Keyboard output circuit - stereo/mono

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No. You connect R+ to L- , L+ to speaker , speaker to R- and you get the sum of the two channels.

Sorry - but that's VERY, VERY bad advice :eek:

On the vast majority of amps the section I've underlined will short out the RH side, most probably blowing the amplifier. In the case of a bridged amplifier you're directly shorting two of the outputs together, likely to blow TWO of the four amplifiers.

In the rare case that the amplifier had output transformers, and the two secondaries were completely isolated, then it would 'work' - but I'm dubious about how it would actually sound?.
 
I understand what you mean, and I agree that usually L- and R- share a common ground in the amplifier. But I did say that the amplifier outputs must not be internally grounded.
We seemed to get away with it, perhaps it was an older solid state amp with output transformers.
If you can avoid the shorting problem, it can sound OK. The voltage sources add together and the internal impedances add together so you do get a factor of two reduction in damping.
 
I understand what you mean, and I agree that usually L- and R- share a common ground in the amplifier. But I did say that the amplifier outputs must not be internally grounded.
We seemed to get away with it, perhaps it was an older solid state amp with output transformers.

Can you name ANY amplifier that has output transformers that aren't earthed at the bottom end as well? - I've been repairing amplifiers professionally for 43 years and don't recall having ever seen one? (most with transformers would have been valve of course, or 100V line PA amps), or even seen a circuit diagram for one.

Do you remember what make and model it was.

But in any case, it was EXTREMELY bad advice to give, even assuming there was ONE model of amplifier that it could possibly apply to.
 
Hi Nigel,

Yes, you have got me wondering. I agree with what you are saying!

Unfortunately, I wasn't there with the amp in question (just over the phone).

I think there are three possibilities:

(a) the amp was an old transformer type with floating outputs. (Does seem very unlikely!)

(b) my friend wired it up so that the two outputs subtract (and is getting the difference of the two channels - as you originally suggested).

(c) one channel blew an output fuse and the output is just the other channel all by itself. (Fortunately my friend said that the amp was 'expendable'!)

Thanks for your clarifications. Actually, I wasn't trying to 'recommend' this, just trying to making a general point about current source signals adding in parallel and voltage source signals adding in series.
 
As has been suggested, the usual way is to feed the outputs from a low impedance driver through resistors. The right channel jack has a NC contact that connects the two outputs together when no plug is inserted. This makes a summing point which is mono on the left output.
 
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