Keysight 34461A Fan Noise

[Ahrenp]

Hi,

Long timer lurker, first time poster. I just got a Keysight 34461A and there seems to be quite a lot of fan noise, contrary to the reviews and posts I've seen about it. Along with what I would call typical 'fan noise/white noise' there also seems to be a louder low frequency hum/buzz. I'm just curious if this is typical for this meter? It's difficult to describe and when I tried to record the sound the hum didn't really pick up. It is somewhat annoying and louder than my nearby PC and Rigol DS1054z.

[TheSteve]

Hmm, that is odd - I have had a DS1054Z and it is much louder then my 34461A

[orbiter]

I don't believe its anything to worry about, I have the 34465A and it's fan is definitely not so quiet, it also has a rather audible whine to it as well.

[DaJMasta]

Fans can actually differ a bit in noise even of the same type, and it takes a very small amount of gunk to change it further.  If the manufacturer isn't making big claims about it being quiet, then there may not be a lot of checking done to be sure of it.

Fans do get noisier if the airflow is obstructed though, so if you have a bird's nest of cables in front of it, that could be contributing.  As for picking it up on a mic, make sure the gain is up and hold it close to the fan, if you can hear it at a distance, odds are good even a cheap mic can pick it up alright.

Don't have a 34461, but do have one on the way, maybe I can report back this week to see if mine is the same (hoping for quiet!  )

[Performa01]

Don't have a 34461, but do have one on the way, maybe I can report back this week to see if mine is the same (hoping for quiet!  )

I guess most of us hope for quiet instruments, rather than getting mini vacuum cleaners.
Unfortunately it appears most T&M manufacturers don't care about our hopes (cheap tiny fans, non-optimized airflow, no temperature control) ...

[slurry]

It should absolutely not be louder than a 1054Z.
I also have a 34461A and it's quiet, of course i can hear it but no more than a laptop.

[Ahrenp]

As for picking it up on a mic, make sure the gain is up and hold it close to the fan, if you can hear it at a distance, odds are good even a cheap mic can pick it up alright.

I was able to pick up the general fan noise, from across the room at that, but not the "hum" portion. Unfortunately I don't have a proper mic and was just recording the audio with my phone.

It should absolutely not be louder than a 1054Z.

If it were the same tone as the Rigol and I had them both running at the same time I probably would not notice the 34461A, but it's just different enough that I can still hear it buzzing away.

I realize they don't make any claims about it being quiet, and if it's normal I can deal with it, because otherwise it's an awesome meter. I would be lying if I said I wasn't a little bit disappointed though.

[billfernandez]

in mine the fan is quiet, but high enough pitched to be annoying.  No low frequency hum or rumble.

[BVH]

My now sold 34461A and my new 34465A have the same higher pitched annoying fan noise.  It's not the loudness but the irritating pitch.  Has no one tried to locate a generic, quieter replacement?

[Messtechniker]

I have the 34465A and it's fan is definitely not so quiet, it also has a rather audible whine to it as well.

Can confirm this for my 34465A. But I am known to be very
sensitive to any kind noise as I am running a small voice recording studio.
My 34465A lives is a separate lab room (house in very quiet rural
surroundings) and I always switch it off when I not need its capabilities
 - i.e. when my Voltcraft VC 940 will do instead. As to the fan in my
Hameg HMC8043 lab power supply, the Hameg people definitely did
a better job - rubber suspension and all so that I can leave it on all
the time (at low to medium loads that is). Anyway. Replacing the fan#
and improving its suspension will inevitably require a recalibration,
as the airflow is an important part of the equation.

[LaurentR]

I have had 3 of the 3446XA and they all had the same, not particularly loud but certainly annoying high-pitch whine.

Between the fact that it's tolerable and the difficulty to predict the effects of a reduces airflow on the calibration, this is one of the few pieces of test equipment I haven't refanned.
Unfortunately, this means that mine are off most of the time because I don't tend to leave them running when I don't really need them because of the whine.

I wish Keysight had picked a less whiny fan. The airflow is not all that high so there should be plenty of quieter fans with similar airflow.

[Ahrenp]

My now sold 34461A and my new 34465A have the same higher pitched annoying fan noise.  It's not the loudness but the irritating pitch.  Has no one tried to locate a generic, quieter replacement?

I ended up putting one of these in mine:
https://www.amazon.ca/gp/product/B00NEMGCIA/

[Messtechniker]

I ended up putting one of these in mine:
https://www.amazon.ca/gp/product/B00NEMGCIA/

What did this do to the calibration?
Measurements with a voltage/resistance reference
before and after the change please.

[Ahrenp]

I ended up putting one of these in mine:
https://www.amazon.ca/gp/product/B00NEMGCIA/

What did this do to the calibration?
Measurements with a voltage/resistance reference
before and after the change please.

I don't have the numbers to post here; I made the change a while ago, so it's up to you if you take my word for it or not, but voltage measurements were unchanged. Make this change at your own risk. Technically it probably should be re-calibrated, but I did not have an issue.

[Messtechniker]

Is anyone of the voltnuts around here prepared to try exactly
this fan with measurements before and after? 
Even so, I would very much prefer an endorsement of exactly this
fan by Keysight themselves. Probably never going to happen.
I myself do not have any voltage or resistance references precise
enough for checking this DMM.

[BVH]

I just installed the Noctua NF-A4x-10, 5 Volt, 40mm quiet fan in my 34465A.  Using a 5 Volt reference, I got the following VDC readings before and after:  Before 5.00062-5.00063.  After 5.00061-5.00062.  The very irritating fan "tone" or "pitch" is completely gone and the air movement noise is about the same.  I can now keep my meter on between 1-3 hour uses.  Definitely worth the change.

[Messtechniker]

Now that's some news! Thank you.
A Noctua NF-A4x-10 for my 34465A has been on
my shopping list for long time now. Since I can not check the
calibration after changing the fan, I was quite hesitant.
In view of your results, I will now go ahead.

[Gandalf_Sr]

I just installed the Noctua NF-A4x-10, 5 Volt, 40mm quiet fan in my 34465A.  Using a 5 Volt reference, I got the following VDC readings before and after:  Before 5.00062-5.00063.  After 5.00061-5.00062.  The very irritating fan "tone" or "pitch" is completely gone and the air movement noise is about the same.  I can now keep my meter on between 1-3 hour uses.  Definitely worth the change.

I have a 34461A that's:
a) Slightly noisier that I'd like
b) Recently calibrated
c) Well out of warranty

I might consider the fan replacement too.  I can get it from Amazon here https://www.amazon.com/Noctua-Cooling-Bearing-NF-A4X10-FLX-5V/dp/B00NEMGCIA

Is that the right one and do you have any other suggestions for vendors?

[iMo]

Sometimes it is enough to wire a 22-68ohm resistor in series with the fan's Vcc such the high pitch disappears (lower rpm).

[Messtechniker]

Well, its not any high pitch which is annoying.
Rather, its the howling sound the stock fan makes.

[iMo]

Cut the fan's Vcc wire, insert a 100-220ohm pot trimmer and adjust such it runs without the disturbing sound.. Replace with a 1W resistor afterwards. It could be the fan's rpm/air_volume is not linear and the fan is running too fast without good reason..
PS: replacing the original fan with a different XYZ one could have the same air pressure result as lowering the rpm (with help of the resistor) with the original one..

[Gandalf_Sr]

Does the 34461A need a 5V or 12V fan?  The Noctua NF-A4x10 5V is a 5V fan but you guys are talking about cutting "the 12V wire".

[iMo]

I fixed "12V" above, no idea what fan's Vcc is in 34461. In my 34401 the fan's Vcc is 0V. Decades back my first job with building a new PC was cutting the fan's wires with all fans in the box
Ideally do set the rpm with the trimmer (at the highest Tamb expected) such the measured external ref voltage will not change while the fan's noise is minimal. A nice Sunday's afternoon exercise

[HighVoltage]

Almost all my HP / Agilent / Keysight gear with a fan have received a 100 Ohm (1W) resistor in line with the power cable.

The fans still turn fast enough for good airflow and the annoying noise is gone.
And as a side effect, the fans will last many more years.

[iMo]

Almost all my HP / Agilent / Keysight gear with a fan have received a 100 Ohm (1W) resistor in line with the power cable.

.. with the fan's Vcc wire..

[BVH]

I had the 34461A before upgrading.  It used the same 5V fan with the identical disturbing "howl".  I bought mine from Amazon.  I reasoned "Why do all the wire cutting and resistor inserting when the higher quality Noctua fan with it's higher quality bearing system is a drop-in replacement."

[iMo]

I had the 34461A before upgrading.  It used the same 5V fan with the identical disturbing "howl".  I bought mine from Amazon.  I reasoned "Why do all the wire cutting and resistor inserting when the higher quality Noctua fan with it's higher quality bearing system is a drop-in replacement."

A quick 5cents fix?..

[EE-digger]

There's also a 0 cent fix.  I held a 1/2" thick pad of loose medical cotton on my 34465A fan exit and it silenced the noise.  A thin piece of open cell foam sheet will do the same thing.  Just be sure it's open cell and has no adhesive on it.  The material used to replace window ac filters is about right.

You're basically building a muffler.  Most fan noise is caused by the shearing of the air between the blades and housing, etc. 

[Messtechniker]

Just replaced the stock fan of my 34465A with a Noctua NF-A4x10 5V fan.
The howl is a bit less now, but still not good enough for me

The air flow provided by the Noctua is noticeably higher by the way.

1 k 0.05 % resistor tolerance measurement after a 1 hour
warm-up immediately after having made the change.

Before: -0.0021 % to -0.0020%
After: -0.0013 % to -0.0012%


Just for the record: list of parts and tools you will need for the replacement:

Parts:
1.) The Noctua NF-A4x10 5V fan, of course
2.) 2 M4x20 mm metal bolts (doubtful if plastic bolts will be better)
3.) 2 M4 metal nuts
4.) 2 M4 spring washers

Tools for disassembly and reassembly of your 3446x:
1.) T15 bit
2.) T20 bit
3.) Screwdriver for the 2 M4 bolts you will be using.

What I might be trying next (should Xmas be too boring):
Inserting a piece of cotton fabric between casing and fan.
Reducing fan speed a bit.
As a last resort: cut away the metal at the fan exit. Something I will definitely not try.

Edited: Minus sign for tolerance values

[Zucca]

The air flow provided by the Noctua is noticeably higher by the way.

Then the 100ohm 1 watt resistor could be a nice solution.

I would also measured the delta T (inside - ambient) before and after swapping the fan or installing the resistor.
I have a similar problem on my Agilent 66309D, which is lauder than a 1969 tractor compared to all others devices on my bench.

I swapped the old fan with a new high quality papst one:
https://www.tme.eu/de/details/612nn/dc-lufter-12v/ebm-papst/612-nn/

I took the old one out and I did an "outside device  test" comparing new and old.
I was so happy to hear almost nothing with the new papst and a stupid rattling noise in the old one.

Unfortunately, after installing the new one in the unit I was not even close to the noise of the outside test, I improved the situation but there was still too much noise.
I am now tempted to try to add some rubber padding where the fan is mounted, to me the fan rotation resonate with that big heat sink.

TLTR: changing the fan with a new high quality one is always good, but sometimes it does not solve the problem.

[Zucca]

I coudn't resist and I open up again my 66309D.

After a quick check I discover that the bottom part of fan was rattling against the PCB, so I put some absorbing material:



the noise is now decent.

Furthermore since the fan is mounted on a very long hollow heat sink, the acoustic is bad by design.
In other words that heat sink seems to work somehow like this:

[Messtechniker]

See graphs of the tolerance measurements on a 1 k resistor
using the stock fan and the Noctua replacement fan.

Findings:
1.) Due to the higher air flow of the Noctua, the 34465A stabilizes faster.
2.) A recalibration will be required.

Measurement conditions:
1.) 34465A off over night.
2.) 34465A switched on early in the morning.
3.) After starting up switched immediately to Ohms.
4.) Start logging immediately after having switched to Ohms.

Instead of these measurements I would rather have liked
to log the internal temperature of my 34465A. But in the manual
there is apparently no SCPI command for logging the internal temperature. 
Or is there a secret SCPI command for this?

If not, maybe this would be - if at all possible - an idea for the next firmware update.

Found the command SYST:TEMP?

Will repeat the measurements in the next few days.

[HighVoltage]

What software are you using for logging?

[Gandalf_Sr]

I get the argument that a fan change would justify the need for a recalibration but, having just had my 34461A recalibrated, I'm now not sure I want to change the fan.  If the 34461A has a temperature-controlled voltage reference (does it?) then, as long as the new fan is made to match the same air flow rate (perhaps using a resistor) then is a recalibration really needed?

Surely the calibration can't be that sensitive to the fan flow rate, it's more likely to be sensitive to the ambient temperature and, even then, Keysight would have to make a $1,200 meter have some sort of allowance for a 'normal' ambient temperature range e.g. 65 - 75 F?

[Messtechniker]

What software are you using for logging?

Home-grown logging based on "Profilab" from here:
https://www.electronic-software-shop.com/index.php?language=en

Been using it for years because I am not interested in programming as such,
but rather want to get measurement related things done easily and quickly. Since my (and everyone's at my sort of age) learning ability is degrading by and by over the years  , a program like Profilab
and others from Abacom for that matter, helps to ease the pain.

[Messtechniker]

If the 34461A has a temperature-controlled voltage reference (does it?) then, as long as the new fan is made to match the same air flow rate (perhaps using a resistor) then is a recalibration really needed?

Yes, I am thinking about a speed dropping resistor. Planning to make a quick and dirty air flow indicator based on  a suspended piece of thick paper to determine the resistor value which will cause the Noctua to produce the same air flow as the stock fan. But first I will be doing some internal temperature logging for both fans.

It would be nice to know how the temperature is measured, i.e. sensor type and location on the pcb. and within the circuitry. Pity no schematic is available as in the old days. Sigh. What I do know is that the temperature is measured in increments of 0.1 deg C. So a bit of averaging will be need to smooth things out a bit.

[iMo]

I made measurements on my 34401A (with the temperature sensor mounted inside).
The voltage reference is the same as yours (LM399). The TC of the meter at 10V range (Tdmm=38..43C) is aprox 5uV/C. Yours will be similar, imho, unless 34461A+ does a TC correction. I do TC correction externally (stm32).
The internal temperature Tdmm is always aprox Tdelta=17.5C higher than ambient Tamb (no fan).
With the fan the delta will be lower, but still Tdmm=Tamb+Tdelta.

[nctnico]

See graphs of the tolerance measurements on a 1 k resistor
using the stock fan and the Noctua replacement fan.

What kind of resistor? If this is a standard one then you are likely seeing the temperature dependance of the resistor. It makes no sense for the DMM to be sensitive to the amount of airflow because this is likely to vary anyway by the environment of the DMM, variations between fans, built up of dust and aging of the fan.

BTW I just ordered the Noctua fan with a low-noise-kit (adapter cables to reduce the speed of the fan).

[Messtechniker]

What kind of resistor? If this is a standard one then you are likely seeing the temperature dependance of the resistor. It makes no sense for the DMM to be sensitive to the amount of airflow because this is likely to vary anyway by the environment of the DMM, variations between fans, built up of dust and aging of the fan.

BTW I just ordered the Noctua fan with a low-noise-kit (adapter cables to reduce the speed of the fan).

Got me! It was an ordinary 1k 0.05% 35 year old metal film resistor. Probably with a TC of 50 ppm/K. It was simply a stopgap measure, because at that time I did not know how to measure the internal temperature of the DMM. Even so, the logging graphs above indicate that the higher airflow of the Noctua fan seems to stabilize the DMM readings more rapidly.

[Performa01]

It was an ordinary 1k 0.05% 35 year old metal film resistor. Probably with a TC of 50 ppm/K.

If a resistor is specified as 0.05% (500ppm) tolerance, it certainly has to have a better TC than 50ppm.

I just had a look at my old stock of precision components and most 0.05% resistors have a temperature coefficient of 4-5ppm; the worst one I could find was 10ppm.

Nevertheless the original objection is valid. You should not use Resistance measurements to verify the calibration of a DMM in the first place, even if you have a good resistor reference with TC<1ppm available. A DMM has its best accuracy and stability when measuring DCV. Furthermore you should use the base DCV range with direct connection (without internal attenuator), which is usually the 10/20VDC range. Quite obviously you need a stable (preferably 10V) reference voltage for this.

[nctnico]

A quick off-the-cuff calculation makes me think that the differences measured with the different fans are well within the 34461A's specification. The 34461A's spec also shows a few PPM of drift per degree Celsius of temperature change. So using a fan with more or less airflow is going to affect the 34461A's readings a little bit as well because the internal temperature will be different.

[Messtechniker]

See graphs of the 34465A internal temperature measurements
using the stock fan and the Noctua replacement fan.
Please note the different logging periods.

Measurement conditions:
1.) 34465A off over night.
2.) 34465A switched on early in the morning.
3.) After starting up, started logging immediately and left the lab.
4.) Did not enter the lab while logging.

Conclusion: not much to be seen here as to the different fans influencing the internal temperature. I'll leave the stock fan installed since there is not much to be gained by replacing it with the Noctua fan.
Cutting noise levels down to signifiant levels will here require a redesign of the air flow outlet, preferably with the assistance of an air flow expert (from the car or aircraft industry). Anyway some insights were gained.

[Messtechniker]

Here now the logs

[radhaz]

You could always get one of these, it has no fan.
https://www.eevblog.com/forum/testgear/iwatsu-voac-7602/

[rernexy]

Yesterday my 34461a started producing an extra sound, like a pulsing at high frequency. That sound can be heard in the first attached mp3 (isolated it using a high pass filter on the original audio).

Has anyone ever heard that in their 34461a, or in a related Agilent/Keysight bench DMM?

The sound it usually made is just the high pitched whine from the fan. That can be heard in the 2nd mp3 (isolated it using a low pass filter on the original audio).

The 3rd mp3 is the original unfiltered audio of the noise my 34461a now makes.
The jpg is a spectrogram of the audio, starting from powerup.

[rernexy]

Welp, it's definitely the fan: https://drive.google.com/file/d/1-zcULG_mnfcH52MxQbIPaJsYwtBfZU2V
Is the new sound just a sign that the fan is broken in now? 

[rernexy]

Dismantled the fan. Surprised it uses 2 bearings.

[rernexy]

Replaced the fan with a Multicomp MC002684, which is actually a Sunon MF40100V2-1000U-A99.
Spectrogram of the new sound is attached.

[NoisyBoy]

Good to see you got it repaired, it sounds like my 34461A with the new fan.

I know there has been some complaint about the fan noise on the Truevolt DMMs, but both of mine have been very quiet.  They are perhaps my quietest fanned equipment in my lab.

[tangram]

My 34465A was making one hell of a racket.  It's just over three years old now and never missed a beat, but it was howling like a husky in heat. 

Bought a Noctua NF-A4x10 5V, Premium Quiet Fan, 5V Version (40x10mm, Brown) from amazon and installed it.  It didn't come with the molex two-pin connector that the board uses, so just chopped the wires off the old fan and spliced them together. 

The one I bought from Amazon was the 3 pin version.  I'm not even sure that there is a two pin version of this fan, but just chopped the third (yellow) wire short and capped it with a little heat-shrink before soldering and heat shrinking the +ve and -ve.

Works a treat, blows a good amount of air and is barely audible.  Meter needed a good hoover inside anyway. it was getting a bit dusty in there.

Happiness is restored 

[J-R]

I picked up a new 34461A and also found the fan noise annoying and wasn't sure about making any modifications.  But being new, I suspected there might be quite a bit of drift during its early life so decided to dig further.

The unit came with both the original brief Keysight calibration data plus the full 3rd party calibration data done 1 month later.  It was clear the Keysight calibration was an adjustment as all values were perfection, and the 3rd party calibration was a verification as many values were off (yet still within spec).  For example, 100 Ohms was 99.9970 Ohms (4-wire, other values were better).
I ran tests with all of my calibrated equipment and unsurprisingly found my old HP 34401A to be a bit better than the new 34461A.
So with that in mind I did not really have any reservations about replacing the fan.

I found the thicker NF-A4X20 5V has higher airflow but lower noise than the NF-A4X10 5V and picked that up.  (Do be aware Noctua has 4 versions each of the "NF-A4X20" & "NF-A4X10" and some sellers are sloppy with the specifications.)
The 20mm fan went in just fine and the noise was reduced but still slightly annoying, so I added a trimmer pot to adjust the fan speed lower (a speed controller might be better).
After the modifications, there is still quite a bit of airflow but the noise is nearly inaudible.

Ran through the tests again and could not locate any differences before/after.
So ultimately I would have no concerns with making this modification on a 34461A.

There is no need to cut any wires to install the fan.  Simply carefully pull the plastic connector base up through the pins on the PCB with some pliers and connect the new fan directly to the board.

[TheSteve]

34460A has no fan.

I think the 34461A only has a fan to meet the specs at elevated environmental temperatures.

My 34461A fan has always be quiet, it is unfortunate they are not all like that.

[J-R]

OK, edited.

I think whether the fan noise is annoying can also depend on where it is placed.  If it's in the middle of a stack you might not notice it.  In my case, it's somewhat more exposed and all of my other equipment is very quiet unless under load.

[TheSteve]

I have a bunch of Keysight gear in the same form factor, all of the other gear is quite a bit louder than the 34461A. Still noise less would be nice.