Fan-cooled small box questionable design ?.

[N Cook]

A commercial mixer amp in a tiny box. By gut-feeling I don't like the
relative absence of air cooling and would like some justification to
increase the , at least output vent size - there is space for much larger
holes, but not necessarily aesthetic.
2 switch mode power supplies plus 2 100W rms D class amps and an 8 channel
mixer in a box only 10x6x2.5 inches. The 2 SMPS are "cooled " by a 12V 40mm
fan blowing the internal air, not positioned near any inlets, over the
heatsinks.
Inlet vents , if all 1/4 inch sockets should have tight fitting plugs in all
of them, are 30 x 2mm diameter holes.
The other fan is 12V, 50mm fan that pulls air over the 2 amps and exhausts
directly to the outside but via 8 small slots .44 inch long and .08 inch
wide that seriously restricts the flow out. The owner reports the exhaust as
being hot rather than warm and i've taken some measurements with thermometer
etc with relatively low continuous sine to external dummy load. I intend
giving the owner a thermometer to take some readings in normal use over an
hour of guitar performance.
I fixed, with sealing tape, a large diameter tube around the exhaust area to
direct into a very thin plastic rubbish sack, with no perforations, to time
and measure the air flow until about 2/3 full and holding the floppy bag up
as well so I'm assuming relatively little back pressure or whatever the term
is.
I did not think, while inside , to note any amp rating as well as 12V, nor
make and model, but generally 50 mm fans would be about 8 to 18 cfm cubic
feet per minute.
But from my calculation over 3 minutes then the flow rate was only 0.78 cfm.
This amp is designed so it can be transported and used , recessed into one
of the speakers, so not much heat lost through the walls of the box I would
have thought, I'm advising, interim, for the owner to use the amp removed
from the speaker.
Any comments on the adequacy of venting?

 

[Eeyore]

A commercial mixer amp in a tiny box. By gut-feeling I don't like the
relative absence of air cooling and would like some justification to
increase the , at least output vent size - there is space for much larger
holes, but not necessarily aesthetic.
2 switch mode power supplies plus 2 100W rms D class amps and an 8 channel
mixer in a box only 10x6x2.5 inches.

Name names !

There's plenty of bad design around especially if it's from Asia. Proper thermal
management is a discipline that's rarely taught to electronics engineers. I had
to learn it by researching for myself.

Graham

 

[N Cook]

A commercial mixer amp in a tiny box. By gut-feeling I don't like the
relative absence of air cooling and would like some justification to
increase the , at least output vent size - there is space for much larger
holes, but not necessarily aesthetic.
2 switch mode power supplies plus 2 100W rms D class amps and an 8 channel
mixer in a box only 10x6x2.5 inches. The 2 SMPS are "cooled " by a 12V 40mm
fan blowing the internal air, not positioned near any inlets, over the
heatsinks.
Inlet vents , if all 1/4 inch sockets should have tight fitting plugs in all
of them, are 30 x 2mm diameter holes.
The other fan is 12V, 50mm fan that pulls air over the 2 amps and exhausts
directly to the outside but via 8 small slots .44 inch long and .08 inch
wide that seriously restricts the flow out. The owner reports the exhaust as
being hot rather than warm and i've taken some measurements with thermometer
etc with relatively low continuous sine to external dummy load. I intend
giving the owner a thermometer to take some readings in normal use over an
hour of guitar performance.
I fixed, with sealing tape, a large diameter tube around the exhaust area to
direct into a very thin plastic rubbish sack, with no perforations, to time
and measure the air flow until about 2/3 full and holding the floppy bag up
as well so I'm assuming relatively little back pressure or whatever the term
is.
I did not think, while inside , to note any amp rating as well as 12V, nor
make and model, but generally 50 mm fans would be about 8 to 18 cfm cubic
feet per minute.
But from my calculation over 3 minutes then the flow rate was only 0.78 cfm.
This amp is designed so it can be transported and used , recessed into one
of the speakers, so not much heat lost through the walls of the box I would
have thought, I'm advising, interim, for the owner to use the amp removed
from the speaker.
Any comments on the adequacy of venting?

an afterthought.
How much added strain is there on an axial fan by stopping down the free
flow rate by more than 90 percent , leading to premature failure even
without being in a hot airstream.
I'm thinking of kitchen extractor fans which are relatively small motors for
their power , relying on the passing airstream of human scale temperature
range passing over them.

 

[Meat Plow]

A commercial mixer amp in a tiny box. By gut-feeling I don't like the
relative absence of air cooling and would like some justification to
increase the , at least output vent size - there is space for much larger
holes, but not necessarily aesthetic.

Justify to who?

 

[N Cook]

Using vol of air per min = watts / (air density*specific heat*temp diff*60)
in metric terms
60 seconds in minute , taking air density = 1.205, Cp= 1.05 and conversion
factor of 35.3 for cu m /min to cf/m and measured outflow of 0.78 cf/m ,
however I plug these measured results in,scaling for higher demand, does not
look good for temp diff at higher output and fan duress apart from the
electronics which could be in circulation dead spots.

400Hz source sine fed into one amp only, delivering 12.5 watts continuously
into an external dummy load. After 40 minutes the exhaust temperature was
still increasing but very slowly, so cancelled then. So heat from 2 switch
mode power supplies and one idle amp with no load and the other one active
plus contributions from the voltage regulators and mixer circuitry etc.
Temperature from bulb of thermometer laying directly over the exhaust vent
rose to 42 degrees C from a room temp of 24 degrees C.

 

[James Sweet]

A commercial mixer amp in a tiny box. By gut-feeling I don't like the
relative absence of air cooling and would like some justification to
increase the , at least output vent size - there is space for much larger
holes, but not necessarily aesthetic.
2 switch mode power supplies plus 2 100W rms D class amps and an 8 channel
mixer in a box only 10x6x2.5 inches. The 2 SMPS are "cooled " by a 12V
40mm
fan blowing the internal air, not positioned near any inlets, over the
heatsinks.
Inlet vents , if all 1/4 inch sockets should have tight fitting plugs in
all
of them, are 30 x 2mm diameter holes.
The other fan is 12V, 50mm fan that pulls air over the 2 amps and exhausts
directly to the outside but via 8 small slots .44 inch long and .08 inch
wide that seriously restricts the flow out. The owner reports the exhaust
as
being hot rather than warm and i've taken some measurements with
thermometer
etc with relatively low continuous sine to external dummy load. I intend
giving the owner a thermometer to take some readings in normal use over an
hour of guitar performance.
I fixed, with sealing tape, a large diameter tube around the exhaust area
to
direct into a very thin plastic rubbish sack, with no perforations, to
time
and measure the air flow until about 2/3 full and holding the floppy bag
up
as well so I'm assuming relatively little back pressure or whatever the
term
is.
I did not think, while inside , to note any amp rating as well as 12V, nor
make and model, but generally 50 mm fans would be about 8 to 18 cfm cubic
feet per minute.
But from my calculation over 3 minutes then the flow rate was only 0.78
cfm.
This amp is designed so it can be transported and used , recessed into one
of the speakers, so not much heat lost through the walls of the box I
would
have thought, I'm advising, interim, for the owner to use the amp removed
from the speaker.
Any comments on the adequacy of venting?

I avoid the use of fans, they draw a lot of crud into the device,
particularly problematic if the mixer is used in a venue where smoking is
allowed. If the thing is not failing in short order, the heat is probably
not an issue.

 

[Eeyore]

Using vol of air per min = watts / (air density*specific heat*temp diff*60)
in metric terms
60 seconds in minute , taking air density = 1.205, Cp= 1.05 and conversion
factor of 35.3 for cu m /min to cf/m and measured outflow of 0.78 cf/m ,

You get what ?

You won't get anything like a fan's rated cfm figure when the airflow's
restricted btw.

So, is it overheating ? Put a thermocouple on the heatsink and see what temp it
is.

Graham

 

[N Cook]

I avoid the use of fans, they draw a lot of crud into the device,
particularly problematic if the mixer is used in a venue where smoking is
allowed. If the thing is not failing in short order, the heat is probably
not an issue.

The other figure is 90W consumption printed on the box at the mains inlet,
presumably 90W plus 2x100W of audio, ie 90 Watts having to be vented from
the box at full audio output.
Plugging 90W in the formula and with 0.78 cf/m then temp diff would be 54
above ambient or 78 degree C with presumably temperature hot spots above
that inside.

 

[Eeyore]

The other figure is 90W consumption printed on the box at the mains inlet,
presumably 90W plus 2x100W of audio, ie 90 Watts having to be vented from
the box at full audio output.

No, the figure on the 'plate' is meant to be the actual power drawn.

Have you been able to measure 2 x 100W continuously ? It may have circuitry to
stop you doing that, I've seen that trick before.

And what is this unit ?

Graham

 

[martin griffith]

You get what ?

You won't get anything like a fan's rated cfm figure when the airflow's
restricted btw.

So, is it overheating ? Put a thermocouple on the heatsink and see what temp it
is.

Graham

Do you know or recommend any app notes for a small class D amp, say
50W, running off a non automotive 12V lead acid battery?


martin

 

[Eeyore]

Do you know or recommend any app notes for a small class D amp, say
50W, running off a non automotive 12V lead acid battery?

12V won't do 50W with direct coupling.

I'd be inclined to use a SMPS to provide suitable +- supplies to do the job then use
any of the many claas D designs thar are kicking around.

Graham

 

[Rich Grise]

.

The other figure is 90W consumption printed on the box at the mains inlet,
presumably 90W plus 2x100W of audio, ie 90 Watts having to be vented from
the box at full audio output.
Plugging 90W in the formula and with 0.78 cf/m then temp diff would be 54
above ambient or 78 degree C with presumably temperature hot spots above
that inside.

Well, is the unit failing? It sounds from your description like it was
designed to run hot.

If it works, don't fix it.

Cheers!
Rich

 

[BH]

On Sat, 23 Jun 2007 18:59:35 +0100, N Cook wrote:

...


Well, is the unit failing? It sounds from your description like it was
designed to run hot.

If it works, don't fix it.

Cheers!
Rich

N Cook:

Why not tell us make & model. There is always a chance that someone
out there has experience with the unit.

Bruce

 

[martin griffith]

12V won't do 50W with direct coupling.

I'd be inclined to use a SMPS to provide suitable +- supplies to do the job then use
any of the many claas D designs thar are kicking around.

Graham

Thats what I thought, I'm using the Phirrips/NXPee BTL stuff at the
mo, with the big Boost caps and they can get a bit warm, and haven't
played with D amps. Any recommendation on TI devices etc. or what to
avoid?
As they all claim to be near perfect!


martin

 

[Eeyore]

Thats what I thought, I'm using the Phirrips/NXPee BTL stuff at the
mo, with the big Boost caps

What are these big boost caps ?

and they can get a bit warm, and haven't played with D amps. Any recommendation on TI
devices etc. or what to avoid?

Honestly I haven't any experience of them.

As they all claim to be near perfect!

But of course !

Graham

 

[James Sweet]

Do you know or recommend any app notes for a small class D amp, say
50W, running off a non automotive 12V lead acid battery?


martin

I think National Semiconductor has some, take a look on their site.

 

[Don Lancaster]

A commercial mixer amp in a tiny box. By gut-feeling I don't like the
relative absence of air cooling and would like some justification to
increase the , at least output vent size - there is space for much larger
holes, but not necessarily aesthetic.
2 switch mode power supplies plus 2 100W rms D class amps and an 8 channel
mixer in a box only 10x6x2.5 inches. The 2 SMPS are "cooled " by a 12V 40mm
fan blowing the internal air, not positioned near any inlets, over the
heatsinks.
Inlet vents , if all 1/4 inch sockets should have tight fitting plugs in all
of them, are 30 x 2mm diameter holes.
The other fan is 12V, 50mm fan that pulls air over the 2 amps and exhausts
directly to the outside but via 8 small slots .44 inch long and .08 inch
wide that seriously restricts the flow out. The owner reports the exhaust as
being hot rather than warm and i've taken some measurements with thermometer
etc with relatively low continuous sine to external dummy load. I intend
giving the owner a thermometer to take some readings in normal use over an
hour of guitar performance.
I fixed, with sealing tape, a large diameter tube around the exhaust area to
direct into a very thin plastic rubbish sack, with no perforations, to time
and measure the air flow until about 2/3 full and holding the floppy bag up
as well so I'm assuming relatively little back pressure or whatever the term
is.
I did not think, while inside , to note any amp rating as well as 12V, nor
make and model, but generally 50 mm fans would be about 8 to 18 cfm cubic
feet per minute.
But from my calculation over 3 minutes then the flow rate was only 0.78 cfm.
This amp is designed so it can be transported and used , recessed into one
of the speakers, so not much heat lost through the walls of the box I would
have thought, I'm advising, interim, for the owner to use the amp removed
from the speaker.
Any comments on the adequacy of venting?

Not on this particular design.

But I conclusively proved that the vent holes on the LinkSys cable modem
are WAAAAY too small. Creating all sorts of long term reliability problems.

And truly bizarre symptoms.

And easily compounded if you stack a router on top of it.
Cisco denies the problem.

The only way I've found these to remain reliable is to lean them at a 45
degree angle with free flow above and below.



--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[martin griffith]

What are these big boost caps ?

Class H where they charge a couple of4700u caps to Vcc, and *add*
them to the supply lines, to handle peaks, like TDA1560

Honestly I haven't any experience of them.



But of course !

Graham

martin

 

[N Cook]

Well, is the unit failing? It sounds from your description like it was
designed to run hot.

If it works, don't fix it.

Cheers!
Rich

Its a Yamaha Stagepas 300 , made 2005
someone's pic, no scaling on any pics I found, but only 10x6x2.5 inch box
http://www.keyboardplayer.com/images/home/0305_stagepas_300.gif

Mosfets knocked out maybe by recovered punch-through in softened,
suspiciously small size 470uF 85 degree C output filter cap.
Interestingly the pic i've referred to above would suggest an enlarged grill
area in the bottom left corner. The bit that looks like a bar code is
probably and extended grill that is not on the one I am dealing with, the
grill is only from the chrome handle to the extreme left corner.
I will probably bend up the corner of the cover where the outlet vent is,
propped up on a 1/4 inch stand-off, and add a bit of grill. So increase the
airflow by double, decrease the temperature by half and reduce the strain on
the main fan.
If it was my amp I would probably cut a hole in the case where the internal
fan is to ameliorate matters as well.

 

[Eeyore]

Class H where they charge a couple of4700u caps to Vcc, and *add*
them to the supply lines, to handle peaks, like TDA1560

Do they indeed ? I wondered. It must have a slightly adverse effect on the audio output I'd
have thought.

Graham