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21st September 2013, 12:28 AM  #1181 
diyAudio Moderator

Thanks Bob! As with everything diy, the more that is revealed the more I realise how little I actually know!!
Also above I forgot to multiply my current by amps for my watts... it should have been 25W class A when I said 10W and here was I thinking only 10W! Also I've realized that I do need a bigger transformer if I want to drive 4 ohm loads the guidelines would have been on the assumption of 8 ohm loads... If I want a hope of delivering the extra current that a 4ohm load can demand then I really need to double the transformers capacity. doh. Tony. 
21st September 2013, 10:03 AM  #1182 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

The transformer for a ClassA amplifier is usually specified as 6times to 10times the maximum ClassA output.
Your 25W ClassA amplifier should require a 150VA to 250VA transformer. A two channel amplifier would be expected to use a 300VA to 500VA transformer. BUT !!! Do you have a 25W ClassA amplifier?
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regards Andrew T. Sent from my desktop computer using a keyboard 
21st September 2013, 11:25 AM  #1183 
diyAudio Moderator

I don't have anything as yet, but that is a damn good question Andrew! My limited understanding of class A is that whatever the bias current is, that is he max current that you will can deliver class A power with. My over simplistic calculation is that if I set bias to 2.5A that into four ohms 2.5A will result in a 10V voltage drop. Power = VI so that means 25W.
Now the BA3 with complementary output stage will in fact deliver more power than the class A power (but fall into class B to do it). But my limited understanding led me to that conclusion regarding the class A power Something tells me however that I've probably completely missed something (or badly botched some calculations) Tony. 
21st September 2013, 01:27 PM  #1184 
diyAudio Moderator

OK so I went and read Nelson's "Leaving class A article" and it seems my assumptions were based on single ended Class A and that my 25W should actually be 50W... that seems high!!! have I really well and truly missed something? It doesn't seem so, based on the table given in the article (if I double the bias current when doing my calculation it fits with the peak class A power in the table).
I guess I can probably drop the bias down a bit If I so choose. I think I'm now more confused than ever Tony. 
21st September 2013, 04:51 PM  #1185 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Winternute.
You really need to do some reading and learning. Single ended ClassA can deliver upto bias current value as the maximum output current. No More ! Push Pull ClassA can deliver upto two times bias current value as the maximum ClassA output. Push Pull can also transition into ClassAB for all output currents that exceed two times the bias current. I am sure (I will guarantee) that these three facts have been posted much more than once on this Forum. Now to output power P = I² * R = V² / R = I * V all currents and all voltages are in constant value DC values. If you want to work with AC currents and AC voltages you must use rms values for currents and voltages. If you are working with sinewave peak values then the formulae change to: P = Ipk² * R / 2 = Vpk² / R / 2 = Vpk * Ipk / 2 These too have been posted on the Forum many times. If you have a Push Pull ClassA amplifier biased to 2.5A then ClassA Ipk ~ 5Apk. Maximum ClassA power = 5² * 4 / 2 for a 4r0 resistive load. i.e. 50W of ClassA into 4r0. The ClassAB power is limited by the arrangement of the amplifier and the PSU. All of this has been posted on this Forum.
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regards Andrew T. Sent from my desktop computer using a keyboard 
21st September 2013, 11:35 PM  #1186 
diyAudio Moderator

Thanks Andrew, I was using the right formulas, and once I read the leaving class A realized that the current for push pull was double that of single ended (I already knew it fell into class B when exceeding the bias, which is why I went for the push pull over the single ended).
I guess I was just a bit surprised that I could get 50W of class A into 4R! that far exceeded anything I was considering doing and the usual stereotypes of class A amps being very low power (due to their inefficiency). This means that I can consider lowering the bias current quite a bit (I still need to consider peak output capability for my transformers). Thanks for clarifying for me. It was mostly a case of I was doing the correct calculation but doubting the results. Tony. 
22nd September 2013, 07:57 AM  #1187 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Your 50W ClassA amplifier would need a transformer of roughly 300VA to 500VA.
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regards Andrew T. Sent from my desktop computer using a keyboard 
22nd September 2013, 10:33 AM  #1188 
diyAudio Moderator

Thanks Andrew, that is what the spice sims tell me as well. I think though that I need to size based on the total power (class A and then class AB when pushing past the class A point) just for headroom reasons. Anyway I've got a better grasp on what is going on now
Tony. 
22nd September 2013, 11:38 AM  #1189 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

normally biased ClassAB generally works well with 1.5Times Maximum power.
If your 50W into 4r0 becomes 100W into 2r0 then only 150VA is necessary. If you now "correct" that to include your high bias, then you usually find the 6times ClassA is sufficient to meet the very high bias ClassAB quite adequately.
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regards Andrew T. Sent from my desktop computer using a keyboard 
22nd December 2013, 10:34 PM  #1190 
diyAudio Member
Join Date: Sep 2009
Location: Santa Fe, Argentina

follow up from a post in the Tubes/Valves section " Vacuum Tube Spice Models"
I've got this fft sims (a push pull tube amp at 1W output) same circuit, same conditions , same everything, xcept diff windowing a default (no windowing) b Blackman c Hann I'm interested to learn which one is most adequate for (analog) audio devices analysis, and , if possible, why
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