MBOW1 3-Way Repair

[Swerd]

It looks like some of the resistors in the photos are wired in parallel or in series to each other. Dennis may have lacked individual resistors of the needed values, so he may have done that to make use of the resistors he had in hand. Nothing wrong with that. Look carefully at two resistors to see if they are wired in parallel or in series.

When resistors are connected in parallel, the total resistance is the reciprocal of the sum of the reciprocals of the individual resistors.

1/Rtotal = 1/R1 + 1/R2 + etc.

For example, a 10 ohm resistor connected in parallel with a 5 ohm resistor will produce:
1/[(1/10) + (1/5)] ohms = 1/(0.1 + 0.2) = 1/0.3 = 3.33 ohms.

Its easier when resistors are connected in series. The total resistance is the sum of the individual resistance values.

Rtotal = R1 + R2 + etc.

See http://en.wikipedia.org/wiki/Resistor

 

[TLS Guy]

You are not trying to tell us Dennis built those crossovers, are you?

 

[Haoleb]

There is nothing wrong with wiring resistors in series or parallel to get the desired value... When you are screwing around in your basement. There is definitely something wrong with that when you send it out the door as a product. I would be very interested to find out who actually is responsible for these. Its beyond a matter of being messy.. its just plain a hazard. you take out an amp with those speakers and who gets stuck with the bill? The consumer. You could even start a fire with that concoction if the conditions were right. Say some of those wires short out inside the speaker. The amplifier output is shorted igniting a resistor or something and the whole place could go up in smoke.

 

[Alex2507]

It looks like some of the resistors in the photos are wired in parallel or in series to each other. Dennis may have lacked individual resistors of the needed values, so he may have done that to make use of the resistors he had in hand. Nothing wrong with that. Look carefully at two resistors to see if they are wired in parallel or in series.

When resistors are connected in parallel, the total resistance is the reciprocal of the sum of the reciprocals of the individual resistors.

1/Rtotal = 1/R1 + 1/R2 + etc.

For example, a 10 ohm resistor connected in parallel with a 5 ohm resistor will produce:
1/[(1/10) + (1/5)] ohms = 1/(0.1 + 0.2) = 1/0.3 = 3.33 ohms.

Its easier when resistors are connected in series. The total resistance is the sum of the individual resistance values.

Rtotal = R1 + R2 + etc.

See http://en.wikipedia.org/wiki/Resistor

Good thing you posted that. I wired something in parallel when it should have been in series. Crap. The ceramic resistors with 2.2 written on them should be two 1 Ohm resistors in series. It says 2.2 because that's what my meter measures them at. It's so easy to make mistakes on this. Glad I caught that before testing my remaining tweeter into giving up the magic smoke. I just measured the resistors to obtain their values because I was too lazy to look up the color codes.

-later-

Fixed wiring screw up and both boards passed testing.

You are not trying to tell us Dennis built those crossovers, are you?

The latest photos are the results my efforts. I don't know who built the original 2-ways and I don't know who modified them into being 3-ways. If it was Dennis I think it would be okay for him to blame Richard. That's how I would handle it. My moral compass may not be true but it is steady.

Here's a little before and after.





... and one more.

 

[Swerd]

You are not trying to tell us Dennis built those crossovers, are you?

I'm defending Dennis until he shows up and does that for himself.

There is nothing wrong with wiring resistors in series or parallel to get the desired value... When you are screwing around in your basement.

I disagree. There is nothing at all wrong with joining resistors if its needed. In fact, using two resisters in parallel (each rated at 5 watts) can allow greater power handling than one resister with the desired resistance value also rated at 5 watts.

I can't disagree with the rest of what you said primarily because a tweeter got blown when it shouldn't have.

Good thing you posted that… I just measured the resistors to obtain their values because I was too lazy to look up the color codes.

I can never remember those color codes either. I usually write their value on them with a sharpie pen as soon as I get them or as soon as I look them up.

If I remember the original thread where Alex bought these speakers, I got the impression that these speakers were originally built as MBOW1 2-ways. I don't know who assembled them. It looks like the cabinets were well made. It is possible that the 2-way crossovers were built (on one board?) and installed inside the cabinets. I'm guessing about this.

Later, the owner decided he wanted the 3-way version instead. The lower cabinets were built and veneered with laminate instead of maple (?) veneer to keep costs down. The woofers look like those Peerless drivers with a multilayer paper/plastic sandwich cone. I can't remember their model name.

Some one built the boards containing the bass and mid range networks on larger boards and installed them in the plinth under the bass cabinet. I'm guessing that Dennis may have done that. At a later time, the owner of these decided to buy Philharmonic 3s, and Dennis took these used speakers as a partial trade.

I have two questions:

1. What's with the different details on the tweeter high pass filter that Alex has?
   
   
2. Did someone take the original 2-way crossover board and cut it, removing the mid woofer net, and keep the tweeter net? This might have been done to keep costs down while converting from 2-way to 3-way? If so, that might account for the ragged appearance of that board.

I think I found an answer for #1. Apparently the MBOW1 2-way DIY design originally featured two different crossovers: an acoustic 2nd order Linkwitz-Riley crossover at about 2600 Hz, and a 4th order Linkwitz-Riley crossed at about 2400 Hz.

Here is the 2nd order crossover for the 2-way:

The 4th order tweeter net can be seen on the MBOW-1 2-way web page if you scroll down some. It looks like this:

Note that the 4th order design, the tweeter is wired in direct polarity with the mid woofer. Also note that the 0.5 Ohm resistor in line with the 0.25 mH tweeter shunt coil really exists. It does not represent the inherent DC resistance of the inductor coil, as I had suggested in an earlier post.

The crossover tweeter net for the Murphyblaster Revised MBOW1 3-way is this:

Alex, which of these do you have for your tweeter high pass filter?

 

[slipperybidness]

I agree, there is nothing wrong with a parallel or series config of resistors, caps, or inductors to get the values that you need.

Especially if the choice is "do it today", or "place an order and wait 2 weeks for parts".

As long as it is done by a skilled hand and done cleanly and correctly, then it's not a problem at all. See my pics in my chipamp thread for a good example where I series-connected 2x 1W resistors to get the required values and Watts.

There is an old mnemonic for remembering the color codes that makes it simple. However, it is vulgar so I won't repeat it here (that's why it's so easy to remember ).

Personally, I read the color codes and ALWAYS double check the values with a multimeter right before I solder them in place.

 

[Alex2507]

I don't have any of those. I have a 20 Ohm resistor and only 2 caps and they are not in series AND my crossovers are different from each other.

Sometimes sh!t just fails. I don't see anything in the xo that would have made it fail either. Me jerking the thing around while my g/f asked me where the Michael Bublehead CD was wasn't in the durability standards.

 

[Alex2507]

Here's the crossovers drawn out.





BTW, I used 5 watt resistors as replacements. They are the short white rectangular ones. That's got to be enough for the high pass, right? The two on the right are in parallel in the photo but that's a mistake that got corrected after the photo. That is the 2.2 Ohm value resistor on the Right XO.

 

[TLS Guy]

Here's the crossovers drawn out.





BTW, I used 5 watt resistors as replacements. They are the short white rectangular ones. That's got to be enough for the high pass, right? The two on the right are in parallel in the photo but that's a mistake that got corrected after the photo. That is the 2.2 Ohm value resistor on the Right XO.

That looks a lot better than before. I personally use bigger resistors then that even in the high pass. I use 10 watt to avoid thermal compression. You don't want L-pad resistors heating to any significant degree at all. The rated value is when it is hot enough to smoke and burn.

 

[KEW]

I'm still waiting to see how Alex makes this repair into a 3-way!