I built 12V6 PP monoblock amps with a differential front ends using a 12SN7. The amplifiers use no feedback. I like to have a mute switch on my power amps and, on these amps i also have both single-ended and balanced inputs. So installed a 3-position switch:
Position 1 - balanced input: source connected to the two 12SN7 grids
Position 2 - mute: 12SN7 grids connected to one another no sources attached
Position 3 - SE input: source + connected to 1 12SN7 grid, source earth connected to other 12SN7 grid plus second grid is connected to circuit ground
Initially the Mute position connected both 12SN7 grids to earth but that gave a large thump when used so I reverted to the above configuration.
All worked fine with single-ended inputs and balanced outputs from my Benchmark DAC1 directly into the power amplifiers. I could switch between SE Mute and Balanced with no problems.
Then I built a transformer coupled preamp (Lundahl LL1680 9:1 driven by a 6J5) the output winding of the LL1680 is floating for balanced output (according Lundahl spec sheet) and one end of the winding is grounded for single-ended output.
When using the preamp in balanced output there is a large thumo when switching between mute and balanced output. There is no thump when using single-ended output switching between mute and the S#E input.
I have tried creating an artificial center tap on the winding with two 1k resistors to earth for balanced, but still a large thump when switching.
What gives? As I say when using my DAC's balanced output this does not happen...?
Position 1 - balanced input: source connected to the two 12SN7 grids
Position 2 - mute: 12SN7 grids connected to one another no sources attached
Position 3 - SE input: source + connected to 1 12SN7 grid, source earth connected to other 12SN7 grid plus second grid is connected to circuit ground
Initially the Mute position connected both 12SN7 grids to earth but that gave a large thump when used so I reverted to the above configuration.
All worked fine with single-ended inputs and balanced outputs from my Benchmark DAC1 directly into the power amplifiers. I could switch between SE Mute and Balanced with no problems.
Then I built a transformer coupled preamp (Lundahl LL1680 9:1 driven by a 6J5) the output winding of the LL1680 is floating for balanced output (according Lundahl spec sheet) and one end of the winding is grounded for single-ended output.
When using the preamp in balanced output there is a large thumo when switching between mute and balanced output. There is no thump when using single-ended output switching between mute and the S#E input.
I have tried creating an artificial center tap on the winding with two 1k resistors to earth for balanced, but still a large thump when switching.
What gives? As I say when using my DAC's balanced output this does not happen...?
Power amp input is a differential pair with a CCS on cathodes connected to a - 12V rail. Grid resistors 100k 0.1% each to circuit ground. Circuit ground connected to amp chassis, amo chassis connected to mains earth. Have a ground lift switch on amps that connects XLR pin 1 to chassis - get a thump regardless of position of switch. Pin 2&3 are hot and cold.
Preamp is on a piece of wood. 6J5 with transformer primary on anode, 255V regulated into primary, 1k8 bypassed cathode resistor. Secondary floating for balanced out. For single-handed one side of secondary is grounded.
Preamp is on a piece of wood. 6J5 with transformer primary on anode, 255V regulated into primary, 1k8 bypassed cathode resistor. Secondary floating for balanced out. For single-handed one side of secondary is grounded.