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Old 26th March 2013, 12:21 PM   #1
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Default use subwoofer amp as full-range?

Hi car audio experts. I am looking to set up an amp for off-the-grid PA (music and speech) to run on a 12V battery. A single channel (mono) is all that is needed. The speakers would be one or two 8-ohm. Amps like the Lepai LP2020 are limited to about 7 true watts at 12V supply with an 8-ohm speaker, double that for a 4-ohm load. So how to get more watts? I'm aiming for something around 100 watts, with the least draw on the battery and hopefully at low cost.

One idea I'm investigating is to use a readily available class D car subwoofer amp. E.g., the Kicker DX250.1 is rated 140 watts RMS into a 4-ohm load. The little info I have found on these amps indicates that they have a switching power supply internally that creates high enough DC voltages to reach the target power output. It seems much easier to use such a pre-built amp than to try and build my own step-up power supply or to patch together such a supply and a separate amp.

But, these subwoofer amps have a built in low pass filter, and I want to use it as a full-range amp. I would think that modifying it to bypass the filter would be easy enough? But can the amp circuit itself handle frequencies up to, say, 15 KHz, or is it inherently unsuitable for that, e.g., due to the use of too low a switching frequency?

If there are similar amps that already are full-range let me know. A stereo one would not hurt, even if I only use one side, as long as it's small and efficient and inexpensive. But if modifying a sub amp is feasible, it may be the ideal solution.
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Old 26th March 2013, 07:28 PM   #2
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In many subwoofer amps there is a master/slave switch in case someone wants to use a pair of these amps in bridge mode.
If you set it at "slave" you bypass the low pass filter as well as all preamp controls.
Personally i have tried that during a repair of an amp ( a rockford T30001bd).
The amp produced clear mid to high frequencies but the zobel resistor started to overheat.
So, maybe this is a mod you need to go through.
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Old 26th March 2013, 11:37 PM   #3
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Default or perhaps the Kicker DX125.2 ?

Thanks for the info. Good point about the zobel, so presumably I'd want to replace the capacitor there with a smaller one.

OTOH, seems like there is a non-subwoofer amp that would fit my needs: the Kicker DX125.2 - it's stereo, but bridgeable. Pros and cons?
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Old 27th March 2013, 12:13 AM   #4
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Location: Louisiana
You have to determine what's most important (cost, efficiency...).

You can buy full range class D amps that produce significant power but they're not cheap (but they will be relatively efficient).

You can buy cheap class AB amps (like the Kicker) but it won't be as efficient as a good quality class D amp.

Most class D subwoofer amps won't produce higher frequencies required for good quality audio, no matter how they are modified. You could, however use a wide-range class D amp for bass and mids and use a small class AB amp to reproduce the higher frequencies. This won't be inexpensive but it will be relatively efficient.
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Old 27th March 2013, 02:01 AM   #5
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Default is it AB or D?

Originally Posted by Perry Babin View Post
... You can buy cheap class AB amps (like the Kicker) but it won't be as efficient as a good quality class D amp.
- some web pages say they are AB, others say D. Kicker's own web page says D. Has anybody figured this out for sure? How about any measurements of their efficiency and their quiescent power draw? I am talking about their smaller models, DX250.1 and DX125.2.
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