Amp noise caused by iMac plugged in on same circuit

[agedhorse]

Right. Computer on one circuit, amp on another(different room), cab plugged into amp. No other interfaces plugged into computer. No bass or cable plugged into amp.

Ok, this eliminates a whole lot of stuff from the problem. Now, it's down to the computer injecting EMC (called conducted emissions) into the AC power system. We have eliminated radiated emissions and EMC modulated ground loop circulating currents from consideration.

I can think of only a few possible causes...

1. There is no protective ground on the AC system, even if there are grounded receptacles. Often switching power supplies use the protective ground as a component in the control of conducted EMC, and if it's not present can result in excessive conducted emissions.

2. The computer's power supply is defective (or counterfeit), resulting is greatly increased conducted emissions. Generally, Apple products are very good in this regard. These emissions can be conducted through the ground, neutral or hot conductor depending on the fault/defect.

3. The amp is highly susceptible to conducted interference, and there is a mechanism within the amp that allows it to pass through the power supply into the audio. In some cases, this can be exaggerated by the way specific circuit currents are steered within the amp. This is most common on vintage amps, especially ungrounded amps, which didn't have to deal with the amount (and frequency range) of EMC present in some of today's products.

Is the noise affected by any of the controls?

When using the battery supplied UPS with the AC power disconnected, there is no longer any path for conducted emissions into the AC power system. The UPS generates the power completely isolated from the AC power system and if often the only effective solution in such cases. There are double-conversion UPS systems that contain this isolation internally, so they can remain plugged into the AC power system with no ill effects.

 

[Jim C]

Andy,
Would you think this problem is RF or power line?
That seems to be the first question.

If it is power line wouldn't the troubleshooting begin with checking the relationship of L1, neutral, and ground?
Wonder if this is an older home where the neutral is bonded to ground in the panel.
Lastly, assuming power is good, is it possible for harmonics build up on the neutral buss due to a switching power supply?

Oops; posted before I read yours

 

[agedhorse]

Andy,
Would you think this problem is RF or power line?
That seems to be the first question.

If it is power line wouldn't the troubleshooting begin with checking the relationship of L1, neutral, and ground?
Wonder if this is an older home where the neutral is bonded to ground in the panel.
Lastly, assuming power is good, is it possible for harmonics build up on the neutral buss due to a switching power supply?

Oops; posted before I read yours

Neutral and protective ground MUST be bonded together within the service panel, and then bonded to building/foundation steel, (metal) water piping, (metal) gas piping, driven electrode, (wired not fiber) telco and CATV/internet, etc.

It is possible for serious defects in the power wiring to contribute.

I seriously doubt the harmonic currents are very high as the SMPS is quite small compared with the neutral ampacity and impedance.

 

[Jim C]

Neutral and protective ground MUST be bonded together within the service panel, and then bonded to building/foundation steel, (metal) water piping, (metal) gas piping, driven electrode, (wired not fiber) telco and CATV/internet, etc.

It is possible for serious defects in the power wiring to contribute.

I seriously doubt the harmonic currents are very high as the SMPS is quite small compared with the neutral ampacity and impedance.

Yes, I've seen some homes where either a ground connection from panel to water pipe or earth ground had either degraded (high resistance) or had been removed. I have seen one where there was not connection to an earth ground of any kind.
Would a standard 3 light outlet tester see this kind of infraction?

Good point about a single PC and harmonic currents being very low.

 

[seamonkey]

Not sure how to edit my original post! But I tried plugging a different amp into the same circuit, with the same result. Seems as if this makes the computer unusable for studio purposes.

But your powered monitors are OK?

The Mac is probably fine. I wouldn't worry about this until you've done a few real recordings.

I can't play your video, but you might just be hearing very low noise levels because your house is quiet or you have good hearing. It may be well below any noise level that you'd worry about on any recording. Try the DI out of the amp into a audio interface and see for yourself the noise floor and dynamic range you'll actually get recording.

Many Macs are used in studios, pro recording studios, and FOH. They do have to pass stringent FCC compliance for home use also. I haven't heard of backlight noise issues since manufacturers dumped fluorescent for LEDs.

Recording studios are going to run separate lines from the breaker box to each outlet just to be sure. But - Normal homes will have several rooms on the same breaker, and so using an outlet in a separate room can be the same circuit. If there's a bad ground on the circuit, it may be the same in several rooms. You likely don't need separate lines but a electrician checking and tightening ground connections might be helpful.

I have plenty of power-line automation sending all kinds of signals to the power lines and my gear does not pick this up. Sitting next to monitors or not. Good equipment is designed to filter out the vast majority of power-line noise itself. No additional filters or so call conditioners needed for me for my gear.

 

[agedhorse]

Yes, I've seen some homes where either a ground connection from panel to water pipe or earth ground had either degraded (high resistance) or had been removed. I have seen one where there was not connection to an earth ground of any kind.
Would a standard 3 light outlet tester see this kind of infraction?

Good point about a single PC and harmonic currents being very low.

There are certainly many possibilities when considering not to code installations, the 3 light testers only identify some of the faults.

It's possible, but not all that likely that a quality power line filter type distribution unit (like the Furman series mode type) MIGHT help. I would strongly recommend trying before buying as it depends on how the interference is entering and the mode involved as to if this will help (or possibly hurt).

15A Classic Series Power Conditioner w/Lights | Furman Power | Purifying power for over 40 years.

 

[James Collins]

I had an issue in my home where all of my passive basses and guitars would generate a loud hum whenever I wasn't touching the strings. I never listened to the amp alone.

My house is 120+ years old and has had various wiring done and redone. My ground connection was weak. I know the electrician talked about a power conditioner, but we had to have some work done anyway that involved replacing the ground that was previously to a water pipe to several grounding rods (I assume that is what they are called). It fixed my problem though.

It's been awhile I don't remember what the voltage difference was between ground and neutral, but I remember it was measurable.

 

[honeyiscool]

Most Macs I’ve used have an ungrounded power supply. I wonder if that does anything.

 

[somebrains]

I live in an old house with knob and tube.
I usually have a Mac, 2-3 monitors, a workstation class PC, networking gear, and a Bass amp on.

My recordings are fine.

We have to record Dev sessions live debugging or coding, product demos, I'm recording videogame sessions for upload, etc.

 

[Groove Doctor]

Take your setup to another separate house and test it. If it goes away it’s your house/wiring, if it doesn’t it’s your setup.

 

[agedhorse]

The grounding system in your house is a SYSTEM, it's designed to return fault current to the designated grounded conductor (neutral) while insuring that under fault conditions the return conductor never raises above ground by more than a few volts, nor does the grounded (metallic) items in your house raise above the defined ground by a few volts either.

There are very strict electrical codes that govern this, and the amp's safety and EMC certification depends in part on this as well. It's common (especially on the internet) for folks who do not really understand this to profess things that make no sense.

If you are interested, I encourage those who want to know the truth about grounding systems to study he National Electrical Code article 250. It's very thorough and well defined, and for good reason.

 

[Jim C]

Take your setup to another separate house and test it. If it goes away it’s your house/wiring, if it doesn’t it’s your setup.

Sometimes the easiest solutions are not so apparent; great idea.
(although having code compliant electrical wiring in your home is of paramount importance regardless of audio issues)

 

[clockworkcorpse]

@agedhorse I am an electrician in Canada. The electrical code here does not allow us to count on the bond as a conductor, but requires us to provide it, or a ground fault interrupter. We are told that the CSA (canadian standards association, they write our code and approve device designs) will not allow any device to be sold in Canada that needs a bond to function properly. For example these newfangled wifi enabled switches are often designed to use the bond in place of neutral as it's common to find no neutral in a switch box. We are now required to provide a neutral to all switch locations for this reason.

My question is, and this is not for sake of argument but for my own knowledge, is it true that there are cord connected devices that require a bond to function properly?

I ask because the standard accepted method to address a non-bonded receptacle is to use a gfci receptacle. This makes any ground fault trip the receptacle instead of relying on the circuit breaker, but the gfci receptacle will not be bonded.

A small point of trivia, a gfci receptacle installed in this case will show as an open bond with a plug tester, and a plug tester with a gfci test button will not trip the receptacle.

 

[agedhorse]

Good question.

The protective ground wire (or conduit if hard piped) is not allowed to be used in place of a neutral here either.

UL and CSA require a grounded chassis to be used in place of a double-insulated construction method, but there is always at lease one insulation system in place plus either the "double" or a grounded chassis (the test procedure is different between the two)

There was also an exception for dryer and range circuits to use the ground as a return for small loads such as timers, but I believe both exemptions are no longer allowed, with the range exemption expiring first.

For EMC purposes, both Canada and the US use the general FCC guidelines and limits (Canada calls the ICES-003) , Canada accepts the FCC verification paperwork as well. If the device is designed to be used with a grounded power source, that's how the EMC tests are made.

 

[clockworkcorpse]

If the device does not need a bond, but is causing interference and noise on the bond, would it be a good solution to build a two prong cord with an unbonded gfci receptacle on the end to isolate either the offending device or the offended one?

I had assumed the noise was caused by harmonics from the switch mode power supply(ies) in the mac, but I guess that would not have made it across the transformer in the old amplifier. I'm still curious about these kinds of issues but It definitely goes beyond my area of expertise.

Thanks taking the time, agedhorse, I always enjoy your advice.

 

[sadwinkey]

Take your setup to another separate house and test it. If it goes away it’s your house/wiring, if it doesn’t it’s your setup.

I took your advice and took the setup to another house and the noise is gone! So it seems that it has something to do with the way my house is wired.

Looks like I'll be hiring an electrician! Any advice on specific wiring issues that would be causing this that I could suggest they look for?

 

[agedhorse]

If the device does not need a bond, but is causing interference and noise on the bond, would it be a good solution to build a two prong cord with an unbonded gfci receptacle on the end to isolate either the offending device or the offended one?

That's one way to legally (kind of) or safely get around the requirement for a grounded power source, but in general this is not often successful or even used.

I had assumed the noise was caused by harmonics from the switch mode power supply(ies) in the mac, but I guess that would not have made it across the transformer in the old amplifier. I'm still curious about these kinds of issues but It definitely goes beyond my area of expertise.

Thanks taking the time, agedhorse, I always enjoy your advice.

Depending on the design of the power transformer, it's quite possible for there to be very good coupling between the primary and secondary at very high frequencies. There are ways to wind a power transformer to improve the isolation but it's not all that common in the MI industry. It is (or was) much more common in the medical equipment industry though, different requirements for both conducted emissions and immunity.

 

[clockworkcorpse]

Haha, so the TC needs a custom box built with an isolation transformer wound 1:1 for medical use, with an isolated ground fault protected bond. Then he can plug his amplifier into that, and all's well.

Edit: nevermind, all TC needs is a boring old electrician.

 

[brianrost]

Any advice on specific wiring issues that would be causing this that I could suggest they look for?

Have him check the grounding throughout the house first.

 

[Rabidhamster]

Hey everyone! Sorry if this isn't the right forum to post this.
So I recently purchased a new iMac, and it works great. But, it seems that having this new computer plugged into the same circuit as my PF-50T is causing the amp to pick up some sort of interference from the computer. I took a video to demonstrate:



You'll notice that as i decrease the brightness of the screen, the noise gets louder and changes. It also seems to change depending on what I'm doing on the computer. If I open up a new tab or even click, the amp seems to buzz accordingly.

As of now, I have the computer plugged into a 1500VA battery backup and the amp plugged into an Isolating power strip. I get the same results when I plug them both straight into the wall; It seems that as long as they're on the same circuit, I get this noise.

I also have some powered monitors that are plugged into the same circuit as well, but they don't have this issue. My question is, is this a computer issue, an amp issue, or just something that happens sometimes and I have to deal with? This is all new equipment, so if it is related to hardware, I would like to get it figured out so I can get the faulty gear replaced

Thanks in advance!

PC fans, mechanical hard drives and power supplies create all sorts of emf noise.
WiFi and Bluetooth adaptors can also be a big culprit, they shouldn’t interfere with audio communications but some cheap or failing ones will have audio band noise as well. try disabling both on your mac temporarily and see if it changes anything.
Do you use a mechanical hard drive or an SSD? If it’s a mechanical hard drive and your house ground isn’t faulty then I’d back up anything you care about now.

I get cell phone noise a lot also, which has a similar but different clicking and in my experience a more abrupt and irritating buzzing than this one.