The 4401 was launched in 1975, two years after the mighty Bose 1801 power amplifier. Esthetically speaking it is a perfect match to the 1801 and the construction has the same high level of quality, rugged and modular. This was the age of quadraphonic records and the 4401 has provisions to receive plug and play modules for the various standards of that time : CD4, SQ Matrix etc...

Looking at the innards of the beast made me realize how under rated was the 4401 amongst competitors like Mc Intosh, Marantz and the like. The Bose is amazingly well built. However, on the audio performance side (or rather lack of) there is room for improvement. First by getting rid of the dreadful opamps the 4401 is plagued with. I am pretty sure that poorer than average transparency and dynamics of the 4401 were down to these  Raytheon RC4136DP which are nothing else  than two µA741s etched on the same substrate. The µA741 is probably one of the worst existing opamps for audio applications and any audiophile will stay away from it at all costs. This opamp is described as a sonic disaster by the most demanding audiophiles. That's the reason why I got hold of six  LM4562 dual opamps, which are the finest audio opamp available today. They  will be installed in the 4401 by the means of Brown Dog adapters as you can see on the pictures. These adapters are designed to accept two dual opamps (SOIC format). Two LM4562s will fit on one adapter and voilà ! Full compatibilty with the RC4136.

For the moment I have only verified that the preamp powers on and checked various voltage test points as per the Service Manual. I have to say it's a faultless device and it's a credit to Bose past glory : 30 years after it left the factory the 4401 is perfectly silent and voltages are up to specs. Of course, all of the electrolytics capacitors will be replaced with audiophile grade ones (low ESR, low noise from ELNA audio and Nichicon Muse). From my experience, secret to lifelike music reproduction and  audio transparency lies in the fastest possible power supply circuitry. And remember : there is no magic in tweaking. I also keep repeating that quality is never an accident.  Of course, noise, distorsion, IMD and THD figures  reports BEFORE and AFTER the mods will be published here, so watch this space !

There are three RC4136 in the 4401 (hence the six LM4562) they are in charge of processing the delicate audio signal. Thanks to Bose engineers, the opamps were  installed on sockets (see pics) which make removal and swap even more easier, although later in my project I decided to use ZIF sockets to make playing with opamps easier ! The rest of the active components in the preamplifier consists in FET transistors used for muting the outputs at power on/ off - no upgrade is needed there, they perfectly play their role. There is also the power supply rectifiers, these will be replaced with STTH Ultrafast soft recovery schottky diodes for the sake of signal/noise ratio.

I have finally decided to replace the ouput transistors as well (black parts looking like mushrooms on this picture) with modern and highly regarded epitaxial planar devices : 2N5416 will replce 2N3644 and 2N3440  will replace 2N3569. These are in TO39 package and will be a perfect fit. They will ensure better reliability and also allow ultimate audio performance : a quick check of  the datasheets for Vcbo, Vceo, Vebo indicates that every single characteristic is at least twice better for the newer transistors. This shall considerably increase the 4401's  driving capabilities with higher output voltage and current swing with less distorsion and avoid clipping. 

I did a first test run, preamplifier right out of the shipping box that is to say in it's original condition. Results can be seen here. It should noted that right channel appears to be dead with horrendous response curve and distorsion figures. This is probably down to a shot capacitor leaking DC in a feedback loop or on a RC4136 power supply rail. Nothing to worry about : after 30 years or so it is no surprise when an electrolytic goes belly-up. This is why the report shows poor general performance, however left channel has excellent distorsion figures. Comparisons will be made with this channel after the refurbishing and upgrades as a reference. After the Bose 4401 has been fully rebuilt the general performance has seen a huge improvement. Comparison with the original Bose specifications shows how modern parts used at the right place can turn a slightly dull unit into an audiophile preamp killer.

The E111 FET transitsors used for muting the ouputs are quite noisy. Most of the 4401's hiss comes from there. These FETs  will be replaced with modern ultra low noise Toshiba 2SK170 devices. I will report possible signal / noise ratio improvement after the transplant.

Related links :

• Bose 4401 Service Manual
• Bose 1801 modifications page
• Bose 1800 Series II
• Bose 901 series II equalizer

Parts list  :

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Some measurements 

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