Opamp suggestion for DC-servo in an Alexander amp

[jcarr]

Per:

>The Alexander amp is a high speed design so I think the physical design is really important. No hard wiring!<

If you combine hardwiring (component leads only, no separate wires) with low-impedance ground and power planes, and design a tight layout, you can make some pretty high-bandwidth circuits. I've designed and built all-discrete 600MHz amplifiers like this.

If anything, I find it easier to go faster using 3-dimensional circuit structures rather than keeping everything on the pcb (2D). A calculated mixture of smd, leaded board and 3D also works well, and is how I normally design.

hth, jonathan carr

[Emsad]

I have also build this amplifier a year ago with six pairs of IRFP240
and IRFP9240.
The driver transistors are ultra fast BF758 and BF762 they have bean used in tv as uhf amplifiers, i think they are discontinued.
I have used OPA604AP from Buur Brown as gain stage instead
of SSM2131.
I did not use DC servo beacose offset was always beatwin +/-
30 to 50mV.You have to place trim pot beatwin pins 1,5 for offset triming.
i dont know why peaple say that amp oscilates, its very stable
i also didnt use zobel network its not nesesery this is high bandwith amp,and you dont want to put low pas filter in the output.
you also must take care of grounding of the driver board,i did
had a problem with input left open it is very sensitive beacose
input capacitor(750pF) is a filter which make this amp goes to
1MHz.
The amp is unbelivably fast(transient reasponse) if you put some
CD with acustic gitar or piano or some pasage with some good solo drums it realy scare you.
The amp reali have total control of the speakers, if you never heard amp with high bandwith and high slew rate(250V/us)then you peaple dont know what you are mising.
i wil send some foto's in a few days (driver and power print).
I am also suprised how few peaple have build this amp.

[mikeks]

Originally posted by jcarr
Per:

. I've designed and built all-discrete 600MHz amplifiers like this.

jonathan carr

Hi Jonathan,

Why do you find it necessary to design such a wide bandwidth into your audio amps.?
mike.