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Class D cooling fan turns on at odd times?

Discussion in 'Amps and Cabs [BG]' started by n1as, Dec 15, 2017.


  1. n1as

    n1as

    Mar 29, 2013
    A few years ago I had an Aguilar TH350. Great amp, but the fan operation was odd. The fan would come on after about 20 minutes even is the amp was being used at very low volume or even just idling.

    I would assume that the amp would get hotter the harder it is driven and at low volume it would not need the fan. Am I wrong?

    Do all class D power amps act like this? Do they build heat and need forced air cooling at all volume levels?
     
  2. lz4005

    lz4005

    Oct 22, 2013
    Apparently.
     
  3. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    There is a temperature sensor in some amps that controls the fan. It could turn the fan on and off or it can control the fan speed. It depends on the design. The temperature inside the amp will dictate when the fan turns on and how fast it runs. Some amps with fans do not have sensors, the fan is engaged when the amp is turned on.

    It is important to ensure that maintenance is performed on all amps. Those with sensors can have dust and gunk built up on the sensor and this can affect the fan’s operation. This can explain why a fan might start operating differently. So have your amp cleaned once in a while.

    Even when an amp is idling, it heats up. It takes some time for the amp to come up to temperature. This could explain why it takes some time for fan to start running.

    If you feel that the amp has an issue, it wouldn’t hurt to have a tech take a look at it.
     
  4. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    This does not occur on all class D amps, just those it does happen on. ;)
     
  5. VicDamone

    VicDamone

    Jun 25, 2000
    +1 beans-

    While both are considered solid state Class D, or switching amplifiers, by their very nature run cool and use much less energy compared to linear solid state amplifiers.

    Because of their 2ohm specification Class D Electric Bass amplifiers may be the only application where one will find the need for fan cooling. Since Class D designs vary as much as tube or linear solid state amplifiers Aguilar would provide the only definitive answer to your question.

    Class D technology is improving rapidly. In high fidelity and sound reinforcement circles the trend in developing dedicated designed and internally mounted amplifiers for each driver is finding a market. Some include digital sound processing DSP that monitors the acoustic environment and can automatically adjust the systems equalization to a desired compensation.

    Imagine a small full range powered cabinet that can actually clearly produce low B and not overload the room.
     
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Most class D PA amps have fans too.
     
    n1as likes this.
  7. n1as

    n1as

    Mar 29, 2013
    An A/B amp gets warmer the more current / power it is asked to produce, right? So they run cool at low volumes and get hot when pushed. Hence their fans stay off for bedroom practice then come on at the gig.

    Are class D the same? Do they get warmer when pushed and run cooler when not pushed?

    I'm still trying to figure out the fan in a brand new amp would need to turn on after sitting idle for 20 minutes. Maybe it was defective and I didn't know it.
     
  8. n1as

    n1as

    Mar 29, 2013
    You're always helpful agedhorse, but sometimes it his helpful with a little 'h'. :)
     
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    You are making generalizations that do not necessarily hold true in both theory as well as the real world.

    There are different modulation schemes that vary the rate of modulation based on output power, these are quite common.

    Cooling approaches are as varied at the manufacturers who develop them, each has their own idea when it comes to this.
     
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    This is because there are different ways of doing things. What you are seeing doesn't surprise me at all, I choose not to do it the way I do, but that's my approach which others may not understand or appreciate.
     
  11. n1as

    n1as

    Mar 29, 2013
    Thanks for the insights. If I may ask a very Mesa-specific question, Should I expect the cooling fan on a D800 or D800+ to come on while doing home practice with a CD? Playing at a medium home-practice level with a typical efficiency music instrument 12" cab?

    BTW, I ended up selling the TH350 and going with a Genz-Benz Shuttle 3.0 partly due to sound (preferred the GB) and partly because the Shuttle had no fan to make noise while I was practicing. The fan in the TH350 was annoying.
     
  12. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    With the Subway D-800 & D-800+, the fan runs at a slow speed all the time. This is how the cooling algorithm is designed to operate.

    The Shuttle 3.0, which I also designed, does not use a fan and is based on a somewhat different SMPS/Class D topology that is inherently higher efficiency, but only applies to lower power levels. Once you get beyond ~300 watts, different factors come into play (especially as power density, in watts/cubic inch, increase).

    Hope this helps.
     
  13. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    I won't claim to know the answer, but MAYBE, in part, it could have to do with weight savings. Big heat sink/less airflow = small heat sink/more air flow (although time is also a factor). I know my class D has the heat sink entirely in chassis, so it's going to want air flow right off the bat. I know at least some SS/not class D amps have huge heat sinks that must weigh as much as a class D head with the fins external to the chassis (essentially, it's the back panel). I was looking at an old used one the other day and the TO-3 (?) outputs were external and had plastic cover I guess so no one gets juiced by them. 4U, huge, heavy, 19" rack mount, heat sink 4U x 19" or so... packing whopping 100 Watts. I don't think it had a fan, not sure. Neat old amp, and a bit under a buck a watt. I think one could do better in price and weight per watt. WW2 era. Maybe WW1.
     
    mikewalker and agedhorse like this.
  14. Fat Freddy

    Fat Freddy Supporting Member

    Feb 23, 2016
    Albany NY
    :rolleyes::D
     
    S-Bigbottom likes this.

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