Congrats on your success of installing the ribbons. For steady hands, definitely a one cup and not a two cups of coffee job. If you maxed out the L pad and it helped (you are getting output from tweeter) then it “sounds” (pun intended) like everything is working as it should. The idea of jumpering around the L pad was just a troubleshooting tip to determine if the L pad was open. If you can hear output from the tweeter when maxed out the L pad is not open.
When the pot is maxed out you have effectively jumpered past the L pad as the resistance is 0 ohms. I forgot how the L pad worked so I removed my external RM-40 xo and disconnected the L pad leads and measured. Putting it here so I can refer to in the future.
The input is coming on pin 3 and the output to the driver is pin 2. When in the extreme low position the resistance between pins 2 and 3 is 8 ohms. When in the mid position 4 ohms, and extreme high position 0 ohms. When in recommended start position of 1 o’clock the resistance is 3.2 ohms. So with our 6 ohm driver in the 0 position the voltage across the driver would be 42% (6/(6+8)) of input voltage, mid position 60% (6/(6+4) of voltage will be across driver, and full throttle 100% of input voltage will be across the driver. 1 o’clock position 65% on input will be across driver. The L pad resistance between pins 1 and 2 is 0 when in the low position, 20 ohms when in mid position, and 40 ohms when in the high position.
Certainly could replace the L pad with a resistor. A reisitor in series with tweeter would matter. It is going to have to be high wattage resistor. Mundorf M-resist Supreme 20W resistor would work nicely and are highly regarded. Certainly are other choices like the Mills MRA12. They are non inductive and should be soncially superior to the L pad. To double the power rating to 40W you would put two resistors in parallel. They come in standard resistance sizes of 3.9, 4.7, 5.6, 6.8 ohms. They are $17.10 each at Madison Sound. The Mills MRA12 resistors are less expensive at $7.10 each. They are 12W so you would need 3 or even 4 in parrallel to get the required wattage. I like the 2 o’clock position which is likely just under 3 ohms, so two 5.6 ohms for 2.8 ohms or a 5.6 and 6.8 for 3.2 as an example. I use an active XO, but still use the XO for the tweeter. I am going to do this for my tweeters in the new year. Thanks for getting me to think about this.
Some of the info above is not quite right. A 8 ohm L pad not only attenuates the power to tweeter but also always tries to maintain an 8 ohm load to the source. To do this there are two resistors, one in series to the tweeter between leads 2 and 3 and another in parrallel with the tweeter. When in low position 8 ohms between 2 and 3 there is 0 ohms between 1 and 2, so 8 ohm load to amp and no voltage across tweeter. When in mid position 4 ohms between 2 and 3 and 20 ohms between 1 and 2 providing 4 ohms plus 20 ohms in parrallel with 6 ohm tweeter a 8.6 ohm load to amp while providing half the voltage to tweeter. When very close to full on position say 1 ohm between 2 and 3 and 40 ohms between 1 and 2 which provides a load of 1+40 ohms in parrallel with tweeter 6 ohms equaling 6.2 ohms (close to 6) and nearly all voltage across tweeter. When you go to max level the reistance between 2 and 3 is 0 and resistance between 1 and 2 goes to open (I mispoke previously and said 40 ohms, goes to 40 at close to max and then open) so the load is then the load of tweeter of 6 ohms. So to maintain Brian's design and exact output level at your optimal position (say 2 o'clock) you need to determine this value for both resistors (measure) and then replace both with a quality resistors. Or you could just use one resistor and maintain a close to 8 ohm load to amp, but to get same spl level you have to do some figuring on its value to compensate for the missing resistor between pins 1 and 2 of the L pad. Pardon the long winded explantion.
Did a test to determine actual power across tweeter. A 1V 6Khz was close to 90dB SPL. So that is .16W. To double the subjective loudness would be 100dB and double again for good measure 110dB would be 17 watts. So a single 20W or even 12 W resistor would be fine.
