[jparkhur]

You are looking for the IN amp impedance.  Many amps have a 100k ohms, 47k ohms, 33k ohms etc.  Remember, RCA and XLR inputs are different usually depending on amplifier.

Yes, You would have to do some internal work if you had an integrated since there is no in/out.

JP

[mboxler]

If your coupling cap value turns out to be lower than the stock input cap in your amp, and you want to put the filter on the input jack of your amp (for less connections), can the stock cap be left in place and the new cap simply added to the input jack, or does the stock cap need to be removed?

Some tube amps already have a coupling cap on the input.  My Decware amp does not.  If your amp already has an input cap, I believe this would be in series with the external one you are adding, and would reduce the effective value.

Still learning, but am I correct?

Mike

[Davey]

You are correct.
However, an internal capacitor would most likely be a much larger value than what you're adding externally.  The lower value capacitor dominates and the net value would probably not be significantly lowered.  It would be good to check the value internal capacitor though.......or perform a response check at the output to verify.

Dave.

[rak313]

Ed.  Fixed and you are correct.    My cap value is a .033uF for a 3 db down at 48 hz but remember the roll starts on octave higher- so about 80 hz or a little higher.

Can you tell me the logic in designing the filter at 48 Hz. I don't understand.  You are trying to crossover at 80 Hz - right?  Yes it's down 3 dB at 48 Hz (and also down 1.3 dB at 80 Hz, 0.4 dB at 150 Hz, 0.1 dB at 300 Hz, etc.).  So there really isn't any starting of it being down until you pick an arbitrary threshold to say now it's starting to be down. Engineers typically use the 3 dB point because it's conveniently the location of the pole.

Is the choice of 48 Hz related to crossing over to an OB design speaker at 80Hz, or is it to a typical "small" (as used with an AVR) speaker?  If I'm trying to understand the logic.

Thanks
 

[Danny Richie]

John,

Dave was correct in answering the questions posted by bdp24. Your small cap value will be very slightly lowered, and not by much.

[jparkhur]

Can you tell me the logic in designing the filter at 48 Hz. I don't understand.  You are trying to crossover at 80 Hz - right?  Yes it's down 3 dB at 48 Hz (and also down 1.3 dB at 80 Hz, 0.4 dB at 150 Hz, 0.1 dB at 300 Hz, etc.).  So there really isn't any starting of it being down until you pick an arbitrary threshold to say now it's starting to be down. Engineers typically use the 3 dB point because it's conveniently the location of the pole.

Is the choice of 48 Hz related to crossing over to an OB design speaker at 80Hz, or is it to a typical "small" (as used with an AVR) speaker?  If I'm trying to understand the logic.



Thanks
 

80 was my starting point because these are specifically made for the LGK driver and speaker and was to be used in conjunction with a sub that can reach those higher limits.  48 came about because that's what value of caps was available to buy and a perfect match to a frequency in this case was not an option.  So 80 was my choice of where to roll these off.

Also, this is just a how to if you need these in your specific system.  Many do and many don't.  It is now a viable option to those that want to take their shot at soldering and some cool passive xo.

Lastly, we all see these type of barrel connector xo at the local PE and they claim specific xo frequencies.  Well they are good if you have the right impedance, if not, you could be off by a 100 hz. 

Hope this helps.    I'm not perfect, just ask my wife. 

[rak313]

80 was my starting point because these are specifically made for the LGK driver and speaker and was to be used in conjunction with a sub that can reach those higher limits.  48 came about because that's what value of caps was available to buy and a perfect match to a frequency in this case was not an option.  So 80 was my choice of where to roll these off.
...

Instead of using whatever your amp's input impedance is - and forcing a cap value, I would add a smaller resistor across the input.



Preamp+ ----|C|---- AMP+
                |
               [R]
                |
Preamp- ----------- AMP-


So the pre-amp + output goes to a cap which goes to the amp + input.  The resistor goes from the amp + input to the amp - input.  This lowers the load impedance (as seen by the cap and preamp) to a value you select.  The resistor value must be large with respect to the preamp output impedance but low with respect to the amps input impedance. 

Here is an example to show the point.  Say the preamp output impedance is anywhere from 60-400 ohms.  Say your amp's input impedance is anywhere in the 30k-100k ohms. 

Now use a 7k resistor across the input with a 0.33 uF cap.  For a 30k input impedance, the filter frequency would be 74Hz. For 100k impedance it would be 85 Hz, (+/- the tolerance of the cap - an additional 5%).
 

[bdp24]

Which input impedance are you looking for, the preamp or the power amp?

does it make a difference if you have separates or if the amp is integrated?

It's the power amp's input impedance that matters. I have mono tube amps with an input impedance of 100K Ohms, and have been using the high-pass section of the old Dahlquist x/o to roll off my mains at 180Hz. The high-pass filter is just a 1st order passive, the frequency selectable on the front panel. Installing the correct value cap to the back side of the XLR input jacks to get the x/o frequency of (-3dB at) 180Hz is a more elegant (and better sounding) way of doing it, of course. The low-pass filter of the Dahlquist is a 2nd order active (it's frequency also selectable on the front panel), and is not necessary because the Rythmik plate amp does the same thing, and without the extra connections and cables. Good-bye Dahlquist, hello capacitors.

[usp1]

Complete newbie question. If I just want a low pass crossover to feed signal to a subwoofer, can I simply cut the positive wire in my interconnect and solder a 0.033uF cap inline to get a roughly 80 hz low pass filter?

-ve ___________________________________ -ve

+ve______________________||____________+ve

                                             0.033

Or am I reading jparkhur's pictures incorrectly. Which is the input rca in the pictures?

[jparkhur]

Complete newbie question. If I just want a low pass crossover to feed signal to a subwoofer, can I simply cut the positive wire in my interconnect and solder a 0.033uF cap inline to get a roughly 80 hz low pass filter?

-ve ___________________________________ -ve

+ve______________________||____________+ve

                                             0.033

No

We are only talking about a high pass in line filter here.  You are talking about a low pass filter for your sub which would be inductors.   If you were to do whAt you gave as an example you would have limited or no bass at the sub. 

Does your amplifier not have a selectable xo built in? 

[usp1]

*** Just went ahead and purchased FMOD cables from Harrison labs to solve my problem****

[bdp24]

Complete newbie question. If I just want a low pass crossover to feed signal to a subwoofer, can I simply cut the positive wire in my interconnect and solder a 0.033uF cap inline to get a roughly 80 hz low pass filter?

-ve ___________________________________ -ve

+ve______________________||____________+ve

                                             0.033

Or am I reading jparkhur's pictures incorrectly. Which is the input rca in the pictures?

If you get a Rythmik/GR sub or two, the x/o is built in. The controls do not only the low-pass filtering, but many other useful things to integrate the sub(s) into your system.

[usp1]

bdp24 - Thanks for the input. The reason for asking that question was that I want to use an old crown xls402 that I already have but does not have a low-pass crossover.

[planet10]

Could you clarify the formula?

A more comprehensive article on the subject of PLLXOs of which Jon's is a simple example.

http://www.t-linespeakers.org/tech/filters/passiveHLxo.html

dave

[planet10]

FMOD cables

A compromised implementation of a PLLXO, optimum requires consideration of the amp input impedance.

dave

[planet10]

Some tube amps already have a coupling cap on the input.  My Decware amp does not.  If your amp already has an input cap, I believe this would be in series with the external one you are adding, and would reduce the effective value.

Mike,

Usually there will be an R to ground before the 1st cap.  The 1st cap is a pole, but will be much lower than the PLLXO cap. If you do not mind poking around in your amp, the slickest way to do the high pass is to replace that cap with a suitably smaller one.

dave

[usp1]

A compromised implementation of a PLLXO, optimum requires consideration of the amp input impedance.

dave

From everything I read about them you are absolutely correct. That is why I wanted to see if I could DIY a crossover myself but not sure why to do. Also, if I want to change amps would i have to change the crossover in that case?

[bdp24]

From everything I read about them you are absolutely correct. That is why I wanted to see if I could DIY a crossover myself but not sure why to do. Also, if I want to change amps would i have to change the crossover in that case?

Only if the new amp has a different input impedance.

[usp1]

Only if the new amp has a different input impedance.

I still have to figure out a low pass filter  since the fmod cable does not seem to be a very good solution. The fmod cable I purchased is supposed to give me a low pass filter with cut-off freq of either 70hz or 50hz depending on cable orientation. Regrdless of which cut-off I use, I can hear vocals if I just power the second voice coil on my Gallo 3.1s. The vocals are significantly attenuated, but I would have expected that with a 50hz cut-off and a 12 db slope I should not hear any vocals at all.

[jparkhur]

You shouldn't hear any, you are correct. But with fmods, who knows what freq and impedance they are actually built for.  Remember the formula above, you may just want to build one for better results.

Sorry it's not working out

Jp

High or low pass, you will get better results with your own creation