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In Reply to: RE: First Watt B4 arrives today - setup questions for Tymp I-Ds posted by Boulder Bob on April 21, 2017 at 05:30:38
That's a horribly expensive and fancy device just to perform a simple first-order crossover. :)
A simple resistor/capacitor network (about five bucks) will do the same thing.
But yes, 1000Hz is where you should set both filters. That's pretty darn close to the 1D stock curves.
Dave.
Follow Ups:
I know the formula for creating a 1st-order high-pass filter via a single capacitor, plugging in the power amps input impedance and the desired x/o frequency. What is the formula for the corresponding 1st-order low-pass filter? Can the low-pass be accomplished just as easily, with a single capacitor on the power amps input jacks, or are more parts required, like a resistor as well as a cap?
Edits: 04/21/17
Same formula. No, you can't just a single (shunt) capacitor at the input jacks because you need a series element (resistor) to create a low-pass filter.
At speaker-level you would use an inductor, but you can't do that at line-level because the L value would be very large.You need to fully characterize both power amps input R and the preamp output R before even attempting this.
Dave.
Edit: Sorry for the duplicate information, Neo beat me to it and posted while I was typing.
Edits: 04/22/17
You need to use a series resistor and shunt cap for the LP, creating a voltage divider. The resistor will introduce a insertion loss. You can match the insertion loss on the HP by also using a resistor but as you said a single series cap is sufficient.
I married the perfect woman. The downside is everything that goes wrong is my fault.
I considered going passive - the input and output impedance specs are not available for the two woofer power amps I will be using, made by Professional Systems Engineering (PSE), one amp bridged per woofer. PSE is no longer in business and I inquired locally with some techs about trying to measure impedance and didn't get anywhere. I am getting the First Watt on a 10 day approval so if it doesn't do what I want, I may investigate some other woofer amps with known specs.
Measuring an unknown input impedance is easily done. I have a little box (with a variable resistor) I constructed many years ago to accomplish this. :)
http://www.sengpielaudio.com/calculator-InputOutputImpedance.htmThe power amplifier output impedance is irrelevant since it will be a small fraction of the speaker load.
Cheers,
Dave.
Edits: 04/21/17
Hmmmm - maybe I should rethink this active vs passive XO conundrum...
Bob, I too got myself a B4. I justified doing so by the fact that I have a number of different speakers with which to use it (old Quads, ET LFT-8b, Maggie Tympani T-IV, a couple of pair of small monitors), all needing different x/o frequencies and perhaps slopes. But your Tympani T-I's can be filtered with just a couple of passive parts, avoiding extra electronics and inter-connects, and their cost. But the B4 is sure a well-designed and versatile product, isn't it?!
You can use this URL
Note that you don't have to know what the input impedance of the bass amp is, so long as it isn't very unusual.
If you want to add level controls you add the bass level control pot after the low pass filter section and the highs level pot before the high pass filter section.
While I agree that the B4 is overkill for a T 1 D crossover, It is convenient and does not require you to learn how to solder and mount parts or calculate filters and components. It is well built and has a small sonic signature.
You won't need a level control on the low-pass section because the RC network will create an insertion loss. Even adding one to the high-pass filter section can get a little tricky because it alters the source resistance.
The t-linespeakers link information is not incorrect, but it's much too "textbook." A person will most likely need to adjust the component values because of capacitor standard values, relative attenuation, etc, etc.
Dave.
Thanks for all the info guys! If a 1st-order high-pass filter is all one requires for the main speakers, it seems like going with that passive filter for them, with an active, adjustable-level low-pass x/o for the subs or woofers would be an effective, low-cost approach for the op. The B4 does that and more, but at around a grand is perhaps not justifiable, cost-wise. In the case of the Tympani T-IV, the old Dahlquist DQ-LP1, with it's passive 1st-order filter (whose hp filter frequency can be changed by the value of the capacitor installed) and an active 3rd-order low-pass (what the T-IV requires for bi-amping) and level control is just what is called for. If anyone is looking for a DQ-LP1, I just happen to have one (in great shape, with original factory carton and instruction manual) I can let go ;-)---Eric.
Edits: 04/22/17 04/22/17
Unfortunately, that unit was not designed for this application.
It was designed primarily for sub/main speaker crossover duties where the main speaker already has a native 2nd-order roll-off. Thus, you would achieve a symmetric 3rd-order slope crossover using the electrical 3rd/1st electrical slopes of the Dahlquist unit.
So, yes, in the case of the T-IV it might work effectively, but NOT in the case we're talking about here with the I-D. Two completely different animals with completely different line-level solutions required.
Dave.
Exactly Dave. For the op's Tympani T-1, a symmetrical 1st-order x/o is required for both the high and low pass filters. At the time of it's original manufacture, the T-1 was being distributed by Audio Research, whose PC-1 passive x/o was designed expressly for that application. It created a 1000Hz x/o point, with four level controls, one each for left and right channels/high and low pass outputs.
I would hang onto that Dahlquist unit. That's a nicely conceived unit and could come in handy in the future.
Dave.
Good point Dave. That's why I still have the DQ-LP1! I kept my copy of Frank Van Alstine's plans for his upgrade modification to it. One thing to do is just not use the high-pass inputs---the capacitor is uses for filtering can just as easily be installed on the power amp input jacks, saving an extra set of inter-connects. Then use the active 3rd-order low-pass inputs only. That control has an adjustable frequency range, though not nearly as wide or precise as that of the First Watt B4.
Edits: 04/24/17 04/24/17
Well, it's hardly going to make any difference unless you have interconnects that are a mile long.
Installing the capacitor internal to the DQ-LP1 is preferred as it allows to use the shunting resistor location so you can fine-tune while still using standard value capacitors.
Dave.
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