Information request for modifying Bose 401s

I am new to this site and am hoping you guys can assist me with some improvement modifications to my pair of Bose 401's I picked up at an estate sale. The cabinets are in perfect condition, the grills are good and original, and the drivers and tweeters are fine (I replaced one tweeter with a replica of the size and specs that was broken.)
As seems to be the case with these speakers, the highs are muddled. Those from the left tweeter almost sound as if the tweeter is blown, but it is new, so that is not the case.
A user on a different Audio site said that he performed the following mods on his 401's and the result was a much improved sound. He has not revisted that site recently, so my questions to him about the specifics of how he did it and where he got the materials remain unanswered. I am hoping someone here can provide those answers.
Here is what he did:
I got a pair of Bose 401s.
One of the cabinets was falling apart and I ended up gluing the cabinet back together. The cabinet workmanship is pretty poor quality. The woofers have no crossover and the tweeter uses an inferior electrolytic capacitor. The wire that Bose uses internally is small gauge, single strand telephone wire.

  • I added a small Neodymium magnet to the center of the magnet structure on all of the speaker drivers. The result was faster transients and reduced "audio blur".
  • I replaced the tweeter's capacitor with a good quality polypropylene 4uF capacitor.
  • I added an inductor with a 4 ohm resistor across the inductor for the main 6 ½ inch woofer and ran the "surround" 6 ½ inch woofer straight.
  • I replaced all of the wires with 16-guage wire.
The Bose 401s sound much better now.

Any help you can provide is greatly appreciated! I want to enjoy these speakers, but right now I am just not getting a good quality audio reproduction. I had a pair of 501 Series IV speakers many years ago and they sounded great; much better than these 401's!

Thank you!


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the highs are muddled

Welcome to the forum!

In the first instance you should replace the tweeter's ageing bipolar electrolytic capacitor with a modern equivalent.

Over the years, electrolytic capacitors can change significantly in capacitance and series resistance values.

Replacement will return the capacitor to the original specifications and hopefully restore the missing 'sparkle' to the tweeter.

I suggest you try this before attempting any other modifications then report back on your findings.

P.S. A photograph of the capacitor will confirm if it is an electrolytic or film type. Bose used a film (polypropylene) type in the 301 Series II I renovated.
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It the 4.0 uf capacitor has two pinched ends, it is electrolytic and suspected of excessive age.
If the ends are not pinched and it is marked PP or MKP somewhere, is it already polyprophylene.
Here is one source of suitable capacitors in Ohio. Another supplier in Florida is Madisound. Also USspeakers
Usual suppliers like digikey, newark, mouser, do not usually stock >100 vac polypro capacitors. The one I listed is a 250 vac, good for >800 w/ch use.
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Personally, without a way to measure said capacitor, I wouldn't just throw parts at it.

A lot of what your hearing is the woofers. One or more of those could be compromised. The tweets are usually crossed fairly high. That said I don't see many of those woofers with issues as they are very similar to the 201's.

I also wouldn't agree with all the "upgrades" from the other person.
Where do I find the caps? With poly-fill so jam-packed inside the cabinet, I am unable to find them. Since the cabinet is not able to be opened without damaging it, please let me know how to locate those caps.


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For your information, the capacitor and protection lamp are wire wrapped to posts screwed into the enclosure wall.

When removing a wire wrapped component, snip its leads at the posts but don't remove the wire which is wrapped round the posts.

The replacement component can then be soldered to the wire wrap instead of directly to the post which, not being copper, may not take solder.

P.S. Work with the speaker on its back so that no strain is placed on the wiring by dangling drivers!