Stereo to mono, is it so difficult?

[Boater Sam]

Have a need to connect one speaker - mono - to a car radio/cd player which is stereo output.

The radio has 4 speaker connections.
The front 2 are used as stereo inside a boat.
The deck speaker outside is a waterproof one. I want to connect this speaker to the 2 rear stereo outlets on the car radio/cd so that we get the output of both channels out of the one speaker.
In other words convert stereo to mono.

Looking around it seems to be fraught with problems but is it?

Sam.

[Karen O]

Hi Sam,

I'm thinking in terms of the radio I had in my Astra more than 20 years ago!

The rear speakers just duplicated the front, except there was a front/back balance control to move the image fore and aft (see - I know a boating term!) If it's a modern radio it might do some fancy surround sound processing on the rear outputs (?)

Car radios often drove the speakers bridge fashion to get more power output from a 12V supply. If this is the case then you might find you can get a mono output from the rear outputs by connecting to + of one and - of the other. That gives you + L - - R = L + R !!!

I hope this helps.

[paulsherwin]

It sounds counterintuitive, but the easiest way to do this is to buy an amp module from a Chinese eBay dealer for 99p and drive the speaker with that. You can drive it from the two stereo outputs bridged and attenuated by a couple of resistors. It does mean you have to obtain power from somewhere, but the amp module doesn't have to be in the speaker itself. You can put it in a little box round the back of the main unit somewhere.

[wd40addict]

I used to feed a mono extension speaker in my kitchen simply by using a pair of 8R resistors to sum the L&R channels then connecting the speaker to the junction of the two resistors and ground. Obviously the damping factor will be poor, but in this kind of application it tends not to matter!

[Boater Sam]

I've been searching for a 1 + 1 to 1 transformer that I can connect both stereo outputs to and the mono speaker on the secondary.
Without any result. Is there such a thing available?

[GrimJosef]

Would you be better off with two separate 1:1 transformers whose secondaries were in series driving the speaker ? I think the thing that would concern me most would be the effect of injecting a signal from a low impedance source back into the output of each channel of the amp. Naively if you were taking the feedback from that point you would be trying to run a system where, say, the R channel was trying faithfully to amplify the R signal only, but its output was being forced to be L+R. If you can't put substantial isolation between each channel's output terminal and the common load then there has to be a problem with the mismatch between input and output doesn't there ?

If you don't want to run a separate amp to the existing speaker, as Paul suggested, the other conceptually simple solution would be to buy and wire in another speaker.

Cheers,

GJ

[radioman]

At one time you could get a single 'speaker driver that had dual voice coils and this would be ideal here - however, I'm not sure if these are still available..

Andy

[Boater Sam]

Two transformers would be a solution, better than simple resistor separated for impedance matching.
I thought a 10 ohm resistor in each output, joined at the single speaker with a 10K ohm resistor across that speaker, sound?
Where would I find transformers capable of driving a couple of watts?
The use of another speaker is not valid, the existing one is mounted outside in the steelwork, not easy to find space for another.
Some ceiling speakers are made with summing transformers but they all appear to be from USA at daft money and they are not weatherproof.

[stuarth]

Could you connect a 1:1 speaker isolating transformer to, say the right channel, then wire its secondary, appropriately phased, in series with the speaker and the left channel output.

Effectively you would have the equivalent of the bridge configuration, but with the inversion on the output of one power stage rather than the input.

Stuart

[Boater Sam]

I'm trying to get my head around what I need to isolate here. Obviously I need DC isolation assuming the radio/cd uses a DC coupled output stage which is most likely.
Stuart's idea gives me that.
But will effectively feeding the output of one channel back into the other cause a problem?
The previous car radio died because some numpty joined both channels together at the speaker, I want to avoid another failure.

[Scimitar]

I think that the safest and best way is to dummy load the outputs and feed the input of a cheap mono amp via a resistor from each channel. It will sum properly and not risk the PAS stages, whilst giving you what you want.

[Boater Sam]

Unfortunately I am going to have to send whatever is required with explicit instructions to a non technical owner, a seperate amp and arranging power for it may be a step too far for him. I need to keep it very simple, join wire A to B etc.

[kalee20]

Yes, just connecting the L and R outputs together is sure to cause distress.

If you want to do it creatively, you can use a transformer, assuming the L and R outputs have a common return (if not, use two transformers).

Basically, if the L and R outputs do the same thing, then the CT has to follow them exactly. If they do different things, the CT follows the average.

I daresay suitable transformers are available, at a price - but I'd be inclined to use a mains transformer with two 15V secondaries (or thereabouts) and ignore the primary. The radio can't put out more than 12V p-p which is about 4V RMS, and even if it goes down to 20Hz, at the lower voltage, the transformer will be fine with that.

[G8HQP Dave]

I would just use some suitable resistors. Assuming 8R impedance, use two 4R resistors. One on each output, speaker fed from their junction. The speaker sees about half the power output which should be fine. Each channel output sees the other channel via 8R, which should be fine. Actually, I would be tempted to use 2.2R resistors, as most genuine stereo signals are largely the same on both channels anyway. It is direct connection which causes problems, so as long as you have some ballast resistance in use it will be OK.

[mhennessy]

Why has no-one commented on Karen's answer? Simple, and elegant, and as the the radio almost certainly has bridged outputs, it's the right way to go

Could you find out what make/model the radio is so we can verify this?

[Refugee]

I don't understand what all the fuss is about as waterproof passive speakers can be had for under £20 per pair from normal auction sites.

[Boater Sam]

Sorry to cause a fuss, just wanted ideas on how best to do it.
As I explained, changing the speaker or adding another is not an option.
I need a simple to install idea for a guy who has few tools, no soldering iron, no meter.
I will have to build it all on a board in a box with just the right wires with connectors sticking out.

[Scimitar]

Sorry to cause a fuss, just wanted ideas on how best to do it.

Why not just dummy load one output and feed to other to the existing speaker? Is he really going to notice the difference on the majority of broadcast material?

[Boater Sam]

Its a cd player as well so yes, he may well find he gets only half the track with some recordings.

[GMB]

Originally Posted by mhennessy

Why has no-one commented on Karen's answer? Simple, and elegant

... but as is often the way, it is probably not so simple in practice.

I searched the net for some example circuits and what I am seeing is various designs of bridge output stages that are not so symmetric, the two parts interact in various ways.

Having said that it would probably work if you also added some resistors across both normal speaker outputs so that the half-of-one and half-of-the-other feed is only a fraction of what the amplifiers think they are driving.