170W/8Ω tri-amplified system

Rohit Balkishan Dubla

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This page describes the 3-way active amplifier system that I built. It is an upgrade to an older system, done by adding another amplifier and crossover to it.

The photo above shows the complete setup. The wooden box contains the three power amplifiers and the pre-amp cum crossover. The power amplifiers are built around the LM3875, STK4132 and LM1875 audio power ICs, each catering to the low, mid & high ranges respectively. LM3875 is a mono amplifier rated for 56W into 8Ω (two for stereo), STK4132 is a stereo amplifier rated for 20W + 20W into 8Ω and LM1875 is a mono amplifier rated for 20W into 8Ω (two for stereo, each configured for 10W into 8Ω). The crossover is an active 12db/oct 3-way Bessel type with the crossover frequencies set to about 312Hz and 3.4kHz. Sitting over it is a Pioneer AM/FM tuner. Above this, on a plank are the JVC L-E30 turntable and its phono pre-amp cum RIAA equaliser (see Phono pre-amp version 2). To the left and right are the speakers bought from a seconds shop. The large floor-standing ones are Jamo make and the ones mounted on the Jamos' sides are from a Panasonic component system (SC-PM17), and were meant to be bi-wired. The tweeters have been disconnected, and the bookshelf speakers are used solely for the mid-range. The Jamo speaker is a 3-way type, and the mid was crossed over at about 4kHz and tweeter at about 9kHz. I removed the 4kHz crossover (which happened to be a single series capacitor) and connected the tweeter amp to it. With the current steup, the Jamo mid is now the lower treble crossed over with the mid-range at about 3.4kHz, and the tweeter forms the upper treble section at 9kHz. However there is no degradation in performance and the overall system now becomes 3-and-a-half way.

The photo above shows the internals of the system. The power transformers are on the left side - the upper one (torroidal) is for the LM3875 amp and the lower one obscured by the rectifiers and filters mounted on a cardboard base over it powers the rest of the circuits. Though not clearly visible there is a 3A fuse (and holder) just below the torroid. Just to the right of the torroid is the delay circuit which mutes the mid-range drivers and woofers for about 2.5 seconds after powering up, to suppress start-up thumps. Towards the lower-right corner can be seen the two crossover boards. The one on the right (green coloured) is the preamp cum 3.4kHz crossover while the one on the left is the 312Hz crossover. The 312Hz crossover's input is fed from the low-pass section of the 3.4kHz crossover and hence the high-pass o/p of the 312Hz crossover becomes a mid-range output. To the left of this (towards bottom-centre) is the LM1875 amp - it is upside down and the PCB is supported on the IC pins which are in turn mounted on the heatsink. The heatsink is fastened to the base using cable-ties. This may look bad, but in practice the weight of the small PCB is easily taken by the IC pins (as long as it is not subjected to violent vibrations). Towards the right-centre and above the crossover boards, is the STK4132 board. To its left is the LM3875 board - the bridge and the filter capacitors are also mounted on-board since it allows for short wires between the filters and supply pins. I have not used supply decoupling capacitors and yet the amp is completely stable - perhaps because of the short wires from the filter caps to the supply pins. The heatsink for the LM3875 board appears smaller than the STK4132 - the LM3875 heatsink is taller (and thicker) which is not obvious due to the way the photo has been taken. The wiring has a lot of joins as can be inferred by the number of insulation tape wraps - that's because I soldered the input/output/power wires on-board and joined the respective wires between the boards to interconnect the system.

The front panel is a thin wooden sheet mounted over the crate's front face using five 3cm tall wooden pieces. This arrangement makes it easy to mount the switches, pots, etc., without having to drill large holes (recesses, rather) in the 10mm thick wood of the crate to fit these. Only small (5-10mm dia) holes are needed to bring out the wires to the front panel. Since the panel is thin (3mm) it is easy to mount the controls on it without a lot of drilling effort.

The above photo shows the rear panel. The speaker terminals are made by mounting electrical switch-board type connectors to the panel. The wires from the amps are brought out from small holes drilled beneath the terminals. This arrangement is very secure since the speaker wires are screwed tightly into the connectors. On the downside it doesn't lend itself to quick (or comfortable) connections/disconnections as standard speaker binding posts would. Again, the need to avoid drilling recesses led me to choose this method. The input RCA jacks are mounted on a thin wooden sheet which in turn is mounted on the rear panel using 1cm thick spacers. The mistakes in marking can be clearly seen ;-).

Subjectively, the sound is very good - bass is smooth and clear without having to turn up the volume. Earlier the volume had to be somewhat high for bass - ok during daytime, but rather loud during the night. This is because the low (or bass) range is extended (using a peaking 2nd order high-pass filter after the low-range o/p of the 312Hz crossover) to about 30Hz (-3dB) with a 3dB peak in SPL at about 70Hz. The peaking filter's parameters are Gpk = 6dB and Fpk = 30Hz.

Specifications

Amplifier

Power (low): 56 + 56W/8Ω.

Power (mid): 20 + 20W/8Ω.

Power (high): 10 + 10W/8Ω.

Pre-amp cum crossover

Pre-amp gain: 6db

Crossover type: 3-way active, 12dB/oct Bessel alignment.

Crossover frequencies: 312Hz and 3.4kHz.

LF eq: Active 12dB/oct peaking high-pass (Gpk = 6dB, Fpk = 30Hz)

Speakers

Woofer: 8Ω, 16.5cm (Jamo)