My very useful Presonus HP4 v1 headphone amp fried last Fall. I found another on eBay for not too much, but I hung onto the old one. Turns out they’re full of this quad amp IC, MC33079P, and one on the input stage failed. The chip is obsolete, but I replaced it with an MC33079PG that seems to be the same. Or at least compatible.
The hardest part was un-soldering a 14-pin IC without destroying the PC board. I added a socket in case something else had (and would again) aggravated the chip, but after several long days’ use it seems to be doing fine.
What spurred me on to do this was that channel 1 of the replacement HP4 failed. A check with a VOM from the power pins to the signal pins shows the same lack of potential (15V DC) across one of the amp’s pins. The power supplies seem good, so I think it is the chip that’s failing, and not the power supply or a current limiting resistor.
Next project is to replace the channel 1 chip on the replacement HP4 and try to make two complete units.
– Apparently these amps are version 1 which use a 16V AC power supply. The latest ones seem to use a 12V DC power supply. I’m not sure what other components have changed. Since I can’t find the original ICs, it’s a good bet, they’ve changed in the newer ones as well.
– 3 months later the repaired HP4 is working great.
– I repaired my other one with a bad channel 1 and a scratchy channel 4. Made the same IC replacement. So far, so good.