Bose 4401 Preamp Restoration!

Raccoon1400

Super Member
Here is the matching preamp for the bose 1801, from BilboBaggins.

The plan to replace the following
-opamps
-transistors
-electrolytic caps
-mylar caps
-carbon comp resistors

First, I am looking at the signal path caps. 8x 5uf axial electrolytic caps, and the output caps 4x 16uf NP caps. (One per channel, there are 4 channels!)

I see two options for the 5uf caps. Panasonic has 5uf polyester caps. And 4.7uf polypropylene caps. Would the 4.7uf ones be close enough? I'd prefer to use the polypropylene ones. You can find the SM on hifiengine, they are on page 10, near the op-amps.

For the output caps, it would be good to use polypropylene as well. But physically fitting them would be hard. Thinking of solens, or maybe wima.

-Raccoon
 

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Looks like it's been gone through already.. like super clean? Doesn't it work?
 
Coupling caps are like hi-pass filters: larger caps allow low freqs to pass. Smaller caps filter the low freqs.

Anyway, I think 4.7 is close enough to 5 uF (6% smaller). I'm sure the originals are 20% tolerance, so 4.7 should be OK. I prefer polypropylene too.

For the 16uF, I'd stay with electrolytic. Film caps will be too large and too expensive. Add some 0.01 , 0.047 or 0.1 uF by-pass if you want.
 
Here is the matching preamp for the bose 1801, from BilboBaggins.

The plan to replace the following
-opamps
-transistors
-electrolytic caps
-mylar caps
-carbon comp resistors

First, I am looking at the signal path caps. 8x 5uf axial electrolytic caps, and the output caps 4x 16uf NP caps. (One per channel, there are 4 channels!)

I see two options for the 5uf caps. Panasonic has 5uf polyester caps. And 4.7uf polypropylene caps. Would the 4.7uf ones be close enough? I'd prefer to use the polypropylene ones. You can find the SM on hifiengine, they are on page 10, near the op-amps.

For the output caps, it would be good to use polypropylene as well. But physically fitting them would be hard. Thinking of solens, or maybe wima.

-Raccoon

Hi !

Good move, good choices.

Here's how mine looks now.

Great sound improvement, which is confirmed by excellent measurements results

MKP is the way to go : for tone control circuitry, coupling and front channels output caps I choosed Wima and SCR polypros.
To allow comparisons with the MKP output caps I used Nichicon Muse electrolytics for the rear channels.
Indeed they do the job very well too and do not occupy a huge space as those big barrel polypros do.

Thierry

P1150481.JPG




Phono board : precision (1%) silvered mica caps for RIAA equalization networkl, 1% metal film resistors, Panasonic FC series electrolytics.
15V Power supply decoupling cap (C3) went from 47µF to 220µF (Panasonic FC series).
RIIA equalization sticks exactly to RIAA specifications (will put a pic online soon).
Now, with volume control knob rotated fully clockwise the phono stage is dead silent :D



The phono board before
P1150474.JPG


The phono board after
P1150477.JPG
 
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The C109, C209, C309 and C409 capacitors are, as elnado stated, part of the input high pass filter; with the stock value of 5µF, the F3 cut-off is 0.32Hz. The stock value is necessarily large to minimize the sonic impact of using an electrolytic as a coupling capacitor; if you replace with a film you can use lower values and still achieve a low enough cut-off frequency (e.g., a 2.2µF film would give a 0.72Hz cut-off). This affords the use of a very high quality film and it will still fit on the board nicely (caps that are over-sized and ill-fitting are prone to adding noise to the circuit.
 
The C109, C209, C309 and C409 capacitors are, as elnado stated, part of the input high pass filter; with the stock value of 5µF, the F3 cut-off is 0.32Hz. The stock value is necessarily large to minimize the sonic impact of using an electrolytic as a coupling capacitor; if you replace with a film you can use lower values and still achieve a low enough cut-off frequency (e.g., a 2.2µF film would give a 0.72Hz cut-off). This affords the use of a very high quality film and it will still fit on the board nicely (caps that are over-sized and ill-fitting are prone to adding noise to the circuit.

I guess this is what you were telling me about doing the math. Makes sense.

What about the caps C112, C212, etc? They are in the signal path but I don't see the filter. I don't see a highpass filter for the output caps either. Maybe I'm just missing it. Can we apply similar logic here?

4.7's are fine. Film output caps would be nice, but will be a bit large...these Dayton's are about 1" diameter and about 2" long: http://www.parts-express.com/dayton-audio-dmpc-15-15uf-250v-polypropylene-capacitor--027-432

But they are pretty cheap.

If it were my amp, I'd want to see how much DC is on that cap, and could I get rid of it altogether...

My meter only reads 1-2mV...
 
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A question about replacing the resistors. The carbon comps are all 1/2W. I imagine the majority of them do not need to be.

Thierry:
What power ratings did you use for resistors? I can see you used 1/4W for the phono. What did you use elsewhere?
I also notice you put heatsinks on the op-amps. Should I do this?
 
A question about replacing the resistors. The carbon comps are all 1/2W. I imagine the majority of them do not need to be.

Thierry:
What power ratings did you use for resistors? I can see you used 1/4W for the phono. What did you use elsewhere?
Standard size for modern 1% metal film resistors is 1/4W and I used them everywhere.
Modern 1/4W metal film can dissipate as muche power as the ancient 2W compressed carbon resistors.
Anyway, in the 4401 there is nothing to worry about resistors power, current is not an issue there and this preamp stays cold lile a stone even after a week left on.

I also notice you put heatsinks on the op-amps. Should I do this?

Just because I like to keep on the safe side but they aren't really necessary.

Thierry
 
Measure from the leads of the cap to chassis ground. We aren't too interested in the voltage across the cap. ;)
 
Standard size for modern 1% metal film resistors is 1/4W and I used them everywhere.
Modern 1/4W metal film can dissipate as muche power as the ancient 2W compressed carbon resistors.
Anyway, in the 4401 there is nothing to worry about resistors power, current is not an issue there and this preamp stays cold lile a stone even after a week left on.



Just because I like to keep on the safe side but they aren't really necessary.

Thierry

+1
It used to be a common practice to use 0.5W rated resistors instead of 0.25W to take advantage of the former's lower temperature coefficient in the actual circuit (results in lower distortion). This is no longer an issue with modern 1% metal film resistors.
 
I guess this is what you were telling me about doing the math. Makes sense.

What about the caps C112, C212, etc? They are in the signal path but I don't see the filter. I don't see a highpass filter for the output caps either. Maybe I'm just missing it. Can we apply similar logic here?

C112, C212, C312 and C412 are indeed directly in the signal path and are part of a high pass filter; the shunt impedances in this case are provided by Q101, Q201, Q301, and Q401. If you can determine the impedance the capacitor "sees", you can apply the same calculation. But I am not sure how one would determine this impedance in this case.

You should also consider replacing the ceramic capacitors in the low pass filter, i.e., C110, C210, C310 and C410. Ceramic caps are notorious for introducing distortion.
 
after doing some research on these pre's I can see why they command a good price even not working. Seem to be very well laid out and accessible and as noted modified. Not to mention quad, but can they be run in 2 channel mode or do you guys run them in quad?
 
after doing some research on these pre's I can see why they command a good price even not working. Seem to be very well laid out and accessible and as noted modified. Not to mention quad, but can they be run in 2 channel mode or do you guys run them in quad?

With a Bose 4401 you get 2 stereo preamplifiers in the same casing or one quad preamp, up to your choice. By instance, if you're lucky enough to own two Bose 1801s you will hook 'em up such that each amp feeds a pair of Bose 901. The famous "wall of sound" :smoke:

Seriously, I use it as a stereo preamp.

Thierry
 
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