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G8MNY > TECH 22.11.24 10:31z 62 Lines 2228 Bytes #151 (0) @ WW
BID : 20399_GB7CIP
Read: GUEST
Subj: Variable Speed Fan for AF PA
Path: SR1BSZ<EA2RCF<LU9DCE<LU4ECL<ED1ZAC<GB7CIP
Sent: 241122/1028Z @:GB7CIP.#32.GBR.EURO #:20399 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Apr 23)
(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 (4R/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 and run the fan, I used a 2W 390R and 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 as 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 and 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" & Variable Speed Thermal Fan"
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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