On 12/14/2023 6:05 PM, John Larkin wrote:> On Thu, 14 Dec 2023 22:40:23 +0000, piglet <erichpwagner@hotmail.com> > wrote: > >> Inspired by the late Jim Thompson's autobias design that we saw in the >> thread "Power Amplifier for 100kHz" I thought his use of a bang-bang >> comparator in setting bias was non-optimal but I liked his idea of >> running the pa in pure class B for large signals and setting a minimum >> quiescent class A current for small signals. >> >> My idea uses JT's way of sensing output device current falling below >> threshold but instead of pumping up a "rubber Vbe diode" I pull the bias >> point apart with resistors and a pair of current mirrors. As an IC >> designer JT was liberal with current mirrors so I hope he would approve. >> >> I expect over the decades many clever ways have been devised at >> eliminating crossover distortion and autobiasing class A/B so I have no >> shame in adding another. Does this look interesting or has it been done >> already? >> >> Here is a pdf schematic: >> >> <https://www.dropbox.com/scl/fi/x1tw0nj4d44pgmutxw36p/classAB_rubber_mirrors_autobias.pdf?rlkey=0xr3fv4l2qswyhrombqtanwvo&raw=1> > > > Most of the audio amp circuits posted here seem to have been designed > before the invention of the opamp. >U were designed before the invention of the opamp
Yet another Class AB autobias
Started by ●December 14, 2023
Reply by ●December 19, 20232023-12-19
Reply by ●December 19, 20232023-12-19
On 12/14/2023 9:20 PM, Fred Bloggs wrote:> On Thursday, December 14, 2023 at 6:05:59 PM UTC-5, John Larkin wrote: >> On Thu, 14 Dec 2023 22:40:23 +0000, piglet <erichp...@hotmail.com> >> wrote: >>> Inspired by the late Jim Thompson's autobias design that we saw in the >>> thread "Power Amplifier for 100kHz" I thought his use of a bang-bang >>> comparator in setting bias was non-optimal but I liked his idea of >>> running the pa in pure class B for large signals and setting a minimum >>> quiescent class A current for small signals. >>> >>> My idea uses JT's way of sensing output device current falling below >>> threshold but instead of pumping up a "rubber Vbe diode" I pull the bias >>> point apart with resistors and a pair of current mirrors. As an IC >>> designer JT was liberal with current mirrors so I hope he would approve. >>> >>> I expect over the decades many clever ways have been devised at >>> eliminating crossover distortion and autobiasing class A/B so I have no >>> shame in adding another. Does this look interesting or has it been done >>> already? >>> >>> Here is a pdf schematic: >>> >>> <https://www.dropbox.com/scl/fi/x1tw0nj4d44pgmutxw36p/classAB_rubber_mirrors_autobias.pdf?rlkey=0xr3fv4l2qswyhrombqtanwvo&raw=1> >> Most of the audio amp circuits posted here seem to have been designed >> before the invention of the opamp. > > They're a step up from tubes...Thank goodness the thread seems to have designed a satisfactory audio amp using a 250 MHz 900 V/usec op amp.
Reply by ●December 19, 20232023-12-19
piglet <erichpwagner@hotmail.com> wrote:> On 19/12/2023 5:20 pm, Phil Hobbs wrote: >> >> I think the novel part of JT's design is that it controls what you >> actually care about, namely the minimum quiescent bias under transient >> conditions. Linear methods only get you the average, which doesn't >> guarantee what happens with a large output signal. Mike's method has a >> large transient error at the zero crossing, which gets worse at smaller >> inputs. >> > > No sure about that: JT's design has two big capacitors (100uF and 22uF I > recall) in the bias control so it will always be sluggish on transients. > My second version has no capacitors in the bias control so should be > very adaptive? > > piglet > > >It’s not as good, though. JT’s gizmo samples the quiescent bias only at the zero crossings, and accumulates the results on the big cap on the base of the rubber diode. That takes account of large signal effects, capacitance, and so on. Jim’s 100 uF is too big—something around 1-5 uF is better. You want just enough to avoid distortion in the low bass. The current version of the zip file I posted has a plot file and screen shot that shows an honest 0.0005 % THD while putting 8 V pp of 20 kHz into 4 ohms. Swapping out the $7 THS4631 for a 50-cent OPA172 still does 0.01% THD, which is much better than your average car speaker. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
