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Help. Is the passive crossover in the PE 250W amp any good, or get active x-over?


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#1 of 8 OFFLINE   Javier_Huerta

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Posted December 13 2002 - 01:19 PM

I was wondering about this. Should I use the passive crossover on my PE 250W amp, or should I get an active crossover?

My system uses MartinLogan Scenarios, an Adcom GFA-5400 amp, Luxman CD player, and a Tempest Adire Alignment sub. I was wondering if I should get an external crossover, and which! I can't find any good ones.

If there is one, by any chance, can I find it in New York? I'll be there next week...

TIA!

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#2 of 8 OFFLINE   Dan Pawlowski

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Posted December 13 2002 - 03:46 PM

I may be wrong but doesn't the PE amp come with an active Xover???

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#3 of 8 OFFLINE   Peter Jessee

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Posted December 13 2002 - 05:42 PM

Quote:
I may be wrong but doesn't the PE amp come with an active Xover???

Yes, the line level crossover is active, not passive. The factory design has high pass Q=1 (about 2-3 dB peak) at 140 hz. Check out this site for information on modifying the crossover for other frequencies and Q's:

http://www.geocities.../PE300-794.html

Peter

#4 of 8 OFFLINE   Eric Massey

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Posted December 13 2002 - 05:54 PM

If I read that page right, I think it said this amp has a high pass filter set at 30hz? Is this the "rumble filter" that they talk about on the partsexpress webpage for this amp? If not, does anyone know what frequency the "rumble filter" is set too and is there any way to change it?

I'm considering using this amp on a sub tuned to 19 hz and it would kind of suck to have a hi pass set on 30hz.

#5 of 8 OFFLINE   Javier_Huerta

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Posted December 14 2002 - 12:33 AM

Thanks a lot for your answers! I checked the PDF manual online at PartsExpress and I couldn't find any references to an active crossover on the line-outputs. It's a good thing to know the subwoofer amp actually has one built-in, although a Q of 1.0 isn't exactly what I'd like for my speakers.

I'll definitely be checking that link, Peter. Thanks for it, although hopefully I won't be needing it, since I'll be using a Behringer UltraCurve to auto-eq my room (still, a Q of 1.0? I don't think I'll be able to leave that alone).

EDIT - Eric, it seems the amp is rolling off the frequencies below 30 Hz! I see the response at 20 Hz is around F3, which basically sucks, since I'm also tuning my subwoofer at a lower frequency (around 17-18 Hz). Check out Peter's link - it seems the only way to move the rumble filter from that frequency is to change R26 from 18K to 100K ohms. I wonder if I still remember how to solder, because this is the first mod I will attempt. Posted Image

Hmm. I just checked the PE manual for the amp. It specifically says it has no active crossover... check it out:

Quote:
Line-out: This line level RCA output provides a full range (20-20Khz) output (emphasis added) and is used to connect "daisy chain" the line level signal to other amplifiers. Note: Line out is only active when a signal is present at the line-in input.

I'd assume the amp has no internal active crossover...

But then, the description in the web page says:

Quote:
It also features a 6dB high pass filter (125 Hz @ 8 ohm) output for satellite speakers and a line level output with a built in electronic high-pass filter.

So, which one is it? Full range output or electronic high-pass filter? Posted Image

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#6 of 8 OFFLINE   Javier_Huerta

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Posted December 14 2002 - 01:09 AM

Edit - double post.
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#7 of 8 OFFLINE   Baldemar Garcia

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Posted December 14 2002 - 03:25 AM

Guys,
Trust John K.'s website. There is a high-pass active filter on the low-level outputs. There is a passive filter on the speaker level outputs (single cap, 100Hz/6dBper octave). The active high-pass filter on the low-level outputs is the same circuitry as the 'rumble' filter, so you could easily calculate/modify as you wish. BTW, the 300-794 no longer has a 30Hz,Q=1.6 rumble filter. They now ship with an Fc=20Hz,Q=1.? configuration, so it's not too bad. Still, I'd modify if I were tuning lower than 20Hz.
bgarcia

#8 of 8 OFFLINE   Peter Jessee

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Posted December 19 2002 - 05:11 PM

Quote:
Trust John K.'s website. There is a high-pass active filter on the low-level outputs. There is a passive filter on the speaker level outputs (single cap, 100Hz/6dBper octave). The active high-pass filter on the low-level outputs is the same circuitry as the 'rumble' filter, so you could easily calculate/modify as you wish. BTW, the 300-794 no longer has a 30Hz,Q=1.6 rumble filter. They now ship with an Fc=20Hz,Q=1.? configuration, so it's not too bad. Still, I'd modify if I were tuning lower than 20Hz.


Here's a link to an Excel spreadsheet that you can use to calculate resistor values and graph the results:

http://marv829.homestead.com/Home.html

PE also has a .pdf file with tables of resistor values for modifying the "rumble filter" and bass boost. There's a link for it on the page for the 300-794.

Peter


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