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Old 14th March 2012, 01:46 AM   #1
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Default 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!

Last edited by zerokelvin99; 14th March 2012 at 06:02 PM.
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Old 14th March 2012, 09:28 AM   #2
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I think I have perhaps posted this in the wrong sub-forum?

Feel free to move it mods :-)

Thanks!
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Old 14th March 2012, 09:43 AM   #3
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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.
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Old 14th March 2012, 10:56 AM   #4
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Hi Tony,

Major oops in my wording (!) the two resistors and capacitor are in parallel. I think my brain stops working at 2 in the morning!

I have drawn a schematic - it's at home, I'll put it online this evening. (Or tomorrow morning to you :-))

Thanks,
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Old 14th March 2012, 11:09 AM   #5
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Even the thread title is wrong d'oh
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Old 14th March 2012, 11:58 AM   #6
AllenB is offline AllenB  Australia
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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.
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Old 14th March 2012, 05:52 PM   #7
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Click the image to open in full size.

Click the image to open in full size.

I have shown the inductor leads in red as they were hard to see, PCB breaks are in blue


Thanks!
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Old 14th March 2012, 07:31 PM   #8
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Are you modelling this with CAD using known electrical impedance and frequency response or are you just winging it with textbook values?

Textbook electrical filters will never get textbook acoustic results because drivers are not textbook.
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Old 14th March 2012, 07:43 PM   #9
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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 !
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Old 14th March 2012, 07:58 PM   #10
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It's the attenuator for the tweeter. The lower the frequency, the more attenuation. Does the tweeter sound loud enough? If not, piggy-back a 10 ohm on top of one of the 33 ohms. You can remove the 1.2 uf capacitor and see if you like that sound better. You have options. Live free.
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