Amp and preamp on same outlet?

Using separate outlets, especially if they are on separate runs back to the breaker panel, will reduce the amount of amplifier-generated electrical noise that may couple back into the preamp. On the other hand, doing that may increase susceptibility to ground loop issues, including high frequency electrical noise as well as low frequency hum. If the connection between preamp and power amp is balanced, the likelihood of ground loop issues is considerably reduced.

All of this is highly dependent on the design of the particular components, and has little if any predictability. So ZD is right, "there's no way to tell unless you try it." But if, as you say, "it sounds great as is," that would seem likely to be the bottom line.

Regards,
-- Al

Thanks, Jim (Jea48). Excellent points, some of which are further emphasized and explained on pages 31 to 35 of this excellent paper, which you had called attention to a while back.

Wrm, thanks also. Best regards,
-- Al

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, 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

John, yes your math is of course correct. And the 700cx does indeed have a max power consumption spec of 6000 watts, which if anything seems a bit low in relation to its specified output power rating of 2800 watts/channel into a 2 ohm load (the 8 ohm and 4 ohm ratings being 700 and 1400 watts respectively).

But it seems safe to assume that it would be an unusual occurrence for the amp to be used in a manner that would require it to draw more than 20 amps x 120 volts = 2400 watts for a long enough time to cause a 20 amp breaker to trip.

Best regards,
-- Al

If Al is still following this thread he can do a better job of translating the info from page 22 of the owner manual than me.

Hi Jim,

I can't do that with any certainty, because as I see it the specs provided in the manual are both ambiguous and inconsistent.

First, output power into the various load impedances is specified as "each channel driven." I would normally expect output power ratings to be based on both channels being driven simultaneously, but as I alluded to earlier the specified maximum input power of 6000 watts seems (given that this is not a class D amp) to be too low to be consistent with the specified output into 2 ohms of 2800 watts, if the 2800 watts (or anything close to that amount) is provided by both channels simultaneously. So perhaps "each channel driven" means "one channel driven," rather than "both channels driven."

Second, the specified maximum power consumption of 6000 watts is identical to the spec for the FPB 750Mc monoblock amp (for which specs are provided on the next page of the manual), with that amp having just slightly higher output power ratings. Same goes for the other two monoblock amps in the FPB series and their stereo counterparts. Yet the power consumption numbers for standby and idle conditions differ between the stereo amps and their monoblock counterparts by approximately a factor of two in most cases, as might be expected.

So I don't know how to reconcile all of that, and it seems that all of these numbers have to be taken with some grains of salt. And adding to the uncertainty is the point you raised earlier about inrush current, which of course is unspecified.

Best regards,
-- Al

John & Jim, ROTFLOL!! Thanks!!

Best regards,
-- Al