Sourcing electrical parts for BFM build

[mrtcat]

Hi all,

I'm currently building a pair of BFM DR280 pa cabs. I'm getting along nicely but cannot for the life of me find any 4 ohm 10-20watt resistors for the high & low pass filters. I've trawled the net and drawn a blank. Can anyone help?

Woodworking skills are fine but I'm a bit green with the electrical side. I can locate 3.9 ohm 10w resistors but you'll probably tell me that to substitute these will result in a full meltdown of life itself. Pls advise

[Pentode]

I suspect the 3.9ohm jobbies will be ok.

Have a word with the peeps on the BFM forum, they'll know for sure!

[mrtcat]

Many thanks Pentode will do!

[icastle]

[quote name='mrtcat' post='1077242' date='Jan 4 2011, 01:02 PM']Hi all,

I'm currently building a pair of BFM DR280 pa cabs. I'm getting along nicely but cannot for the life of me find any 4 ohm 10-20watt resistors for the high & low pass filters. I've trawled the net and drawn a blank. Can anyone help?

Woodworking skills are fine but I'm a bit green with the electrical side. I can locate 3.9 ohm 10w resistors but you'll probably tell me that to substitute these will result in a full meltdown of life itself. Pls advise [/quote]

Big wirewound resistors (the type you are looking at) are not manufactured to close tolerances - they will be + or - 10% of the marked value. A 0.1Ω resistance difference will make no difference to the overall performance.

[stevie]

Not only that, but 3.9 ohms is a preferred value and therefore much easier to obtain than 4 ohms.

[flyfisher]

Try RS Components or Farnell Electronic Components.

As mentioned above all resistors have a tolerance and any decent circuit design will take that into account. In fact, I'm rather surprised that you have a circuit that specifies a 4 ohm resistor, since that is not a 'preferred' value (standard resistors are not available in every single possible value, otherwise there would be millions of them, so the industry has settled on a specific range of 'preferred' values and electronic engineers work to these values).

3.9 ohms ohms is -2.5% of 4 ohms but typical high-power aluminium-cased resistors are 5%, so could actually be 3.8 to 4.2 ohms anyway. In other words 3.9 ohms should be fine.

Occasionally it is necessary to use a non-preferred resistor value. In such cases, designers may use a combination of series and parallel resistors to create the required resistance. Or, if cost is not an issue, use a variable resistor and adjust-on-test.

If you are really paranoid about using a 4 ohm resistance in your circuit then you could consider using a 3.3 and 4.7 in series to give 8 ohms and then add a similar pair in parallel to give 4 ohms in total.

However, such a level of paranoia would also require you to calculate all the tolerance possibilities and it's unlikely you'd end up with 4.0 ohms anyway.

And all of that is before we even consider that loudspeakers actually present a complex load to an audio amplifier (hence the use of impedance rather than resistance) and there is no such thing as a "4-ohm" loudspeaker anyway. It's just another bit of shorthand to hide all the underlying complexity, but is perfectly acceptable for rules-of-thumb.


Hmm. Probably too much information really. I'd just use a 3.9 ohm resistor.

[mrtcat]

Many thanks to all for taking the time to help. I'm going to go with the 3.9 ohm as the BFM forum also has other people recommending the same thing and you all clearly know what your talking about as Ive heard nothing to contradict whatsoever.
Again thanks to all for the help.