I will soon take position of a fully-balanced Audio Research LS5 preamp. I know I can use adapters to run single-ended gear, but I was wondering if it was possible to modify my amp to accept a true balanced connection? The schematic is attached. I don't know enough about reading schematics to know if it's doable not.
Not easily, because the "phase inverter" V2 stage is within the overall feedback loop from the 8 ohm tap.
Even if you added another duplicate V1 stage, with feedback from the 0 ohm tap (driving V2 pin 7),
the original V1 feedback from the 8 ohm tap would not be balanced with it properly.
So the original feedback loop would have to be changed to the (not externally connected) 16 ohm tap instead.
Probably not a good project to attempt.
Even if you added another duplicate V1 stage, with feedback from the 0 ohm tap (driving V2 pin 7),
the original V1 feedback from the 8 ohm tap would not be balanced with it properly.
So the original feedback loop would have to be changed to the (not externally connected) 16 ohm tap instead.
Probably not a good project to attempt.
Piece of cake: lose C3 and R7. Then build up a second input similar to the circuit around V1. That will mean adding an extra dual triode to the chassis somewhere - don't know if there is room for that?
But basically you are now creating a 100% balanced amp and no longer need the long tail pair phase inverter.
I would also consider sharing the cathode resistor R4 and reducing the value by half. And the same for the R28 feedback resistor.
But basically you are now creating a 100% balanced amp and no longer need the long tail pair phase inverter.
I would also consider sharing the cathode resistor R4 and reducing the value by half. And the same for the R28 feedback resistor.
The "phase inverter" circuit has gain (and phase), and cannot simply be deleted
without making an entirely different amplifier with higher distortion and less output.
without making an entirely different amplifier with higher distortion and less output.
Are you feeding this from balanced? Can always add an input transformer for the BAL —> SE conversion, with the benefit of galvanic isolation.
Yes, if you must use these two components together, use a BAL/SE converter of some type.
Since it will only operate at line level, it is possible to keep a good level of performance.
Since it will only operate at line level, it is possible to keep a good level of performance.
A 1:1 transformer for BAL/SE is passive and wideband.
You don't want to short anything, just use the preamp positive phase output, if a transformer is not acceptable.
You don't want to short anything, just use the preamp positive phase output, if a transformer is not acceptable.
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Come to think of it, you may want to keep two seperate feedback loops. Same values for cathode resistors and feedback resistors for the new input triode - but tap the "0" termianal for the feedback loop. As you can see, ARC have ground at the "4 ohm" tied to zero/chassis ground.
Good luck.
Good luck.
If you dont want an additional triode, you can use a balanced input transformer to have a balanced pre-amp out turn into a single ended input.
All of ARC's original amps (D51, D75, D76, D150 from the 1970s) had balanced feedback from the 16 and 0 ohm taps,
with the 4 ohm tap grounded. But these amps were single ended input, with "phase inverter" outside of the feedback loop.
Makes sense. But adding a 4th dual triode to VS55 will require a new hole to be drilled in the chassis. That could end up ugly.
The easiest is to leave the amp as is. A single triode feeding a long-tail pair driver/inverter is a great amplifier topology. Typically sounds good.
Cary SLI-80 makes use of a similar approach.
The easiest is to leave the amp as is. A single triode feeding a long-tail pair driver/inverter is a great amplifier topology. Typically sounds good.
Cary SLI-80 makes use of a similar approach.
All those images were grabbed off random searches. No idea what that switch is for in this context.