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Mark E. Skeels | 6 Sep 14:30

[EE]recording studio hum

Greetings, PICsters.

I have a friend who has a small personal recording studio consisting of 
a rack of various preamps and effects devices, and a smalll TASCAM 2 
channel all in one hard drive recording unit.

He lives in a multi-unit apartment building; maybe 6 to 8 apartments in 
a two story residential structure.

Some time ago he began to experience periodic 60 Hz hum in his 
headphones and on his recordings.

I stopped by last night to take a look.

We disconnected the rack units and all other outputs/inputs so that only 
the mics were connected directly to the recording unit for test purposes.

We isolated it to the compressor cycle of a window air conditioner in a 
nearby apartment.

It occurs ONLY when the compressor cycles on, not when the AC fan runs 
alone.

It occurs ONLY when a mic is plugged into the unit, not when the inputs 
are unused (no source connected).

It does not occur with any of the other window air conditioner in any 
other apartments, as far as we could tell.

Moving the AC power line plug to other outlets in the apartment does not
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Geo | 6 Sep 14:44

Re: [EE]recording studio hum


> It occurs ONLY when the compressor cycles on, not when the AC fan runs 
> alone.
> 
> It occurs ONLY when a mic is plugged into the unit, not when the inputs 
> are unused (no source connected).

Can you hear it if the live microphone is placed in a soundproof isolating box - 
i.e. is the microphone picking up a mechanical 60Hz vibration?
Or fit a dummy load instead of the microphone.

George Smith

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Mark E. Skeels | 6 Sep 14:55

Re: [EE]recording studio hum

Thanks for reminding me....no, isolating the mic acoustically DOES NOT 
get rid of the sound.

Mark

Geo wrote:
>> It occurs ONLY when the compressor cycles on, not when the AC fan runs 
>> alone.
>>
>> It occurs ONLY when a mic is plugged into the unit, not when the inputs 
>> are unused (no source connected).
>>     
>
> Can you hear it if the live microphone is placed in a soundproof isolating box - 
> i.e. is the microphone picking up a mechanical 60Hz vibration?
> Or fit a dummy load instead of the microphone.
>
> George Smith
>
>   
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Herbert Graf | 6 Sep 15:34

Re: [EE]recording studio hum

On Sat, 2008-09-06 at 13:44 +0100, Geo wrote:
> > It occurs ONLY when the compressor cycles on, not when the AC fan runs 
> > alone.
> > 
> > It occurs ONLY when a mic is plugged into the unit, not when the inputs 
> > are unused (no source connected).
> 
> Can you hear it if the live microphone is placed in a soundproof isolating box - 
> i.e. is the microphone picking up a mechanical 60Hz vibration?
> Or fit a dummy load instead of the microphone.

Could it be the mic is picking up the hum electromagnetically?
Compressors are large coils, dynamic mics have a relatively large coil,
you could have some EM coupling going on (think cordless toothbrush type
effect).

I once had a case where it turned out a particular mic line was the
perfect length to catch a local AM radio station, that was a tough one
to debug.

Try a condenser type mic to see if the problem goes away.

TTYL

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Mark E. Skeels | 6 Sep 15:47

Re: [EE]recording studio hum

Thanks, Herbert, for your suggestion, but it occurs with both dynamic 
and condensor mics...

Mark

Herbert Graf wrote:
> On Sat, 2008-09-06 at 13:44 +0100, Geo wrote:
>   
>>> It occurs ONLY when the compressor cycles on, not when the AC fan runs 
>>> alone.
>>>
>>> It occurs ONLY when a mic is plugged into the unit, not when the inputs 
>>> are unused (no source connected).
>>>       
>> Can you hear it if the live microphone is placed in a soundproof isolating box - 
>> i.e. is the microphone picking up a mechanical 60Hz vibration?
>> Or fit a dummy load instead of the microphone.
>>     
>
> Could it be the mic is picking up the hum electromagnetically?
> Compressors are large coils, dynamic mics have a relatively large coil,
> you could have some EM coupling going on (think cordless toothbrush type
> effect).
>
> I once had a case where it turned out a particular mic line was the
> perfect length to catch a local AM radio station, that was a tough one
> to debug.
>
> Try a condenser type mic to see if the problem goes away.
>
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Olin Lathrop | 6 Sep 15:51

Re: [EE]recording studio hum

Mark E. Skeels wrote:
> It occurs ONLY when the compressor cycles on, not when the AC fan runs
> alone.
>
> It occurs ONLY when a mic is plugged into the unit, not when the
> inputs are unused (no source connected).

My first knee jerk reaction is that this is magnetic interference.  It could
be caused by the compressor motor itself, or by unballanced wiring leading
to it, or maybe a nearby power transformer that feeds the circuit the
compressor is on.

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Gerhard Fiedler | 6 Sep 16:03

Re: [EE]recording studio hum

Mark E. Skeels wrote:

> I am guessing maybe a loose screw on an outlet or some other undesired 
> resistance maybe in the neutral?

Did you try isolating (for testing only) the ground prong? And/or running
the equipment off an isolation transformer (of course without the ground
connected through)?

Gerhard

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Mark E. Skeels | 6 Sep 16:59

Re: [EE]recording studio hum

Thanks, I will try that.

Mark

Gerhard Fiedler wrote:
> Mark E. Skeels wrote:
>
>   
>> I am guessing maybe a loose screw on an outlet or some other undesired 
>> resistance maybe in the neutral?
>>     
>
> Did you try isolating (for testing only) the ground prong? And/or running
> the equipment off an isolation transformer (of course without the ground
> connected through)?
>
> Gerhard
>
>   
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Forrest W Christian | 7 Sep 12:26

Re: [EE]recording studio hum

Gerhard Fiedler wrote:
> Did you try isolating (for testing only) the ground prong? And/or 
> running the equipment off an isolation transformer (of course without 
> the ground connected through)?
I was going to suggest trying running the gear with the ground isolated 
as well.   

I actually have several two-to-three prong adapters in my live sound/dj 
equipment for those hums which are caused by power-ground weirdness.   
I've also run into problems when I do live sound with hum caused by 
simply plugging the the amplifier stack and/or powered speakers at the 
Front of House into a different circuit than the mixers and equipment in 
the rear.   Sometimes lifting ground helps - sometimes not.

My laptop always runs with power ground isolated - there's enough 
unshielded power supply noise, that if it's grounded I get nice 
"switching power supply noise" inducted into the entire setup...

I also have seen situations a 60 hz hum will get worse/better with 
microphones plugged/unplugged as well - there's enough stray hum getting 
in just from the low 60 Hz EM field which seems to pervade all of our 
environments today.    Usually these seem to be very position dependent 
though...  That is, if you plug it in, and get the 60hz hum, if you move 
the wires it will get better/worse.

-forrest
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Bob Blick | 6 Sep 16:50
Favicon

Re: [EE]recording studio hum

Assuming the mic preamps are actually balanced(not all mixers with XLR
inputs have balanced preamps!) since you said you used two different
mixers, then it is most likely cheap mic cable.

In a past life I did engineering in 24 track studios and saw every
possible problem.

Cheerful regards,

Bob

Mark E. Skeels wrote:
> Greetings, PICsters.
> 
> I have a friend who has a small personal recording studio consisting of 
> a rack of various preamps and effects devices, and a smalll TASCAM 2 
> channel all in one hard drive recording unit.
> 
> He lives in a multi-unit apartment building; maybe 6 to 8 apartments in 
> a two story residential structure.
> 
> Some time ago he began to experience periodic 60 Hz hum in his 
> headphones and on his recordings.
> 
> I stopped by last night to take a look.
> 
> We disconnected the rack units and all other outputs/inputs so that only 
> the mics were connected directly to the recording unit for test purposes.
> 
> We isolated it to the compressor cycle of a window air conditioner in a
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Mark E. Skeels | 6 Sep 17:02

Re: [EE]recording studio hum

Thanks, I will try a high quality mic cable.

Mark

Bob Blick wrote:
> Assuming the mic preamps are actually balanced(not all mixers with XLR
> inputs have balanced preamps!) since you said you used two different
> mixers, then it is most likely cheap mic cable.
>
> In a past life I did engineering in 24 track studios and saw every
> possible problem.
>
> Cheerful regards,
>
> Bob
>
>
> Mark E. Skeels wrote:
>   
>> Greetings, PICsters.
>>
>> I have a friend who has a small personal recording studio consisting of 
>> a rack of various preamps and effects devices, and a smalll TASCAM 2 
>> channel all in one hard drive recording unit.
>>
>> He lives in a multi-unit apartment building; maybe 6 to 8 apartments in 
>> a two story residential structure.
>>
>> Some time ago he began to experience periodic 60 Hz hum in his 
>> headphones and on his recordings.
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Dave Tweed | 6 Sep 17:12

[EE] recording studio hum

Mark E. Skeels wrote:
> I have a friend who has a small personal recording studio consisting of 
> a rack of various preamps and effects devices, and a smalll TASCAM 2 
> channel all in one hard drive recording unit.
> 
> He lives in a multi-unit apartment building; maybe 6 to 8 apartments in 
> a two story residential structure.
> 
> Some time ago he began to experience periodic 60 Hz hum in his 
> headphones and on his recordings.
> 
> I stopped by last night to take a look.

It sounds like you've covered most of the obvious paths. Now you need
to look for the more subtle things. I once attended an AES presentation
by Bill Whitlock of Jensen Transformers about fixing problems like these
in audio systems, and he really knows his stuff. He explains it better
than I can -- check out some of his white papers here:

   http://www.jensen-transformers.com/apps_wp.html

Some aspects of your story are puzzling -- for example, the hum only
appears when a microphone is plugged in, but occurs identically (I presume)
with any type of microphone, any type of cable, and any physical position
of the microphone and cable (?).

This indicates that the power line is the source, but the coupling occurs
somewhere in the microphone preamp, or perhaps in the phantom power supply?
Something is unbalanced somewhere.
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Vitaliy | 6 Sep 19:34

Re: [EE] recording studio hum

Dave Tweed wrote:
> My guess would be that the air conditioner controls the compressor with a
> triac, and this is producing some high-frequency harmonics on the power
> line. The compressor motor itself should be an induction motor, and not
> producing harmonic distortion. A brute force low-pass filter on the AC
> line, or maybe a ferroresonant transformer, would help confirm this.

You mean, like a 0.1uF cap across the 110V lines? Maybe with a choke in 
series with the load?

How about an off-the-shelf line conditioner, or an online UPS?

Vitaliy 

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Dave Tweed | 6 Sep 20:37

Re: [EE] recording studio hum

Vitaliy wrote:
> Dave Tweed wrote:
> > My guess would be that the air conditioner controls the compressor with
> > a triac, and this is producing some high-frequency harmonics on the
> > power line. The compressor motor itself should be an induction motor,
> > and not producing harmonic distortion. A brute force low-pass filter on
> > the AC line, or maybe a ferroresonant transformer, would help confirm
> > this.
> 
> You mean, like a 0.1uF cap across the 110V lines? Maybe with a choke in 
> series with the load?

I was thinking of something involving chokes and capacitors (pi network?),
with a fairly steep cutoff above 60 Hz in both common and differential
modes.

> How about an off-the-shelf line conditioner,

Depends on what's inside. Just a handfull of MOVs for surge suppression
won't have any effect.

> ... or an online UPS? 

Only if it's pure sinewave output, otherwise it'll just confuse the
issue. Something like the Galaxy Audio "Far Outlet" would be ideal, but
unfortunately, it's been discontinued and is only available used.

-- Dave Tweed
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Bob Blick | 6 Sep 21:04
Favicon

Re: [EE] recording studio hum

If you read the original post. the solutions you are suggesting are
likely to do nothing, or worse, have side effects with other equipment.
Unless you can shut up the source of the interference(it's a neighbor,
so forget it) you need to deal with where it's getting in.

There's no noise until a mic is plugged in. It's a bad cable, or if they
are using a snake, maybe that's bad. They've already eliminated almost
everything else, including the preamp, by substitution. It's not ground
current because the mic is at an unconnected end of the chain. If you
buy mic cables at music stores there are some brands that are really
terrible and do just what he was describing.

Plug the mic in with two cables in series and see if the problem gets
worse. If it does, look no further, go test different cables at the
store if they have a PA system.

Cheerful regards,

Bob

Dave Tweed wrote:
> Vitaliy wrote:
>> Dave Tweed wrote:
>>> My guess would be that the air conditioner controls the compressor with
>>> a triac, and this is producing some high-frequency harmonics on the
>>> power line. The compressor motor itself should be an induction motor,
>>> and not producing harmonic distortion. A brute force low-pass filter on
>>> the AC line, or maybe a ferroresonant transformer, would help confirm
>>> this.
>> You mean, like a 0.1uF cap across the 110V lines? Maybe with a choke in
(Continue reading)

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Brendan Gillatt | 6 Sep 21:27

Re: [EE] recording studio hum


Bob Blick wrote:
> There's no noise until a mic is plugged in. It's a bad cable, or if they
> are using a snake, maybe that's bad. They've already eliminated almost
> everything else, including the preamp, by substitution. It's not ground
> current because the mic is at an unconnected end of the chain. If you
> buy mic cables at music stores there are some brands that are really
> terrible and do just what he was describing.

Yup I agree with that wholly: I used to have a no-name guitar lead that
had a 50Hz noise floor about level with what my bass was actually
producing. Unsurprisingly, it only cost me a few quid.
--
Brendan Gillatt | GPG Key: 0xBF6A0D94
brendan {a} brendangillatt (dot) co (dot) uk
http://www.brendangillatt.co.uk
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Dave Tweed | 6 Sep 22:14

Re: [EE] recording studio hum

Bob Blick wrote:
> If you read the original post. the solutions you are suggesting are
> likely to do nothing, or worse, have side effects with other equipment.
> Unless you can shut up the source of the interference(it's a neighbor,
> so forget it) you need to deal with where it's getting in.

I'm sorry if I confused you. I was suggesting diagnostic tools, not
solutions. Until we know how the unwanted signal is getting into the
system, it's premature to suggest solutions.

Actually, from the OP's first post and immediate followups, it wasn't
entirely clear what the invariants were with respect to the microphones
and cables, or whether different microphone/cable positions had been
tried. I was trying to suggest clarifying that issue in my first message.

Also, it would be interesting to know what changed when the noise started
to appear. Was that particular A/C newly installed, or did something change
in the recording setup?

-- Dave Tweed
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Mark E. Skeels | 6 Sep 22:56

Re: [EE] recording studio hum

A bit more info.....

It never occurred to me to suspect the cables or the mic positions. We 
did not move them at all. But before I got there, my friend tried 
different mics in different rooms with different preamps (a Yamaha 
powered mixer).

We did not determine the connection between the AC and the hum until 
last night. But we did verify that other AC units in the same apartment 
did not cause the problem.

Nothing in the recording setup changed.

Also, this is definitely 60 Hz hum, nothing else. (at least, that I can 
detect with my ears....)

Mark

Dave Tweed wrote:
> Bob Blick wrote:
>   
>> If you read the original post. the solutions you are suggesting are
>> likely to do nothing, or worse, have side effects with other equipment.
>> Unless you can shut up the source of the interference(it's a neighbor,
>> so forget it) you need to deal with where it's getting in.
>>     
>
> I'm sorry if I confused you. I was suggesting diagnostic tools, not
> solutions. Until we know how the unwanted signal is getting into the
> system, it's premature to suggest solutions.
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Steve Smith | 7 Sep 03:03

RE: [EE] recording studio hum

I guess you may have already covered this one but have you eliminated 'earth
loop hum' This can occur due to multiple earths on the audio section. We
often found this was due to an earth on the input to a power amp as well as
the main star point within the mixer. It can also be intermittent and
illusive when you are trying to trace its source.

Steve

-----Original Message-----
From: piclist-bounces <at> mit.edu [mailto:piclist-bounces <at> mit.edu] On Behalf Of
Dave Tweed
Sent: 06 September 2008 16:12
To: piclist <at> mit.edu
Subject: [EE] recording studio hum

Mark E. Skeels wrote:
> I have a friend who has a small personal recording studio consisting of 
> a rack of various preamps and effects devices, and a smalll TASCAM 2 
> channel all in one hard drive recording unit.
> 
> He lives in a multi-unit apartment building; maybe 6 to 8 apartments in 
> a two story residential structure.
> 
> Some time ago he began to experience periodic 60 Hz hum in his 
> headphones and on his recordings.
> 
> I stopped by last night to take a look.

It sounds like you've covered most of the obvious paths. Now you need
to look for the more subtle things. I once attended an AES presentation
(Continue reading)

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Richard Prosser | 7 Sep 09:59

Re: [EE] recording studio hum

Mark,

Is the AC unit "different" in any way? Some newer ones use "inverter"
technology. As far as I can figure, this involves an AC-DC pfc stage
which could emit rf. This is modulated by the 60Hz so you hear 60 (or
120)Hz hum.

Have you tried wrapping the cable through a ferrite toroid for a few turns?

Also - is it actally 60Hz or 120Hz you can hear? Might be clue as to
how it's being generatedf and entering the system.

RP

> Mark E. Skeels wrote:
>> I have a friend who has a small personal recording studio consisting of
>> a rack of various preamps and effects devices, and a smalll TASCAM 2
>> channel all in one hard drive recording unit.
>>
>> He lives in a multi-unit apartment building; maybe 6 to 8 apartments in
>> a two story residential structure.
>>
>> Some time ago he began to experience periodic 60 Hz hum in his
>> headphones and on his recordings.
>>
>> I stopped by last night to take a look.
>
> It sounds like you've covered most of the obvious paths. Now you need
> to look for the more subtle things. I once attended an AES presentation
> by Bill Whitlock of Jensen Transformers about fixing problems like these
(Continue reading)

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Gmane