16W Class A - inspired by JLH
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Mark Johnson
diyAudio Member

Join Date: May 2011
Location: Silicon Valley
Quote:
 Originally Posted by john_ellis The correct equation for the feedback ratio is, of course, F=Rg/(Rg+1/(1/Zcf+1/Rf)) where Zcf is the impedance of the feedback capacitor, Rf the feedback resistor and Rg the "grounding" resistor. (This is a complex number expression, just in case someone does not realise).
This starts to become important at very low frequencies. The simplifying assumption (Cf=Infinity) rather than (Cf=1000 microfarads), introduces a magnitude error of 0.1 dB at 1.5 Hertz. And the error gets larger and larger, as the frequency falls farther and farther below 1.5 Hz.

The simplifying assumption also introduces a phase error. Calculating phase using the simplifying assumption introduces an error of 8 degrees at 1.5 Hz, and the error is 1 degree at 13 Hz.

 12th November 2018, 04:35 PM #92 john_ellis   diyAudio Member   Join Date: Sep 2006 Indeed. The equation dealt with not the LF time constant but the HF. This was to explain the apparent 10MHz unity loop gain frequency. The full expression including the LF time constant is F=(Rg+Zcg)/(Rg+Zcg+1/(1/Zcf+1/Rf)) where Zcg is the impedance of the decoupling capacitor (1mF) in series with Rg. There will indeed be phase errors at LF as with nearly all feedback networks using this approach, but errors of 0.1dB/1 degree well below audio band is probably not too relevant. Last edited by john_ellis; 12th November 2018 at 04:47 PM.
mjona
diyAudio Member

Join Date: Jul 2007
Location: Upper Hutt
Quote:
 Originally Posted by PRR Not in the least. JLH would not approve. mjona said "any". I happened to have the PL400 plan handy. mjona is correct: here it is "required" for current-sharing.
In fact JLH designed a quasi-complementary amplifier for Hi-Fi News and illustrated this in two books he wrote. These style of amplifiers are not picture perfect but they work.

This style of output stage might have been used in the present design. I see this approach as hard to ignore but it has been overlooked.

 16th November 2018, 04:33 PM #94 john_ellis   diyAudio Member   Join Date: Sep 2006 Project update ... I have started to populate a prototype PCB based on Prasi's layout. There is a slight problem I didn't spot earlier (one reason to check the PCB before signing off). Will need to obtain some parts I don't have "in stock" so should be ready to test in about a week. Last edited by john_ellis; 16th November 2018 at 04:38 PM.

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