B&W 803D crossover caps

biggy79

I don't think george reads your posts carefully. You are correct, there is a 'bypassing" cap (ie one parallel to the FST driver) in the crossover network. It is a Mundorf silver gold or silver gold oil, depending on what product you have or version of crossover. It is an upgrade (at least according to Mundorf, who makes these capacitors) over the any MKP version, which is in series with the FST driver.

The EVO versions are the only ones that will fit (47 uFd) as a substitute for that MKP 47 uFd. They will improve the sound considerably, although at very high cost. I do not understand, from a sound quality perspective, why B&W would use an MKP version instead of an EVO (either oil or SGO). From a COST perspective, perfectly understandable.

Don't forget the series coupling resistors in the MR CR network. The service manual shows a couple of values being used (and mine had even different values). Plus or minus 0.5 ohms in this coupling resistor can make a significant difference in the 800-1.2 KHz region.

The changes from 47 to 51 uFd in the D2 may not have anything to do with "smoothness", but a shift in crossover frequencies due to driver changes from previous versions. 

With respect to the "woofer" caps (there are two MKP 100 uFd in PARALLEL with the four drivers), remember these are there to roll off the bass drivers at higher frequencies. I don't think Mundorf makes 100 uFd SGO caps, and as these are there to attenuate signals across the woofers, their impact on sound quality is minimized (unless you have an lower midrange peaking problem). My tests with pink noise do not show any such peaking (nor do I hear one), so I leave these alone. I have heard of people "bypassing" these with lower value (like 1-10 uFd) SGO type caps, but I doubt if you gain much, because the frequencies (and impedance) needed to impact bass drivers. For example, a 10 uFd cap will be lower than 4 ohms at frequencies above about 4 KHz. That is quite a bit higher than the 200 Hz cross over frequency for the bass drivers, so these bypass caps will have little to no impact on the bass drivers at or below the crossover frequencies. 

george- download a version of the service manual from B&W and check out the crossover schematics before you comment.

There are various differences in the tweeters in the 800/802/803 series products. Whether some sound "better" than others may be a matter of personal taste or sensitivity.

(there are two MKP 100 uFd in PARALLEL with the four drivers)

Two cap of the same paralleled in series in the signal path is fine, as both are identical as both have the same time constants.

It only becomes a problem when the two are different values or makes, as both are behaving differently at a certain frequency.

EG:
A series cap 20uf seeing an 8ohm load will roll off -3db at 1khz and go out to beyond many mhz.
A series cap 2uf seeing an 8ohm load will roll off -3db at 10khz and go out to beyond many mhz
If the 2uf was use to by-pass the 20uf both will be letting through the frequencies higher than 10khz at the same time creating smearing effect because each have different sound characteristics.

I have proved this to myself many times, with caps in series with the music signal.
  
The only time a bypass cap should be use is in power supply decoupling.

Cheers George

Sorry, I used "nF" instead of "uF".

@dhl93449

Do you like my project (to "add oil" to the 4.7uF TW coupling cap and to the 10uF MR bypassing cap -since I have 803D, not 803D2- and to replace the 47uF MR coupling cap with the Supreme EVO Oil) ?

What differences could I expect using Supreme EVO Silver Gold Oil instead of Supreme EVO (Aluminium) Oil in the MR?

What values for TW and MR resistors do you suggest?

Ok, I will keep the MKP caps on the bass section.

Many thanks

Wait a minute, maybe I have misunderstood some things:

- does the Supreme EVO Oil not fit in the MR crossover?
Can I use only the standard EVO Oil?

- in the D2, the 10uF bypassing cap is  "Supreme Oil" ... is it a custom model only for B&W speakers, not on the market?

Anyway, my 803D uses Supreme Silver Gold (without oil) both in series with the tweeter (4.7uF) and in parallel with the midrange (10uF).

Replacing both caps with Supreme Silver Gold Oil should give more smoothness and refinement  without changing too much the original project, I guess.

Replacing the MKP 47uF coupling cap in the MR crossover  with the EVO Oil (not the Supreme EVO Oil ?) should give a GREAT improvement in mid frequencies.

@dhl93449  tell me if now I am right

biggy

That is what I did. Silver gold Oil 4.7 uFd for the coupling tweeter; 47 uFd EVO SGO for the mid coupling cap; SGO 10 uFd for the mid bypassing cap.  
Whether you use SGO or just "oil" (those are aluminum metallized instead of gold/silver) is a matter of personal taste (and budget). I never found SGO "harsh" or "bright". Maybe you can try the "oil" version first, since they are much lower cost. I just think the silver/gold metallization is lower impedance since gold and silver are much better conductors, but the aluminum may be applied by Mundorf in a thicker layer, which compensates for lower conductivity of the metal itself. Specs wise I don't think there are major differences, but longevity wise I think the gold/silver are better.
As far as the resistors go, you will have to see exactly what B&W put in your crossovers. As I said, they vary by production year and model. I substituted Caddok MP9100 series for the versions used by B&W; they are a pin/pin exact replacement, and fit in the pc board holes (and on the heatsink) precisely. I would avoid the exotic resistors that will not fit on the pc board. Running long wires to mount these can undo the benefits of them in the first place. Plus, you need to replace the OEM parts with versions that have the same or better power rating.