All,

A few weeks back, at the Lone Star Audio Fest, a forum member (TC) gave me his broken down and shorted AudioSource AMP-100. His only request for me was to fix it and "pay it forward". Well, fixing it was easy, but I wasn't satisfied with the performance.

There was a significant volume difference between the left and right channels and the audio sounded a bit muddied and vague. So, to pay it forward to the community, I set forth on an "optimization" project. This has taken me several weeks of off-and-on testing and listening, but I think I've nailed down the changes necessary to turn this sleeper into a killer amp-on-a-budget. I tried to stick with changes that were BOM options only. I didn't tackle any modifications to the layout. I was NOT impressed with the layout of this board, as several optimizations could be had. However, I wanted to keep this fairly easy to accomplish for someone with a good soldering iron, a soldapullt, and some time on their hands.

My initial tests on the AudioPrecision unit basically confirmed my listening concerns - a 0.6dB difference between the left and right channels, a relatively poor frequency response (0.1dB drop at 20Hz and 1dB drop at 20KHz and falling off rapidly past that), and a THD+N graph that was all over the place. After spending quite a bit of time reverse engineering the circuit, the main issues became immediately evident. There's quite a bit of RF suppression with high-value capacitors, which leads to high frequency droop and phase issues. Also, some of the cap values used in the circuit were acting as a high-pass filter, causing the droop evident in the bass frequencies.

Anyways, enough chatting. I'll break these changes up into a few posts. Here's the changes that I propose for the rear panel. Note that there are two options - one if you want to keep the "auto switching circuit" and one if you don't. The only real advantage to not keeping the auto-switching circuit is that it costs slightly less and it removes the turn-on thump you hear when the amp first gets power.

Rear Panel Rework (Keeping the auto-switching circuit):
  1. Change the pull-down resistors from 100K to 10K to improve the thermal noise. R426, R427, R428, R429 change to 10K, 1%. [Mouser PN 271-10K-RC]
  2. Because pull-down resistors changed, also need to change value/quality of series caps. Change C411, C412, C413, C414 to 100uF, 16V Nichicon KA. Use KA because of temperature. [Mouser PN UKA1C101MDD1TD]
  3. C408, C409 are the supply caps for the buffer op-amps. Change to audio quality caps, use Nichicon KA for temperature. Ordered 470uF, 16V. [Mouser PN UKA1C471MPD1TD]
  4. IC4, IC5 are the buffering op-amps for the left and right signals. Change these to LM4562 for higher quality and higher GBW (helps avoid phasing issues). [Mouser PN LM4562NA/NOPB] Could also possibly use OPA2134, but bandwidth is not as high.
  5. Relay DY1 needs to change to higher quality relay with lower contact resistance. Use Panasonic DS series. [Mouser PN DS2Y-S-DC12V]

Rear Panel Rework (Deleting the auto-switching circuit):
  1. This rework will delete the auto-switching circuit. It costs less, but you lose the functionality of the LINE IN 1 jacks.
  2. Remove the excess circuitry associated with the LINE IN 1 switching. This involves removing the DY1 relay, C411, C412, R426, R427, and IC5.
  3. Change the pull-down resistors from 100K to 10K to improve the thermal noise. R428 and R429 change to 10K, 1%. [Mouser PN 271-10K-RC]
  4. Because pull-down resistors changed, also need to change value/quality of series caps. Change C413 and C414 to 100uF, 16V Nichicon KA. Use KA because of temperature. [Mouser PN UKA1C101MDD1TD]
  5. C408, C409 are the supply caps for the buffer op-amps. Change to audio quality caps, use Nichicon KA for temperature. Ordered 470uF, 16V. [Mouser PN UKA1C471MPD1TD]
  6. Change IC4 to LM4562 for higher quality and higher GBW (helps avoid phasing issues). [Mouser PN LM4562NA/NOPB] Could also possibly use OPA2134, but bandwidth is not as high.
  7. Because IC4 handles the “left circuit” and IC5 handles the “right channel”, we have to do some modifications after removing IC5 and the relay:
    • Using the attached relay pinout diagram at the bottom:
    • Run a wire from pin 4 to pin 6 of the removed relay
    • Run a wire from pin 13 to pin 8 of the removed relay
    • Connect a wire from R429 (the side closest to C413) to the location where R426 was (on the side closest to C411). Remember, R426 should be removed.

Relay Pinout:
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Final Rear Panel showing all components while keeping the autoswitch:
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Final Rear Panel with autoswitch deleted:
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