Can the Pre-Amp HISSSS be filtered out ?

Discussion in 'Amp Central Station' started by PCollen, Apr 3, 2020.

  1. PCollen

    PCollen Friend of Leo's Silver Supporter

    Posts:
    2,796
    Joined:
    May 7, 2010
    Location:
    Man of the World
    I built a KOMET 60 main board and coupled it to a Marshall 50W power section. It sounds great except for the exceptional amount of pre-amp hiss, expected with an amp having 3 stages of gain feeding the PI. Is there a trick, maybe from the Boogie playbook, to filter out some of this hiss ? Like maybe a very small cap to ground ?
     
  2. Archtop Bill

    Archtop Bill Tele-Holic

    Posts:
    503
    Joined:
    Jan 11, 2015
    Location:
    Asheville, The Paris of the South
    Try a volume pedal, noise gate or both between the two.
     
  3. D'tar

    D'tar Friend of Leo's

    Posts:
    2,582
    Joined:
    Jan 11, 2013
    Location:
    WNY
    You could try a 100pf cap parallel to the plate resistor or reduce gain?
     
  4. muchxs

    muchxs Doctor of Teleocity

    Posts:
    13,121
    Joined:
    Aug 14, 2004
    Location:
    New England
    The trick to building a high gain amp that doesn't hiss is to use preamp tubes that don't hiss, old Telefunken smooth plates for example.

    Telefunken Elektroakustik uses a jig to screen tubes.for critical mic preamp applications. It's a little headphone amplifier with a MST of gain. No audible noise means the tube is selected for mic preamp applications. Audible noise means it's sold as a guitar amp tube, no big deal if you need a tube for your trem oscillator.

    There actually is an ultra low noise 12AX7 substitute, the 12AD7. They're exceedingly rare. If you have $200 for a tube and you want zero noise that's the one to have.

    Play Three Card Monty with the tubes you have. There's a chance one is quieter than the others in V1.
     
  5. printer2

    printer2 Poster Extraordinaire

    Posts:
    6,666
    Joined:
    May 24, 2010
    Location:
    Canada
    In the preamp use metal film resistors, 1W minimum, 2W for lower noise yet. If you can turn down the volume and all the noise goes away (within reason) then you only have to concern yourself with the preceding stage. Once I figure out a keeper of an amp I will be using 2W'ers up front.
     
  6. PCollen

    PCollen Friend of Leo's Silver Supporter

    Posts:
    2,796
    Joined:
    May 7, 2010
    Location:
    Man of the World
    I'm using 1/2W MF's throughout the preamp, except V1 Rk has a 1/2W CF as I reduced gain by swapping from 2.7K to 1.5K (and increased Ck to 22uf, as well). I'll try the parallel cap trick. The noise does go WAY down with volume down ...tone stack and volume is off the plate of V1, a la Fender; same circuit topology as TW Express.
     
  7. LightningPhil

    LightningPhil Tele-Meister

    Posts:
    288
    Joined:
    Dec 1, 2018
    Location:
    Nottingham, UK
    The general trick is to generate less hiss. +1 point for metal film resistors and +2 points for wire wound. Make sure you use grid stoppers too as these will each act as a low pass filter as well as a method of stopping RF. With 3 correctly chosen grid stoppers, a 18dB per octave filter can be implemented. Suggest aiming to pass 10KHz. Bit low for a clean amp, but probably the right ballpark for high gain.

    If you link a schematic I can whittle away an hour or so of quarantine pondering suggestions.
     
  8. printer2

    printer2 Poster Extraordinaire

    Posts:
    6,666
    Joined:
    May 24, 2010
    Location:
    Canada
    So you really only need to work on the first stage. You can use parallel resistors, the greater the surface area the current can go through the lower the noise. That is why higher wattage resistors are quieter. You can parallel lower wattage resistors to get the wattage up, of course Ohm's Law still applies. At some point the tube characteristics that Muchxs outlined is the limiting factor. Hi gain shrinks the dynamic range.
     
  9. muchxs

    muchxs Doctor of Teleocity

    Posts:
    13,121
    Joined:
    Aug 14, 2004
    Location:
    New England
    It's been decades since we needed anything like Dolby noise reduction. Dolby was designed to reduce tape hiss. Digital doens't need noise reduction unless you're listening to the data stream. Unless you're a robot data sounds like pure noise.
     
  10. bparnell57

    bparnell57 Poster Extraordinaire

    Posts:
    5,989
    Joined:
    Feb 10, 2014
    Location:
    Philadelphia, PA
    Low noise tubes are your best bet. Tube Depot offers testing on new tubes, or get a variety of tubes and roll V1 until the hiss lessens.
     
  11. guitarist232345

    guitarist232345 Tele-Meister

    Age:
    23
    Posts:
    199
    Joined:
    Nov 21, 2019
    Location:
    london
    Noise gate pedal is a must, also more shielding in the amp?
     
  12. printer2

    printer2 Poster Extraordinaire

    Posts:
    6,666
    Joined:
    May 24, 2010
    Location:
    Canada
    My first project with transistors when I was a kid. Copied a DNR circuit from somewhere to use with my 8 track recorder. Never worked. Should have taken it as an omen and forgot about electronics. Instead look where I have ended up.

    I was thinking of this thread and of a tube that I picked up as a low noise CM3 Siemens tube front end to an amp. These tubes were built as repeaters for trans-Atlantic cables, a price list from 1992 has them at about $200. Low noise long lasting precision made tubes, not like you can go change them. I went for the 20V one, they have a 6V one but hifi guys pay top dollar for those. I'll futz around a PS for it. Need a loctal socket for it though.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.