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starrman93306 3rd November 2004 07:52 PM

resistors in series
 
I believe I need an l-pad to fulfill my tweeter level attenuation requirements. The calculator I have used predicts that R1 (6 Ohms)could see 34.92 watts, which is higher than anything I can find at Parts Express (I love Parts Express). Can I use 3 12 watt/2 ohm resistors is series? Or am I somehow 'unclear on the concept.'?

Regards to all, and if I need to post this to another area, someone please let me know

Doug Starr
Bakersfield, CA

rpapps 4th November 2004 02:50 AM

Resistors in series
 
Hi
Three 18 ohm, 12 watt resistors in parallel will give you 6 ohms rated at 36 watts.
Cheers
Rob

Tim__x 4th November 2004 03:31 AM

Both ways will work.

Stuart Easson 5th November 2004 01:51 AM

too big?
 
Doug,

I could be misunderstanding the numbers, and/or your intentions, but if the 6 ohms you describe is in series with the tweeter, and represents the bulk of the impedance seen by the high pass crossover you don't need anything like a 36 watt resistor. If the resistor were ever to see 36 watts the (8ohm) tweeter would be subjected to approx 48...it would die a horribly, stinky thermal death, the normal dynamic tweeter is only a 5-10w device. If you can find a 5-10w resistor use it and you'll do fine...

Stuart

starrman93306 8th November 2004 07:36 PM

resistors in series
 
Thanks all for the replies.

Stuart's comments make me wonder if it is "I" that do not understand fully what is going on here.

The basic problem is that I am going to use a 4 X 75 watt RMS amplifier to bi-amp a set of components. The head unit has 4V outputs. The crossover and amp, both have gain adjustments, so there should be a fair bit of adjustability in the system. But I don't know how to figure out if I can get the correct level match between the tweets and woofers by using just the gain adjustments. In other words, I'm afraid that to get 15 watts or less to the tweeters and also adequate power to the woofers, that I will have to have the gains all the way up on one side and all the way down on the other side. I don't even know if this would be a problem; I'm just guessing that it is less than ideal.

That's the rationale for an l-pad, but as Stuart has pointed out, some of the numbers are surprising. Incidentally, the calulator I used was at http://www.eatel.net/~amptech/elecdisc/lpad.htm. The inputs I am using are:

speaker impedance 6 ohms
desired attenuation 6 decibles
amplifier power/channel 75 watts

Regards,

Doug Starr

Stuart Easson 8th November 2004 08:50 PM

Doug,

The calculator is correct, but is being too literal...

If you really had 75w going to the tweeter, you would need a huge resistor to lower the level by the amount specified...the reality is however that if you have a 75w signal in the tweeters passband, you will have like 2kw going to the woofer (I'm exagerating here, I haven't done the math). Tweeters cannot stand 75w, they will die instantly. The typical tweeter is a 5-10w device, designers rely on the crossover and typical signal spectrum to make that reality possible.

If you are going fully active, you don't need an inline resistor at all, it will only make things worse. You simply adjust the gain so that the tweeter sees 6db less power than the woofer. This would be 1/2 of the voltage. As a 'pleasant' side effect you could use an amp of 1/4 of the power, and it will still be more powerful than strictly necessary...

If you have an scope or even a good multimeter, disconnect the speakers, get a steady signal playing fullrange through the amp on all channels and adjust the output levels to make the low channels see the correct voltage (2:1) relative to the tweeters, the average gain control will be log and this will be much easier than it sounds, you won't need to be all the way up on one and all the way down on the other, probably one will be about half the other, or perhaps a quarter.

Fully active has some huge advantages, you get 3db more output for free, the tweeter can use lower powered amps, the woofers have better damping, distortion is isolated to the individual channels etc, etc. Do it if you can, you definitely don't need inline resistors.

Good luck

Stuart

Stuart Easson 8th November 2004 08:56 PM

caveats...
 
Doug,

One thing to bear in mind, when you connect the tweeter directly to the amp, it has no protection, if you somehow cause many watts to appear at the output of the tweeter channel, at any frequency, it will kill the drive unit instantly. Be real careful about your, presumably active, crossover settings and make sure you start all tests with the head unit volume really, really low...

If you are in any doubt, put a really small resistor, like 1/4w, 1 ohm in series, it will make almost no difference to the drive unit level, but will die faster than the drive unit, that way you can test everything, once you are satisfied, remove the resistors...believe me it's worth it, don't ask me how I know...

Stuart


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