Stereo to mono, is it so difficult?

[GrimJosef]

It sounds to me as though Karen's solution will work in terms of generating the sum voltage that is needed. Are we sure that the individual amps will be happy passing a current which may (OK, in a pathological case - imagine a heavy, loud drumbeat in the L channel and nothing at all in the R channel) be completely unrelated to the voltage applied at their input ?

Cheers,

GJ

And if you need another mono output there is the -L and +R combination, a win win.

[mhennessy]

Originally Posted by GrimJosef

Are we sure that the individual amps will be happy passing a current which may (OK, in a pathological case - imagine a heavy, loud drumbeat in the L channel and nothing at all in the R channel) be completely unrelated to the voltage applied at their input ?

They're just big op-amps; they won't know (or care) where the current is coming from. They'll simply sink or source whatever current is necessary to keep both inputs at the same potential.

Moreover, if R stays at the midpoint, then they won't get anywhere near their current limit; should L hit +12V or 0V, only half the prospective current is flowing through the output devices.

The only place where this won't work is where you have a quasi-floating differential output stage. These are sometimes seen at line level, but I've yet to see one used for speakers. That's because you have to sense the output current - which involves a series resistor which will rob the output of voltage swing - and use a careful mix of negative and positive feedback of voltage and current, which is hard enough when the load is substantially resistive - it would be a "brave" engineer who attempted to do that with a loudspeaker (quite apart from the fact that there would be zero advantages whatsoever).

[stuarth]

The 1:1 transformer would work with bridge or single ended outputs. I had not appreciated how common bridge outputs were these days, my Motorola doesn’t have one!

As many have said, if it’s a bridge output, you don’t need anything extra, you just connect the speaker to the appropriate speaker terminals. You can find out which type of output stage you have, just connect a test speaker through a DC blocking capacitor between ground and each of the 4 speaker terminals, L+, L-, R+, & R- in turn. If only one of each L & R terminals produce sound, you have single ended outputs, if all 4 terminals produce sound, you have bridge outputs.

Stuart

[julie_m]

I actually built a mobile PA amplifier using a bridged circuit for my O-level project .....

Basically, the speaker is driven by two power amplifiers in series with one another.

Consider a simple push-pull output stage on a 12V supply. There is a PNP transistor trying to pull the output up to +12V, and an NPN transistor trying to pull it down to 0V. From this junction we feed the speaker via a large capacitor for DC blocking. The inputs are biased such that under no signal conditions, the output will go to 6V. With a higher voltage applied to the "+" input than applied to the "-" input, the output will assume a voltage greater than +6V; with a higher voltage applied to the "-" input than the "+" input, the output will assume a voltage smaller than +6V. The output voltage will be equal to the difference between the inputs times the gain of the amplifier, plus 6V. Now when the circuit is first powered on, the speaker DC blocking capacitor will charge up to 6V; and because it has a large value, it will take a long time for the voltage to change appreciably from 6V; and in any case, the voltage at the amplifier output will have changed by then anyway. The voltage at the negative plate of the capacitor can reach a maximum of +6V when the amplifier output is at +12V, and a minimum of -6V when the amplifier output is at 0V, so the peak-to-peak drive voltage is +12V (and even the best real-life amplifiers can't quite get all the way to the power rails, so it will be less in practice).

Now consider another amplifier arranged so that when the output of the first amplifier goes above +6V, its output goes below +6V by an equal amount (so 7V → 5V, 3V → 9V, &c). Normally its output will be at +6V, so we can connect our speaker directly between the two amplifiers with no DC blocking capacitor (first saving straight away!) And the voltage swing is effectively doubled to 24V: the speaker terminals can be at +12V and 0V or 0V and +12V respectively, so you can get anything from +12V to -12V across it; which corresponds to 24V peak to peak. And twice the voltage with the same impedance gives us four times the power!

Anyway. If the source device has bridged outputs (which is entirely plausible for modern, low-voltage equipment), then it would be possible to connect a single speaker between one phase of one of the stereo channels and the opposite phase of the other channel, i.e. "L+" and "R-" or "R+" and "L-"; and it would then see the sum of the L and R signals. However, it's possible that if someone has been pinching the pennies, then the outputs may not go properly in opposition with one another unless there is a DC path from "L+" to "L-" and "R+" to "R-".

[mhennessy]

However, it's possible that if someone has been pinching the pennies, then the outputs may not go properly in opposition with one another unless there is a DC path from "L+" to "L-" and "R+" to "R-".

Hi Julie,

Would you have an example of such a circuit?

If such "outliers" exist in the wild, I'd really like to know about them for when questions like this come up in future.

Thanks in advance,

Mark

[Boater Sam]

The Kef transformer arrived today and was fitted.
He is very happy with the result, clean sound, plenty of volume and no problems with the radio.

The previous radio had the speaker connected directly between the L and R channel outputs and had failed, the output to all the speakers being muffled and distorted with little volume. Hence the need to replace it.

I thank you all for what has become an interesting discussion, I have learnt a lot from it.

So in answer to the original question, Yes, it seems it can be difficult especially to the unwary to produce mono from stereo.

Sam.

[julie_m]

Originally Posted by mhennessy

Hi Julie,

Would you have an example of such a circuit? [i.e., relying on a load being connected for bridge operation]

If such "outliers" exist in the wild, I'd really like to know about them for when questions like this come up in future.

I don't know of any commercial amplifiers actually using such a circuit, but I used to work for a company that would have jumped at the chance to save a few resistors. When you've seen as many boards as I have with interference suppression components not fitted, and relays with a contact omitted to give just a normally-open or normally-closed action when a changeover action is not required, it's hard not to suppose someone might have tried another cost-reduction measure.

If you or I were designing a bridged amplifier, we probably would balk at leaving it in an unstable state if the speaker became disconnected, just out of sheer force of habit. But in industry, margins are tight (a new set of tyres for a Maserati costs a fortune these days .....) and someone is always asking "What can we leave out and it still work?" In a vehicle, the speaker connections are part of the fixed wiring, so there is no reason to suppose it will ever be operating under any conditions other than those for which it was originally designed. (And in any case, if the unit was meant for fitting in a car with only two speakers, they would have omitted the unnecessary amplifier ICs and associated passives.)

So I would initially expect a simple cross-connection from "L+" to "R-" to work fine; but if it didn't, and the "inverted" outputs seemed to be missing, I'd have a good suspicion as to why.

[mhennessy]

Hi Julie,

I certainly know about commercial pressures, and have fought with the bean counters in the past.

However, I guess what I'm struggling with here is this: I can't really envisage how you'd make a functional bridged amplifier that does rely on the speaker being present.

That's why I was intrigued by what GMB said in a couple of posts about circuits with interaction between the two halves. I'd love to see examples of these circuits.

I'm just curious about how such a circuit could be made to work - you can learn a lot from unusual designs.

Thanks all the same,

Mark

One "outlier" would be a simple gain of -1 amp connected to the other output. I can't see any advantage to this and it would work -R +L anyway.

[mhennessy]

That's rarely done because the power amp ICs are almost never stable at unity gain. The TDA2030, for example, needs to be used at 24dB (x16) at least.

However, that approach is fairly common for line-level balanced interconnects. There's no fundamental problem with it - it works well in practice

[Boater Sam]

The radio and CD continue to function fine with the Kef transformer. It would seem that they are now or shortly will be unobtainium.

The guy is very happy, the bit of expense was not a problem, cheaper than wrecking another car unit. His order turned up next day, thanks for finding the advertisement David [Sound Man].
As he is not technically savvy, it was the best solution. He had no problem fitting 6 wires to it.

Its always difficult advising at a distance to someone with unknown skill levels, it seems to have worked out well this time.
Thanks all for the interest.
Sam.