[I.Greyhound Fan]

I have a problem that I need help with.  I have a Parasound A21 amp that since day 1, goes into protection mode if I turn it on and then turn on any one of my preamps.  It trips a few seconds after I turn on the preamp, and then turns back on a few seconds later.  It does not happen if I remove the IC's from the preamp.  I have tried different several different cables and different preamps.  The cables and preamps all work fine with other amps.  I had it in to Parasound and they said there was nothing wrong with it.   Any thoughts.  The preamps are tube.  I think it may be due to DC off set.

[Big Red Machine]

Sure sounds like a ground wire is touching where it is not supposed to be internally assuming the cables are not shorted somewhere. Have you checked with a meter the cables for grins because the other gear may bot be susceptible and a short is being masked somehow.

[I.Greyhound Fan]

I just called Parasound and talked with their head service guy and he agrees that it is due to the DC off set from the tube preamps and it is a known issue.   I just have to keep turning on the tube preamps first and wait about 15 seconds before turning on the A21.  I find it interesting that when I sent the amp in 6 years ago that they could not tell me that.

[Folsom]

If the unit isn't an auto biasing tube preamp, it could be out of bias and have more DC offset.

[sfox7076]

DC offset is my first thought too.  Had the same issue (well, similar) with a tube preamp I built and NCore amps.  Anoying, but not the end of the world.  Do you tube preamps have a mute function on start up or B+ delay?

[jea48]

I have a problem that I need help with.  I have a Parasound A21 amp that since day 1, goes into protection mode if I turn it on and then turn on any one of my preamps.  It trips a few seconds after I turn on the preamp, and then turns back on a few seconds later.  It does not happen if I remove the IC's from the preamp.  I have tried different several different cables and different preamps.  The cables and preamps all work fine with other amps.  I had it in to Parasound and they said there was nothing wrong with it.   Any thoughts.  The preamps are tube.  I think it may be due to DC off set.

 Power amp,
 last on, first off.

You should never turn on a power amp first. I am surprised the Parasound tech didn't tell you that. Unless you didn't tell him you were turning it on first.

It probably is a good thing the amp went into protection mode. If not it could have sent a hell of loud thump to the speakers.
Jim

[sebrof]

Power amp,
 last on, first off.

You should never turn on a power amp first. I am surprised the Parasound tech didn't tell you that. Unless you didn't tell him you were turning it on first.

It probably is a good thing the amp went into protection mode. If not it could have sent a hell of loud thump to the speakers.
Jim

Yes, this is the first thing I thought when I read the OP.
I'm not sure what my amp does if I turn it on before the preamp because I don't think I've ever done that. I even make sure others in the house know to never do that.

[I.Greyhound Fan]

Thanks guys.  We have been turning on the Preamps first and the amps last.  But occasionally you forget.  I always turn my amp off first.

[*Scotty*]

This turn on sequence is old school audiophile SOP. A more sophistcated preamp design would have added a relay with a time delay at the output which pulls the signal to ground through a resistor until the preamp has stabilized. The problem comes from the charging of the output coupling cap which can produce amplifier and speaker destroying consequences. Were it not for the protection circuitry in your amplfier bad things might of happened.As the relay goes between the signal and ground and is open when the preamp is in operation there is no sonic penalty due to its presence.
Scotty

[avahifi]

Scotty, this is the way we design all our small signal devices (preamps and DACs).  Time delay turn on and instant turn off using a relay circuit.  Thus no on or off transients and no chance of damaging speakers or power amplifiers.

Frank

[JLM]

For better or worse, many tube designers pride themselves on simple (some as simple as possible) design (with no embellishments like protection circuits). 

Good for you that the Parasound protected your speakers and amazed that no one at Parasound didn't think to ask the order you turn on (old school rules for old school gear). 

[Steve]

Many good manufacturers provide an automatic mute, relay here (retired now, when I was designing), when designing/manufacturing, that protects against thumps and protection circuit activation. Parallel and the output is shorted until relay is activated.

Even then and now I always suggest powering on the preamplifier first, then the amplifier. A good habit to learn, which you already know.

Cheers
Steve