Acoustic a20 bad balanced out

[KCman]

Thanks for the info and examples J M

Until this problem with the Acoustic(and this thread), I never thought about phantom hurting an amp like that.

My SWR bass 350 doesn't even have caps at the output, only a couple series resistors. If time allows, that would be a nice upgrade.

[J M Fahey]

Thanks for the info and examples J M

Until this problem with the Acoustic(and this thread), I never thought about phantom hurting an amp like that.

My SWR bass 350 doesn't even have caps at the output, only a couple series resistors. If time allows, that would be a nice upgrade.

SWR deals with that by stating "do not connect line out to a Phantom powered input" which doesnīt really depend on *you* , if Soundman enables it, itīs on all or none.

In principle most Op Amps survive ... same way as most cars and buses running this road do not fall into the abyss



And lacking output caps is gross, because even with Op Amps standing the turn on pulse, Phantom injects staggering (in this case) 7 mA DC into each Op Amp output, either grossly misbiasing it or straight blocking it, since standard Op Amps such as TL072 can supply around 5mA peak either way, just look at the datasheets.

So at least add series caps, and if possible clamping diodes too.

[Randall]

I just had the same amp with the same problem, and thanks to this thread, the same fix. U103 on the DI output board replacement.

Now if I can figure out where the white neutral wire with a push on connector coming off the AC inlet board goes. The amp powers up and works, so where could a neutral wire be needed, the power switch? Nothing on the other side of the switch that I can see.

[KCman]

Acoustic never did send me any service material. The amp wasn’t mine, so it’s long gone.

I take a bunch of pics with my phone.

[Randall]

I ended up just sticking the extra neutral w/ push on connector to the unused side of the power switch. The wire was just long enough, and I found no other place for it. Amp is gone and working fine now.