Posted by dnewman (A) on April 26, 2007 at 18:59:58

Finished building my 2A3 Paramounts a month back. Enjoying them
immensely. However, there's a very minor annoyance.

One channel -- the channel I mirrored -- has virtually no audible hum (97 dB speakers).
If I measure the hum with the input jack shorted, I get 0.75 mV across the binding posts.
Great. Pat myself on the back, etc. And a big thank you to Doc, Paul, Mike, and Eileen
for such a great product!!! (Having had had a small company once, I truly envy them the
fun they're having.)

However, the stock -- non-mirrored channel -- has 1.1mV of hum.
Sorry, but that's a challenge which I cannot ignore. And, I'll be the first to admit that
had both channels measured at 1.1 mV, I would have been happier than a clam and
called it quits. Well, I'm merely happy as a clam....

The hum doesn't follow the tubes. More importantly it vanishes abruptly when power
is turned off. (As opposed to slowly fading along with any output signal.) So, I figure
I'm picking up something from the power or filament supplies. (And yes the hum is 60
Hz.) The hum is present even when the channel is isolated from all other equipment.
And yes, I have to stand close to the speaker to hear it. I do believe that it increases
slightly after several hours of operation, but that may just be psychological -- I haven't
tried measuring it.

Unless anyone has a better idea, I reckon I'll put a pickup coil on my oscope and see if
I can pinpoint the source. Feel free to point out that surely I have better ways to spend
my time. But please don't tell me to maximize the hum on the 0.75 mV channel so as
to get as close to 1.1mV as possible ;-)

Dan

• Re: Admittedly minor hum (1.1 vs 0.75 mV) - caffeinator 15:00:36 04/27/07 (0)
  In Reply to: Admittedly minor hum (1.1 vs 0.75 mV) posted by dnewman on April 26, 2007 at 18:59:58:

  I had exactly the same experience with my (non-mirrored) 300B Paramounts...one was about 0.7 mV, the other more like 1 mV, iirc. To my eye, they were nearly identical in layout and wire dress...I tried swapping tubes from one side to the other, but no change...I just chalked it up to normal variation in all the components and moved on.

  FWIW, I can't hear hum from either channel through my Straight 8's, which are fairly sensitive, so I think even at 1.1mV you're not likely to notice it a whole lot. Of course, my hearing isn't the best either...so ymmv...
  
  

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• Layout and lead dress - Paul Joppa 11:35:18 04/27/07 (3)
  In Reply to: Admittedly minor hum (1.1 vs 0.75 mV) posted by dnewman on April 26, 2007 at 18:59:58:

  Those are the usual variables, once you have eliminated the obvious as Nutube talks about. Compare the two sides, especially looking at signal current loops and twisted pairs.
  
  

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  • Re: Layout and lead dress - dnewman 12:31:17 04/27/07 (2)
    In Reply to: Layout and lead dress posted by Paul Joppa on April 27, 2007 at 11:35:18:

    Glad to hear everyone's thoughts as they confirm at that I was
    doing the right sort of checks already. Since Nutube's list checks
    out okay (and I had previously reflowed ground paths), I'm
    suspecting it's an induced current picked up from either a twisted
    pair or proximity to the psu. I'll break out the oscope and pickup
    coil. But first, I may try injecting a small signal into each of the tp's
    and see if I can pick it up that way with a tone tracer or the scope.
    I'm mostly curious if the technique will work. (Safer than doing it
    with the amp plugged into the mains!) However, if this small hum
    is coming from the filament wiring, then my puny injected signal
    probably won't show it.

    Thanks,
    Dan
    
    

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    • Re: Layout and lead dress - dnewman 18:24:00 04/29/07 (1)
      In Reply to: Re: Layout and lead dress posted by dnewman on April 27, 2007 at 12:31:17:

      Reduced hum from 1.1 mV to 0.90mV at which point (and as suggested), I will
      chalk the rest up to component variances and that the other channel with 0.75mV
      hum has a slightly different layout as it is a mirror layout.

      First twisted pair I played with was the AC input from IEC connecto & switch to
      the power tran. I coerced it into lying closer to the chasis plate which also got
      it a tad further from the pair exiting the PSU board going to the filament choke.
      I then did some empirical adjustment with the twisted pair filament supply to the
      12AT7.

      Now to find something else to futz with whilst enjoying my Paramounts.

      Cheers,
      Dan
      
      

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      • something else to futz with ... - Paul Joppa 19:13:58 04/29/07 (0)
        In Reply to: Re: Layout and lead dress posted by dnewman on April 29, 2007 at 18:24:00:

        I'm embarassed to say, but I have not tried the following. Parampounts are selling pretty well, and I have yet to get my hands on a pair for extended lab and listening...

        Anyhow, the LR filter on the filament of the 300B is not a "pure DC", it reduces the AC component by about 16dB relative to AC heating. I chose this because of the bad reputation that capacitors have in filament supplies. However, it is quite possible, even probable, that a large electrolytic cap across the heater pins (A1-A4) will result in hum reduction. You may also lose some "life" from the sound, or maybe not - there arer too many different and incompatible opinions on the web to be confident. Worth a try just to see what you hear though! I'd start with 10000uF, 6 to 16v rating.

        There will always be some hum pickup from the driver heaters, even with a grounded center tap as in Paramount and Paramour. Some 12AT7s will be more immune to this than others. We are also working on a DC supply for the driver (hope to use it for some delay relays as well).

        So there is plenty to futz with when you need it! :^)
        
        

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• My 2 cents.. - Nutube 03:29:02 04/27/07 (0)
  In Reply to: Admittedly minor hum (1.1 vs 0.75 mV) posted by dnewman on April 26, 2007 at 18:59:58:

  1. Preamp tubes; driver tubes... ?
  Lately when I get a hummy ( low frquency)tube my sub is the first to let me know.
  2. Unit too close to another amp or device
  3. Re-solder grounds
  I know all about obssesed with driving the last ounce of unnecessary noise...
  good luck
  
  
  

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