JBL

Stereo amplifier

On the JBL 12W amp, I traced out enough of the circuit to discover the purpose of the two jacks labeled "No Connection" in the picture on your web page.

The "No Connection" phone jack that is next to the power jack is a stereo headphone output.

When you plug in headphones, the speaker outputs are disconnected. The little black block with an "M" embossed on it is the switch.

There is a 100 ohm resistor in series with each channel of the headphone output.

It sounds pretty good with my (32 ohm) Grado SR-80 headphones. With the volume up all the way, it is loud, but not dangerously so.

The same 1000 uF DC-blocking capacitor is shared between the two headphone outputs - it's in the shared ground return path rather than having separate ones in the left/right signal paths. That would cause some crosstalk at very low frequencies - below 30 Hz or so - but that could be seen as an advantage for headphone listening. The speaker outputs have separate 1000 uF DC-blocking capacitors.

The other "No Connection" jack - next to the input jack - is a low-level mono output, connected between tip and sleeve. Sleeve is connected to the same signal ground node as the input jack. Tip is driven from an LF347 opamp section through a 470 ohm resistor in series with a 1 uF capacitor. The output is the sum of the left and right inputs. Its level changes with the volume control pot. The frequency response is flat across the audio band (into a hi-Z scope input; the 1uF capacitor would make the low end roll off if the load impedance were less than about 5K). The output level at the onset of easily-audible distortion is 40 mV peak-to-peak. I expect it's intended for an external powered center-channel speaker or subwoofer. There's a switch block embossed with "K" next to this jack, but I haven't discovered its purpose.

On my unit, the "Bass" knob does not appear to affect the frequency response of either the headphone output or the mono low-level output. I expect the same is true for speaker outputs, since they are driven from the same power-amp chip outputs as the headphone, albeit without the 100 ohm resistors.

Mitch Bradley