Question? Does a preamp see the same impedance whether an amp is on or off?

[nj pheonix]

Title pretty much sums it up. Also, I was thinking it might not be a one size fits all answer.?
Thanks in advance!

 

[BinaryMike]

SS amps often present a very low and nonlinear load impedance when powered down. Most tube amps stay about the same or even reduce their load capacitance due to Miller effect in triode input stages.

 

[nj pheonix]

Thanks for the education.
Being how common wisdom is power on pre first then amp. And turn amp off first then pre, it'd be reasonable to assume the impedance never gets that low regardless ?

 

[BinaryMike]

The folklore arises because some preamps make horrific thumps at power-up time. Letting the preamp stabilize prior to power amp switch-on effectively mutes that thump. Overloading the preamp output line is a non-issue with regard to reliability --- some power amps even short-circuit their input jacks for a while at power-up time. There is a serious problem with paralleling power amp inputs, however, if you plan to sometimes leave an amplifier powered down during normal system operation. In that case, the preamp may deliver a highly distorted signal to the active amplifier channels.

 

[nj pheonix]

I wasn't considering that now, though I have in the past. I never really looked to see how preamps with multiple outs isolated them from each other. I suppose it varies

 

[gadget73]

my modified PAS makes ugly noises at power-on. If I run it with a tube amp, the PAS comes up fast enough that the noises pass before the amp passes signal so no worries. Can't turn the SS amp on before the tube preamp though.

But otherwise agree with what Mike said. Tube circuits are a resistor and a capacitor formed inside the tube. Might be a tiny change tube-side but probably not enough to be worth talking about. Going into an SS amp its really hard to say. Nonpowered solid state devices tend to behave more like diodes.

 

[restorer-john]

Short answer is no. Distortion is a byproduct of a preamp driving unbiased front ends in power amps/tape decks/ eqs etc.

Here's more:

http://audiokarma.org/forums/index.php?threads/cd-distortion.748033/page-2#post-10149923

http://audiokarma.org/forums/index....about-cable-evaluations.742729/#post-10068004

 

[BinaryMike]

I wasn't considering that now, though I have in the past. I never really looked to see how preamps with multiple outs isolated them from each other. I suppose it varies

I think most preamp manufacturers expect users to know enough to add a proper distribution amp when connecting multiple power amps with low input impedances. They're dreaming, of course.

On a related topic; many preamps don't buffer the tape output. Few of us use tape decks these days, but in the past, if an unpowered solid-state tape deck was attached, it would often cause serious distortion to all signals passing through the preamp. The situation is undoubtedly similar with modern digital recording devices and computers.

 

[nj pheonix]

The other stuff was interesting reading. Wasn't on my radar but it makes sense. I know Mac put out a bulletin that certain tube preamps could put out dc at power on and damage direct coupled power amps (or speakers). so it was important to power on pre 1st. Sometimes if i change components in a piece i like to run it a bit before i evaluate. I really was just looking to see if the pre would get loaded similarly if i left the power amp off. No reason to burn tubes or electric if there is no benefit. I'm taking from this discussion that should work fine. Thanks. I really got more than i was looking for (but that's a good thing!)

 

[ConradH]

BinaryMike makes a good point about tape outs. I'm careful to never leave anything connected to tape outs that isn't in use.