200W+50W Bi-Amp Power Section and Crossover Replacement For Advent Powered Speakers.

So this thing is assembled and working, tested at about 95% non-fail. The owner is happy, and I would like to thank him again for giving me the opportunity to design his replacement amp. I think it looks pretty sexy all assembled and glowing.

Before:

After:

The replacement boards are for a late 70s bi-amplified powered speaker made by Advent.  They were funded by the speaker’s owner, Christopher Jensen, who wanted a set of amplifiers with more stability in high temperature conditions; The finished boards represent a couple months of collaborative effort. Our solution was using five National Semiconductor LM3886 Overture chip-amps (a higher output version of the LM3875 “Gainclone” chips): one chip (50W RMS) for the tweeter driver and four in bridged-parallel configuration (200W RMS) to drive the woofer. Later it was decided to build the crossover into the system instead of using the original system’s crossover board.

The design for the high channel is a straight forward LM3886 non-inverting amplifier with an AC coupled input circuit. For the bridged parallel amplifier we used the Fig. 11 BPA-200 design from National’s BPA-200 Application Note, which includes the servo amplifiers for output voltage offset compensation and uses the LM3886 in both inverting and non-inverting modes.

The boards were done in 2oz. copper and black soldermask, with integrated fuses and reverse voltage protection diodes. All high power speaker connections were done with .250″ Fast-On connector tabs. Signal inputs use RCA jacks. Rails for the LM3886 are about 1/2″ wide, running along the edges of the PCB, with a pair of 1000uF Panasonic FM series caps on each rail at each chip-amp. 2200uF FC series caps will fit into the same location. Power for the servos and buffer amp was provided by fixed +15V and -15V linear voltage regulators. The unregulated supply was also replaced, using a 500VA Avel toroid transformer, and very large electrolytic capacitors (they’re like soda cans).

The crossover circuit is integrated into the amp as a module with 0.1″ pitch headers for signal and power and locations for standoff mounting. The power section board can be used with or without the crossover, both amplifiers and the crossover have RCA input connectors. Two different crossover module boards were designed: one based off a 2-pole Linkwitz-Riley active crossover circuit, another using a 4-pole Linkwitz-Riley active crossover. Both include an input buffer before the active filters, and a simple comparator circuit with adjustable reference for driving a clip detector LED.

During design, a potentiometer for volume was added to the inputs when the crossover module was added. This makes the LPF designed into input change roll-off frequency with adjustments of volume, although in later designs this can be fixed by replacing the amp’s input resistor with the attenuator pot, unlike how it is now, before the input circuit filters and coupling. The LPF in the input circuit can also be sensitive to different source impedance. This circuit can easily be eliminated with 1206 jumper and omitting a capacitor. Also Christopher used a cap in series with the feedback divider on the high channel power amp to get its output voltage offset similar to the less than 1mV values of the servo-stabilized bridged-parallel low channel amp. He stood the 1206 resistor and capacitor up so both they fit onto the resistor pad. For this to work on the lower frequency amp or full range amps, you would need a very large value bi-polar capacitor.

I’m told pictures of the re-assembled powered speaker will be available as soon as the silicone sealer stuff dries. I guess for the rear panel, or speakers, or something. Those pics and possibly also circuit schematic and PCB layouts in later posts.

Posted by rmrubin October 2008

Comments are closed.