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Here's an interesting problem. I built a little drop-in FX loop unit for a Fender Blues Jr based on what was in the Blues Deluxe: a TL072-buffered serial loop that is bypassed when nothing is in the return jack. Here's the relevant part of the Blues Deluxe schem, which I duplicated in my build:
It's well balanced and quiet except for one problem. When the amp is first turned on, it's fine, with the fx loop connected to a patch from in to out it's also fine, however when I then unplug everything from the loop it gets this faint background squealing noise- like a parasitic oscillation- that won't go away unless I power down the amp completely (to drain the +/-15V supply, presumably). Moving the FX loop around changes the pitch of the squeal. The two leads to the unit are both shielded - a double core lead for the send and return with the shield as the loop's ground connection and a 2nd lead with the core to the +15V rail and the shielding to the -15V.
Why would it only have this problem once something is plugged in and then unplugged? I have two identical units in two amps which are both doing the same thing so it's not a faulty switch in the return jack. I've tried moving both the 15V and send/return lines around while the squeal is going but it doesn't really go away- it just changes pitch. If I plug a patch cable in, it goes away but immediately comes back when unplugged again. It's almost as if part of the TL072 is not turning off...?
It's well balanced and quiet except for one problem. When the amp is first turned on, it's fine, with the fx loop connected to a patch from in to out it's also fine, however when I then unplug everything from the loop it gets this faint background squealing noise- like a parasitic oscillation- that won't go away unless I power down the amp completely (to drain the +/-15V supply, presumably). Moving the FX loop around changes the pitch of the squeal. The two leads to the unit are both shielded - a double core lead for the send and return with the shield as the loop's ground connection and a 2nd lead with the core to the +15V rail and the shielding to the -15V.
Why would it only have this problem once something is plugged in and then unplugged? I have two identical units in two amps which are both doing the same thing so it's not a faulty switch in the return jack. I've tried moving both the 15V and send/return lines around while the squeal is going but it doesn't really go away- it just changes pitch. If I plug a patch cable in, it goes away but immediately comes back when unplugged again. It's almost as if part of the TL072 is not turning off...?
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