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Power Supply Load for Class AB Amplifiers
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Author:  samc [ 08 Sep 2019, 19:57 ]
Post subject:  Power Supply Load for Class AB Amplifiers

Hello Everyone,

I'm a little uncertain about how to estimate the current loads for a Class AB amplifier. I've noticed in the models I've developed of my current design that the maximum current used by the AB power stage of the amplifier is basically the peak current for a single tube on the composite load line I drew. This is because one tube is not conducting when the other tube is at max plate current.

In Matt Renaud's article on power supply design on this website, his recommendation is to use the peak current for the power stage as the design load. However, his article seems to be more in the context of Class A amplifier operation. I noticed on another site, vtadiy.com, a recommendation to use the quiescent current for the Power output stage, with the commentary that the B+ voltage for the power stage will then drop a bit at maximum input signal amplitude since the power stage is designed for a lesser current. The phase splitter and voltage gain stages of my design do not seem as impacted by this choice since: 1) they're Class A and 2) their current loads are very small anyways.

The two selections of design values are quite a bit different. For the 6v6 output stage I'm working on, there is a nearly 4X difference in output stage impedance, defined as Vb+ / I_load = Z_load. The component values for the precursor filter stages are meaningfully impacted by the change in model output impedance, if I try to target the same Vb+ .

Would someone with a bit of Class AB power supply design experience be able to help me choose an appropriate load current value for my power supply design?

Author:  Suncalc [ 10 Sep 2019, 11:15 ]
Post subject:  Re: Power Supply Load for Class AB Amplifiers

samc wrote:
Would someone with a bit of Class AB power supply design experience be able to help me choose an appropriate load current value for my power supply design?
Herbert Reich covers this topic in section 7-26 of "Theory and Applications of Electron Tubes" (full text here: http://www.tubebooks.org/Books/reich.pdf Warning: 37MB download). See this excerpt:
Attachment:
Figure 7-20.png

Attachment:
Section 7-26 Excerpt.png

I would suggest using Reich's method for determining the power input for your power stage.

I hope this helps.

Author:  ILoveHiFi [ 10 Sep 2019, 23:14 ]
Post subject:  Re: Power Supply Load for Class AB Amplifiers

For solid state amplifiers, if your amp can output 2x 100w RMS into speakers.
Then you need a power supply that will handle 200w*110% = 220W

For valves, if its rated for 2x20W RMS (class AB)
Then power supply needs to handle 40W*130% = 52W

The peak power of 20W RMS is 40W (doubble)
THis means the power supply needs to handle a peak power of 80*130% = 104W

The components only need to be rated for the RMS power, which is 52W.

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