[sdiy] Combined output / headphone jack?

[Mattias Rickardsson]

Hi Brian,

>> The Ensoniq SQ-80 also has the two jacks labeled
>> Left/Phones
>> Right/Mono.
>> Haven't checked the schematics yet, but I'd be most curious how they
handle a balanced TRS plug or two.
>
On Mon, 14 Oct 2024 at 23:52, brianw <brianw at audiobanshee.com> wrote:

> The Ensoniq SQ-80 is far too old to support balanced TRS.


I'm not sure about the timeline, too old for what? Perhaps we might be
talking about different things here.


> On that model, the headphones would be stereo unbalanced, and the line
> output would be unbalanced TS connections.
>
> In fact, it's still quite rare today (or so it seems) to find a synth that
> supports balanced outputs, but I agree that it's worth pursuing. I seem to
> recall that the Roland Jupiter-6 has balanced outputs, but they're on XLR
> male jacks, not TRS.
>
> How many modern synths have balanced outputs? I think the Access Virus
> might.


I guess you mean "proper" symmetrically balanced outputs with
opposing-polarity signals on hot & cold. Not sure if/where that is
done, and if it even makes much sense in synths. :-)

But don't most synths use "impedance balanced" output jacks these days?
With actual signal only on the hot (tip) but identical impedance on hot and
cold (tip and ring) for improved CMRR when driving balanced inputs. Good
way of avoiding some USB-related noises and more. It's so easy and cheap to
add those R's and C's and the TRS instead of TS jacks, and it falls back to
the good old unbalanced connection when used with a TS plug or an
unbalanced input.

Then users will sometimes use TRS cables connected to balanced inputs, and
the output configurations start to matter.

Which of those have TRS jacks for dual-purpose line outputs and headphone
> outputs?
>
> For balanced, Tip and Ring are opposite polarity. For headphones, that
> would sound awful. I suspect that there would need to be some sort of
> manual switch to flip to stereo (unbalanced) for headphones or mono
> balanced otherwise. The challenge is that there is no way to automatically
> detect the difference between a headphone plug and a TRS balanced plug.
>
> Brian
>
> p.s. Re-reading your message, I realize that you probably don't expect the
> SQ-80 to properly support a balanced TRS plug that gets inserted, but
> you're wondering how that would be "handled." I would expect that the
> signal would be very faint on the other end of the cable, if it's actually
> connected to a balanced input. The faint signal would occur for sounds
> panned to center, and they would get louder as the sounds are panned away
> from center, but would still be as much as 6 dB quieter than a proper
> balanced signal.
>

I was rather thinking of a user plugging the synth into a balanced input of
a mixer or audio interface using a balanced TRS cable.

A normal unbalanced output with TS jack handles this well, giving an
unbalanced connection.
But a TRS jack supposed to act as either an unbalanced output for TS cables
or a TRS stereo headphone output for TRS cables doesn't handle the TRS
balanced plug. It takes the Left and Right signals as Hot and Cold and ends
up very weak in the balanced input.

Would be nice with a stereo pair of TRS jacks that could act both as
- a stereo pair of unbalanced outputs (when two TS plugs are inserted)
- a headphone jack (when a TRS plug is inserted in jack 1 and nothing
inserted in jack 2)
- a mono jack (in jack 2 when nothing is connected in jack 1)
- a stereo pair of [impedance] balanced outputs (when two TRS plugs are
inserted)   <-  this :-)

Will have to sketch on that some sunny day.

/mr
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