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Faulty Head Unit?

Sub amp

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#1 Jay87

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Posted 08 August 2014 - 07:36 PM

I have just wired up an amp and sub, having a few troubles with it though......

I'm getting feedback/noise through the sub, no engine noise though, it sounds as if you can hear the cd load/spin, I only get the noise with a cd, the radio seems ok and the aux input is fine too.

Iv'e checked the earth for the amp that's fine i'm getting no noise though the sub unless the rca cables are plugged into the headunit when playing a cd.

Iv'e made sure all the earth's I can see for the headunit and in the engine bay are ok and even tried to re-earth the head unit, but with no luck.

Another problem is the amp not turning off with the headunit via the remote wire, I have an adapter for the steering wheel controls and figured out this was causing the problem, when i disconnect them and wire it up without the controls the amp turns off when the head unit does.

I'm thinking it just could be a faulty head unit, it's just an old single din panasonic one from my old car. Does any one else have any thoughts on what it could be?

Cheers.

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#2 Jay87

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Posted 09 August 2014 - 06:40 PM

Any takers?

#3 BOF

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Posted 15 August 2014 - 03:12 PM

I'm getting feedback/noise through the sub, no engine noise though, it sounds as if you can hear the cd load/spin, I only get the noise with a cd, the radio seems ok and the aux input is fine too.

 

 

Is what you are saying that you get the normal/expected sound through the sub, plus this extra noise whenever the CD loads, or are you saying that you do not get the normal sound but only this load/spin noise?

Is what you have added just a subwoofer/amp or are there any other speakers (ie, mid, tweeters) involved?

 

(I think it probably is connected to the earth, but let's try to find out what is happening first.)



#4 Jay87

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Posted 15 August 2014 - 08:19 PM

 
 
Is what you are saying that you get the normal/expected sound through the sub, plus this extra noise whenever the CD loads, or are you saying that you do not get the normal sound but only this load/spin noise?

Is what you have added just a subwoofer/amp or are there any other speakers (ie, mid, tweeters) involved?
 
(I think it probably is connected to the earth, but let's try to find out what is happening first.)


I'm getting the normal sound through the sub but with the added noise the headunit is making. It sounds like some kind of bad earth/ignition
noise. But it's strange as I can only hear it with a cd. I can hear the disc spin/load as I said, sounds a bit like the old dial up internet noise to start off with. Then a constant noise in the background when music is playing.

If it was a bad earth wouldn't i be getting the noise all of the time? It's only with a cd. The radio is ok and so in the aux in-put too.

There's nothing else running from the amp. I have after market tweeters and front/rear door speakers but they're running off the headunit at the moment.

So just 1 amp powering 1 sub.

Nice one for the reply too.

#5 BOF

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Posted 16 August 2014 - 11:31 AM

Well, as far as I can tell, it is likely to be a 'bad' earth, but not in the way that you mean.

 

Firstly, the CD mechanics take a fairly high current and the current has a reasonably high amount of high frequency content. That's (partly) why you hear this, and not the other stuff.

 

Secondly, between the two earths, they are 'bad' in the sense of not being the same, rather than being bad in the more sense of having an absolutely bad voltage drop on each one, which would limit the power output. That the earth on the amplifier is 'wrong', as far as the output from the HU is concerned, injects a signal into the input of the amplifier (because the signal is referenced to the 'wrong' ground).

 

There are several ways around this; probably not all of the following will work, and you might have to combine them.

 

A big capacitor across power and ground at the HU. Actually, a big capacitor and and a smaller capacitor in parallel would be even better. Critical here is how you wire it and keeping the wires short is vital. This may also help the sound quality a little, so is worth doing for other reasons. (People used to use 'ultracapacitors' for this, but those are so big that they can present other problems and seem to be harder to find, these days.)

 

The second thing is to take the ground cable from the amplifier back to the HU, rather than grounding it locally. It'll be a long cable, which is a disadvantage, but ground by the HU is the right place for it.

 

A small capacitor across the input at the amplifier will help, but whether you can really do enough good without limiting the higher frequencies (not absolutely high, but...) out of the sub is questionable. This is the kind of thing that you might try if it is 'almost good enough, but not quite'.

 

There might be some mileage out of taking the power to the amp back to the power terminal at the HU end, or there might not - difficult to tell. If doing this, a big sized capacitor back at the amp end, between power and ground could help...or make things worse. Another possibility with this scheme could be to add a series choke (usually a couple of turns through a clamp on ferrite), and that might help a little - it is probably only worth trying this if you already have a capacitor across the power at the amp end.

 

Is the amp from from a pre-out at the HU end? If so, using the highest output from the HU and attenuating at Amp end will help a little. If not, you should already have an attenuator somewhere, and referencing this to ground at the Amp end will help a little.



#6 Jay87

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Posted 16 August 2014 - 08:24 PM

I'll try grounding the amp to the same ground as the head unit as you said, before I try anything else. Any idea where the stock ground is located? As far as I can find there is one behind the dash and one by the b pillar for audio. I can't find the one behind the dash at all. So if I can't find it I'll try to find a new ground again.

The amp is connected from the pre-outs at the headunit.....so a high level output may work better instead?

I did go to a car audio place by where I work and he said it was the headunit, but he didn't really care too much and had a feeling he was just trying to get me to spend money on a new one.

#7 BOF

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Posted 17 August 2014 - 09:42 AM

Any idea where the stock ground is located?

 

No, I don't. But, if you say what car you've got, someone with the same car might. Otherwise, you are left tracing wires, I'm afraid.

 

The amp is connected from the pre-outs at the headunit.....so a high level output may work better instead?

 

High level outputs (ie, speaker level) are a bit ugly, so in normal circumstances I'd advise avoiding them. Some pre-amp outputs are higher level than others, so, if you have the option, the highest level pre-amp output and attenuate at the amp end, either by turning its input sensitivity down, or a separate attenuator.

 

(Technically, there is another solution involving a differential amplifier in front of the power amplifier, but I don't know of anywhere you can just get 'a diff amp in a box' that'll solve your problem. You'd end up building it yourself, or getting someone to do it, and that's probably more messing about than is strictly necessary.)

 

But, if nothing else works, you might end up at high level outs from your HU and a separate attenuator at the input of the power amp. I don't like that solution, but if nothing else works...

 

I did go to a car audio place by where I work and he said it was the headunit, but he didn't really care too much and had a feeling he was just trying to get me to spend money on a new one.

 

Plausible. Most audio fitters are pretty poor, and have zero idea of the fundamentals of what they are doing, but have a fair bit of knowledge of what has worked for them in the past and what hasn't. If that means they always advise that you buy one of the HUs that they now fit, then its not a disadvantage to them to do that - at least its a HU that they know, and the cost isn't their problem.

 

If you actually wanted the features of the new HU, fair enough, but otherwise it is awfully close to just throwing money at the problem.



#8 Jay87

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Posted 19 August 2014 - 06:37 PM

Thanks for the advice/help, hopefully I'll crack it sooner or later.

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