Bose 4401 Preamp Restoration!

You were a big influence on my decision to get Bose and a great help on the restoration of these awesome classics. Your threads and input are a tremendous aid to all Bose 180X and 4401 owners. Thanks Thierry!

I will be moving next summer, and I am looking forward to placing this system in a better listening environment. It is to be our last home before assisted living, so we are looking for a one storey single home with all the wanna haves included or added. Very excited about the potential.
 
I recently restored another one of these that did not have the noise issue. I noticed there was a shield around the AC inlets that Bilbo's unit was missing. So Bilbo brought his unit back and I made a matching shield out of a heavy copper foil between two layers of cardstock. I made sure the copper was grounded too. I thought this explanation made a lot of sense, however it does not seem to have made much of an improvement if any. Any thoughts?

Here are pictures of the original shield and my replacement.
 

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Hi,

My own 4401 has the original shield which is made out of cardboard, exactly the same as on your right photo.

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It's function is to prevent unwanted contact between tech's hands or tools and high voltage power supply parts.

Cardboard is totally unable to protect audio circuits from RFI or EMI. If there's noise in Bilbo's unit, it must be down to a faulty part or circuitry elsewehre.

Is it a hum, a buzz or hiss ? Is it volume level dependent ? Did you check that all of the power supply parts (capacitors, diodes, resistors, voltages) match specs ?

Thierry
 
Hi,

My own 4401 has the original shield which is made out of cardboard, exactly the same as on your right photo.
It's function is to prevent unwanted contact between tech's hands or tools and high voltage power supply parts.
Cardboard is totally unable to protect audio circuits from RFI or EMI. If there's noise in Bilbo's unit, it must be down to a faulty part or circuitry elsewehre.
Is it a hum, a buzz or hiss ? Is it volume level dependent ? Did you check that all of the power supply parts (capacitors, diodes, resistors, voltages) match specs ?

Thierry
I figured maybe there was a foil inside the original cover too, I guess not.

Since we replaced pretty much every component, I'm not sure what could be bad.
It was a hiss. There is always some hiss though my speakers but it is a lot more noticeable with this unit. But it isn't horrendous. Not volume dependent.

I did previously notice a little extra noise with the scope on one of the 15V voltage rails but never found why, or even if it was the problem.

I also noticed I used the panasonic ECQ caps inside Bilbo's unit, but I used the better ECW polypropylene caps in the one I just did.
 
If hiss isn't too bad - comparable to white or pink noise - it is normal, and not surprising considering that there are carbon resistors all round in there. I cured it on mine by replacing all resistors with metal film ones. Tedious and long work, but worth the effort.

Noise also depends on how much power has your amplifier and how sensitive it is. the 1801 owners manual advised to adjust it's gain potentiometers to find best compomise between noise threshold and sensitivity. I suppose you'll have to live with this. You can also engage MUTE pushbutton, this will reduce the 4401 output by 20dB, which can then be compensated by the volume control.
 
If hiss isn't too bad - comparable to white or pink noise - it is normal, and not surprising considering that there are carbon resistors all round in there. I cured it on mine by replacing all resistors with metal film ones. Tedious and long work, but worth the effort.

Noise also depends on how much power has your amplifier and how sensitive it is. the 1801 owners manual advised to adjust it's gain potentiometers to find best compomise between noise threshold and sensitivity. I suppose you'll have to live with this. You can also engage MUTE pushbutton, this will reduce the 4401 output by 20dB, which can then be compensated by the volume control.
In this thread we replaced pretty much every component, the resistors were changed to metal film, the signal path caps were changed to film caps, opamps were upgraded, etc. Even ceramic caps were upgraded. Both this preamp and the one I just restored were both played in my system, and Bilbo's does have a bit higher noise. My system is Tannoy HPD drivers with a Kenwood 700m power amp.
 
Okay, seems acceptable. Sorry, I had forgotten about the mods you did (thread started 4 years ago !) congratulations for your perserverance.
 
Okay, seems acceptable. Sorry, I had forgotten about the mods you did (thread started 4 years ago !) congratulations for your perserverance.
I noticed I used the panasonic ECQ caps in the signal path in Bilbo's unit, but I used the better ECW polypropylene caps in the last one I did. Would that affect the noise floor?
 
My 4401 has the CD-4 and SQ decoder boards. But there is another connector on the bottom and position on the selector switch for a third decoder. Does anyone know if Bose ever made a board for QS? Or another format besides SQ and CD-4?
 
Does anyone know what could be wrong the muting circuit? I'm stumped on this one. See post 51 for the scope output when muting is engaged.

Hi Raccoon1400. It’s been a long time since this post regarding the muting circuit problem. I just finished the restoration of my 4401 and encountered the same problem. In my case I replaced the E111s with matched Linear Systems LSK170 FETs. Fortunately the E111s where in good working condition so the only fix I could find was to go back with the E111s.

Is the LSK170 a direct replacement to the E111 or the 2SK170?

Perhaps this is the same problem you had.
 
Welcome to AK, Nicoleto!

I am the owner of the described 4401. Listening to it right now actually. As it has been quite a while since I read this, I'll re-read it so I can respond. Hopefully @Raccoon1400 will jump in.
 
Hi Raccoon1400. It’s been a long time since this post regarding the muting circuit problem. I just finished the restoration of my 4401 and encountered the same problem. In my case I replaced the E111s with matched Linear Systems LSK170 FETs. Fortunately the E111s where in good working condition so the only fix I could find was to go back with the E111s.

Is the LSK170 a direct replacement to the E111 or the 2SK170?

Perhaps this is the same problem you had.
I've restored several of these units now and have had issues with the muting circuit in many of them. It seems to be a finicky design. I just remove the fets bypassing the circuit.
 
T
I've restored several of these units now and have had issues with the muting circuit in many of them. It seems to be a finicky design. I just remove the fets bypassing the circuit.
Thanks @Raccoon1400 and @BilboBaggins for your replies. Fortunately, in my case, the original E111s were working fine so I kept them in place to have the muting function working correctly. However, I did use a pair of LSK170 FETs to replace two bad E111s in the Phono stage.

Apparently, albeit higher noise floor in the phono stage, everything seems to be working OK (Resistors, caps, Op-amps, etc. have been upgraded). However, I wonder if the higher noise floor in the phono stage is caused by the FETS.
 
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