Setting amplifier gain on a sub

[jronnquist]

Hello, 

 

Not sure if anyone out there is into their sound systems enough to know (or care) about this, but I thought I'd ask anyway. 

 

I currently have two amps connected to the RCA outputs on my Kenwood head unit. The amp running my door speakers is a 900W Kenwood 4-channel unit. The amp has a RMS output of 70W per channel at 4 Ohms and the JBL speakers are rated at 50W RMS. I know the output voltage of the head unit's RCA ports is 4 volts, and I have set the input voltage to 4V on the amp. As both the mids and the tweeters in the doors have their own crossovers, I've left both the LPF and HPF off on the amp, and left the bass boost on zero. With this setup I can turn the volume to max (35) with no distortion at all and I'm happy with it. 

 

The problem is the second amp, which is built into the sub and rated at 400W RMS (1200W peak). This too has adjusters for gain, LPF and bass boost, but unlike the Kenwood amp the value for gain simply starts at "min" and goes to "max". I'm assuming this is because FLI is a boy racer Halfords product rather than a serious piece of kit. Still, it does work and I'd like to be able to set the gain to match the head unit in the same way I did for the Kenwood, if only to prolong it's existence until I can get a better one. Any tips? 

 

As for the Frequency setting on the sub amp, I've set this to the lowest level (50Hz) on the logic that the door speakers' frequency response is 50Hz - 20kHz, and this would mean the sub will only pick up where the rest of the system leaves off. Is that right?

[ryan-re]

Setting the gain properly needs a specific bit of kit (osciliscope). With a 4v headunit you'd be looking to set it around the 10 o'clock position.

 

The crossover settings are very user dependant. Id suggest raising it to 80hz so you get a crossover point.

 

The speakers will play what you tell them. The fact their responce is 50-20khz doesnt mean they will play to 50hz. What they will play to depends how you set it up. 

 

Right now you have a full range signal going to the crossover then the crossover is doing its bit. Thats fine but you are relying on the crossover filtering all low frequencys and its likely some will slip through the net as its not a brick wall.

Id set the HPF on the amp for the front speakers to 80hz to help them out.

 

Do you have HPF and LPF on your headunit? Whats are these set to?

[jronnquist]

Thanks for the reply! And the very useful information. 

 

So 10 o'clock for the gain on the amp. Right on. I suppose I could always take it to someone with an osciliscope and the smarts to use it, but for this low end setup I'm just going to run with what I'm sure is sound advice. Dito for 80hz on the LPF.

 

When I say door speakers I should have mentioned that they are components. The JBL spec simply states that the range is 50-20khz, so I'm asuming that means for both combined, and that each has a crossover to filter out what the other is playing. That's why I've left all the filters on the amp off. The headunit has a cornucopia of settings in addition to the standard equalizer, including a fancy menu where you can set car size, speaker set up and size, and filters for all of these. But I have ignored all this for the same reason. Surely the only way it would make any sense to set filters either on the head unit or the amp would be if i had 8 channels going to each individual speaker in the doors. Have I got that right?   

 

 

[ryan-re]

The crossover is a filter but you still need to filter out the lows at the amp and headunit side. The crossover and hpf are not brick walls like I said. 

 

Right now you are relying on the crossover having a steep cut off at the lower end to stop sending sub frequencies to your midbass speaker's. 

 

You have 2 hpf to help prevent this. I'd use them if I were you

[jronnquist]

Sorry, my mental machinery has finally caught up with what you are saying. And yes, that does make a lot of sense. So what levels would you suggest I set the LPFs on the sub for the door speakers? The filters settings run from 50 to 200 Hz.

 

 

[ryan-re]

Hpf try 80hz and lpf 80hz. It's not set in stone, try different combinations. 

 

 

[jronnquist]

On this particular amp you can only have one or the other. So I'm guessing start at 80Hz for the LPF, yes?

[ryan-re]

If the amp is for the speakers use the hpf

 

The sub amp use the lpf 

[jronnquist]

Okay, I got them mixed up. Nice one. I'll set these up tomorrow and see what happens. Thanks a lot for your help. 

[JonBRapid]

Hi not sure how your getting on with the set-up... Have a look on youtube at setting up an amp using a multimeter... download JL Audio or JBL app on it they have calculator tools to help you work ou the rms watts and volts (saves you from doing the maths!) download a high-quality frequency tones 50hz, 80hz, 1k and a full sweep and some different varieties of music from police, phill colins, some D&B from LTJ Bukem a bit of pop and some classical and see how it sounds

 

Peak watts is a load of rubbish, my JL-Audio C3's are powered by a 55w Alpine amp (ok they're being run active, an amp channel for each speaker and there is no crossovers just an audison bit 10d) but they sound sweet.... using the multimeter you can limit the power going to the speakers so they'd get the 50w... giving them 70 will just shorten the life cycle  if you send a distorted signal to the amps they'll amplify the distortion... my head unit doesn't go over 17... I think that's around a third of the total -  the bit 10d has some funky lights that light up if the signal is clipping (Distorted).

 

Hope its going well

[ryan-re]

The multimeter method doesn't really work though. 

I've seen amps, setup with a dmm checked with a scope and a lot of adjustments were needed