Question about driver attenuation

mlucitt

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#3
I just did one and the "+" signal path from the amp was through the capacitor first (which is part of a high pass filter), then the series resistor; so the resistor does not have to work so hard because it is only handling the high frequency tweeter signal. The "-" signal path from the amp goes directly to the tweeter.
I'm not sure why you have an 8 Ohm resistor in parallel with the tweeter, I typically see an inductor there. My component breakdown (for a 8 Ohm tweeter) was a 6uF nonpolarized capacitor, 4 Ohm 5W resistor, and 1.2mH inductor. This gives a second order 12db/octave Butterworth high pass filter with a 2kHz cutoff. And the second order crossover has a 180 degree phase shift, so one of the drivers is connected backwards (plus to minus and minus to plus).
 

Mohawk

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#4
I should have posted more info on what I want to accomplish, my current setup is Tri amped through a DSP that handles the crossover points and EQ so I'm not building a crossover just attenuation & protection from any DC

My JBL 2035 15" bass drivers are 98 dB efficient and the 2245 compression drivers with horns are 118 dB.
So, I need (want) to attenuate the compression drivers and also make sure they have a protection cap just to "catch" any DC

The pic I posted is generic off the web and not the values I'm actually using . Its a basic circuit to try to maintain the 16 ohm driver load while reducing its overall efficiency (dB) basically a fixed L-Pad.
I'm just not sure if the cap needs to be last before the driver or before the series parallel attenuation network

What difference could it make ?
 

George S.

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#6
There are videos on YouTube showing how to take frequency sweeps of the room using REW, setting the crossovers and output gain via the sliders. Thinking you only really need the tweeter protection capacitors.
 

Mohawk

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#7
Hi George,

I am using the 4x10HD and yes the gain structure is controlled via the software, its nearly identical to your 2x4 just more of it !
When the 2245 drivers and horns are connected directly to the one amp there isn't any attenuation / filter affects from passive components
and I have a reasonable "hiss" at idle because they are 118 dB efficient .... If I can explain it that way ...
By attenuating the circuit I can reduce the idle noise and not limit the frequency response.
And, as you said adjust the gain with the DSP to suit !
I have done some basic testing of the resistor networks and with a 10 ohm in series and a 10 ohm in parallel I get about -8dB ( calculated ) at idle and now at about 2 to 3 feet back I can't hear any hiss.
Before, in a quiet room you can tell the amps are on , with the resistor networks in beautifully gone!
No issues with the amps , all 3 are 100% WOPL .
Below are the calculators I used to get started, Does it make any difference if the protection cap is before or after the resistor network..... Hmmmmm I dunno'....

M
 

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George S.

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#8
Nice! Yes, I understand perfectly what your saying. I sometimes wondered what, or if, any passive components other than tweeter protection would be needed in a system such as you have.
Glad you got that tweeter tamed.
What sort of 10 ohm resistors did you use? I saw a while back that Parts Express is now carrying a new line of crossover resistors that aren't the wire wound cemented type.
 

Mohawk

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#9

Gepetto

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#10
If you are just looking to block DC since you are bi-amped and the frequency tailoring is done before the power amp, just go very large on the cap value so you take it out of the equation at the frequencies you are passing. Then it does not matter at all where you place the cap. It will look like a short circuit to the frequencies involved.
 

Gepetto

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#12
If you also want to protect your resistor network from DC runaway then put the capacitor as you show in your second sketch above.
 

mlucitt

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#13
So you are attenuating AND blocking DC. Two separate exercises, much clearer now. If you use a 10 Ohm resistor in parallel with the driver, make sure it is rated at 50W as JBL recommends, because it is going to get hot. JBL says to make the shunt resistor across the HF driver ("to shunt the reactive component of the compression driver's impedance below the horn cutoff") 2-3 times the value of the HF driver rated impedance; that would help to keep it from getting so hot and provide some attenuation.
What are your two crossover point frequencies? I used 2500Hz for the HF driver and got 8uF/50V.
 

Mohawk

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#14
Interesting stuff indeed ! Do you have a JBL service or tech bulletin about shunt resistors in your stash of documents ?

The plan is actually to go 4 way active ...

18 "JBL 2241's -300 Hz
10" JBL 2123's 300 - 1.3k
2445 Comp drivers & 2390 Horns/ lenses 1.3k - 9k
2405 Slot tweeters 9k +

This is what I think will start with and "tune" from there....

Did you see the Protection Cap docs I posted ?

Thoughts ?

M
 

mlucitt

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#15
M, just the JBL pdf you posted. I had seen it previously, but did not have a copy, until now. Thanks!
The info about the shunt resistor is at the end of the document.
 
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