I enjoy contesting…. and often do so with a second person as a logger. A couple years back, I picked up a behringer HA400 to split audio between 2 sets of headphones. It worked great except for one thing- it disconnects the speaker when I plug it into the headphone jack.
I tried running a speaker on one output, but it just doesn’t drive it hard enough for larger speakers.
On a whim, I popped it open, and it’s a simple design- One input amp and 4 adjustable amps for the headphones.
Simple reverse engineering- The amps have a feedback loop… which is a good standard design.
To boost the output on one channel, I bypassed the feedback loop with three 10k resistors in parallel. A 1k resistor would have been preferred, but I didn’t have any in hand… so I can tweak the design later.
After a little searching, one of the guys at work brought me a 1k resistor… fits much better without worry of shorting.
I have noticed some distortion when I completely crank the volume – but I suspect this is more an issue of overdriving the input amplifier (there’s an input amp, and 4 output amps). The goal here is to comfortably drive a speaker for listening without a headset – not to blast Queen’s ’86 Wembley concert for the whole room. I believe this might be bypassing the capacitor (C9 in the image above), which provides some filtering, but for now, it seems workable. As I write this, i realize that my mod only handled one half (in this case, the tip) of the audio circuits. Generally speaking, radio outputs are mono in nature, but some newer radios do have stereo outputs for various uses. I’ll likely go back in and add another resistor across R16 and C10, just to make it complete. That being said, my speaker only has a mono plug at the moment.