I read 1-2mv ac at the OPT speaker connections & 1-2dBC (1" from the speaker throat) without any input signal.
Is there a better way to determine hum with just a meter ?
Is zero hum possible with a 2A3 amp?
Is there a better way to determine hum with just a meter ?
Is zero hum possible with a 2A3 amp?
Is that with volume and tone set for max output, and input shorted, and highest speaker impedance setting?
If you have feedback loop then you could always disconnect that.
Are you trying to trim the hum?
If you have feedback loop then you could always disconnect that.
Are you trying to trim the hum?
Hum
trobbins,
There is no volume or tone control on this basic stereo amp (Moondog 2A3),
I took new readings this morning while it's quiet:
59dBC ambient, 61dBC, with amp on, taken 1" from the speaker throat.
13mV ac using the 16 ohm tap.
Can a 2A3 amp have zero mV & zero dBC hum ?
After thinking about it, I realize I may have caused the hum problem with my DIY amp. I have a separate PS for the B+ but added four filament transformers close to the tubes in the cramped-for-space amp chassis. I'll be rebuilding the PS to relocate the filament transformers & rectifiers now in the amp.
Anything else I can do?
trobbins,
There is no volume or tone control on this basic stereo amp (Moondog 2A3),
I took new readings this morning while it's quiet:
59dBC ambient, 61dBC, with amp on, taken 1" from the speaker throat.
13mV ac using the 16 ohm tap.
Can a 2A3 amp have zero mV & zero dBC hum ?
After thinking about it, I realize I may have caused the hum problem with my DIY amp. I have a separate PS for the B+ but added four filament transformers close to the tubes in the cramped-for-space amp chassis. I'll be rebuilding the PS to relocate the filament transformers & rectifiers now in the amp.
Anything else I can do?
To get zero hum you need to use a battery DC supply in the middle of a field, well away from any buildings or mains wiring.
To get low hum you need to take care with component and wire placement, and the grounding scheme. This assumes a good design; with a poor design you can't reduce hum. Do a search on here, as this topic comes up quite regularly.
To get low hum you need to take care with component and wire placement, and the grounding scheme. This assumes a good design; with a poor design you can't reduce hum. Do a search on here, as this topic comes up quite regularly.
And by careful component placement, he means the placement of the various components within the amplifier chassis/circuit. Not the stereo components (CD, amp, etc).
Zero hum is not possible in a 50/60 Hz environment. But you don't need zero. You "just" need inaudible.
I find a spectrum analyzer or wave analyzer to be a better tool for measuring hum as it tells you the frequency as well as the harmonic content. A good external sound card can be a reasonable substitute for those, provided, that the sound card itself doesn't contribute any significant amount of hum itself. Many of the cheap ones do...
~Tom
You also need to have some reference level to measure against. I use 1-watt RMS as the output reference level. As we're talking about low power SE amps, a 45 is good for 2-watt, the 2A3 for 4-watts and a 300B for 8-watts. I would use 1-watt as the reference in all of the above and consider the additional power as headroom.
In any case for my own acceptance, I prefer 80dB signal-to-noise referenced to 1-watt output. For an 8-ohm load, 1-watt is 2.83 VRMS into 8-ohms. The equivalent output noise to achieve 80dB would be 0.000283 VRMS, or 283 microvolts. For a 16-ohm load, it would 4.0 VRMS for 1-watt and 400 microvolts maximum output noise. This is much lower than what you're posting initially, your equivalent s/n assuming 8-ohms is 69dB for 1mv output noise.
Regards, KM
kmaier,
Thanks for the explanation. I finally know what my goal is for a minimum hum.
With a little luck, relocating the filament transformers will do the trick.
Thanks for the explanation. I finally know what my goal is for a minimum hum.
With a little luck, relocating the filament transformers will do the trick.
'Low hum' would depend on your loudspeaker and room, but -80dB seems a good place to start from. That is -80dB from your normal listening level, which may be below 1W. If you can't hear the hum, it is low enough.
Moving it may or may not help. You are assuming the coupling is via a magnetic feild from the transformers. It could be more likely the wires inside the chassis. Longer wires make better antenna so think about the overall design. The worst thing is to have wires parallel to each other.
Is there ripple in the B+ power supply?
How is the system grounded. Using a star? is there any way at all current can be flowing in the ground. No ground loops. Are you using shield wire in the signal path?
ChrisA,
I don't have equipment to see if there's B+ ripple. I have star grounded everything.
The circuit is :
input at one end
to V1
to V2
to V3
to OPTs
to B+ input
However I ran out of space & mounted the filament transformers & rectifiers next to the tubes. I used a 10 ga. solid copper wire the length of the chassis for the star ground that's also connected to the ac ground & chassis.
I don't have equipment to see if there's B+ ripple. I have star grounded everything.
The circuit is :
input at one end
to V1
to V2
to V3
to OPTs
to B+ input
However I ran out of space & mounted the filament transformers & rectifiers next to the tubes. I used a 10 ga. solid copper wire the length of the chassis for the star ground that's also connected to the ac ground & chassis.
All you need is a volt meter with a AC volts setting. An AC meter will be AC coupled and ignore the large DC component. You don't need a scope.
No AC meter? Then connect a (say) 100 uF cap between the meter and the B+ then rectify the AC with a small signal diode, like a 1n34 or something and measure the rectified DC and then some analysis will tell you the ripple.
If the problem is magnetic coupling it's likely from power transformer to output transformer. You can test this theory even with no tubes installed if you can load up the power transformer so it has full current.
But it is really worth it to invest in a meter that can measure AC voltages. They are not expensive and would make debugging this much easier. Just sort the input jack then probe backwards from the OPT looking for any kind of AC signal.
When you are looking for hum of low audio signals even one of the software osciloscopes that ues a PC sound card is good enough for signals up to about 10KHz. This can cost as little as "free"
ChrisA,
Oh, I have the basic meters such as vac, vdc, ohms, etc. along with lots of air volume, pressure & temperature instruments. In my former life,I was a trouble shooter for industrial equipment installations in automotive assembly plants that also included problems with noise.
Electronic/radio/audio circuits are new to me. I've only been interested in building amps for the past year.
I do appreciate your help slanted towards my naivite' in electronics.
I should buy a textbook of "Amplifiers for Dummies" for reference.
Maybe, then, I wouldn't ask so many dumb questions.
Frank
Oh, I have the basic meters such as vac, vdc, ohms, etc. along with lots of air volume, pressure & temperature instruments. In my former life,I was a trouble shooter for industrial equipment installations in automotive assembly plants that also included problems with noise.
Electronic/radio/audio circuits are new to me. I've only been interested in building amps for the past year.
I do appreciate your help slanted towards my naivite' in electronics.
I should buy a textbook of "Amplifiers for Dummies" for reference.
Maybe, then, I wouldn't ask so many dumb questions.
Frank
Don't buy a book. There are many classic texts from the tube era with expired copywrite. on this site. Some are for beginners others are graduate level engineering text books
Technical books online
A good on-line text is this one
http://www.angelfire.com/planet/funwithtransistors/Book_Contents.html
and here:
http://www.freewebs.com/valvewizard/
The above would take years to read.
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Inaudible at the speaker works pretty well. Very quite is 100uV at the output.
1mV or less would be OK for a tube product. A decent true RMS meter will get pretty close. A sound card is even better to look at the spectrum.
Then you have to factor in the speaker of course. This is just my opinion using what I have seen over 25 years using an AP.
1mV or less would be OK for a tube product. A decent true RMS meter will get pretty close. A sound card is even better to look at the spectrum.
Then you have to factor in the speaker of course. This is just my opinion using what I have seen over 25 years using an AP.
kmaier,
"The equivalent output noise to achieve 80dB would be 0.000283 VRMS, or 283 microvolts. For a 16-ohm load, it would 4.0 VRMS for 1-watt and 400 microvolts maximum output noise."
Shouldn't that 283 microvolts read 2.83mV? I hope it was a mistake or I'll never hope to reach that low of a hum level.
ChrisA,
You were right about the filament transformers locations. I haven't moved them,yet, but I did add a pair of 100R resistors from each leg of the V1 & V2 filament transformers. The hum dropped to 3.3mV @ 8 ohm & 5.4mV @ 16 ohm.
Is the next step to raise the V1 & V2 filament voltage above ground?
Is there anything I can do to the 2A3 DHT tubes? They already have humpots.
The (fast response) noise level scale of dBC I was using is incorrect for this application. The norm is the (slow response) dBA. I now have 1.5 dBA of hum.
Frank
"The equivalent output noise to achieve 80dB would be 0.000283 VRMS, or 283 microvolts. For a 16-ohm load, it would 4.0 VRMS for 1-watt and 400 microvolts maximum output noise."
Shouldn't that 283 microvolts read 2.83mV? I hope it was a mistake or I'll never hope to reach that low of a hum level.
ChrisA,
You were right about the filament transformers locations. I haven't moved them,yet, but I did add a pair of 100R resistors from each leg of the V1 & V2 filament transformers. The hum dropped to 3.3mV @ 8 ohm & 5.4mV @ 16 ohm.
Is the next step to raise the V1 & V2 filament voltage above ground?
Is there anything I can do to the 2A3 DHT tubes? They already have humpots.
The (fast response) noise level scale of dBC I was using is incorrect for this application. The norm is the (slow response) dBA. I now have 1.5 dBA of hum.
Frank
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Hi Dobias,
mV is for millivolts, or thousands of a volt, so 2.83 mV would be 0.00283 V.
Regards, KM
mV is for millivolts, or thousands of a volt, so 2.83 mV would be 0.00283 V.
Regards, KM
kmaier,
I'm aware that 2.83mV is 0.00283V, but your stated goal to achieve 80dB is 1/10th of that.
Have you really obtained that low of a hum level?
Frank
I'm aware that 2.83mV is 0.00283V, but your stated goal to achieve 80dB is 1/10th of that.
Have you really obtained that low of a hum level?
Frank
Yes, my 45 SET amps are running at better that 80dB s/n at 1-watt. I have a couple pairs of 45 ST glass triodes that are down at 90dB. I use AC filaments with a fixed DC balance and adjustable AC balance.
Not all tubes will spec this well as I have many pairs that are only in the mid 70dB range and some that are below 70dB. I match sets of tubes for SETs based on noise, gain, distortion, bias characteristics and mechanical construction.
Regards, KM
Copyright or no copyright, I think books still have a place in electrical engineering. I own several of the classic texts that I've picked up for pennies on the dollar on [THAT AUCTION SITE] or elsewhere. I can flip through a book and find inspiration. I can't flip through a pdf in the same way.
With my 87 dB/W*m efficient speakers, I find that I desire output hum/noise to be below 1 mV RMS preferably below 500 uV RMS. At those levels I can no longer hear the hum when I stick my head in the speaker - never mind at the listening position.
~Tom
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