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 14th February 2006, 04:04 AM #1 Wizard of Kelts diyAudio Moderator Emeritus   Join Date: Sep 2001 Location: Connecticut, The Nutmeg State Resistor Values For L Pad? We are crossing over a Vifa D27 tweeter at 3200 Hz. We have need of an L pad. The inductance is negligible, and the voice coil resistance is 4.6 ohms. Could someone calculate the resistor values to cut the SPL A) 4 dB B) 6 dB? Also, can someone calculate the values for those two SPL cuts which would leave the tweeter with an ohmage of 8 ohms instead the present 6 ohms? Thanks in advance for any help on this. __________________ "A friend will help you move. A really good friend will help you move a body." -Anonymous
 14th February 2006, 05:31 AM #2 diyAudio Member   Join Date: Nov 2004 Location: Québec, Québec Z = 6 Ohms A = 4 dB R1 = 2.21 Ohms R2 = 10.26 Ohms Z = 6 Ohms A = 6 dB R1 = 2.99 Ohms R2 = 6.03 Ohms http://www.lalena.com/audio/calculator/lpad/ __________________ DIYaudio for President !
 14th February 2006, 06:07 AM #3 diyAudio Member   Join Date: Nov 2004 Location: Québec, Québec Now to have 8 ohms load... You add 2 ohms so it will take 25% of the power. You lose 1.2493873660829995313244988619387 dB. Z = 6 Ohms A = 2.75 dB + 1.25 dB R1 = 1.63 Ohms + 2 ohms = 3.63 ohms R2 = 16.11 Ohms ----- Z = 6 Ohms A = 4.75 dB + 1.25 dB R1 = 2.53 Ohms + 2 ohms = 4.53 ohms R2 = 8.24 Ohms Tell me if my math is incorrect. __________________ DIYaudio for President !
 14th February 2006, 11:16 AM #4 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Hi Simon, could you post the spreadsheet and/or formulae you are using? How do you calculate to allllll those sig. fig.? __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 14th February 2006, 08:05 PM #6 diyAudio Member   Join Date: Jan 2004 Location: Saint Etienne Passive CrossoverDesign Calculator A usefull Excel spreadshit.
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Join Date: Nov 2004
Location: Québec, Québec
Quote:
 Originally posted by AndrewT Hi Simon, could you post the spreadsheet and/or formulae you are using? How do you calculate to allllll those sig. fig.?
I used the little calculator to calculate the cuts with 6 ohms impedance.

To modify the L-Pad to end up with a 8 ohms load, I used usual formulas.

Power formula dB to linear : x dB = 10 * log y
Power formula : W = V * I
Voltage formula : V = R * I

Now you play a bit ...
W = R * I^2
W = 8 * I^2 but 2 ohms are passive so W = 6 * I^2 in the tweeter, so you lost 25% of the power in the resistor.

With full power you get : 10 * log 1 = 0 dB
With 75% power you get :
10 * log 0.75 = -1,2493873660829995313244988619387 dB

Now I safely approximated it to -1.25 dB so I used the calculator like before but for 4.75 dB cuts at 6 ohms and 2.75 dB cuts at 6 ohms since I already cut 1.25 dB and already added 2 ohms in series.

Calculator give me a L-Pad with a 6 ohms total impedance.
I add my 2 ohms resistance to the resistance in series. I have now a 8 ohms impedance L-Pad.

BTW, yeah with a spreadsheet it would be easy...

If I understood your last question : Why I got so many numbers after the dot? calc.exe (windows xp calculator in scientific mode will give you that many)
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