ICE Question regards taking audio feed from rear speaker

[mrmr96]

Hi,
I'm installing a sub and amp in my car, but the stock headunit has no pre-outs and I don't want to change it. So I figured that as my Amp can take a speaker level input, I could take an audio feed from the rear speakers.

Question is: Should I just take a feed from one of the speakers, or both, or should I take the + from one and the - from the other?

I've not installed an amp using a speaker feed before, so not sure what the 'normal' method is. All advice gratefully recieved.

Thanks

[mrmr96]

Update, I've had a look at the manual, and despite being a mono amp (for a sub) it can take a stereo input, so I think I'll take a feed from both the speakers. (Two pairs of +/-wires, so 4 wires total.)

However, the amp has a specific design of input connector for the speaker level inputs:


The rest of that diagram shows the input directly from the head unit (i.e. no rear speaker in parallel.)

So two more questions:
1 - So will it be ok to take a feed from each speaker in parallel with the speaker that's already there? Or will that fubar the headunit if I take off in parallel with the speaker?

2 - That connector in the diagram, I don't have. Is it likely to be a 'standard' connector, or will I have to contact the Amp manufacture?

Cheers

[DrDeAtH]

Take the feed in parallell from each speaker, it will work but the signal will not be as 'clean' as one taken through a hi -lo converter (which would then enable you to use RCA inputs) easiest way to connect up would be scotchlok connectors, that way you can remove the equipment later on without major hassles and rewiring. Make sure you get a good earth point for the amp, with the earth lead as short as you are sensibly able to. Under the rear seat is a good place to earth, there are usually spare tapped nuts to use.

[mrmr96]

DrDeAtH said:

Take the feed in parallell from each speaker, it will work but the signal will not be as 'clean' as one taken through a hi -lo converter (which would then enable you to use RCA inputs) easiest way to connect up would be scotchlok connectors, that way you can remove the equipment later on without major hassles and rewiring. Make sure you get a good earth point for the amp, with the earth lead as short as you are sensibly able to. Under the rear seat is a good place to earth, there are usually spare tapped nuts to use.

Cheers for the answer. Parallel with the speaker if def ok then? (This is my main worry at the min, because if the speaker is 4 or 8 Ohm or whatever, then if I put the hi-lo or amp in parallel then maybe that would drop the resistance of the circuit, lowering the load presented to the stock headunit and poss blowing it?) If the hi-lo or my amp have 'near infinite' (aka very high) resistance then:
1/R[total] = 1/R[speaker]+1/R[hi=low or amp]
so 1/R[total] = 1/R[speaker]+1/infinity
so 1/R[total] = 1/R[speaker]+0
so 1/R[total] = 1/R[speaker]
so R[total] = R[speaker]
Hence no impact from dropping the hi-lo or amp in parallel, IF that is the case.

On the other hand if speaker were 4 Ohm, and the hi-lo was ALSO 4 Ohm then
1/R[total] = 1/R[speaker]+1/R[hi=low or amp]
so 1/R[total] = 1/4+1/4
so 1/R[total] = 1/2
so R[total] = 2

So it would only present a 2Ohm load (rather than the 4Ohm which the stock headunit was expecting) and this could cause a potential problem.

So need to be REALLY certain on the 'put in parallel' point!!


Also, why would a 'hi-lo' convertor be better than using the hi level inputs on the amp? (Presume the amp has an inbuilt hi-lo convertor circuit built in, which will probably be better quality than the halfords/maplin hi-lo jobbie?)

(Cheers for tip on amp, it will be connected directly to the battery, which is in the boot of this car. I'll take an ignition switched live as a feed for the 'remote turn on' for the amp. Should be easy to find such a feed as the fusebox is also in the boot. The idea will be that I won't need to run any wires through the car, since everything's in the boot - including the speakers in the rear shelf which I want to take the feed from.)

[DrDeAtH]

The hi level input on the amp will be of high impedance(approx 10k ohm), so will not upset the head unit outputs

If you do not have the plug to go into the amp, then you may have to use the converter.
If the plug area is quite large you MAY be able to squeeze some small spade (or pin) terminals onto the ones in the amp, as long as you can insulate them with some heatshrink tubing.

[mrmr96]

DrDeAtH said:

The hi level input on the amp will be of high impedance(approx 10k ohm), so will not upset the head unit outputs

If you do not have the plug to go into the amp, then you may have to use the converter.
If the plug area is quite large you MAY be able to squeeze some small spade (or pin) terminals onto the ones in the amp, as long as you can insulate them with some heatshrink tubing.

Great - that's just what I needed to know regards the high input on the amp. Am I to assume that the hi-lo convertor would work on the same basis (i.e. have c10k ohm impedance)?

Yeah - I've had a look on ebay/halfords/maplin and I can't see the harness I need to take the input directly into my amp, so it might be that I have to end up using the hi-lo convertor anyway, as the are abundantly available, and as you say it's just RCA leads then (which I already have from a previous install.)

[DrDeAtH]

Look on ebay and amazon... you can get them for under a tenner, other than that,if you want to install asap...... seek out your local car audio place, they should have one for about £15.

just remember to warm up the sub on low volume for about 5 mins before doing any set up testing (especially if sub is new, allow 10 mins)