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Old 7th February 2014, 02:27 AM   #1
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Default Change/Bypass High Pass Filter?

Hello, forum noob here. I'm a physicist but do mechanics, not electronics, so go easy on me.

My son and I are putting together a relatively affordable stereo system for his wakeboat, an older Ski Nautique. The system will consist of the following:

200W x 4 ch. @ ? ohms amp driving four speakers in the boat
125W x 2 ch. @ 4 ohms amp driving two tower speakers
self-amplified subwoofer

My question is about the first item. The amp is an XTP Trifecta 4200. It is being "re-purposed". XTP was designing a system with this amp and tower speakers which had midwoofers and HLCD horn tweeters driven by separate channels of the amp. So channels 1 and 2 were to drive the midwoofers, and 3 and 4 the tweeters. The speakers were never mass produced, and the amps sold off. I actually got 2 new amps for 27 bucks each at an auction.

The manual is awful. I don't know if the 200W is peak or RMS (don't really care), and if the amp is rated for 4 ohms or 2 ohms minimum (do care).

Channels 1 and 2 have a high pass filter (HPF) that is adjustable between 70Hz and 300Hz, which works great for our use. Channels 3 and 4, meant to drive the tweeters, have a HPF adjustable between 2300Hz and 3000Hz, which is of course terrible for our use, where we are trying to drive combination speakers, each with a midwoofer and dome tweeter (and I assume their own crossover inside).


What I have done on the test bench:

1. Tried bridging. Bridged 3 and 4 still go through 2300Hz HPF.

2. So now running two 4 ohm speakers in parallel on each of channels 1 and 2. Not sure if the 2 ohm impedance will burn out the amp long term. Oh well, I have a spare amp.


HERE'S WHAT I WOULD LIKE TO DO:

I want to change the channel 3 and 4 HPF to be just like the channel 1 and 2 HPF. Move it down to 70Hz to 300Hz.

OR, move it to a fixed 100Hz HPF.

OR, just bypass it completely.


PROBLEMS:

a) I cannot locate a circuit diagram.

b) I know what HPF circuit diagrams look like on paper... beyond that, I have no idea what I'm doing.


Do I have a prayer? If not, I'll leave the speakers in parallel and cross my fingers. It's a shame to ride two channels hard while two are loafing, though, just because of that doggone HPF.

Thanks in advance.

Steve
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Old 7th February 2014, 11:40 PM   #2
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Hi folks,

Lots of views but no replies yet. Do you think I'm posting this in the wrong place? I guess I'm looking for some thoughts from an EE type person.

Thanks.

Steve
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Old 8th February 2014, 06:37 PM   #3
rsavas is offline rsavas  Canada
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Location: Ontario
Quote:
XTP Trifecta 4200.
need a service manual or schematics in order to make it easy to figure out.
I take it that you want to disable the HPF for the tweeters. I guess the HPF can be by-paased somehow. Changing the comps you will need a schematic or reverse engineer the design (draw schematic form the layout) which is a PITA.
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Old 8th February 2014, 08:17 PM   #4
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Thanks for the reply.

I got a few replies on another forum (not sure if this forum allows posting to other forums, so for now I won't link to it). The folks there suggested that I use the amp as it was designed--as a true bi-amp. So I'm going to open up the speakers and see if it's easy to access the mid and tweeter wires, to bypass the speaker internal crossover circuit. If so, then I would parallel the front & rear left mids from amp ch 1, front & rear right mids from amp ch 2, front & rear left tweeters from amp ch 3, and front & rear right tweeters from amp ch 4.

It actually seems like a lot of messing around to end up with a bi-amp setup, but it would allow ch 3&4 to handle the high frequencies and so drive ch 1&2 less for a given volume. I have since come across a very interesting bi-amping article, which is a little bit self-contradictory: it touts the benefits of active bi-amping, but states that the lower frequencies contain most of the power. So I'm wondering how much of the power goes to the mids, and how much to the tweeters... i.e. is it worth re-wiring the speakers and running double cables if only 1/8 or 1/4 of the wattage is offloaded from ch 1&2 to ch 3&4 for the tweeters. I'll take a look at one of the speaker's internals and see what's involved.
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Old 10th February 2014, 02:58 AM   #5
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After a lot of discussion on the other forum, and a good bit of testing of the amp, we've decided not to go the bi-amp route. Too complicated for probably no perceivable gain. So I'm back to wondering if I can modify the Ch 3&4 HPF to match the Ch 1&2 HPF. Or I will just run a pair of speakers in parallel on each of Ch 1 and 2. That only presents 2 ohms per channel, though, so we'll see if we burn up the amp.

Would it be typical that an "amp guy" could look at a photo of an amp circuit board (if I could even get to it) and say, "yup, change out that resistor and that capacitor and you're golden"?

Thanks.

Steve
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Old 10th February 2014, 01:11 PM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by boaterdad
Would it be typical that an "amp guy" could look at a photo of an amp circuit board (if I could even get to it) and say, "yup, change out that resistor and that capacitor and you're golden"?
Only an 'amp guy' who was very familiar with the particular item. The rest of us would need a circuit diagram, because photos don't tell us much.
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Old 17th July 2014, 02:21 AM   #7
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Follow-up: I ended up just running a pair of speakers in parallel on Ch 1, and a pair in parallel on Ch 2, and it's been working fine in the boat.

Of course, I forgot to replace the alternator with a high-output model and ended up doing so while on vacation, but that's another story..............

Thanks for your input.
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