2. amps-preamps
  3. 1014719740

Help My

I just bought a used Conrad Johnson PF-R pre-amp here on Audiogon and I'm having some problems. My amp (Citation7.1) keeps overloading (the red indicator light goes on and off and the unit shuts itself off altogether then comes back on It does this even when there is no music playing through it. If I turn the amp on (the CJ is always on) and just let it sit there, after about five or ten minutes the problem begins. I do not have this amp problem with any other pre-amp hooked up to it, it only occurs when the Conrad Johnson is connected to it. It seems really strange that this would happen even when there is no music playing. Is there some kind of short circuit going on in the CJ? I wanted some expert advice before I contact the seller about this problem. I should mention that the pre-amp itself never seems to show any signs of distress and when I'm playing music it sounds fantastic.

Thanks,

Steve
say811

Showing 2 responses by bob_bundus

bob_bundus
1,834 posts
 
kr's question about possible DC on the preamp output is something to definitely check into; DC will put most amps immediately into protect mode. You'll probably need a scope for this test but a voltmeter might catch it as well. But before you go to the repair shop check your interconnects. A loose shield connection that makes / breaks at random could cause that amp to want to protect itself as well.
bob_bundus
1,834 posts
 
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.
More to discover