Amp and preamp on same outlet?

Tim (Mitch2), this post by Atmasphere dated 6-3-14 is one of those in which he has explained the brief high current spikes which characterize the majority of the current draw of most power amplifiers, as Jim (Jea48) indicated above.

To put things in perspective, a 50 foot run of 12 gauge Romex will have a total resistance for both conductors (100 feet total) of around 0.16 ohms. A spike of say 30 amps (just my not particularly well informed guess as to a representative number) would result for a brief instant in a voltage drop of 30 x 0.16 = 4.8 volts across that resistance. There would be additional effects on the high frequency components of the spike due to the inductance of the wiring.

I wouldn't expect that voltage drop **in itself** to have a major effect on a preamp that may be powered via the same run as the amp, in part because (in contrast to most power amps) most preamps have regulated power supplies. But note that Ralph (Atmasphere) refers to the spike having frequency components in the 30 to 100 kHz area, and perhaps even at higher frequencies. That is what I had in mind in my initial post in this thread when I referred to putting the two components on separate lines as having the upside of "reducing the amount of amplifier-generated electrical noise that may couple back into the preamp."

Regarding Audiolabyrinth's Krell 700CX specifically, I would be hesitant to extrapolate what is likely to happen with most amps from experiences with that amp. It is an understatement to characterize it as a monster compared to most other amps, as some of its specs will make clear:

Maximum rated power per channel into 8 ohms, apparently with both channels driven: 700 watts
Into 4 ohms: 1400 watts
Into 2 ohms: 2800 watts

Power consumption at idle: 430 watts
Maximum power consumption: 6000 watts

Weight: 180 pounds

It is not a Class A amp, btw, as is made clear by the difference between its idle and max power consumption numbers, and as might be expected based on its huge output power capability.

I'm perhaps exaggerating only somewhat in saying that I would sooner expect that amp, when playing highly dynamic music at high volume through low efficiency low impedance speakers, to be more likely to cause a neighborhood-wide brown-out than to NOT have an effect on a preamp powered from the same outlet :-)

Best regards,
-- Al

Thanks Al for the link. Interesting comments by Ralph and some reasons why power cords can sound different from each other. I had not seen that post before. Not sure if the voltage drop part is as applicable to my current Class D amps, which don't draw as much as my former Class A Claytons, or other big amps I have owned, but I went ahead and plugged the amps back into the second 20A circuit just in case I feel the need to rock out :~)

Al,

Thanks for finding that particular post by Atmasphere. I believe he has at least one more out there he either posted here on Agon or AA where he goes into more detail the effect a voltage sag can have on the filter caps of a power supply. The part that is the same is this statement of Ralph's,

The more insidious problem is high frequency bandwidth. The power supplies of most amplifiers have a power transformer, a set of rectifiers, and a set of filter capacitors. The rectifiers only conduct when the power transformer output is higher than that of the filter caps. So:

When the caps are fully charged the amp is able to play. As it does so, the caps are discharged until the AC line voltage waveform gets high enough again that the rectifiers in the power supply are able to conduct. Depending on the state of charge of the filter capacitors, this might only be for a few microseconds or it might be a few milliseconds. Either way, the charge is a spike which has very steep sides- and requires some bandwidth to make it happen.

Thanks Al for your example of VD, voltage drop, due to a sudden demand draw of current placed on the branch circuit wiring. Your example should be plain enough for anyone reading it. I would like you to also give an example where the branch circuit wiring is #14 gauge wire which would be a more real world example for the majority of homes in the US where guys are plugging their audio systems into a 15 amp convenience outlet circuit in the living room or a den.

As for the length of the branch circuit I would be willing to bet the length of the 15 amp branch circuit wiring feeding a living room is longer than 50' on average. Of course with that said we have no way of knowing where in the entire length of the branch circuit the audio equipment is plugged into a receptacle. It could be close to the electrical panel or it could at the farthest end or somewhere in the middle.

Something else, though it is impossible to calculate, is the wiring method used by the residential electrician when he made up the joints, wiring connections, feeding in and out of each convenience electrical duplex receptacle outlet wall box. Did the electrician directly connect the in and out branch circuit hot and neutral wires together respectively and extend out pigtails to feed the duplex receptacle? Or did he use the terminal side screws on the cheapo residential grade device to make the in and out connections, daisy chain, of the branch circuit? Or worse yet did he stab the wires in the back of the cheapo residential grade receptacles relying on the spring tension clip inside the receptacle to make a good electrical connection?

All the above can throw a monkey wrench into the equation, especially when corrosion or an ever so slightly loose connection in the branch wiring is thrown into the mix. In this instance VD could/will be greater because the resistance through a corroded connection can change due to the load placed on it.
Jim

Thanks, Jim. All good points.

I would like you to also give an example where the branch circuit wiring is #14 gauge wire which would be a more real world example for the majority of homes in the US where guys are plugging their audio systems into a 15 amp convenience outlet circuit in the living room or a den.

The resistance of 100 feet of 14 gauge wire, corresponding to a 50 foot run, is about 0.25 ohms. For the 30 amp current spike I hypothesized, that would result in a voltage drop of 0.25 x 30 = 7.5 volts. A 75 foot run would increase that by 50%, to 11.25 volts.

As you indicated, less than optimal connections could worsen that significantly.

Best regards,
-- Al

Al,

Thanks for your response to my previous post.
Jim