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G8MNY > TECH 04.09.14 09:19l 60 Lines 2189 Bytes #999 (0) @ WW
BID : 9480_GB7CIP
Read: GUEST
Subj: Variable Speed Fan for AF PA
Path: IW8PGT<IZ3LSV<I0OJJ<GB7CIP
Sent: 140904/0612Z @:GB7CIP.#32.GBR.EURO #:9480 [Caterham Surrey GBR] $:9480_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Nov 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I have often been annoyed by fan noise from powerful AF PA gear, or the
opposite, red hot gear with inadequate cooling.
Here is a solution I adopted for a 100W RMS (4ê/100V line) PA amp that had the
heatsinks inside a metal box, with little chance of proper ventilation.
SPLIT RAIL SYSTEM
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄo-oÄÄÂÄÄ +32V @ 4A
R1 ³ ³
³ 1N4004 ÚÄÄÁÄÄÄ¿ ³
ÃÄÄÄÄÄÂÄÄR2ÄÄ´<ÃÄÄÄÄÂÄij PA ³ ³
_³_ ³ ³ ³OUTPUT³ ³
/ + \ ³ ³ ³ STAGE³ ³+ Transformer
³ 12V ³ +³1000uF ³\__³ ÀÄÄÂÄÄÄÙ === & Bridge
\FAN/ === ³LS_ ³ ³ Rectifier
~³~ ³25V ³/ ³ ³ ³
ÀÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄ)ÄÄÄÄÄÄÂÁÄÄ 0V
³ ³+
³ ===
³ ³
ÀÄÄo-oÄÁÄÄ -32V @ 4A
The small fan was from a scrap PC PSU (mounted inside on the top panel spaced
off 3mm to reduce blade hole noise), to push air into the box, the only exit
was from holes near the heatsinks.
R1 is select to just start & run the fan, I used a 2W 390R & that gave about 9V
on the fan. R2 is selected to speed up the with typical program to give the fan
12-14V, as you like it. the value I used was 5R6 5W.
Although this approach is not a good as a thermal switch, it is simple & the
added DC load to speed up the fan a bit, on a 100W PA is insignificant.
BRIDGED or STEREO
In this case use two diodes into R2 from each phase/channel to share the load.
SINGLE RAIL SYSTEM
This should work as well, as long as the 1/2 rail point is not too high above
the fan voltage.
RESULT
The fan starts up & runs very quietly on 9V & gently runs faster from 50W peak
to very fast at 200W peak (clipping).
Of course it is possible to only power the fan from the LS voltages, & this is
one way to cool a Test Dummy Load!
See TECH bul "PA & PSU Fan Noise"
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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