The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Coupling caps to isolate DC

    Share

    daveshel

    Posts : 147
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Coupling caps to isolate DC

    Post by daveshel on Wed Apr 13, 2016 12:00 am

    I understand that sometimes preamps have some DC in their output, and that the fix for this is to add some extra coupling caps to the output of the preamp or the input of the amp. I am unclear as to what kind of symptoms this will cause. I'm wondering if it's better to add them to the preamp or the amp, and if there's any downside of using them when they're not needed.

    Background: I have a Van Alstine Super PAS Three that's 20+ years old. I used it with my Van Alstine U-70 until I built my ST-35 w/EFB (and dumped that U-70 as fast as I could). Later I got ahold of a pair of MK-IIIs that  are essentially stock but with a lot of upgraded esoteric parts, and one of them makes some farting and popping sounds for a few minutes until it warms up. At the same time, I'm working on a rebuild of an old stock PAS2, on which I've been told I'll need to add some coupling caps after I remove the tone controls. So I'm feeling the need to get a better understanding of this area as to when this is needed - and whether I should add it to the preamps or the amps.


    Last edited by daveshel on Wed Apr 13, 2016 9:54 am; edited 1 time in total

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Coupling caps to isolate DC

    Post by peterh on Wed Apr 13, 2016 1:45 am

    daveshel wrote:I understand that sometimes preamps have some DC in their output, and that the fix for this is to add some extra coupling caps to the output of the preamp or the input of the amp. I am unclear as to what kind of symptoms this will cause. I'm wondering if it's better to add them to the preamp or the amp, and if there's any downside of using them when they're not needed.

    Background: I have a Van Alstine Super PAS Three that's 20+ years old. I used it with my Van Alstine U-70 until I built my ST-35 w/EFB (and dumped that U-70 as fast as I could). Later I got ahold of a pair of MK-IIIs that  are essentially stock but with a lot of upgraded esoteric parts, and one of them makes some farting and popping sounds for a few minutes until it warms up. At the same time, I'm working on a rebuild of an old stock PAS2, on which I've been told I'll need to add some coupling caps after I remove the tone controls. So I'm feeling the need to get a better understanding of this area as to when this is needed - and whether I should add it to the preamps or the amps.
    It ought to be a "rule" that may be expressed as :
    "Be liberal with what to receive, assume bad things and do not be harmed with it, at the same
    time be careful with what you transmit, never transmit anything that can harm or is out of spec"
    Interpreted on an amp would be : "protect inputs from DC, transients and signals outside the spectra, on outputs make sure no DC , no turn off/on transients, and no disturbing high frequency "

    An amp like the ST-70 breaks this, it has no protecting cap on input. The result is that
    if feeded with a signal with DC component the 7199 will have wring bias on the first tube, this will affect sound. PAS 3 before "X" also breaks this as it emits a DC component which
    might/will upset the power amp.
    The fact that dynaco who made many well designed amps got away with this is that
    the effects are minor in this case ( it will affect working point of 7199 but not fatally. A slip
    in an other brilliant series of amps.

    daveshel

    Posts : 147
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: Coupling caps to isolate DC

    Post by daveshel on Thu Apr 14, 2016 12:08 am

    Turns out that my Super PAS Three has 3.3 uFd output coupling caps, so this isn't the likely cause of my farting and popping.

    But still I am unclear as to what issues might be caused by DC in a preamp's output, and how to best deploy output coupling caps in my PAS2 rebuild.

    Tube Nube

    Posts : 641
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: Coupling caps to isolate DC

    Post by Tube Nube on Thu Apr 14, 2016 8:44 am

    I suspect it's a tube thats making all the racket.

    vtshopdog

    Posts : 69
    Join date : 2015-07-11
    Location : UT, USA

    Re: Coupling caps to isolate DC

    Post by vtshopdog on Thu Apr 14, 2016 4:26 pm

    Cracked or cold solder joints can make bad sounds come and go with warming or cooling. Maybe look at tube socket joints under magnification, possibly rectifier first??

    daveshel

    Posts : 147
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: Coupling caps to isolate DC

    Post by daveshel on Thu Apr 28, 2016 1:04 am

    I started out switching the tubes with the partner unit one by one (or in pairs with the output tubes), but the noise stayed with the same unit. Then I re-soldered all the solder joints near the rectifier tube, and it was still there. Finally, I re-soldered everything and it seems to have finally gone away. Good call, vtshopdog.

    While I was at it I wondered why one of the pair had a fifth screw on the speaker terminal strip, marked 70V. While under the deck, I saw that this terminal was connected to a yellow lead from the OPT. (I didn't examine it's 'mate' to see if there was a corresponding lead terminated somewhere else.) Anyone familiar with this vestigial organ?

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Coupling caps to isolate DC

    Post by GP49 on Thu Apr 28, 2016 6:49 am

    The Mark III was made in an extra-cost 70 volt version for sound distribution and industrial use. It had a special output transformer with the 70V tap; otherwise it was identical to the standard version.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Coupling caps to isolate DC

    Post by PeterCapo on Thu Apr 28, 2016 10:06 am

    daveshel wrote:But still I am unclear as to what issues might be caused by DC in a preamp's output, and how to best deploy output coupling caps in my PAS2 rebuild.

    Dave Gillespie stated in his recent landmark study of the original Stereo 70 that using it with a PAS with DC on its outputs would cause low frequency oscillations.  Interestingly, someone, a fellow audiophile and DIYer, wrote to me saying his woofers were pumping after he tried the Curcio instructions for bypassing the tone controls, which omit the 1.0uF output coupling capacitor.  He was using it with a Stereo 70 that, as we know, lacks an input coupling capacitor.  I gave him a procedure for bypassing the TCs that included the 1.0uF.  He used it and reported the woofer pumping stopped.

    Why not deploy output coupling caps the same way Dynaco did in their X-mod for the PAS?

    daveshel

    Posts : 147
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: Coupling caps to isolate DC

    Post by daveshel on Thu Apr 28, 2016 11:03 am

    GP49 wrote:The Mark III was made in an extra-cost 70 volt version for sound distribution and industrial use.  It had a special output transformer with the 70V tap; otherwise it was identical to the standard version.

    Interesting. Though, I wonder - a transformer is already a pretty dense entity. How do they include the extra windings without increasing the physical dimensions of the transformer?

    daveshel

    Posts : 147
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: Coupling caps to isolate DC

    Post by daveshel on Thu Apr 28, 2016 11:10 am

    PeterCapo wrote:
    daveshel wrote:But still I am unclear as to what issues might be caused by DC in a preamp's output, and how to best deploy output coupling caps in my PAS2 rebuild.

    Dave Gillespie stated in his recent landmark study of the original Stereo 70 that using it with a PAS with DC on its outputs would cause low frequency oscillations.  Interestingly, someone, a fellow audiophile and DIYer, wrote to me saying his woofers were pumping after he tried the Curcio instructions for bypassing the tone controls, which omit the 1.0uF output coupling capacitor.  He was using it with a Stereo 70 that, as we know, lacks an input coupling capacitor.  I gave him a procedure for bypassing the TCs that included the 1.0uF.  He used it and reported the woofer pumping stopped.

    Why not deploy output coupling caps the same way Dynaco did in their X-mod for the PAS?

    After listening to my original audio boards for a few days, with the tone controls still in place, I'm going to try removing them before I take the plunge and finish the rebuild in one swell foop (I was getting killed on shipping doing it gradually). I have a couple of 1 MFD electrolytics I can use as coupling caps. If I understand correctly, I'm going to connect these between the output wire from the linestage board and the output terminal, with the + side of the cap on the terminal?

    How did the X-mod deploy them?

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Coupling caps to isolate DC

    Post by PeterCapo on Thu Apr 28, 2016 11:22 am

    Polypropylene would probably be better. There can be variations on this theme, but this schematic excerpt emulates PAS with X-mod tone controls centered.  Note it is for use with amplifiers having a Z-in of 100K or higher.  To use it with amps having a lower Z-in could require additional adjustment - see the study of the PAS line section at the Audioregenesis website http://www.audioregenesis.com/documents/PAS_Line.pdf


    daveshel

    Posts : 147
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: Coupling caps to isolate DC

    Post by daveshel on Thu Apr 28, 2016 12:00 pm

    That's a bit over my head.

    Can I use my meter to see if I've got a couple of volts DC on my outputs?

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Coupling caps to isolate DC

    Post by PeterCapo on Thu Apr 28, 2016 12:03 pm

    daveshel wrote:Can I use my meter to see if I've got a couple of volts DC on my outputs?

    I don't see why not.  Measure from center conductor to ground shell.

    daveshel

    Posts : 147
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: Coupling caps to isolate DC

    Post by daveshel on Thu Apr 28, 2016 9:56 pm

    Followed the Curcio procedure. It's a bit of a pain because it has you soldering wires from the bottom into an eyelet that has the main out wire connected from the top. Of course I pushed one out and had to finagle it back together.

    I didn'd read any DC with my meter, nor do I hear any bass pulsing or any other negative effects.

    Positive effects were pretty marginal as well. There is still a little bit of hiss, independent of the volume control - maybe less that before tone control removal but it's hard to tell when there wasn't that much in the first place.

    Sponsored content

    Re: Coupling caps to isolate DC

    Post by Sponsored content Today at 8:06 am


      Current date/time is Tue Dec 06, 2016 8:06 am