Amp Hum - Are musicians being screwed?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Datsyuk, Jul 6, 2006.

  1. Datsyuk

    Datsyuk Member

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    I've got a question for the more technically minded folks out there. Can someone please tell me why alleged "high end" tube amps produce incredible levels of hum when consumer grade home stereo tube power amps (Carver....) do not. My dad is a professor of electrical engineering and has always made it clear to me that when he looks at the highly coveted gear that I save up and buy, that the components and design are generally absolutely pathetic. It is his position that musicians are being taken advantage of through endless hype and marketing. (Vintage this, vintage that) I must admit that if I am to believe a lot of the music equipment manufacture's claims it would appear that all superior technological developments stopped by 1970.

    Look at the amount of super expensive Fender guitars is currently producing whose entire marketing hype is based around meticulous recreations of instruments that were designed in the 50's. We haven't been able to build on and improve on those designs over the last 50 years? Were still using cables and jacks that were designed and used by switchboard operators 60 years ago? The golden tone of single coil pick ups that create hum and buzz should be revered be all?

    Lastly, back to the tube amp, and by the way I love mine, is it a question of shielding, the quality of transformers etc that causes hum. Home stereo consumers would never tolerate the humming that we boutique amp users put up with all the time. Is there a fair and honest answer to my gripe?
    I'll ponder your responses for years to come every-time I send all of my money to Visa to pay for the best "guitar" amp that money can buy. Peace B
     
  2. Lyle Caldwell

    Lyle Caldwell Member

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    I won't get into the amp hum, other than to say a well maintained tube amp doesn't hum. Unshielded guitars plugged into a well maintained tube amp will hum.

    As to the price and progress isssue, remember that a Fender Twin Reverb in 1964 cost about $2800 in today's money. And people now want to buy a reissue of the Twin Reverb for less than $1000.

    Something's gotta give, you know? You can still buy an equivalent amp to a '64 TR, but it will cost about, well, $2800.
     
  3. John Ward

    John Ward Member

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    I have also been wondering about this issue lately. I recently finished a Sunn 200s-inspired bass amp for my cousin. The power section is about the same as a Dynaco hifi power amp. The high voltage DC supply to the power tube plates is filtered through a cap -> choke -> cap and there is almost no hum when the amp is dimed. Most guitar amps have the power tube plate supply coming from the first cap and let the push-pull output transformer take care of cancelling out the hum. So without a large choke, guitar amps are lighter and cheaper and have more hum. I don't think there is any benefit in tone to light filtering as found in guitar amps.
    John
     
  4. Reeek

    Reeek Member

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    Guitar amps sound better without the hi-fi components. If guitar tube amps were built as ultra linear as audio tubes amps we'd all be pissed off and SS amps would have a semi-better chance at competing.

    I like guitar tube amps just as they are available today. The most hi-fi I would ever go personally, is a vintage Sunn tube amp and its only their trannies that are ultra linear to my knowledge.
     
  5. John Ward

    John Ward Member

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    I agree, almost all guitar amps run in pentode mode, probably the best exception is the Rt66 which runs ultralinear. It would be possible to filter the DC supply better before the power tube plates. I don't think it would change the tone, it would reduce hum, but it would make the amp more expensive. Any pentode amps have an additional filter cap before the plates?
    Anyway in high gain amps most of the hum is usually coming from the preamp. Noise such as hum from the heater supply is amplified more in guitar preamps than in hifi amps because there is generally more gain.
    John
     
  6. brad347

    brad347 Member

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    1. guitar amps have to amplify low-level input signals, and not line-level signals. This requires higher gain which will usually mean more noise

    2. Most of the sound of the vintage amps lies in the design of obsolete components. Your home stereo has no such requirements. Bear in mind that a more resonant, more durable violin top could be made from carbon fiber. In fact, the Adamas guitars use(d) that technology. Yes, they sounded 'okay.' No, they did not sound anything like a vintage Martin. Valid approach but the established sound in everyone's ear is what's in demand.

    3. everything designed has cost as at least a partial consideration. In consumer audio (music reproduction) and musical instrument amplifiers (music sourcing) the priorities are different. In consumer audio sacrificing a little 'warmth' or 'depth' of sound for a lower noise floor is a fair trade. In guitar amps, the opposite is true.
     
  7. BBQLS1

    BBQLS1 Member

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    I'd like to add this:

    In home stereo components the goal is to reproduce the sound with no color added.

    In guitar amps, the amp is supposed to color the sound.

    Are we being taken advantage of? Yes, but I don't think anymore so than people who buy high end audio gear.

    Alot of the stuff used to build our beloved tube amps are only being used in tube amps, the rest of the world has gone on to better components, but when you do that for guitar amps, it changes the sound in a way that most don't like. It's like trying to get a transistor to clip like a tube does, they just don't work that way. If the rest of the world was using the components that they were in the 50-60's, amps would be cheaper.
     
  8. Scumback Speakers

    Scumback Speakers Gold Supporting Member

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    IME, I've found that eliminating ground loops and implementing solid grounding strategies for the chassis components has been one of the most beneficial things you can do. As I learned from HAD five years ago, "where you put" your ground can also make a big difference. I noticed a difference when a star grounding point was moved from one side of the power tranny to the other, and when another star grounding point was moved a certain distance from the V1 socket.

    The biggest improvement is in how you handle isolation of your input jacks, though, which is why I'm pretty sure my amps are so quiet, even when on 11 (and not on standby! LOL), and the "Nuke" mode engaged.

    I've always hated that 60 cycle hum, and all that good crap that goes along with it in my older amps, and that's the first thing I have my amp designer dial out when I get an amp, it drives me nuts.
     
  9. brad347

    brad347 Member

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    Southbay... would you share some of your techniques or do you consider that a 'trade secret'
     
  10. r9player

    r9player Silver Supporting Member

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    Most of the amps I have are very quiet unless I go into 'high volume or high gain' but if I don't plug in a guitar and crank it up I hardly have any hum at all.
     
  11. donnyjaguar

    donnyjaguar Member

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    Low-level inputs and very high gain make keeping hum out more challenging, but not impossible. My stereo premplifier is all tube (Audio Research) and I assure you there is no hum on the moving coil input. However....if you look at the design you'll see DC filaments on all the tubes and shielded cables used throughout. The high voltage supplies are also fully regulated and the power transformer is ludicrously oversized as are the filtering capacitors. If you incorporated these features into a guitar amplifier it would jack the cost up. That and there will always be some wanker who will claim it has "bad tone" because its too far removed from the "cool vibe" of amplifiers of yesteryear.

    That said, I was able to get 90% of the built-in factory hum from my old Fender. :) Like Southbayampworks said, this has a lot to do with the locations of grounds. There are other tricks too like making wires longer so you can route them closer to other leads, or the chassis itself, and cancel hum that way.

    DJ
     
  12. rmconner80

    rmconner80 Cantankerous Luddite Silver Supporting Member

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    If anyone is being led astry by hype and taken advantage of by corporations... it's the hi-fi guys. I.e. "directional silver wire" et. al. and a whole list of voodoo high dollar components that have no real measurable effect.

    As far as amp design, overfiltering in the PS makes an amp hum less from less ripple in the DC, but it also drastically changes the tone, in my opinion. In general, I can't stand the stiffness and boominess of an overfiltered guitar amp.
     
  13. donnyjaguar

    donnyjaguar Member

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    Ah yes, but its still perfectly possible to engineer in better filtering and reduce hum without making it sound like a hi-fi amp. In my opinion, the tetrode output stages keep guitar amplifiers safely out of the hi-fi realm. :) I'm guessing much of the voltage-sag that causes the pseudo-compression of a tube amplifier is from the phase-splitter pulling down the supplies to the voltage amplifiers downstream.
     
  14. Scumback Speakers

    Scumback Speakers Gold Supporting Member

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    Sorry, I don't know it all, but my amp designer is the guy to ask.

    While in fact I do know many details of it, my amp designer considers some of what's been done with the Scumbag amps to be "proprietary", other parts are the result of extensive R&D I paid for, and some of it came from suggestions from HAD...who as we all know, wouldn't be thrilled about me divulging any of it online.

    It's quite possibly the quietest amp you've ever heard on 10...uh...11... :) at least as far as getting some decent gain out of WITHOUT the hum.

    Maybe I should do a clip of that! :D

    Sorry I couldn't divulge anything brad347...
     
  15. brad347

    brad347 Member

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  16. K-man

    K-man Member

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    I don't think a guitar would sound good through hi-fi equipment. Don't expect an electrical engineer to understand that.

    My father-in-law is a retired electrical engineer. I asked him for some help with an OD pedal I was building. All he kept telling me is "That is a stupid design. Why would you want to drive the transistor into distortion?" Then I was talking to him about tubes, and he told me "Why are they using tubes? You can do the same thing with transistors and they are much more reliable."
     
  17. John Phillips

    John Phillips Member

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    Guitar amps are not the same as hi-fi amps. It's that simple.

    The goals are different, and any attempt to judge one by the standards of the other is missing the point... period.

    (A shorter way of saying what other posts already have :).)
     
  18. Fuchsaudio

    Fuchsaudio Member

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    Anyone who's played an old Silverface Twin with the pull master, knows what an Engineer designed guitar amp circuit sounds like ! Maybe it looked good on a scope, but sounds like dreck. The sound and feel of a guitar amplifier is pretty different from audio gear. I work with a EE who often jokes with me about "sending most of his life getting rid of distortion and noise, only to end up working for a company that prides itself on making things distort...!"

    Ironically, the earliest Sunns amps actually used Dynaco power amp modules (Mark-3 and Mark-4's), into which they attached a guitar or bass preamp. The later Sunn's stayed with Dynaco power and output transformers and chokes for quite some time, although built into their own chassis.

    Depending on the final sonic goal, some audio output transformers can sound awesome as guitar amps. Some of the early ODS-50's used Dynaco A-470 outputs and sounded awesome. Andy Marshall and I were discussing how we both collect classic Scott, Fisher, and Dynaco output transformers, some of which make awesome guitar output iron. Some of the earlier Ampegs used Acrosound outputs in them, which were (and still are) amazing sounding transformers.

    Anyway: If a guitar amp is properly grounded internally, and you take the right steps like DC tube filaments or hum balance controls, or elevating the filaments above ground on DC bias etc, you need not tolerate hum in a guitar amp. I run our amps on "eleven" for 40 hours before shipping them, boosts on, into a real speaker, and all I hear is hiss unless something is wrong. Sometimes a nearby cell phone will make them go nuts, but NO hum.

    More often than not, the noise that would enter through a pickup and/or cables and effects add more noise then a well designed amp would (or should). The only exception might be a single ended class-A no feedback design, which can sometimes have a higher residual noise to it.:JAM
     
  19. TubeAmpNut

    TubeAmpNut Member

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    My thoughts exactly. This discussion reminds me of a talk I had with my EE professor 15 years ago. I was building a stomp box and wanted to 'shape' the distortion products and needed his advice. He looked at me with his hands on his hips and beamed, "why the hell would you try and design in distortion?!!" Some guys just don't get it.

    I think the reason 'old school' designs have lasted as long as they have is simple: They sound good.

    Harmonic distortion rules.

    BK
     
  20. VacuumVoodoo

    VacuumVoodoo Member

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    I object strongly.:puh There's quite a number of us electronics engineers who have no problem understanding this and who also know why it's so. Some are even accomplished guitarists.

    There are no real secrets to designing a quiet free from hum amplifier, be it guitar, microphone or any other. Several articles and books on tube amp design and construction have been published well over 50 yrs ago dealing with the subject.
    Proper grounding and supply decoupling, proper lead dress like not running heater wires parallel with signal, separating protective earth from system ground physically but not electrically. Separate ground buses for signal and supply grounds etc. All this is considered common knowledge by experienced electronics engineers. Have a look at how tube ocilloscopes of the 50s & 60s were built - high input impedance & high gain amplifiers, no hum. Many guitar amp builders have mastered these techniques. One doesn't have to be a graduate engineer with a fancy diploma to learn through hands-on experience.

    There's one aspect not mentioned here: absolute level of hum & noise doesn't mean or say much. It's the ratio between the useful signal and hum that is important. So what if you open your amp to noon and hear some hum - it will be drowned by the sound and masking mechanism in your brain.
    But, yes, audible hum from amps is disturbing to many, myslef included, I'm allergic to it. Hum injected into the amp by guitar pickups is on the other hand often unavoidable and difficult to deal with.

    As for hum in PP output stages, yes, it can be brought down to negligible levels. But hum cancellation mechanism is inherent to symmetry of the PP stage + output transformer and cancels only the ripple present on power tubes' plate supply. No problem here to have a nice sag and very low hum level. Any hum comming from outside of the PP stage itself will be amplified. Hum in - hum out. It's back to proper grounding etc.

    So why are guitar amps out there that hum, buzz and let you listen to chinese medium wave radio? I dare say it's sloppy engineering. If they tell you that "it's normal in guitar amplifiers" it only means that somebody didn't do the homework properly.
    Now guess why you so seldom see hum & noise levels in the tech specs of guitar amps.
    Right, got it off my chest:AOK
     

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