Steve notify the seller immediately describing exactly what's going on & our theories behind the intermittent fault. DC on ANY component's audio output line is *bad news*. Think of the audio signal as AC (an alternating voltage actually). Think of a battery as DC (a direct non-alternating voltage). (The "C" in AC or DC stands for 'current' of course referring to basic electrical household Current, AC).
When a non-alternating DC voltage (this is not the AC music signal) appears on the inputs of a direct coupled amp, the amp will amplify that DC as well. This high DC voltage apppearing at the amp output to the speaker will destroy a woofers' voice coil almost instantly; that's why DC is so bad when appearing on a signal line. This is an indication of an intermittent failing component within the preamp, probably nothing real serious buy needing repair nevertheless. The amp has sensing circuitry built in which sees the DC & goes into protect mode, to protect not only the speakers but the amp itself as well.
The seller may have been using this preamp with a tube power amp. Tube equipment is usually not direct coupled, rather it is capacitor coupled, which blocks the DC component. That's why he may not have known about this problem. The answer is to repair the preamp (replace the failing part) not to add a coupling capacitor. Caps are sometimes necessary but direct coupling is always superior when you can have it.
My own system is completely direct coupled from the signal inputs thru to the speakers.
When a non-alternating DC voltage (this is not the AC music signal) appears on the inputs of a direct coupled amp, the amp will amplify that DC as well. This high DC voltage apppearing at the amp output to the speaker will destroy a woofers' voice coil almost instantly; that's why DC is so bad when appearing on a signal line. This is an indication of an intermittent failing component within the preamp, probably nothing real serious buy needing repair nevertheless. The amp has sensing circuitry built in which sees the DC & goes into protect mode, to protect not only the speakers but the amp itself as well.
The seller may have been using this preamp with a tube power amp. Tube equipment is usually not direct coupled, rather it is capacitor coupled, which blocks the DC component. That's why he may not have known about this problem. The answer is to repair the preamp (replace the failing part) not to add a coupling capacitor. Caps are sometimes necessary but direct coupling is always superior when you can have it.
My own system is completely direct coupled from the signal inputs thru to the speakers.