Trying to find out what's wrong with the internal amplifier of my old Klipsch subwoofer SW-12. I'm a novice in electronics.


This is the second question related to this subject.


The schematic of this model is on page 15-17 of its service manual:


The subwoofer consists of three things: enclosure (wooden box), an amplifier with inputs and outputs connectors, and a front control panel consisting of three controls [LEVEL, PHASE ADJUST, LOW PASS] and an LED connected to the amplifier through two cables (6 wires and 5 wire) and two sets of connectors.

(They are shown on Sheet 1 of the schematic above.)

Can turning on the amplifier (with the driver connected) without connecting the three controls and the LED to the amplifier damage the amplifier or the driver? In other words, is it safe to test the amplifier without connecting the three controls and the LED? While testing, the low level LINE IN may OR may not be connected to an audio source such as an AV receiver. No speaker input or output terminals on the back of the subwoofer's amplifier are connected to anything.

The three controls on the front are made with 4 pots: 1 for LEVEL, 1 for PHASE ADJUST, and 2 for LOW PASS.

  • \$\begingroup\$ The PA is AC coupled with C29 so no problem. However trim the offset adjust R50 pot and feedback [K] must be connected to get 0V out. Current sense on R112 feeds [H] & [R] to disable Relay RLY1 (OV overload) \$\endgroup\$ Feb 2, 2017 at 22:26
  • \$\begingroup\$ check all big caps for Ac ripple or ESR or C value or slight bloating lids \$\endgroup\$ Feb 2, 2017 at 22:31
  • \$\begingroup\$ Tony Stewart: "However trim the offset adjust R50 pot and feedback [K] must be connected to get 0V out." I tested the amp without any source signal inputs with the controls and LED disconnected AND without doing what you wrote. Do you think I damaged the amp? By testing, I mean just leaving the amp on for a few hours to see if the noises come on. \$\endgroup\$
    – zeron
    Feb 6, 2017 at 16:37
  • \$\begingroup\$ Tony Stewart: "no." Thank you. Because the two original noises (static & crackling) disappeared after the last test while a new noise started (more regular, like water dripping from a faucet every 20~ 40 seconds), I thought I might have damaged the amplifier. Prior to this, I touched up about 20 solder joints that didn't look quite robust and changed one power resistor [R90] because it was reading ~426 instead of specs' 750. However, the new R90 is smaller in size. That may be causing a problem. I'll post a question about this. \$\endgroup\$
    – zeron
    Feb 7, 2017 at 21:14

1 Answer 1


Yes, it is safe for the amplifier circuitry to turn on the amp with these three controls and the LED disconnected.

But be careful. There are dangerously high voltages in that amplifier. +/- 81V is enough to kill you.

  • \$\begingroup\$ I only looked quickly at the schematic, But I don't think the amp is going to work at all with out the "level" input pot connected. Maybe no harm.. \$\endgroup\$ Feb 2, 2017 at 21:19
  • \$\begingroup\$ The original problem was two kinds of noises (static and crackling kinds) coming out of the speaker after the amp is warmed up. I tested once without the three controls and the LED connected. The noises came on as before. \$\endgroup\$
    – zeron
    Feb 2, 2017 at 22:16
  • 1
    \$\begingroup\$ With the "level" pot disconnected there won't be a path for the audio, but the amp won't be damaged and you can still check the DC voltages. If you are trying to trace the audio signal, yes, it will stop at the missing "level" pot. \$\endgroup\$ Feb 3, 2017 at 0:36

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