1801
modifications by Thierry Martin
dedicated to Bill Tobert, a friend from Georgetown, Texas and owner of several Bose 1801 amps.
Here are the upgrades and modifications performed on a faithful 30 years old Bose 1801 power amplifier. Among other tweaks was the considerable increase of power supply energy reserve, thus the facelift done by using a Bose 1800 faceplate to allow enough space for the additional reservoir capacitors. The power supply now totals 88000µF / 90V. The boards have also been completely refurbished and some areas were redesigned, particularly the input buffer power supply. The output stage is using Motorola MJ15024 transistors (250W x 28) instead of the original Bose output transistors, old RCA TO3s. The input stage has received a Burr Brown OPA627 opamp and benefits from redesigned power supply circuitry. Polypropylene decoupling caps have been installed in selected places, they can be seen on some pictures (big black axial caps).
None of the original parts were kept apart from the power transformer, the printed
circuit boards and the chassis : new caps, new resistors, new transistors, new
everything. The original bridge rectifier has been replaced with two 60A Schottky
ultrafast power diodes from ST. A relay operated DC ouput protection device
has been installed in the very unlikely case something would go wrong. Speakers
connection is delayed by 6 sec. at startup.This amplifier now compares easily
to the best audiophile amps available with huge power available for any
use. It is mainly used with a
pair of Bose 901 series II which are a perfect match. Actually these speakers
are so power hungry that they NEED such an amp. Here's a link to the Bose
901 series II EQUALIZER .Alternatively the 1801/1800 serves as bass amp
in a multiamplified system, for this usage it is impossible to beat. If needed
this amp can be used as a monoblock, paralleling the outputs will provide 1000
watts of pure power on a 8 ohms load ! Just invert signal phase on inputs
and connect load between "hot" outputs of both channels, it's as simple as this.
A matching preamplifier was marketed shortly after launch of the 1801, it was the Bose 4401 that I was lucky enough to be given as a present by my friend Bill Tolbert.
Since these mods I have acquired a Bose 1800 Series II which is the immediate successor of the Bose 1800. A more compact design makes it easier to transport - well, you need to have a good back with 31Kgs of copper and aluminium in your hands - but it still has a large amount of power available for any use, professional or domestic.
Link : the Bose 1801 Service Manual
Click picture to view normal size image.
