[Ernst]

Hello All,

Greetings and well wishes. A few months back I asked the group for
advise regarding construction of a high power tube amp, with good
results I might add.

I'm down to the power supply. Here's the thing: DC is DC right?
So if I cobble up a switching supply to provide B+, bias, and
filament, then there should be no problem.

200KHZ ripple in my view should be no problem to filter. Only thing
is that I plan to section each voltage to require little or no
regulation, i.e., I would not place the B+ line in a 350-600V
regulator; conversely, I would apply design center 450VDC up front,
skip the regulator, and apply the filter network.

Does this make sense?

Regards,

Ernst

[robert casey]

Ernst wrote:
Hello All,

Greetings and well wishes. A few months back I asked the group for
advise regarding construction of a high power tube amp, with good
results I might add.

I'm down to the power supply. Here's the thing: DC is DC right?
So if I cobble up a switching supply to provide B+, bias, and
filament, then there should be no problem.

200KHZ ripple in my view should be no problem to filter. Only thing
is that I plan to section each voltage to require little or no
regulation, i.e., I would not place the B+ line in a 350-600V
regulator; conversely, I would apply design center 450VDC up front,
skip the regulator, and apply the filter network.

Does this make sense?


If you regulate the bias supply (this I think would be the most critical
of voltages in the entire amp) the rest of the supplies should track
reasonably well enough.

Are you using output tubes with indirectly heated cathodes, or directly
heated filaments? If the latter, you might be able to use the high
frequency switching frequency AC of appropriate voltage directly on the
filament. But some filament "battery radio" tubes were designed to
expect one end of their filaments to see the positive side of the
filament supply, but I don't think they did that with power tubes
usually used in audio amps.

[[email protected]]

Ernst wrote:

Hello All,

Greetings and well wishes. A few months back I asked the group for
advise regarding construction of a high power tube amp, with good
results I might add.

I'm down to the power supply. Here's the thing: DC is DC right?
So if I cobble up a switching supply to provide B+, bias, and
filament, then there should be no problem.


Depends. If you use a single transformer then only one of the dc outputs
regulates the entire supply. This effectively means only one rail is
regulated. The other supply rails' regulation depends on the transformer
regulation and the filter alone. If that is good enough for you then yes
there should be no problem.

Ian

[Patrick Turner]

Ernst wrote:

Hello All,

Greetings and well wishes. A few months back I asked the group for
advise regarding construction of a high power tube amp, with good
results I might add.

I'm down to the power supply. Here's the thing: DC is DC right?
So if I cobble up a switching supply to provide B+, bias, and
filament, then there should be no problem.

200KHZ ripple in my view should be no problem to filter. Only thing
is that I plan to section each voltage to require little or no
regulation, i.e., I would not place the B+ line in a 350-600V
regulator; conversely, I would apply design center 450VDC up front,
skip the regulator, and apply the filter network.

Does this make sense?

Regards,

Ernst

DC is DC.

Smoke is smoke, right?

When you have designed, built, and tested a SMPS for tube amp DC supply,
let us all know, but so far not a living soul uses SMPS for tube amps.

Patrick Turner.

[John Byrns]

In article ,
flipper wrote:

On Sun, 24 Jun 2007 14:38:11 GMT, Patrick Turner
wrote:

When you have designed, built, and tested a SMPS for tube amp DC supply,
let us all know, but so far not a living soul uses SMPS for tube amps.

I've seen some references to SMPS being used in 'car' tube amps.

How else are you going to do a "car" tube amp without a SMPS of some
sort?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[E M F]

Ernst scribbled;

"Ernst" wrote in message
oups.com...
Hello All,

Greetings and well wishes. A few months back I asked the group for
advise regarding construction of a high power tube amp, with good
results I might add.

I'm down to the power supply. Here's the thing: DC is DC right?
So if I cobble up a switching supply to provide B+, bias, and
filament, then there should be no problem.

200KHZ ripple in my view should be no problem to filter. Only thing
is that I plan to section each voltage to require little or no
regulation, i.e., I would not place the B+ line in a 350-600V
regulator; conversely, I would apply design center 450VDC up front,
skip the regulator, and apply the filter network.

Does this make sense?

Nope....

Regards,

Ernst

SMPS output wide band noise and are not easy to filter, If you
use several supplies @ 200KHZ you will detect the difference
frequency which will fall into the audio band......
don't waste your time....SMPS = Some of the most awful
junk devised by man..........GC

[John Byrns]

In article ,
flipper wrote:

On Sun, 24 Jun 2007 20:24:32 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Sun, 24 Jun 2007 14:38:11 GMT, Patrick Turner
wrote:

When you have designed, built, and tested a SMPS for tube amp DC supply,
let us all know, but so far not a living soul uses SMPS for tube amps.

I've seen some references to SMPS being used in 'car' tube amps.

How else are you going to do a "car" tube amp without a SMPS of some
sort?

The means most people remember was vibrators, which is technically a
SMPS, but they also used dynamotors. The Motorola FMTRU-140D was
dynamotor powered.

Good point, I don't know how I could forget dynamotors, although they
weren't much used in consumer equipment.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Eeyore]

flipper wrote:

For example, a notebook of mine has significant SMPS noise on the
audio.

I doubt very much it's the SMPS at fault.

Integrated sound is usually crap because of all the noise it picks up from the
computer generally (especaially video it seems)

Graham

[Eeyore]

flipper wrote:

On Sun, 24 Jun 2007 20:24:32 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Sun, 24 Jun 2007 14:38:11 GMT, Patrick Turner
wrote:

When you have designed, built, and tested a SMPS for tube amp DC supply,
let us all know, but so far not a living soul uses SMPS for tube amps.

I've seen some references to SMPS being used in 'car' tube amps.

How else are you going to do a "car" tube amp without a SMPS of some
sort?

The means most people remember was vibrators, which is technically a
SMPS, but they also used dynamotors. The Motorola FMTRU-140D was
dynamotor powered.

Mechanical vibrators didn't operate at anywhere near the frequency of
modern SMPS but contact sparking presents a set of it's own problems
and the *first* thing you checked on a dead radio was the vibrator.
Odds were 90% that was the problem.

Vibrators are no use for large powers.

Dynamotors are huge.

Graham

[Patrick Turner]

flipper wrote:

On Sun, 24 Jun 2007 14:38:11 GMT, Patrick Turner
wrote:



Ernst wrote:

Hello All,

Greetings and well wishes. A few months back I asked the group for
advise regarding construction of a high power tube amp, with good
results I might add.

I'm down to the power supply. Here's the thing: DC is DC right?
So if I cobble up a switching supply to provide B+, bias, and
filament, then there should be no problem.

200KHZ ripple in my view should be no problem to filter. Only thing
is that I plan to section each voltage to require little or no
regulation, i.e., I would not place the B+ line in a 350-600V
regulator; conversely, I would apply design center 450VDC up front,
skip the regulator, and apply the filter network.

Does this make sense?

Not complete sense.

The SMPS must offer the same levels of safety as the isolation tranny
linear supply.

To get isolation with SMPS, the mains voltage is rectified
to make say +340V dc, then a chopper circuit drains pulses from the
input cap and this feeds a HF tranny of small size and low weight and
it's this tranny
that offers the isolation. A very small section of circutry remains
connected to the mains
and this can be shielded or isolated.

SMPS have been used for 20 years in TV sets and in PCs for years,
and in many amps, including state of the art Halcro.

Nobody has tried to do SMPS with tube amps because it'd never be
cheaper than a linear because of production run numbers, and not because
it cannot
be done technically.

To make a 500V dc supply at 1 amp capability with SMPS
probably isn't too difficult, but nobody has ever bothered.

Regulation od the PS is usually achieved by some FB mechanism
controlling the chopper chip
which switches the mosfet on and off.
I am over simplifying, so to find out what is possible build one
yourself.
There have been no answers for 7 years I have been here about SMPS.
Not a single person has tendered a fully tested schematic here.

Patrick Turner.



Regards,

Ernst

DC is DC.

Smoke is smoke, right?

I'm not so sure.

The potential problems I see are that switchers have high transient
current surges, which makes filtering a bit more problematic that it
would at first seem, and operate at high frequency, creating a
potential EMI problem.

I would imagine all are solvable but not necessarily trivial.

For example, a notebook of mine has significant SMPS noise on the
audio. Now, granted, it's not intended as an 'audiophile' device but
it's 'typical' filtering just don't do the job well at all even though
those IC suckers are supposed to have gobs of 'supply rejection'.
Which suggests to me that just because 'DC' comes out doesn't
necessarily make it great for an amp.

Could be wrong, though.

When you have designed, built, and tested a SMPS for tube amp DC supply,
let us all know, but so far not a living soul uses SMPS for tube amps.

I've seen some references to SMPS being used in 'car' tube amps.

Patrick Turner.

[Patrick Turner]

John Byrns wrote:

In article ,
flipper wrote:

On Sun, 24 Jun 2007 14:38:11 GMT, Patrick Turner
wrote:

When you have designed, built, and tested a SMPS for tube amp DC supply,
let us all know, but so far not a living soul uses SMPS for tube amps.

I've seen some references to SMPS being used in 'car' tube amps.

How else are you going to do a "car" tube amp without a SMPS of some
sort?

Regards,

John Byrns

Years ago before SMPS, they used a vibrator
to get from 12Vdc to +350V.

It was a magnetic buzzer type of device which switched the 12V at
about 200Hz and created square waves which could be transformed up and
rectified.
RDH4 shows how it was done.

As soon as transitors that were reliable enough were made they used them
to switch the
12V electronically, so no vibrator contacts to wear out.

As years passed the SS devices for switching went faster, so switching F
went up to about 500kHz,
allowing a very small tranny indeed, with good reliability.

Patrick Turner.

--
Surf my web pages at, http://fmamradios.com/

[Ian Iveson]

Ernst wrote

Greetings and well wishes.

Hail.

A few months back I asked the group for
advise regarding construction of a high power tube amp, with good
results I might add.

Care to share the schematic? A brief description in words would help
if you can't post a diagram.

I'm down to the power supply. Here's the thing: DC is DC right?
So if I cobble up a switching supply to provide B+, bias, and
filament, then there should be no problem.

Cobble? You're kidding...

200KHZ ripple in my view should be no problem to filter. Only thing
is that I plan to section each voltage to require little or no
regulation, i.e., I would not place the B+ line in a 350-600V
regulator; conversely, I would apply design center 450VDC up front,
skip the regulator, and apply the filter network.

Does this make sense?

Maybe, depending on what species of SMPS you are "cobbling", and
whether your amp is class A or AB. But it's not a trivial pursuit.

Care to share your SMPS circuit? A brief description in words would
help if you can't post a diagram.

cheers, Ian

[John Byrns]

In article ,
Patrick Turner wrote:


SMPS have been used for 20 years in TV sets and in PCs for years,
and in many amps, including state of the art Halcro.

Actually SMPS have been used in Television sets for nearly twice that
long. I had a Television set 36 years ago that used a SMPS, and IIRC
the design had come out several years earlier. The power supply had a
design error, or rather a component specification error, the factory
capacitor on the heater supply for the kinescope was not up to the job
and would fail in a relatively short time.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Eeyore]

flipper wrote:

Eeyore wrote:
flipper wrote:

For example, a notebook of mine has significant SMPS noise on the
audio.

I doubt very much it's the SMPS at fault.

Integrated sound is usually crap because of all the noise it picks up from the
computer generally (especaially video it seems)

I am aware of that source but with the notebook I can remove the SMPS
brick so I'm pretty sure that's where it comes from.

You can usually tell computer and video induced noise because it'll
vary in sync with activity.

Fair enough.

You have a uniquely bad SMPS in that case. Filtering the output from them is
normally trivially simple.

Graham

[robert casey]

DC is DC.

Smoke is smoke, right?

When you have designed, built, and tested a SMPS for tube amp DC supply,
let us all know, but so far not a living soul uses SMPS for tube amps.


Designing and building a SMPS is significantly more difficult than an
old fashioned linear supply. And most DIYers would rather spend their
time and effort on the actual audio amp circuits anyway. Sure, a good
SMPS would be smaller and such, but the old fashioned 50 or 60Hz power
transformer, rectifier, choke and filter cap topology is well understood
and easily thrown together. Power supplies are rather boring anyway.

I think there are vendors that sell SMPSs suitable for tube work. But
if it fails, it would be a bit of a PITA to repair it.

[robert casey]

Dynamotors are huge.

Graham


It could be planted inside the trunk, or maybe you could add an extra
alternator to the car engine that directly produces a few hundred volts
(I was going to say "generator" but those use brushes that would spark
and make noise). But keeping the voltage steady would be a bit of a
PITA, so the dynamotor looks more attractive...

Dynamotors are not that efficient, but that's not that serious in a car.

I've seen dynamotors the size of about 10cm diameter and 25cm long, but
I don't recall the output power of one like that.

[robert casey]

John Byrns wrote:



Actually SMPS have been used in Television sets for nearly twice that
long. I had a Television set 36 years ago that used a SMPS, and IIRC
the design had come out several years earlier.

In a sense, TV sets used forms of SMPSs since day one of consumer TV
sets. Namely the flyback transformer, used partly to develop the very
high voltage (10KV or higher) for the picture tube. And also to get
around 600VDC for the vertical circuits and focus voltages for the
picture tube. These things ran at the horizontal deflection frequency,
15734Hz.

Antique Electronic Supply sells a solid state replacement for vibrators
for tube car radios. Today you could adapt something like that along
with a vibrator transformer to create B+

[Eeyore]

flipper wrote:

Eeyore wrote:
flipper wrote:
Eeyore wrote:
flipper wrote:

For example, a notebook of mine has significant SMPS noise on the
audio.

I doubt very much it's the SMPS at fault.

Integrated sound is usually crap because of all the noise it picks up from the
computer generally (especaially video it seems)

I am aware of that source but with the notebook I can remove the SMPS
brick so I'm pretty sure that's where it comes from.

You can usually tell computer and video induced noise because it'll
vary in sync with activity.

Fair enough.

You have a uniquely bad SMPS in that case.

I think so too. However, it's probably as good a 'slapping something
together', as was mentioned

Filtering the output from them is
normally trivially simple.

That I disagree with. It would seems simple because one figures it's
'easy' to filter a higher frequency but the heavy current spikes
introduce ground bump and other problems, like lead inductances and
capacitor ESR, etc.

These aren't real problems if designed competently.

Graham

[Ian Iveson]

Eeyore said:

These aren't real problems if designed competently.

Meaningless and unhelpful tautology.

But here is a chance for you to demonstrate your competence. Post a
design for a single SMPS suitable for a typical valve amp of, say,
80W.

If it's so easy, it shouldn't take you long.

Ian

[Eeyore]

Ian Iveson wrote:

Eeyore said:

These aren't real problems if designed competently.

Meaningless and unhelpful tautology.

Fact.


But here is a chance for you to demonstrate your competence. Post a
design for a single SMPS suitable for a typical valve amp of, say,
80W.

If it's so easy, it shouldn't take you long.

I didn't say it was easy to design an entire SMPS. In fact it's quite
complicated, not least the transformer design.

Furthermore, a circuit schematic is only half the story. A vast amount of SMPS
design is to do with proper layout.

Filtering the output however is not complicated at all.

Graham

[robert casey]

I think so too. However, it's probably as good a 'slapping something
together', as was mentioned

That's what probably was done over in China when they designed that
SMPS...

[Chris Hornbeck]

On Mon, 25 Jun 2007 16:06:05 -0500, flipper wrote:

You have a uniquely bad SMPS in that case.

I think so too. However, it's probably as good a 'slapping something
together', as was mentioned

Maybe not too unique though. Questions about audio noise
in laptop computers posted to rec.audio.pro often seem
lately to end with blame placed on the SMPS itself.

The mechanism is necessarily unclear, but contamination
of signal ground might reasonably be assumed. This is
always an issue, and one that the OP will need to address
also.

Filtering the output from them is
normally trivially simple.

That I disagree with. It would seems simple because one figures it's
'easy' to filter a higher frequency but the heavy current spikes
introduce ground bump and other problems, like lead inductances and
capacitor ESR, etc.

It's a trivial issue if its importance is recognized, but
laptop computer makers might likely feel otherwise. All
corners *must* be cut; like that.

Thanks, as always,

Chris Hornbeck
"The French don't have a word for 'entrepreneur'." -George W. Bush

[Eeyore]

Chris Hornbeck wrote:

flipper wrote:

You have a uniquely bad SMPS in that case.

I think so too. However, it's probably as good a 'slapping something
together', as was mentioned

Maybe not too unique though. Questions about audio noise
in laptop computers posted to rec.audio.pro often seem
lately to end with blame placed on the SMPS itself.

Almost certainly due to the Y cap that's there do deal with regulatory
requirements for EMI emissions that creates a leakage current to ground.

Graham

[Chris Hornbeck]

On Mon, 25 Jun 2007 23:50:26 GMT, Eeyore
wrote:

Maybe not too unique though. Questions about audio noise
in laptop computers posted to rec.audio.pro often seem
lately to end with blame placed on the SMPS itself.

Almost certainly due to the Y cap that's there do deal with regulatory
requirements for EMI emissions that creates a leakage current to ground.

That's certainly a good thing to look into first.
ISTR though that some folks had similar issues with
SMPS supplies having only two-wire Edison connections,
and while "free-standing", ei. unconnected to anything
else Edison, leaving the statutory minimums.

It's an interesting problem, and just one of many
that the OP is about to discover. That's the great
thing about life.

Thanks, as always,

Chris Hornbeck
"The French don't have a word for 'entrepreneur'." -George W. Bush

[Eeyore]

Chris Hornbeck wrote:

On Mon, 25 Jun 2007 23:50:26 GMT, Eeyore
wrote:

Maybe not too unique though. Questions about audio noise
in laptop computers posted to rec.audio.pro often seem
lately to end with blame placed on the SMPS itself.

Almost certainly due to the Y cap that's there do deal with regulatory
requirements for EMI emissions that creates a leakage current to ground.

That's certainly a good thing to look into first.
ISTR though that some folks had similar issues with
SMPS supplies having only two-wire Edison connections,
and while "free-standing", ei. unconnected to anything
else Edison, leaving the statutory minimums.

The Y cap causes this problem regardless of whether the SMPS is connected to the
mains via 2 or 3 wires.

It's actually a connection from an internal live point to the secondary side.
And yes, it's perfectly legal when a safety (Y) capacitor is used.

Graham

[Chris Hornbeck]

On Tue, 26 Jun 2007 00:18:35 GMT, Eeyore
wrote:

The Y cap causes this problem regardless of whether the SMPS is connected to the
mains via 2 or 3 wires.

It's actually a connection from an internal live point to the secondary side.
And yes, it's perfectly legal when a safety (Y) capacitor is used.

Ak, OK. Now I see your thrust better.

Any shunt filtering will induce *some* noise into
system ground, and, no primary-to-secondary isolation
is perfect. Very good points.

Thanks, as always,

Chris Hornbeck
"The French don't have a word for 'entrepreneur'." -George W. Bush

[Patrick Turner]

robert casey wrote:


DC is DC.

Smoke is smoke, right?

When you have designed, built, and tested a SMPS for tube amp DC supply,
let us all know, but so far not a living soul uses SMPS for tube amps.


Designing and building a SMPS is significantly more difficult than an
old fashioned linear supply. And most DIYers would rather spend their
time and effort on the actual audio amp circuits anyway. Sure, a good
SMPS would be smaller and such, but the old fashioned 50 or 60Hz power
transformer, rectifier, choke and filter cap topology is well understood
and easily thrown together. Power supplies are rather boring anyway.

I think there are vendors that sell SMPSs suitable for tube work. But
if it fails, it would be a bit of a PITA to repair it.

Last time I did a google search there was nobody is selling a SMPS
capable of +500V at 1 A.

There are a few transmitters being sold with SMPS using tubes though,
Eimac?

Patrick Turner.

[robert casey]

Patrick Turner wrote:




Last time I did a google search there was nobody is selling a SMPS
capable of +500V at 1 A.


Nothing that powerful. What I do remember were supplies that would do
about 200V @ 100ma or such. Not much power...

[Ernst]

On Jun 25, 5:45 pm, flipper wrote:
I'm just suggesting that the OP might need to put a bit more thought
into it than simply 'slapping something together'.


I don't recall saying anything about "slapping something together"
Your words, not mine. If I were going to just take a shade-tree shot
at it, I would not have asked the question.

Ernst

[Ernst]

On Jun 23, 4:45 pm, robert casey wrote:
Ernst wrote:
Hello All,

Greetings and well wishes. A few months back I asked the group for
advise regarding construction of a high power tube amp, with good
results I might add.

I'm down to the power supply. Here's the thing: DC is DC right?
So if I cobble up a switching supply to provide B+, bias, and
filament, then there should be no problem.

200KHZ ripple in my view should be no problem to filter. Only thing
is that I plan to section each voltage to require little or no
regulation, i.e., I would not place the B+ line in a 350-600V
regulator; conversely, I would apply design center 450VDC up front,
skip the regulator, and apply the filter network.

Does this make sense?

If you regulate the bias supply (this I think would be the most critical
of voltages in the entire amp) the rest of the supplies should track
reasonably well enough.

Are you using output tubes with indirectly heated cathodes, or directly
heated filaments? If the latter, you might be able to use the high
frequency switching frequency AC of appropriate voltage directly on the
filament. But some filament "battery radio" tubes were designed to
expect one end of their filaments to see the positive side of the
filament supply, but I don't think they did that with power tubes
usually used in audio amps.- Hide quoted text -

Good points. I'm not so sure about introducing RF into the
filaments. I think the safe way to go is to use DC.

[Ernst]

On Jun 25, 9:37 am, "Ian Iveson"
wrote:
Maybe, depending on what species of SMPS you are "cobbling", and
whether your amp is class A or AB. But it's not a trivial pursuit.

Care to share your SMPS circuit? A brief description in words would
help if you can't post a diagram.


I wish I hadn't used that word.

Anyway, some of the examples I have seen are he
http://noel.feld.cvut.cz/hw/philips/acrobat/8140.pdf The basis is an
old SG3524 switching regulator chip. Seems like the +/- 50V example
would be a good starting place, then down to transformer windings and
filter networks to suit. More power? Parallel devices (switched
MOSFETS). There is another application note (AN120) from Philips
floating around that goes more into depth on theory.

Ernst

[Ernst]

On Jun 26, 1:23 am, flipper wrote:
On Tue, 26 Jun 2007 04:25:06 GMT, robert casey
wrote:

Patrick Turner wrote:

Last time I did a google search there was nobody is selling a SMPS
capable of +500V at 1 A.

Nothing that powerful. What I do remember were supplies that would do
about 200V @ 100ma or such. Not much power...

Here's one with a bit more oomph that you can supposedly buy a PCB
and/or kits of.

http://www.audiohobbyist.com/projects/smps.htm

http://www.audiohobbyist.com/diypart...kit/carpsu.htm

Schematic

http://www.audiohobbyist.com/diypart...EW-SMPSmod.pdf

Thanks.

[Ernst]

On Jun 26, 8:48 pm, flipper wrote:
Sorry, Just my version of "cobble."

No prob. I neglected to mention that Amidon Associates http://www.amidoncorp.com/
provides plenty of toroid transformer material and info to boot. They
primarily serve Amateur Radio, but are a good resource.

Ernst

[Sergey Kubushin]

Ernst wrote:
On Jun 26, 8:48 pm, flipper wrote:
Sorry, Just my version of "cobble."

No prob. I neglected to mention that Amidon Associates http://www.amidoncorp.co
m/
provides plenty of toroid transformer material and info to boot. They
primarily serve Amateur Radio, but are a good resource.

Thanks for a link. They look really good.

---
************************************************** ****************
* KSI@home KOI8 Net The impossible we do immediately. *
* Las Vegas NV, USA Miracles require 24-hour notice. *
************************************************** ****************

--
Posted via a free Usenet account from http://www.teranews.com

[Arny Krueger]

"flipper" wrote in message

On Mon, 25 Jun 2007 02:47:59 GMT, Eeyore
wrote:



flipper wrote:

For example, a notebook of mine has significant SMPS
noise on the audio.

I doubt very much it's the SMPS at fault.

Integrated sound is usually crap because of all the
noise it picks up from the computer generally
(especaially video it seems)

I am aware of that source but with the notebook I can
remove the SMPS brick so I'm pretty sure that's where it
comes from.

Maybe. The SMPS might stimulate a certain audible problem with noise, but
the exposure to that kind of noise pickup would be an inherent design fault
of the computer's innards.

You can usually tell computer and video induced noise
because it'll vary in sync with activity.

Agreed.