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 Post subject: Hum, Buzz and Hiss
PostPosted: 03 Oct 2012, 20:36 
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If you have built any audio gear in the past which has a power supply and help amplifier, buffer or control the audio signal, noise, in the form of Hum, Buzz or Hiss, can be an issue. You can add "crackle" and "pop" to the list of noises possible. We would all love to build gear with none of these "demons" present in our creations. But in truth, audible or not, most of these noises exist in some form and at some level or another. Our job is to ensure that these are kept to an absolute minimum or at best can't be heard from our listening position.

I use the the one meter rule. If you can't here any noise, at one meter from the speaker, with your ear inline with any of the speaker drivers, the noise doesn't exist. This doesn't stop me pushing my ear up to the drivers listening deep into the darkness to see if I can detect any noise at all. My quietest amp is the Suncalc designed 300BSE Silver Dragon. It generates as much noise when it is on as when it is off with my ear resting on the dust caps of my sensitive speakers. To the ear it is dead silent!

Please add to this thread relating noise issues and how you fixed them or just seeking help on how to purge the demons from you system. My first entry is about hum but not from amps, preamps or any other active divice but induced by cheap cables: The first was related to a bad hum from my Fender Champ guitar amp I built. After putting up with hum for a few months I bought a new (and expensive) guitar lead. Hum gone. When I lent the amp out everyone was astounded how quiet the amp was. At home it hummed. Difference was the others were using quality guitar leads. Mine tested fine but had poor shielding.

Second hum, not related to active components, was again leads. When I setup my 2 channel home theater system I got hum only when using my recorder, not from the Oppo BDP-93 blue Ray player. While re-arranging the system I swapped out the $2 audio lead with a much heavier one. Hum gone. Don't always blame your amps etc. Look for the simple fixes first then checkout your gear. A cable swap is a five minute job trying to chase hum in an amp can be frustrating and time consuming. Don't look for something you have not proven is there.

My 807 amp (Luciano) and 6EM7 (Paris) amps both have the tiniest amount of hum but this can only be heard with my ear against the drivers. Definitely un-hearable at 1m (one meter rule) and no-way audible from my listening position 4m away. I'm not even going to try and remove it.

Got a noise issue?

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 Post subject: Re: Hum, Buzz and Hiss
PostPosted: 03 Oct 2012, 23:02 
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Joined: 04 Jun 2008, 20:59
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Hi Mark, I really hate hum, buzz, noise and any of the other odd things that might come from a system. I am not as forgiving as you are though. I want absolutely nothing at the speaker with your ear right on it. I find that this level is about -90dbv. One thing that I have found is that the grounding both inside a piece of equipment and between pieces of equipment is extremely important to a quiet system. I use a modified "buss" and "star" arrangement inside gear. The active circuity has its own non power carrying ground buss and the power supply has one that carries no signals. They join at a central location, usually at the input jack ground. I have found no other practical way to get to the -90dbv level. Externally it is paramount to avoid ground loops between pieces of equipment. I don't want to write a book here on this (lots of others already have) but beware of mixing chassis and signal grounds and various shields on cables. They can cause a loop where you least expect it. Check for multiple ground paths to signal entry and exit points. If there is more than one way a "ground" can go, then you are ripe for a hum based loop. Next on my list is clean power supplies. I have seen many poorly designed power supplies. Small wonder there is noise. For folks that don't understand filtering well, just ask on this forum. There is lots of expertise here, and we are glad to help. :)

Good listening
Bruce

BTW and a bit OT...nice speaker project Mark

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 Post subject: Re: Hum, Buzz and Hiss
PostPosted: 05 Oct 2012, 08:47 
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One strange but understandable hum I had from an early tube build was fixed by running a thin wire from the main PS earth to the RCA ground connection. But here is the strange part. I had earthed the top plate of the amp to both the mains and PS earth. This practice I perform with all my amps but I know most don't like it. I was relying on the 1mm Al top plate to carry earth to the RCA ground side. But it didn't do the job well.

Only when I ran a wire direct from the PS earth to the RCA did the hum go away. Don't rely on heavy metal components to carry an important earth to critical components, give it its own earth.

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 Post subject: Re: Hum, Buzz and Hiss
PostPosted: 05 Oct 2012, 12:57 
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Hi Mark, Yes indeed. I ran into that some time back and now never use any part of the chassis or covers as part of the signal / PS ground. It is only connected to the earth ground and then via a X2 capacitor and parallel 120R resistor to the main junction of the signal and power grounds. BTW another way to do the connection I have seen is to use a bridge rectifier and parallel resistor. It seems funny that in order for the "ground" to work it must not in this case be a good low impedance one. I have studied all the techie stuff on it and just now take it on faith.

Good listening
Bruce

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 Post subject: Re: Hum, Buzz and Hiss
PostPosted: 07 Oct 2012, 05:54 
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In all of my builds I try to stop any outside electrical rubbish entering. To this end I add a ferrite choke to the captured power cord then under the bonnet a power filter. In Australia private companies now own our power infrastructure (once owned by the government). At one time an 80% capacity was maximum load on our power transformers and grid now it's100%. So noise, DC and non-sine wave AC rules.

I have found the choke and filters work and once I started using these simple and cheap passive devices (many builds ago) my amps really started sounding better. They became quieter and didn't pop or click when lights and heaters in my home were turned on and off. There was greater detail in the mid-range as well. I know with the Asian based toroid power trannis sold through the store I work at will buzz if there is DC on the line.

So of course DC filtering and smoothing is necessary but stopping noise getting in is easy and better than trying to filter it out.

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 Post subject: Re: Hum, Buzz and Hiss
PostPosted: 07 Oct 2012, 06:30 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
I also hate sibilance!

But that is more likely to be a recording issue...


Cheers,

Miguel


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 Post subject: Re: Hum, Buzz and Hiss
PostPosted: 07 Oct 2012, 16:51 
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I have a Diana Krell double LP which has shocking sibilance particularly side one. I have found various tube builds handle this differently. With my last few builds, which handle sibilance extremely well, even this shockingly "sibilance" recording is listenable. But funny who some amps handle it better than others.

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 Post subject: Re: Hum, Buzz and Hiss
PostPosted: 07 Oct 2012, 17:54 
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Joined: 06 Apr 2009, 10:08
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"Hum, Buzz, and Hiss" That's actually a pretty big menu. Different sonic defects, usually from far different sources.

Hum - Hum is almost always an injection of the primary line frequency or 2x the line frequency into the signal path. Hum usually originates in one of three places. First is insufficient power supply filtering. Easy to fix; add filtering. Just remember that one needs to add filter stages and not just bigger and bigger caps. Second source is ground loops and bad grounds. When interconnecting equipment, float all signal grounds except one. Keep signal and case/safety grounds separate except for one point (even when interconnecting equipment). And use either heavy ground busses or star ground schemes for signal grounds. Third source, and hardly worth a mention in my mind, is heater hum. This comes usually from poor lead dressing within the chassis. Heater circuits should always be twisted pair, should be kept well away from the signal lines, and should enter the tube socket area at near perpendicular paths to the signal wires. In rare cases heater hum can bleed through to the signal chain due to cathode coupling in unipotential cathode tubes. But this is rare and usually can only be seen when gains get over 50dBv to 60dBv. Look at the noise specs on the 12AX7 for typical numbers.

HIss - Hiss is either thermal or shot noise coming from a formed component (usually a resistor). First rule is don't use carbon based resistors; film or otherwise. For signal paths use quality meal film resistors, for power applications use wire wound. This will usually lick most hiss problems. However, there is another source of hiss that can be a bugger to find. This is high frequency oscillation. Even at very low voltage levels this can creep in. Always check for oscillation sources (pentode/tetrode screen supply circuits can be a real problem). On high gain tubes, series screen, control grid, and plate resistors can help a lot. Especially when using transmitter tetrodes like the 807 or 1625. And watch those totem pole configurations like SRPP, White Cathode Follower, and µ-Follower. These can oscillate at high frequency (>1MHz) like nobody's business.

Buzz - This is one of three things; a bad tube, a bad ground, or sand in the signal path. The first is simple, swap tubes and see if it follows channels or goes away. The second, look for finicky grounds with a VTVM (or other high input impedance meter) and visually examine all ground solder joints. For sand this is a little different (Yes I know I just stomped on a bunch of people's toes. And yes I know if done properly sand can be used and result in a dead quiet circuit. But the key word here is "properly".). So first, always snub SS diodes with small signal caps to shunt the high frequency noise, as close as possible to the device. Second, only use linear analog SS devices. Most voltage regulators are fine but never use a switching supply and never use an active control circuit that self oscillates to maintain control. Both are bad news for noise reduction.

Follow these rules and hum, hiss, and buzz should be rare problems in your circuit. Ok; now everyone I've offended, please feel free to let fly. :)

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 Post subject: Re: Hum, Buzz and Hiss
PostPosted: 07 Oct 2012, 20:31 
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Hi, Another source of his and noise is having too wide a bandwidth in a high gain circuit. I like to select the bandwidth at no more than twice the max frequency that is to be reproduced. The equivalent noise of a tube can also be a factor in high gain circuits. One way to get around hum induced into the cathode circuits is to elevate the heaters (with a DC reference voltage) to slightly above the the potential on the cathode. Also tube selection can help. I find that tubes wit spiral wound heaters are less prone to introducing hum into circuits. The actual type of circuit can be a factor as well. It is one reason I like to use SRPP stages. They have a fairly high degree of noise rejection from the power supply.

Good listening
Bruce

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 Post subject: Re: Hum, Buzz and Hiss
PostPosted: 08 Oct 2012, 02:18 
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Another source of hum your hand crafted equipment may not be responsible for is one piece of gear getting "jiggy" with another. For me when I had one of my 4S preamps (in earthed Hammond Al case) sitting on top of thick natural marble toward the right hand side of my Opp BDP-93SE I got a hum induced in my system. When I slide the tube preamp to the left of the Oppo the hum vanished. I'm blaming the Oppo here.

You would think I would get hum from my $2.5 home grown very short single strands of lose wire-warp wire into $1.25 flimsy RCAs interconnects. But no! No shielding, no hum and do degradation of audio signal.

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