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Old 16th August 2015, 11:54 AM   #1
Welcome is offline Welcome  France
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Default What normally causes buzzing?

Hey guys.

I seem to have some serious issues with buzzing. Lots of tube amps I've come across has it. DIY or otherwise.

Same in both channels, independent of volume knob (same level at all times, even with no volume).

Any pointers? Cannot find anything decent online.
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Old 16th August 2015, 12:06 PM   #2
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Ground loop ? Are the devices all connected to PE ?

You say buzzing but if it is hum (so the mains frequency of 50 or 60 Hz) then it could be a ground loop.
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Old 16th August 2015, 01:09 PM   #3
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Buzz (as opposed to hum) almost always means that you have a grounding error involving the smoothing cap in the power supply. Depending on the rectifier/filter topology, the connection between the smoothing cap and the return (commonly the power transformer center tap) carries high current spikes- do not connect anything else within that loop.
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Old 16th August 2015, 01:30 PM   #4
Keit is offline Keit  Australia
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If the buzzing is in the speakers, and not a mechnical buzz comming from the transformer laminations, and is independent of the volume control, then:-

1) If it is a diy amp, look for power supply wiring routing errors, eg main smothing cap not earthed correctly, power trans centre tap not routed directly to the main smoothing cap -ve.

2) If it is a diy amp, it may be that the output transformer(s) is not oriented correctly and is picking up stray field from the power transformer.

3) If an old factory made amp, is the power transformer frame and interwinding screen properly earthed to chassis?

4) Perhaps one or more tubes has/have excessive heater-cathode leakage.

5) Perhaps one or more tubes have been changed, and the person that did it forgot or didn't realise that the hum-bucking trimpot(s) (if fitted) need to be readjusted to suit the replacement tube(s).

6) It can happen in old equipment that low gain tubes have been swapped out for high gain tubes by well meaning hobbyist restorers/repairers/owners. Eg swapping a 12AX7 for a 12AU7, or EL34 in place of a 6L6. Just because the pin connections are the same, it doesn't follow that the tubes are equivalent and can be interchanged. The increased gain may make a normally unnoticeable hum or buzz quite noticeable, if the amp has a low amount or no neg feedback.

Last edited by Keit; 16th August 2015 at 01:35 PM.
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Old 16th August 2015, 05:17 PM   #5
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Quote:
Originally Posted by Welcome View Post
Hey guys.

I seem to have some serious issues with buzzing. Lots of tube amps I've come across has it. DIY or otherwise.

Same in both channels, independent of volume knob (same level at all times, even with no volume).

Any pointers? Cannot find anything decent online.
If I understand correctly it occurs with many different devices be it ready made or DIY. So I take you also tried recent/new devices that well behave when used at other places (then they are simply OK) ? Could it be your source that causes this ? Did you try other sources ?

I know my english is awful but please explain matters a bit more precise if you want a good advice. Do you use different power amps with always the same preamp and DAC etc. ? It simply is strange when EVERY amp buzzes. I would automatically think the amps are out of the quotation if you have tried them elsewhere with fine results.
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Last edited by jean-paul; 16th August 2015 at 05:19 PM.
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Old 16th August 2015, 06:56 PM   #6
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It's a buzzing sound, like bzzzz, through both speakers. I've encountered it several times, seems like a tube amp curse to me.

It goes away somewhat if I touch the particular amp I am dealing with now (Philips AG 9016).

I think the problem is what SY said: the first PSU smoothing cap. The original multi-section electrolytic has been replaced with discrete capacitors.

It's wired like this:

To clarify the image a bit, the speaker minus connectors are not connected to anything else, only to the bridge rectifier minus pole, and the power transformer CT is connected directly to the speaker minus connectors.

Click the image to open in full size.


And here's the full schematic, which does not show each individual wire and how they are supposed to be installed:

http://i.imgur.com/EG2t5MI.png
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Old 16th August 2015, 08:07 PM   #7
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I just tried putting an additional 47 uF capacitor directly on top of the bridge rectifier - no difference.
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Old 16th August 2015, 08:54 PM   #8
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Most likely, a grounding issue or not enough filtering in the power supply. Second most likely, the heater wiring not twisted and getting near high impedance areas of the circuit.

Minimal hum/buzz is usually achieved by having what's called a "star center" ground point, where all ground connections return to, and this will also be the one connection to the chassis. But it's not that simple. The power supply needs to have it's grounds all connected together independantly, and then run a wire from that to what will be the star center ground point for everything else. This minimizes the large charging currents of the main filter cap(s) modulating the other grounds differentially. I prefer to have one ground return to star center from each stage or section of circuitry, rather than every single ground point.

Rf coming in at either the input or the AC line could get demodulated (detected), but more typically makes random noise rather than a consistent buzz (although with near by light dimmers a buzz could be somewhat consistent).

Putting .01u caps across the bridge rectifer diodes will cut rectifier switching noise.

Raising the heater circuit, relative to circuit ground reduces hum and buzz by reverse biasing the heater/cathode electrodes, so heater buzz isn't attracted to the cathode acting as a plate. A resistive divider off B+ with a bypass cap to circuit ground is how I do that. Usually roughly an 820k and a 220K and 10uF, with the tap going to either side of the heater winding (or center tap of heater winding if there is one).

Another thing is that the wires taking the 6.3VAC around to all the tube heaters will radiate significantly less hum/buzz if they are twisted fairly tightly. that way the radiated fields largely cancel out.

Also, single ended tube circuits often have very poor rejection of power supply hum and noise, so the power supply might need to have one more stage of RC filtering.

I also try to keep input impedances no higher than I really need. It appears to be traditional for tube circuits to have a 1Mohm R from grid to ground. Sometimes there is a good reason for that. Other times you can reduce that to 100K or lower, and it will pick up less hum.

Also, many tube amps I've seen put the power transformer too close to the tubes, or in guitar amps sometimes the speaker magnet is so close to the power transformer that the magnet will cause premature core saturation in the transformer.
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Old 16th August 2015, 09:42 PM   #9
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Quote:
Originally Posted by Welcome View Post
I seem to have some serious issues with buzzing. Lots of tube amps I've come across has it. DIY or otherwise.

Same in both channels, independent of volume knob (same level at all times, even with no volume).
Please teach me proper english. It can not be that all amps have the same problem ?!?! If you are using old stuff I am sure you recap them don't you ? Had more than one AG9016 and none of them buzzed...
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Old 16th August 2015, 09:53 PM   #10
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