Help! No Low Pass Filter on My Grundig PA120 Amp, What Should I Do?

I recently installed a 12" sub (rms unknown, 4ohm) for the first time with a Grundig PA120 amp (rms 120). But the amp doesn’t seem to have a low pass filter. As I increase the gain, the high frequencies also increase. I checked the documentation, but there was no mention of a filter. The operating range listed is 4hz-75khz.

A while ago, in uni, I tried to build a low pass filter with a -3db at 1800hz (originally at 1500hz, but didn’t realize the inductor resistance wasn’t zero). This was for credits, not for the sub specifically. Now I’m wondering, should I try connecting it and running it that way? I tuned the amp without a load to around 20 volts, and it sounds the clearest when the sub is connected.

Also, the amp’s light pulses, and the gain doesn’t seem to make much of a difference. I’ve checked the ground and positive connections. Any ideas or tips to improve this setup?

Just toss it into the bin.

Shae said:
Just toss it into the bin.

Nah, I’m studying electrical engineering, so I don’t want to just throw it out over a small issue. I want to figure out how to fix or improve it. There’s got to be a solution for this. I’d feel like a terrible electrician if I couldn’t figure something out.

@Reilly
How is the amp wired in your car?

Shae said:
@Reilly
How is the amp wired in your car?

It’s a BMW E90 with a non-hifi system. As far as I know, the signal to the subs is unfiltered, and there’s a filter at the sub itself, but I can’t access it. I’m not completely sure how the signal gets there. The E90 has separate subs under the seats.

I took the signal from the wires that go into the sub, which is mostly full range. I connected it to the hi-low converter and then to the amp, positive to positive, negative to negative. The setup is mono. Nothing too complicated unless I missed something. Yes, there’s a fuse.

I’m using the positive from the battery and the negative from the battery as well for convenience.

Not sure what else to add right now. The hi-low converter also has a remote wire, so I took the positive/negative from the amp connection and connected the remote to the remote.

@Reilly
Ah, you said it’s a PA amp, I assumed you were using an inverter or something.

Shae said:
@Reilly
Ah, you said it’s a PA amp, I assumed you were using an inverter or something.

The amp’s model is PA120, which likely stands for power amplifier, I think.

Reilly said:

Shae said:
@Reilly
Ah, you said it’s a PA amp, I assumed you were using an inverter or something.

The amp’s model is PA120, which likely stands for power amplifier, I think.

Oh, okay. Well, not much can be done about it unfortunately. A passive crossover at 80Hz would work, placed in line with the speaker wire going to the amp.

@Shae
Yeah, I might try modeling one, or copy someone else’s design.

But I’m also considering using the low pass filter I made in uni for a project. Its crossover is at around 1800Hz, so it might be an upgrade over the current situation. If it doesn’t blow anything up, I’ll at least get some unique experience.