Autocom - an update and a question

[Blokko]

Following on from a couple of offline queries, I thought I’d do an update on this post here.

 

When I connected my original Autocom unit to the car battery (by hooking into the supply coming off the ciggy lighter) I was getting all sorts of interference problems. Before the first unit went kaput, RFI / EMI was apparent at all times - in the background (loud enough to be heard above the music) with the pitch being related to the engine revs (as Ian James described - sounds like a distant supercharger).

 

The Autocom unit has since been replaced for a new box and I wanted to make sure that I didn’t get the same RFI / EMI problems. As a starter for 10 I have looped the cable coming off the ciggy lighter around a ferrite ringa couple of times. Rather than fixing the unit to the bulkhead (by the fusebox) as before, I’ve temporarily placed it in the passenger side of the Jill Judd tunnel bag.

 

I can report that at the moment the interference is still there intermittently, but very much suppressed. It is present occasionally on trailing throttle and sometimes on hard acceleration (but only up to about 3k revs). The pitch / noise is reduced to the extent that I can’t really notice it at all with music playing. At this stage I cannot attribute the interference suppression to the ferrite ring - it may be that housing the unit in the saddle bag has made the difference (or that the last unit was a total duffer).

 

When I get back off my touring holiday, the first thing I’m going to try is to fix the unit back to the bulkhead using Velcro to see if the fixing position of it makes any difference. If it does and the interference returns I’ll be onto solution #2 - the bridge rectifier.

 

Now to my question - I think the sound quality from my Autocom headsets is really crappy - the bass in particular is very woolly. To me the sound is not as good as a bog standard set of iPod headphones. Is this something that other Autocom users have found, or have I got duff headsets as well?

 

-----Steve - Orange and Black ex Academy - my other car is a very smelly Mondeo.

[Trotsky]

Hi Blokko, I was told by many Autocom owners that I Would probably get interferance as you state when I wired my unit to the cars electrical system and that I would need some sort of suppressor fitting , however, I have now wired it up correct polarity wise to the cig plug in, ( believe me Im no sparky, just twisted the end together and electrical tape ) and I can say that I have absolutely no interferance, the cig lighter socket is under the dash and I have run the cables under transmission tunnel cover to the unit which is sat in a mesh pocket on bulkhead betwean seats and connected to a Garmin nuvi 760 which also has my music on SD card.I dont know if its because the unit is so far away from the electrical source or no but pleased to say no interferance, happy with the sound quality, not brilliant but certainly acceptable. I have the peltor type headsets tho which both myself and passengers find uncomfortable after an hour or so, would prefer some nice moulded in ear type but not sure if they are availableto go with mic and cabling etc,

 

Trotsky N7 HPC

[Paul Deslandes]

You're hearing alternator whine which is present on the +12V system on the car and results from the fact that the alternator generates ac and has a high level of ripple on the output, even though its been through a rectifier. You have either to eliminate the ripple or reduce the unit's susceptibility to it.

 

A ferrite will not have sufficient inductance to make much difference although it will work better if you put each of the two supply wires, pos and neg, through separate cores. Putting them both through the same core will have no affect whatsoever on supply line ripple although it will help to reduce radio interference. You should then add a large electrolytic capacitor on the amplifier side between the two wires.

 

I have a Terratrip intercom which was awful in this respect and, in the end, I modified the layout of the pc board inside which totally eliminated the problem.

 

As a first step try connecting the unit directly to the battery terminals pos and neg with no other connection to the car supply or bodywork, as the battery should be smoothing the alternator supply. This may be why Trotsky has had success with his install if the cigy socket has both terminals going directly back to the battery, rather than connected to chassis somewhere else.

 

I'm not sure what a bridge rectifier would do. You've already got dc, albeit with lots of ripple. All the bridge will do is reduce the voltage by a Volt or two.

 

Paul

 

 

[myothercarsa2cv]

Have the bog standard Terratrip, and fitted as per the instructions straight on to the battery terminals, I didn't think it was too bad... I barely use it though, as I'm usually on my own ☹️

 

What did you change to make it better, Paul?

 

John

_________________________

 

Bugsy: '82 2cv6 (Back on the road!!! 😬)

Talloulah: '08 1.6K Classic (Grubby )

[Paul Deslandes]

John

I don't know if you're into electronics but the amplifier chip is a twin power amp TDA2822 which has pos and neg inputs on each channel.

 

The neg input is decoupled to ground via a 100 microfarad cap and the pos input, which has the mic input via a 50k pot, is connected to ground via a 10k resistor. The problem is that the ground connections of each of these components are inches apart on the pc. The noisy dc supply current circulating around the board induces a differential input voltage between the pos and neg inputs, hence the output whine.

 

The solution was to cut the ground connection for either the cap or resistor, I forget which, and reconnect so that they are at the same point on the ground track. It completely eliminated the problem.

 

If you want to have a go let me know and I'll try to look out my notes of what I did.

 

Its a great little intercom once the whine is fixed and I hooked up the audio from my TomTom to the auxiliary input/output socket for a trip around France which was great.

 

Paul

 

 

[MarchHare2008]

Anyone know what the battery life would be like if I just stick to the 9 volt duracell? I've just bought a couple of Autocom headsets for a trip to France at the end of this month and I wasn't planning to wire it in.

 

 

[Stationary M25 Traveller]

I just extended the power supply lead that came with it, and connected one lead to a switched live on the ignition switch and the other to the earth behind the dash on the passenger side. Tucked the wire up under the tunnel top on the driver side. Simple job. No worries about battery life. If you do choose to stay with PP3 battery power, remember to unplug the drivers headset plug from the unit itself to switch off the power when you leave the car.

 

edit - AutoCom unit lives in the triangular pouch of the Soft Bits for Sevens Draught Reducer - between the seats.

 

 

Edited by - Stationary M25 Traveller on 14 Apr 2009 13:47:03

[Foggy]

Snap!!

 

Did exactly the same and no interference at all! 😬

 

 

 

 

 

Foggy the Silver Seven !!

[Trotsky]

Battery life on mine was hopeless , about 2hrs, but I did have music playing constantly, best duracells as well!!

 

Trotsky N7 HPC

[MarchHare2008]

Ah, right, looks like I'll be doing a bit of wiring this weekend then!

 

 

[Blokko]

Hi Paul

 

The plan was to use a 4700µF capacitor in conjunction with the bridge rectifier like this.

 

Would you say that this is not worth bothering with?

 

 

-----Steve - Orange and Black ex Academy - my other car is a very smelly Mondeo.

[bobt]

If positioning of the unit does make a difference, mine is stuck to the back of the bulkhead in the boot using velcro and experiences no interference at all. The unit is hard wired (via small connector) with power and ground leads going through the tunnel back up into the dash and spliced into the instrument supply.

 

Rob

[Johnty Lyons]

If your running extra electronics on your se7en It makes sound sense to run in an Aux fuse box to feed everything.

I took 2* 44strand cables direct from car battery,The + through an inline fuse at battery, and ran them through a 30 Amp relay activated by ign sw The neg to a new Busbar and the+ supplies power to an Aux fuse box which directly powers Autocom, Two way radio, Map light, Garmin. and Aux socket.

It also means when i get out and sw off ign I'm sure everything is OFF.

 

---

jj

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Membership No.3927.

240BHP 1900cc K Series 40th Anniversary

 

 

Edited by - Johnty Lyons on 14 Apr 2009 20:26:50

[Paul Deslandes]

Steve , the capacitor is used after a bridge, as in your diagram, or other full wave rectifier to smooth the ripple from the rectified ac going in. As you have noisy dc going in, the bridge won't do anything other than reduce the voltage. The capacitor across the supply is worth a try but it didn't stop the wine on mine but it did reduce it slightly. Make sure the -ve side isn't connected to chassis after the capacitor. Ideally the cap should be as close as reasonably possible to the amplifier, directly across the supply going in, i.e. not on the end of a long pair of wires.

Best of luck

 

Paul