SE-BAL-SE Switching

[Joshua_G]

A line amp I'm going to build will have both SE and balanced input and output. The input and output should be selected individually, either SE or balanced. That is, the amp should work either SE to SE, balanced to balanced, SE to balanced or balanced to SE – all by means of a switch at the input and another switch at the output.

The switching schematics is attached.
Will this circuit work properly under all input and output conditions?

[Wavebourn]

Originally Posted by Joshua_G

Will this circuit work properly under all input and output conditions?

No.

[Joshua_G]

Originally Posted by Wavebourn

No.

Thanks.
What changes are needed for proper operation under all conditions?
Are changes needed on the input, or output, or both?
I made one change, added load resistor for SE output.

[Wavebourn]

I would start from drawing values on the schematic. Most probably for proper operations under all conditions it is better to replace R3 by CCS.

However, you have to keep in mind that amplification factor will depend on which input and which output were used. I don't know, is it proper operations, or not, according to your requirements.

[Joshua_G]

Thanks.
The schematic attached here is a simplification of the actual circuit. R3, R8 and R9 in the attached schematic, each represent a CCS.
I'm aware of the changes in amplification factor between SE and balanced operation. Those changes will be adjusted in the final circuit by switching resistors.
Other than changes in amplification, will this circuit operate properly under all input and output conditions?

[atmasphere]

Why are you bothering with the switching? You don't need it...

[Joshua_G]

Originally Posted by atmasphere

Why are you bothering with the switching? You don't need it...

Switching is needed, at least on the minus of the signal.

[atmasphere]

Why? At the most you only need to tie the unused input to ground if you are running single-ended input. It makes no difference at the output. Usually the way you do the input is with a U-shaped jumper that installs between pin 1 and pin 3 of the XLR.

[Joshua_G]

Please check again. Some switching is necessary both on input and output.

[Wavebourn]

Originally Posted by atmasphere

Why? At the most you only need to tie the unused input to ground if you are running single-ended input. It makes no difference at the output. Usually the way you do the input is with a U-shaped jumper that installs between pin 1 and pin 3 of the XLR.

If not to ground unused input of a diffpair there will be higher noises and worse THD measurements. However, if not to use a diffpair at all noises will be lower, sound cleaner, but may be THD figure not so nice looking.
I would definitely use a single tube and a pair of transformers instead of what is drawn, however if to speak about high-end approach. Otherwise, LEDs under all tubes would be nice.