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Old 15th June 2004, 02:43 AM   #1
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Location: santa rosa
Unhappy amp help please

maybe this isn't the right forum for this but here we go anyways. I just bought a 60 watt suboofer amp off of this website http://www.allelectronics.com
and when i got it the amp had no line in jacks, only speaker level input so my question is, is there a way to modify it internally to accept a line input? I opened it up and the thick speaker wire comes from the input to a small circuit board where the highs and mids go out on thicker wire to the speaker-out wires, but the low end gets routed through a small ribbon cable to the amping section, there has to be something in that small circuit board that shrinks the bass signal down to a reasonable level..... I did see a big resistor but it couldn't have been much bigger than a 5watt one, looked like a large run-o-the mill carbon. anyways, thanks in advance for any help i recieve.

p.s. this amp is for bass shakers in my couch, I don't have enough bass in my sub It is a 15" dayton with 2 15" PR's in a 3 cuft box, double thick 3/4 MDF. yummy. I will post pics when it is completly finished, right now it is just embarrasing.

Mike
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Old 15th June 2004, 09:46 AM   #2
sreten is offline sreten  United Kingdom
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What you say all sounds correct.

Except your not looking for a "big" resistor.

A voltage divider, e.g. 20kR and 1kR will draw minimal power
from the loudspeaker input (~0.1W) and will look like normal
1W resistors. A lower resistance divider would need higher
power resistors, also check the input impedance of the
amplifier proper.

sreten.
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Old 15th June 2004, 11:23 PM   #3
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thanks sreten, i am almost sure they seperated and then attenuated the bass signal before sending it to the amplifier, i just have to figure out how to bypass the attenuation part, i want to run a line-in signal (i think they are 5 millivolts, not sure i think it varies) straight to the amp without frying it. I am splitting my sub output on my reciever, one goes to my big sub, and the other will go under my couch to this amp at a low voltage. but it is going to lower it even more because it is setup for a speaker level input sheesh. it can't be that difficult to figure this stuff out... i think i need another look inside the amp for sure! thanks a lot!

Mike

p.s. this amp came with zero paperwork, how would i figure out the input impedence? I have a trusty fluke at my disposal.
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Old 15th June 2004, 11:39 PM   #4
sreten is offline sreten  United Kingdom
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You will need to add even more more low pass filtering for
the shakers IMO, they should only come in well below your
subwoofer for good effect IMO.

5mV is way too low for line level, 100mV to 500mV is ballpark.

Basic input impedance is measurable the correct side of any
input coupling capaciitor.

sreten.
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Old 16th June 2004, 02:53 AM   #5
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yeah the lowest xover freq it has is 80hz. I would be totally lost if i tried to x it over lower, i can do loudspeaker xovers, i usually hand wind the inductors myself and measure them, but this is a whole other ball game! Any tips or ideas? I am competent electronically, i was a navy electronics tech for 4 years so i know the basics. though i have to admit i was confused by your input impedence measuring statement..... so i measure + and - but being sure to be on the opposite side of the input cap? makes sense i suppose.... but that is all there is to it? I really do need to crack that amp open and have a look..... gotta wait until after work..
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Old 16th June 2004, 12:17 PM   #6
sreten is offline sreten  United Kingdom
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For the input to the shaker amplifier put a 10KR resistor
in series and then a capacitor to ground to roll off bass.

Capacitor value is 0.33 uF ballpark for 50Hz 1st order.

Note you are simply adding this shaping to the sub
output so frequencies above 80Hz are already rolled off.

Just try a few capacitor values to see which sounds best.

One side of the input capacitor to ground should be the input
loading resistor, unless the amplifier topology is unusual.

sreten.
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Old 17th June 2004, 01:41 AM   #7
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ok i half-assed traced it, it got pretty complicated and i had to wake up early. the negative side splits right away into 3 branches, 1 is a 17.47k resistor, another is a 495 ohm resistor that looks kinda big... right after this resistor the signal goes into the amp board. the third branch connects to the positive side with a 7.4k resistor... confusing, i could read it better if it had a dang schematic. the positive side splits into 2 paths, one is a .339M ohm and the other connects to the negative side via the 7.4k resistor. geeez, i need some one on one in person help here..... or a schematic!!!! the board also has 2 small ic chips that i don't want to kill. i just gotta be careful. I will experiment and keep updating on my progress. thanks.

Mike
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Old 17th June 2004, 02:38 AM   #8
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maybe it would help if i were to post a pic of the circuit board.....
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Old 29th June 2004, 04:13 AM   #9
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OK I think i may have found the right spot, there were 2 positions that i found that worked, 1 worked very well, so well in fat that my other sub ceased to operate. I have 1 line out of my reciever going to my normal sub, and the other going to my new amp, well when i touched the wires to the solder points on the underside of the board the amp would operate fine but my sub would shut off.... scary thing the other position works okay, but it isn't as loud, and i can hear an AM radio station through it! the XO filters most of it out but i can still hear it. any ideas peoples? DO i need to take it to someone at an electronics repair shop? thanks.

mike
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