VHF - remote speaker problem

[dave_gibsea]

Hi,

I need some advice. I have wired up a cockpit speaker for my VHF, the radio has a jack socket for this which I have used. It works fine except that when the speaker is connected the main speaker on the radio in the cabin does not. I can see this would be a good idea (say) if at sea at night with crew sleeping below but I would like both to work.

On taking the covers off the set I can see three wires going to the jack socket. I imagine only two carry the speaker connection and one is something to do with turning off the main radio speaker. So, do I cut one of the wires? if so which? Any advice on what to do next would be most welcome.

Cheers,

Dave
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[nct1]

Hi

I believe that the VHF radio is designed to only output on the external speaker if connected, killing the internal speaker output.

This is certainly the case with mine, and I have heard others comment that this is normal.

To my mind it seems a daft design, but there you go.

I shall watch with interest to see whether anyone comes up with a workaround.

 

[KREW2]

Hi
I am just about to leave to go to my Gib'Sea connect a jack plug onto the speaker extension wires. Having now just read your post I was wondering if it is the insertion of the jackplug that disconnects the main speaker? or if an inline switch will do the job, getting to the back of the radio every time I want to unplug is going to be fiddly.
regards KW

 

[Pierrot]

That is what you expect. The amp in the radio will be designed to deliver its power into a given load (say 4 Ohms). Connecting two speakers in parallel would halve the impedance - which could blow the amp. Connecting two in series would double the impedance, so reduce the volume - probably to unacceptable levels.

In summary there is no easy way to use an external speaker at the same time as the internal one.

If you do want speakers in the cabin and on deck the best solution is to use two external speakers, wired in parallel and plugged into the jack. Each speaker needs to be twice the impedance of the internal speaker

 

[discovery2]

You are correct in that only two wires are required for the external speaker. However, instead of cutting a wire, two actually need to be joined together!

With the covers removed, if you insert the jack plug in the socket, you should see the tip of the plug push against, and open a small contact. The opening of the contact disconnects the internal speaker and connects the external one. If a small bridge wire is soldered to these two contacts, the internal speaker will remain functioning when the plug is inserted.

HOWEVER, I am <u>not</u> an electronics expert and those who are may advise against this procedure as it may/will affect the impedance (technical word!) of the speaker output and may damage the set. I would advise that you find out the impedance of both the internal and external speakers - they are quite likely to be either 4 or 8 ohms, and will almost certainly be stated this in the manuals/packaging - and consult someone who is more knowledgeable about electronics. A call to the manufacturer of the set may also be helpful.

Hope this is of some help.
David

 

[GilesC]

As mentioned elsewhere, its probably not worth risking having both connected at the same time (although you may well get away with it).

I solved this conumdrum by getting 2 external speakers - a waterproof one for the cockpit, and another in the cabin (a v.cheap one off e-bay), and a switch between them (v.cheap from Maplins), driven from the external speaker jack. That way, there's no risk of blowing the amp, and a choice of internal or external speaker, and no risk of damaging the unit. I did that two years ago with no problems - just a bit more fuss initially, but its still working well. The switch and internal speaker cost about £10 between them, which I thought was money well spent compared to possibly over a hundred for a new VHF!

 

[Talulah]

Not all VHF radios are like this. i.e. The Simrad RD68 continues to output on its internal speaker even when a remote speaker is plugged in.

 

[Plum]

This is what NASA Marine advised me

I had the same problem with my NASA SX35 VHF and asked NASA for advice. Here is their reply:-
*************************************
Hi,
An external speaker does mute the internal speaker (as specification dictates). One solution is to connect two 8 ohm speakers in parallel.

Regards,
Tony Kocher
Nasa Marine
******************************************

 

[fireball]

At normal volume levels having speakers in parallel is not too much of an issue.
As said above quite a few VHF units do disconnect the internal speaker when you plug into the external socket - this can be very helpful, but not always.

If your happy with your internal speaker and just want to connect an external one in addition then you just have to have a jump wire over the two connectors of the jack socket - if your crafty you could add a switch into this jump wire - allowing you to decide if you want internal on or off - bearing in mind, when you disconnect the external speaker the internal one will be on anyway.
If you add an inline switch to the external speaker too you then get the choice without having to un/plug all the time....

We weren't happy with our internal speaker - it is based on the bottom of the unit and shuved up inside the nav instrument panel - so we have used the external speaker socket to plug in a Nokia speaker from a car kit (it was free ok!). We will tap into this line for a cockpit speaker and probably put a POT in so we can control the relative volume of both.

 

[EdEssery]

Re: This is what NASA Marine advised me

This is correct - most VHF sets I have seen have an output spec. is 4-8 ohms. One 8 ohm speaker or two 8 ohm speakers in parallel are both within this spec. This means you need two external speakers one in the cockpit and one in the saloon to replace the disabled internal speaker. It's then very easy to design a switch circuit with which you can enable one or other speaker or both but never allow both to be switched off as this would blow the output stage of the VHF. I have done this on my boat and can provide a circuit diagram. The necessary switches can be bought from Maplins very cheaply.

 

[fireball]

Re: This is what NASA Marine advised me

Not sure how running an amp into infinity ohms will blow it ... running it into zero ohms maybe!! Then again - not much point in turning all the speakers off whilst still having the radio on!

 

[Strathglass]

Re: This is what NASA Marine advised me

When the output of an amplifier is not loaded then in some situations it is possible for the output circuitry to generate excessive voltages which can damage the audio output stages.

In most cases hanging another loudspeaker on the output of a VHF radio will not do any harm. In mist instances probably the leads to the second speaker will be of a reasonable length and the impedance (not the DC resistance) of these leads at audio frequencies will be several ohms. This will be in series with the impedance of the second speaker. (Modern domestic HIFI speakers now run two pairs of wires to each speaker to try and nullify this effect)

What this means is that adding an additional speaker with a long pair of thinnish wires to the rear of a VHF will only present an additional load of something like 8 ohms. This would not be a problem on most sets.

The sugestions for using external switching of the speakers is a good idea