and now pre is running with no hum whatsoever,yay! cheers

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