6290 amp questions

[Jakob]

Hi!
I'm thinking of getting a 6290 amp to drive my 18" subs. But I'm not a fan "fan". I guess these amps were made to work in rather harsch conditions such as 4 amps stacked on each other in a small space delivering 300 watts continuos for hours, and in such a case, I understand why there are fans. But I will probably only use a third of their power potential and have it standing airy. With less heat produced maybe You could just disconnect the fan? The heat protection circuit should still work. Anyone tried this?
Of course I could get a non-fan 6260 amp but I think I need more headroom.
Any suggestions?

regards, and a MERRY CHRISTMAS!

[subwoof]

The 6290 is basicly a 6260 with a second power transformer and an extended chassis that houses the fan. Since the heatsink is somewhat enshrouded a fan was needed ( along with the extra transformer which creates more heat ).

If you replaced the fan with a super quiet "whisper" type, and added a 10watt 600 ohm resistor in series with it, you would be unable to hear the fan under any condition. If you used a temp switch ( mounted to the heatsink ) across the resistor the fan would go to high speed *only* when it got real hot. This is how the crown PSA2 / SA2 does it.

There are 3 versions of the 62XX series - I believe the last generation had this but the fan is an industrial noisy type.

Asking the last chance temp circuit to save you is like asking your cars emergency brake to power slide on a down hill mountain road. With that much heat ( speed ) built up you're asking for trouble.



sub

[Jakob]

Thank You sub. You seem to have a lot of experience with this. What would the resistor in series with the fan do? Lower the speed of it? Any suggestion on a better fan? What marks a whisper type fan?

Thanks!!

[Zilch]

Available from Digikey, quieter feathered edge blades, less air delivery. Series resistor runs them slower.

http://www.comairrotron.com/ac_family.asp?FamilyID=69 57 cfm, 42 dB

http://www.comairrotron.com/ac_family.asp?FamilyID=71 72 cfm, 35 dB


DC versions also available....

[subwoof]

Fan 101

To have the fan run slower but keep most of it's torque you need to lower the *frequency* of the AC source since it's an induction motor with a fixed number of poles.

Since this is damn near imposible to do without using an expensive amplifier / oscillator combination that would be as massive as the amp itself...adding the resistor is an industry accepted fudge.

Another method that is also used on large wall fans is to add a 5 to 30 uF 250V mylar capacitor in series. This has the advantage of no heat production but at higher cost. This creates a small phase shift and tricks the motor poles into playing "catch-up" with the AC line frequency.

Since this is a very small fan, experimenting with the cap size will be needed if you go this route.

Sound 101

Remember if you turn the system up REAL loud, the fan noise will be masked by falling debris, ruffling of drapes and the distant spousal scream.

Lubricating fluid is recommended.

[pelly3s]

while all this is going on about the fan I have a little question maybe someone can help me out with. i need to know if anyone knows where to get knobs for the 62xx amps. i got one for free and it didn't have any knobs.

[Zilch]

Call or eMail JBL Pro Parts.

"Customer Service" is on the JBL Pro website.

OR, wade through the 91-page parts list there. It's alphabetical by part description. Lots of interesting stuff in stock....

[Dave Zan]

I think this amp is class B with fixed power supply rails. One point to be aware of is that in such cases the maximum heat production is NOT at full power - contrary to most expectation.
The worst case is a 25% of full power square wave.
For sine waves waves it's about one third power.
For music probably more.
I expect that's still more than you would typically use - so you should be Ok -but it's trapped a few people in the past.
There was an amp power specification standard that demanded that the amp be prepared for the full power test with a one third power sine wave. No one had noticed the problem!