PLLXO help

[Bob Brines]

OK. I'm ready to embark on a PLLXO. I ran some numbers through the spreadsheet and notice that the cap values are several orders of magnitude smaller than speaker level XO's. Questions:

1. In my ignorance, it appears that the low valued caps will have to be ceramics or styrene type bypass caps. True?

2. I have no idea what the amp Z is. I suppose that I can simply assume 100kΩ, see what happens and adjust values acordingly?

3. I intend to use a plate amp to cross the sub. I'll start with 300Hz 1st ordrfor the high-pass.

4. Using a pot for the XO resistor, what kind of frequency range can I expect for a given cap?

Thanks,
Bob

 

[planet10]

Bob, is this for the Topping?

The amp Z is essentially the R to ground at the input...

For hipass, i have been using this R as the shunt element of the RC filter and just using a cap in front for 1st order... adding an additional shunt R in front decreases the net input R and degrades potential performance if you want to reach for 2nd order and also requires a larger cap.

One of the advantages of PLLXO is that it uses much smaller caps then a passive XO, allowing you to get a much better cap for your budget -- no point in using anything but good poly caps in these.

dave

 

[Bob Brines]

Bob, is this for the Topping?

Yes

The amp Z is essentially the R to ground at the input...

OK. I can do that. The Topping reads 49kΩ, and old Yami 100kΩ and my p-p tubes 87kΩ

For hipass, i have been using this R as the shunt element of the RC filter and just using a cap in front for 1st order... adding an additional shunt R in front decreases the net input R and degrades potential performance if you want to reach for 2nd order and also requires a larger cap.

Putting 49kΩ into the spreadsheet gives 10.8nF. Unless I don't understand the units, which of course is possible, the smallest foil and film cap is 0.01μF, or 10pF and 10^3 too big. Where would you suggest that I sourse the proper caps?

Bob

 

[planet10]

0.01 is close enuff...

Solen Electronique Inc.

Solen Electronique Inc.

You can fine tune with a shunt R across the input R.

dave

 

[PeteMcK]

re:'Using a pot for the XO resistor, what kind of frequency range can I expect for a given cap?' - you can use a resistor in series with the pot to set the range

 

[Bob Brines]

0.01 is close enuff...

Ah! So the units on the spreadsheet are really μF. I'll buy some parts for a patch box and see what happens.

Thanks,
Bob

 

[901Fixer]

Doesn't it go:
milli = 10^-3
micro -6
nano -9
pico -12

Pico being smaller than nano that is. .01 micro = 10 nano?

 

[Rev. Billy Bob]

Here's on online converter that may help.
Online Conversion - Electric Capacitance

eta -When I ran 49 kohms through my first order calculator @ 300hz, my result was C1 = 1.0816326530612E-8 farads, so I think you are on the right track

 

[Bob Brines]

Thanks, guys.

Bob

 

[pforeman]

Bob, is this for the Topping?

The amp Z is essentially the R to ground at the input...

dave

Is this also true when using a volume pot. Or is the input impedance now the impedance of the volume pot
Paul

 

[planet10]

More or less...in some circuits the impedance will change depending on the setting of the pot. If the volume pot is in parallel with another R to ground that has to be taken into consideration.

dave