Tick/Whining through speakers...

[40330]

I got a Mazda 3 that has the original Bose Head Unit in it. I spliced the audio between the Head Unit and Bose Amp under the driver seat. Ran the RCA on the driver side and ran the power on the passenger. I repositioned the ground for the amp which got rid of the alternator whine... but I still have the ignition ticking.

Next thing I'm thinking of is to lift the RCA ground at the sub amp. Cause the RCA is what is inducing the noise through all the speakers. I can unplug the RCAs and the ticking goes away.

Any ideas would be great.

Also, can running the power cable across the engine bay induce noise? I have it running by the spark plug wires and alternator.

Thanks,

Matt

 

[Perry Babin]

What is the DC voltage on the center conductor and the shield of the RCA you spiced into the stock wiring?

Place the black meter probe on the amplifier's ground terminal. Place the red meter probe on the point where you need to measure the voltage.

 

[40330]

Unplugged RCAs from sub amp. Checked the DC voltage between the center RCA pin and the Sub amp ground terminal and ended up with +6.68V.

Checked the Voltage from Shield and Center on RCA. 0V.

-MR

 

[Perry Babin]

Unless the amp has balanced inputs, the DC voltage on the shield could be causing the problem. A ground loop isolator may solve your problem.

 

[40330]

Ok, well I did some reading and the output of the Head Unit is differential... it is not line level. So that's probably why I'm getting all the ground issues cause the HU is above the chassis ground... I was almost thinking I could just splice into the HU at the audio IC and make my own line RCA outs...

-MR

 

[Perry Babin]

Differential means that signals of opposite phase are on the individual conductors. It's still line (preamp) level.

 

[40330]

Yeah I made a GLI but the response and volume was but out alot. I found this link and it seems like the posters have had really good frequency response out of the project...

Here is a pic of the Difference in response. The lower extended frequency response curve is with the newer RCA outs and the other is normal out.

 

[Perry Babin]

Are you sure you made a ground loop isolator?

 

[40330]

One of the guys I work with had some 1:1 transformers that were used in a line level balanced out circuit for an old Kurzweil K250...

Which the more I think about it... I think the quality for what I need they probably weren't the greatest thing to use. But it did fix the ticking issue. Only If I could make/buy a isolator or just a work around the grounding issue. I understand what is happening... besides I don't understand why there is +6V riding on the + lines on the RCAs between the HU and Bose Amp...

Decisions.... Could you explain why that +6V is there. I don't understand that...

I have a 2 yr in Computer and Electronic Eng. but this whole above ground voltage on + terminal of RCA slice isn't make much sense to me.

I've made my own Tube guitar amps and with those you must make sure you don't have ground loops. The preamp/input section must not me grounded anywhere close to the driver/output stage or you will get nasty 60 hz filiment hum or line hum.

Anyways I'm rambling.

LMk what you think...

Thanks for your time so far too.. Really appreciate it!

-MR

 

[Perry Babin]

The head unit uses the available supply voltage which is only positive voltage (0-12v). To allow the audio waveform to be able to swing symmetrically above and below the reference point, the reference point has to be set at the middle of the supply voltage. This means it will be at about 5-6v DC. Standard head units block the DC with capacitors but with capacitors, you have to include muting transistors. They don't need those components so they don't include them and the DC passes to the amp.

 

[40330]

Alright ... I got rid of the buzzing by lift the RCA ground.... BUT my amp is a legacy la360 2ch bridgeable... I went ahead and turned it on, which now it kinda thumps when the car powers the amp up... I ran some audio. Sweeped left and right... the channels were phasing each other out even when I took the Left and Right - and + and swapped each channels center terminal... What I'm thinking is the amp itself doesn't sum the Left and Right channels when using bridge mode... it ends up phasing them out... confusing...

I may just end up taking the left RCA, I won't ground it and I'll split the signal to go to the left and right inputs at the amp... or just use the left RCA. I tried getting some info from the manual but when it starts explaining bridging it doesn't mention the la360 and just mentions the 4 channel la660.... bleh.

Whatcha think?>

-MR

 

[Perry Babin]

I'd suggest that you try a standard GLI.

The amp is oscillating because it has no ground reference.

 

[Therum]

You need an PAC SVEN-2 or 4 The new version is the SOEM-2 or 4. I used it in my BMW when I bypassed the factory amps and still used the factory head unit. Its simple and it works. It also allows you to use a after market head unit with factory amps that uses a balanced input.

 

[40330]

What I ended up doing is leaving the stock bose sub in the car.

Then I spliced off the left front balance line after the HU and before the bose amp.

I ran both the + and - through a RCA cable to the aftermarket sub amp at which I took the + wire and soldered it to the RCA center pin. The - I left disconnected at the aftermarket amp.

Then I soldered a wire and ran it from the RCA ground of the aftermarket sub amp to the chassis of the HU.

After all this I don't get any oscillation and all the ground loops/weird noises have vanished. Sounds great now.

Thanks for the support guys.

-MR