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I have made a preamplifier with filter but before asking that question I decided to open a specific question about a MOSFET audio power amplifier. It seems an old one and I cannot find its data-sheet so I couldn’t share it. Below is its right inputs and right outputs:

enter image description here

From its banana jack outputs it is powering a 8Ohm 60W speaker. Speaker is used for some experimental setup at low frequencies from 0.1Hz to 100Hz max.

I noticed that they are using sym input which is coming from a function generator set to much higher than line voltages like 1V. One of the reason is that same function generator output also goes to a scope and a DAQ. So basically they don’t use typical audio inputs.

The preamplifier I made will replace the function generator and will be coupled to sym input. But I don’t know the input impedance of this input.

Regarding the photo my question is; what is sym input used for? I couldn’t find what sym and asym stand for? And what can be the ground lift switch?

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  • \$\begingroup\$ Symmetric and asymmetric. Ground lift disconnects the ground pins on the input connectors from the amplifier's local ground, to reduce problems with ground loops in large venues. \$\endgroup\$
    – TimWescott
    Commented Oct 20, 2018 at 17:35
  • \$\begingroup\$ Does that lift break the safety earth you mean? \$\endgroup\$
    – user1245
    Commented Oct 20, 2018 at 17:46
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    \$\begingroup\$ Isnt that another way to say differential or balanced? Those are xlr differential connectors \$\endgroup\$
    – Passerby
    Commented Oct 20, 2018 at 17:55
  • \$\begingroup\$ @Passerby Can be but I don’t know. Can we find the voltage levels for these inputs? \$\endgroup\$
    – user1245
    Commented Oct 20, 2018 at 18:00
  • \$\begingroup\$ I'm guessing here, but balanced audio is typically mic level voltage. \$\endgroup\$
    – Passerby
    Commented Oct 20, 2018 at 18:04

2 Answers 2

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Possible internal wiring.

  • The asymmetrical input would be signal and ground - similar to a guitar jack.
  • The symmetrical input would be for a differential signal - similar to a balanced microphone for low level signals or long distance. A transformer can be used to unbalance the input internally.
  • Earth loops can be a problem with audio electronics. Typically this happens if two pieces of equipment are connected to mains earth but also connected to each other via a shielded cable. The earth lift switch breaks this loop by disconnecting the audio ground from the mains earth. The metal chassis would normally remain earthed.

The only problem with my last point is that the ASYM jack isn't isolated from the chassis which appears to be metal.

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  • \$\begingroup\$ It looks like a rubber washer or insert on the asym jack. \$\endgroup\$
    – Passerby
    Commented Oct 21, 2018 at 2:07
  • \$\begingroup\$ Also, you don't address the question on possible voltage. \$\endgroup\$
    – Passerby
    Commented Oct 21, 2018 at 2:08
  • \$\begingroup\$ @Transistor I didnt know that the sym has input has such isolation. Does that also mean the transformet also filters DC bias if there is any? \$\endgroup\$
    – user1245
    Commented Oct 21, 2018 at 3:22
  • \$\begingroup\$ @atomant: I don't know either. Transformer inputs were common forty years ago but it could be an op-amp differential input instead. The transformer is the easiest way to visualise the inputs but given that you say the frequency response goes down to 0.1 Hz the transformer input is unlikely. \$\endgroup\$
    – Transistor
    Commented Oct 21, 2018 at 9:45
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Asym (short for asymmetric) input is also known as an unbalanced input.

Sym (short for symmetrical) input is also known as a balanced input

The Unbalanced input most likely has an input sensitivity of either -10 dBu or -10 dBv (depending on the age of the amplifier and where it was manufactured) for full output.

The Balanced input most likely has an input sensitivity of +4 dBu for full output.

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