That spec brings up another question, this one a hazy memory from long ago. There was an amp that put out less wattage into 4 ohms than it did into 8 ohms. If I remember correctly, the figure was half the watts into 4 ohms compared to the watts at 8 ohms. And I thought it was a class "A" tube amp, although I can't be sure.
I know, that's strange.
Has anyone ever heard of something like that? jj
I know, that's strange.
Has anyone ever heard of something like that? jj
I built à 2x 10 W true class A amplifier in 1965 using power germanium transistors AD 149.
Yes, the power drawn from the PSU is constant wether at high or low volume.
Yes, the power drawn from the PSU is constant wether at high or low volume.
The GM70 amp I just sent back consumed 380W with no signal. Allegedly it produced 38W/channel, although I believe that was an ambitious figure. Of course the heaters were 3A at 20V, so that's 120W there!
" .... Refer back to my post #232 (not a Class A amp though). ..."
Yes, I remember. Poor communication skills again. I should have asked, "Has anyone ever heard of a class "A" tube amp that behaves like this?"
My bad. 🙂 jj
Yes, I remember. Poor communication skills again. I should have asked, "Has anyone ever heard of a class "A" tube amp that behaves like this?"
My bad. 🙂 jj
Here's a definition of Class A operation: “Grid bias and alternating grid voltages are such that plate current in a tube flows at all times”.
And here's an explanation of why the plate current has a constant value:
In a single-ended, pure Class A amplifier, the plate current runs at 50% of maximum even at idle when there’s no incoming signal. When a sinusoidal signal voltage of a particular frequency is applied to the input grid, it causes the current flow to fluctuate above and below that 50% of maximum.
The increases and decreases in current flow are equal and opposite around that 50% midpoint so at one instant there will be more current flowing, but, in the next instant, there will be an equal amount less flowing. Thus the total current flow over any period of time remains constant at 50% of maximum.
I have taken and adapted this information from the following source: What is Class A and how does it differ from Class AB? – MESA/Boogie Support
The discussion in the above source also extends to the pure Class A, Push-Pull amplifier where 100% of the maximum current flows at idle, 50% through each tube.
And here's an explanation of why the plate current has a constant value:
In a single-ended, pure Class A amplifier, the plate current runs at 50% of maximum even at idle when there’s no incoming signal. When a sinusoidal signal voltage of a particular frequency is applied to the input grid, it causes the current flow to fluctuate above and below that 50% of maximum.
The increases and decreases in current flow are equal and opposite around that 50% midpoint so at one instant there will be more current flowing, but, in the next instant, there will be an equal amount less flowing. Thus the total current flow over any period of time remains constant at 50% of maximum.
I have taken and adapted this information from the following source: What is Class A and how does it differ from Class AB? – MESA/Boogie Support
The discussion in the above source also extends to the pure Class A, Push-Pull amplifier where 100% of the maximum current flows at idle, 50% through each tube.
I would need more to go on than your "hazy memory from long ago"! 🙂
Here's the last word on Class A: The Last Word on Class A
I believe the Hafler 500 is a class A amp and it draws 240W at idle and 1800W to produce 510 watts output.
Made me look:
https://www.hafler.com/pdf/archive/DH-500_amp_man.pdf
With that much idling power, it may well be running Class A for the first 50 watts.
The Hafler 500 sounds like an AB amp with heavy biassing into Class A. I have had several amps, SS and valve, with anything from 8 to 15 watts in Class A.
I believe the difference in idle and full output draw from the wall would definitely indicate this amp is not class "A". A true class "A" amplifier will, to the best of my imperfect knowledge, draw the same power from the wall under any signal condition. Please see posts # 235 and 237. jj
Is this what jj was referring to, I wonder. So, there are two kinds of class AB amp that might call themselves class A as I see it. Those that are built as class A but given the freedom to go into B for headroom purposes and which do that with ease.. and the kind that are essentially class AB and have been overbiased. There may be a middleground, I expect DIYers will tend toward the first kind.
^ The BA3 with complimentary output stage is an example of the first kind. You can decide how much class A you want, and how much will end up Class AB for headroom. It all depends on the bias you set.
If I ever get round to building mine, I'll probably be wimpy and only run it around 10W class A to save power and heat generation. I suspect most of the time I would not be going outside of that range anyway.
Tony.
If I ever get round to building mine, I'll probably be wimpy and only run it around 10W class A to save power and heat generation. I suspect most of the time I would not be going outside of that range anyway.
Tony.
To that I would add the Hifisonix Ovation series by diyAudio member Bonsai, which run the gamut from full class A to high powered AB in a few well considered variants, based on current feedback as used by Jean Hiraga.
http://hifisonix.com/wordpress/wp-content/uploads/2018/11/kx-Amplifier-Oct-2018.pdf
http://hifisonix.com/wordpress/wp-content/uploads/2018/11/kx-Amplifier-Oct-2018.pdf
" .... there are two kinds of class AB amp that might call themselves class A as I see it. Those that are built as class A but given the freedom [will] go into B for headroom purposes .... "
Now THAT makes a great deal of sense! The gentleman to whom I referred seemed to insinuate that there was something nefarious afoot when he said that most weren't true class "A". However, he's even older than I am, His ideas may be grounded in an older and much more conservative (hidebound?) attitude.
If I remember correctly ( I'm not absolutely sure that I do) he said that a true class "A" amp will not go into class "B", but instead go into clipping when it reaches max output. Does that sound familiar?
Again .... thank you. jj
Now THAT makes a great deal of sense! The gentleman to whom I referred seemed to insinuate that there was something nefarious afoot when he said that most weren't true class "A". However, he's even older than I am, His ideas may be grounded in an older and much more conservative (hidebound?) attitude.
If I remember correctly ( I'm not absolutely sure that I do) he said that a true class "A" amp will not go into class "B", but instead go into clipping when it reaches max output. Does that sound familiar?
Again .... thank you. jj
The other possibility is that he considers a single ended amplifier as the only "true" class A.
Tony.
Tony.
In a single-ended pure Class A amplifier, the current fluctuations occur around the mid-point i.e. the 50% of maximum which is the same as the idle current.
Maximum undistorted signal output would occur when the input signal drives the tube so it conducts a current that fluctuates between 100% and 0%.
Beyond those limits must lie clipping!