Balanced Output

[SerpentRuss]

I am finishing up a current build that is a 5-watt SE head and I thought about adding a balanced output jack like I saw on another's schematic. It seems like a bit of a shortcut, no dedicated transformer, but I wondered if anyone has done something like this. As a side note, I am including a switched load resistor so I can run the head without a cabinet and I plan on putting in the headphone jack like the attached drawing shows. This I've done before.

I do have a couple of nice direct boxes, so I could always feed a balanced output using the headphone jack and one of them, but I thought this might be interesting to try.

The output is created by using a stereo jack and one dropping resistor from the speaker positive to the tip, another equal dropping resistor from the speaker negative for the ring. The sleeve is left unconnected, so the ground reference is lifted to the next device.

What do you think of this method, does it have any merit?

 

[robrob]

EDIT: I just realized that the Balanced Out jack is a TRRS (stereo) jack using only the two ring terminals which would give you a balanced signal path. I would ground the shield terminal for noise protection (shown as NC or no connection in the schematic).

It won't be balanced with that type of line out jack. Balanced requires two conductors with the same impedance. A guitar or speaker type shielded cable has different impedance on the tip and shield. If it is not balanced you won't get common mode noise rejection--the reason balanced is used.

Unless a speaker is plugged in the 10 ohm resistor to ground (probably a no-speaker-safety-load) would unbalance the output. Remove the 10 ohm resistor and you're good to go except that this setup will damage the output transformer without a speaker because it doesn't use a shorting switch like most amps.

I have a few pages on balanced headphone mods: https://robrobinette.com/HeadphoneMods.htm

 

[24 track]

EDIT: I just realized that the Balanced Out jack is a TRRS (stereo) jack using only the two ring terminals which would give you a balanced signal path. I would ground the shield terminal for noise protection (shown as NC or no connection in the schematic).

It won't be balanced with that type of line out jack. Balanced requires two conductors with the same impedance. A guitar or speaker type shielded cable has different impedance on the tip and shield. If it is not balanced you won't get common mode noise rejection--the reason balanced is used.

Unless a speaker is plugged in the 10 ohm resistor to ground (probably a no-speaker-safety-load) would unbalance the output. Remove the 10 ohm resistor and you're good to go except that this setup will damage the output transformer without a speaker because it doesn't use a shorting switch like most amps.

I have a few pages on balanced headphone mods: https://robrobinette.com/HeadphoneMods.htm

nice stuff there Rob thanks for posting!

 

[Tom_]

Unless a speaker is plugged in the 10 ohm resistor to ground (probably a no-speaker-safety-load) would unbalance the output. Remove the 10 ohm resistor and you're good to go except that this setup will damage the output transformer without a speaker because it doesn't use a shorting switch like most amps.

Hi Rob,
I'm not seeing the ground connection on the 10R resistor (assuming it's an isolated socket and not grounding through the chassis)... it looks to me as though the transformer output is floating w.r.t. ground; the 10R resistor is there to guarantee a load when the balanced or headphone outputs are being used without a loudspeaker and both of these outputs are floating w.r.t. the chassis. Am I misreading the diagram?
Thanks,
Tom

 

[SerpentRuss]

Hi Rob,
I'm not seeing the ground connection on the 10R resistor (assuming it's an isolated socket and not grounding through the chassis)... it looks to me as though the transformer output is floating w.r.t. ground; the 10R resistor is there to guarantee a load when the balanced or headphone outputs are being used without a loudspeaker and both of these outputs are floating w.r.t. the chassis. Am I misreading the diagram?
Thanks,
Tom

I don't believe the builder of this amp grounded the speaker. I have an internal photo of the build and he used an Amphenol stereo jack for the speaker output and it looks like he left it floating. Of course, it is one photo of a point to point build, but that's what I'm seeing. Speaker jack is lower left, balanced out is right below it, headphone jack (connected with ethernet cable) is upper right.

 

[robrob]

Hi Rob,
I'm not seeing the ground connection on the 10R resistor (assuming it's an isolated socket and not grounding through the chassis).

I believe you are right. I was assuming the shield connector was grounded. You know what happens when you assume.