Resistor in parallel with cap = Lpad? (Now with pictures)
Hello,
I am currently modifying a crossover, learning as I go. However, I have hit a brick wall :-(
It's a 1.8Khz crossover, with a basic 12db/oct for the bass and 24db/oct for the tweeter. I am bringing this crossover down to around 1.3khz by simply replacing all the capacitors with those 1.4x the value. (From my research this seems to be okay??)
However, there is a part of the circuit which doesn't seem to tally with anything online! After the HF protection bulb there is a 1.2J, 250v capacitor and two 15W, 33R resistors in parallel. I take it this acts as the lpad, but what are the formulae? How do they worK??
Thanks for taking your time to read this, and apologies if I appear stupid!
Hello,
I am currently modifying a crossover, learning as I go. However, I have hit a brick wall :-(
It's a 1.8Khz crossover, with a basic 12db/oct for the bass and 24db/oct for the tweeter. I am bringing this crossover down to around 1.3khz by simply replacing all the capacitors with those 1.4x the value. (From my research this seems to be okay??)
However, there is a part of the circuit which doesn't seem to tally with anything online! After the HF protection bulb there is a 1.2J, 250v capacitor and two 15W, 33R resistors in parallel. I take it this acts as the lpad, but what are the formulae? How do they worK??
Thanks for taking your time to read this, and apologies if I appear stupid!
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moved
I don't understand either, two 33R resistors in series for 66R total? ALL in series with the cap and driver?
an lpad is constructed with a low value resistor in series with the driver and a higher value resistor in parallel with the driver.
You can just put a resistor in series as well say a 2.2R for some cut. but 33R (or 66R) seems very excessive! can you fraw and attach the schematic of the crossover?
Tony.
I don't understand either, two 33R resistors in series for 66R total? ALL in series with the cap and driver?
an lpad is constructed with a low value resistor in series with the driver and a higher value resistor in parallel with the driver.
You can just put a resistor in series as well say a 2.2R for some cut. but 33R (or 66R) seems very excessive! can you fraw and attach the schematic of the crossover?
Tony.
Are you suggesting there is a protection bulb, then these three components going from after the bulb to ground, then your fourth order filter then the tweeter?
A big guess here but they could be tailoring the effect of the bulb by warming it to a set point, whether to adjust its protection or alter it's padding effect or adjusting distortion.
I should stop speculating. I'll just wait for the schematic.
A big guess here but they could be tailoring the effect of the bulb by warming it to a set point, whether to adjust its protection or alter it's padding effect or adjusting distortion.
I should stop speculating. I'll just wait for the schematic.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I have shown the inductor leads in red as they were hard to see, PCB breaks are in blue
Thanks!
Hi,
I am modifying an existing crossover from a Turbosound TXD-151 to change the crossover from 1k9 (measured) to 1k2. Impedance is the same for both HF and LF. I will need to pad the HF by an additional 2dB due to increased efficiency of compression driver.
I am hoping to replace the 15J and 3.3J resistors with higher values to lower the Xover point !
I am modifying an existing crossover from a Turbosound TXD-151 to change the crossover from 1k9 (measured) to 1k2. Impedance is the same for both HF and LF. I will need to pad the HF by an additional 2dB due to increased efficiency of compression driver
.I am hoping to replace the 15J and 3.3J resistors with higher values to lower the Xover point !
10 watt is OK for the piggy back resistor. Most often a tweeter is attenuated with 1 ohm or so, so this 16.5 ohm looks very silent to me. Is the tweeter very silent? If you put one 10 ohm on each 33 ohm, that will make a total of about 4 ohm, which is still high. As long as you go with 10 watt resistors you can mix and match as you please. The 1.2 uf capacitor is like a resistor that varies with frequency. At 20 kHz it's about 6 ohm, at 2 kHz it's about 60 ohm. I would remove it, unless you are playing a horn tweeter, where it might do good to shape the sound. But try without. If the tweeter is much too silent, try a 5 ohm resistors on the 33 ohm. Buy four at once, two for each speaker.
The capacitor is allowing a rise in output at higher frequencies. As well as fitting in to the crossover as a whole, its value is dependent on the value of the resistors. Strawberry, think about the impedance at the point between those resistors and the filter. There is a reason that this ought to be working as it is.
Okay, I'll give that a go. The "tweeter", ie compression driver, has a sensitivity of 113dB 1w/1m (BMS 4544) while the old one was down at 110dB (P.audio BMD450). The woofer (JBL 2226) is far down at 97dB.. It's actually a little bright at the moment for my liking, even though it is ±2 55hz-18khz. 4dB extra padding should suffice.
The resistors do need to be uprated though, those 15w get too hot at full volume. I'll experiment to find the correct value then place two 25w in their places.
The resistors do need to be uprated though, those 15w get too hot at full volume. I'll experiment to find the correct value then place two 25w in their places.
OK, a compression driver. 16.5 ohm makes sense then. Well, remove the capacitor then and see what you think of that sound. It will be less loud, perhaps to your liking. If too quiet, put a 0.47 uf capacitor there instead, 100V or more. The resistors look fine. They do get hot. Do you know how much power feeds the speakers?
That's right zerokelvin.
The load created by the filter changes with frequency, the padding resistors have been modified with the capacitor to also change with frequency which seems to have been done to restore an even handed attenuation.
Reducing the capacitance would probably reduce the top end response. Increasing the resistance would probably reduce the lower tweeter response. I would ordinarily keep both in proportion, and don't take big steps. However leaning more on the cap or the resistor may be warranted depending on what you mean by bright. I'm sure you are the best one to find that out for yourself.
The load created by the filter changes with frequency, the padding resistors have been modified with the capacitor to also change with frequency which seems to have been done to restore an even handed attenuation.
Reducing the capacitance would probably reduce the top end response. Increasing the resistance would probably reduce the lower tweeter response. I would ordinarily keep both in proportion, and don't take big steps. However leaning more on the cap or the resistor may be warranted depending on what you mean by bright. I'm sure you are the best one to find that out for yourself.
600wrms to 800wrms depending which amplifier I run it off. The crossover is only rated to 350wrms so it's a little on the edge as is.
I'll do some experimentation with the two values and see how they turn out. Do you have any equations to help guide me? I have researched parallel RC circuits, but can't seem to find the link between frequency and impedance.
I'll do some experimentation with the two values and see how they turn out. Do you have any equations to help guide me? I have researched parallel RC circuits, but can't seem to find the link between frequency and impedance.
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