Thread: Nexxia radio fitting kits for P38A

Originally Posted by p38arover

Not really. When I first sketched it I thought it was an L-pad - then I realised I had drawn it the wrong way about. The revised sketch is attached.

At first glance it doesn't make sense unless the negative lead into the door amps is grounded (it probably is).

I pulled a front door amp out today. It is labelled Harmon Motive so i assume Harmon Kardon. The speakers only have LR labelling, not the HK labels I've seen in other forums.

The speaker feeds from the HU are capacitively coupled via 4.7uF caps on both the + and - leads. I wonder if it uses balanced inputs. I guess I'll need to trace the diagram out.

I cannot find any info on the 'net relating to the amps, e.g., specs or circuit schematics.

I've looked at the circuit for the Clarion head unit (I am only referring to the PU9836A) and the output stages are balanced, i.e., the output stages for the +ve and -ve lines are the same (mirror imaged) and neither the + or - leads are earthed.

I have traced a small section of the Harmon Motive front door amplifier and it appears that the inputs go to the + and - inputs of an op amp (part of a quad op amp BA14741F) wired as a differential amp. The inputs are also balanced. I suspect (but, as I'm not an electronics engineer, I'm not sure) that any buzz and hum coming in on both inputs will be cancelled assuming they are in phase (Common Mode Rejection?).

I need to trace a bit more - I shouldn't drink red wine whilst doing it!

Unless I'm wildly wrong, the design of the attenuator interface from Nexxia cannot work to properly interface a new head unit to the door amps. Also, the Nexxia unit has capacitors in circuit but for reasons unknown. Both the output of the Clarion HU and the inputs of the Harmon door amps are capacitor coupled.

Instead of earthing (grounding) the negative input, I have decided to try a resistor (value to be determined) to ground on the negative wire at the head unit end (pseudo-balancing).

In theory, that should still leave it capable of picking up the same interference as on the +ve line and, hopefully, they should cancel out in the differential input amp of the door amplifier.

<later>

I had a quick play with pseudo-balancing and the results were very, very encouraging.

I now have more drive into the door amps than I was getting from the Nexxia interface with significantly less buzz. In fact, the buzz is almost inaudible. I have only done one channel.

Next week, when I get back home from the bike races, I'll try trimming the balancing resistor (I'll install a pot and adjust it) and I'll do all channels.

This is the Nexxia interface (posted earlier) which I don't believe can possibly work:



What I have done is disconnect the door amp -ve lead from the 750 ohm resistor and connected it to an 8.2Kohm resistor to ground. The only reason I picked an 8.2K was that I made a mistake - I meant to get a 4.7Kohm resistor out of the drawer. I'll reduce it in value for the next test.

I tried a potentiometer to ground in the -ve line to the door amp and tweaked it for minimum buzz. The good news is that the buzz was completely nulled out. I have more than sufficient drive to the door amps - probably more than from the original Clarion.

Now the bad news.

• There is the very slightest trace of a medium pitched whistle. I knew I shouldn't have put the door trim back on. I need to pop it off to put a CRO across the mid-range speaker. The whistle is inaudible with the engine running.
• To get best nulling of the buzz, the resistor to ground needs to be adjusted. One front door required 10 Kohm and the other 5.6 Kohm.
• I feel the system may be distorting so I need to check further.

Those figures are for my car so another may vary.

More experimenting tomorrow. It looks like I'll end up throwing away a fair bit of the Nexxia kit.

I'm annoyed (mild term) that I spent good money (with postage, it wasn't cheap) for the Nexxia kit which patently doesn't work and they should have known that it wouldn't work. Had I been able to buy the 10 pin connector for the radio I certainly wouldn't have bought the Nexxia kit.

Ok, status as of today.

I took these Nexxia interfaces:




and modified them to this design (two per PC board):






I then adjusted each potentiometer (variable resistor) to minimise the buzz in each door. I only modded the front and rear interfaces as there is no audible buzz in the sub-woofer.

Results?

No audible buzz or whistle from the front left speakers.
Extremely low level buzz from the mid-range speaker in the right door but a little more from the tweeter.
Extremely low level buzz from the mids in the rear doors.

All of the variable resistors are set at different points which was expected. If this was to be made for sale, then the installer would need to adjust the pots for best results.

With the engine running, the noise is inaudible and I'll try it like this for a while to see if I feel it's OK. It is certainly far superior to the original Nexxia product.

I also tried the test tones and I was happy that there was no significant distortion in the amps - what I'd been hearing was distortion on the tracks on the iPod I was using for a sound source.

I have ample drive to the door amps, in fact, I think I have higher sound pressure levels from the speakers than I did before. It is much louder than i'd ever want to hear.

Ron, you never cease to amaze...!!