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Old 06-12-2021, 11:04 PM   #29
FR-S2GT86
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Originally Posted by soundman98 View Post
i was really hoping we could figure out a way to safely bypass some of the filtering in an easy 'solder wire here and here' method

I'm certain that if somebody were to remove those input resistors from the board, one could install a pot to replace them to at least give you some sort of control over the input signal level so that it would make it easier to integrate an aftermarket head unit into the system. Bypass the resistors and add something like this on the RCA outputs of the head unit that feed it a signal:

https://cerwinvegamobile.com/stroker...er-slmlc1.html
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Old 06-13-2021, 08:48 AM   #30
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but the amp gets a line level signal, where that volume control is for preamp level signals

if the amp got a low level input signal, there's a hundred different ways to modulate its usefulness without even opening that housing. the sticking point is the high level inputs
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Old 06-13-2021, 03:45 PM   #31
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Originally Posted by soundman98 View Post
i was really hoping we could figure out a way to safely bypass some of the filtering in an easy 'solder wire here and here' method
If the goal is to modify the factory amp to effectively bypass the input filters and use it as a full audio spectrum amp, I have a suggestion BUT PROCEED AT YOUR OWN RISK. I HAVE NOT TESTED THIS MOD AND YOU MAY DAMAGE YOUR AMP IF YOU ARE NOT CAREFUL.

The amp has both a High Pass Filter (HPF) and a Low Pass Filter (LPF) at it's input stage to limit the overall frequency response of the amp. A High Pass Filter can also be thought of as a "low frequency blocking filter". In looking at the graph provided by @FR-S2GT86 we can see that the amp has a usable low frequency response to below 20Hz and therefore the HPF does not require any modification.

A Low Pass Filter (LPF) is also a "high frequency blocking filter". The graph shows that the amp in stock form is only usable to about 350Hz and all the mid and high frequency audio is effectively gone. The LPF needs to be modified to allow the full spectrum to pass.

See the diagram below of a "non-inverting op-amp low pass filter" which I think is what our amp is using. This circuit places a capacitor across the feedback resistor. This has the effect, as the frequency rises, of increasing the level of negative feedback and reducing the gain of the op-amp at mid and high frequencies.

More info here about op-amp filters:

https://www.electronics-notes.com/ar...ive-filter.php

It seems reasonable that if the capacitor were to be disabled or eliminated that the gain of the stage would remain constant across the frequency spectrum.

This could be accomplished by either unsoldering C111 and C112 or cutting the circuit etching as indicated by the yellow lines (See the circuit board photo below). Either method could be reversed if desired but would require reasonable skill with a soldering iron.

If anyone attempts this, please post your results so we can further unravel the mystery of how this amp can be used.
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Last edited by LimitedSlip; 06-13-2021 at 07:15 PM.
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Old 06-14-2021, 11:57 AM   #32
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but the amp gets a line level signal, where that volume control is for preamp level signals

if the amp got a low level input signal, there's a hundred different ways to modulate its usefulness without even opening that housing. the sticking point is the high level inputs

We know we have 8.2k-ohm input attenuating resistors on the amplifier board. These resistors are there so the speaker-level signal doesn't overdrive the amplifier inputs, in effect, our fixed gain control.

If we knew what the maximum unclipped speaker level voltage is at 50Hz out of the OEM head unit into the amp, (a test CD, voltage meter and a distortion detection meter or an O-scope could give us this information) we could assume that to be the maximum intended input signal voltage into the amp that would prevent damage to the amp's internal electronics.

We know that aftermarket head units have RCA signal voltages anywhere from .2V on some cheaper units on up to 6.5V on some of the high end models. If one could remove the attenuating resistors, one could insert, for example, a varistor with a range of 500-ohms to 8.2K-ohms in that same circuit trace, so that an RCA-level signal OR a speaker-level signal can be used, and would allow for adjustment.

I don't have my factory head unit in the system anymore and I just sent my amp test harness to another member here so I don't think I'm going to be tackling this project anytime soon.
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Old 06-14-2021, 06:48 PM   #33
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I have no idea what's going one past the first two posts, but thanks for putting it out here guys.

You can never have too much information out there ~
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