Hum noise on my swr sm400s - diyAudio
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Old 9th July 2008, 12:20 PM   #1
davbass is offline davbass  Italy
diyAudio Member
Join Date: Jul 2008
Default Hum noise on my swr sm400s


I've replaced the 12ax7 tube on my sm400s, but it produces
a little 50Hz hum noise in hi gain condition on preamp section (some volume level then 'shhhhh' noise).. this hum increase when i boost the bass on eq section.

another costant hum noise (test condition: input of power amp section shorted and master = zero) come from the two power amps modules

a mechanical vibration (50Hz) also compare on all chassis.

can anyone help me to eliminate hum in preamp section and in power amps sections?

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Old 9th July 2008, 04:18 PM   #2
diyAudio Member
Join Date: Jun 2008
I have an sm900 that I have to work on some and noise is an issue. But for me I can hear the fan thru the amplifier AND i can hear the posts something(s) are picking up noise they shouldn't.

The tube could be microphonic or I, we, could have a couple bad opamps, or even the power supply could use some updating.

Theres a guy named dave around here who is a real good swr repair guy, a guru pretty much.

hoping he chimes in when he can.
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Old 10th July 2008, 12:47 PM   #3
davbass is offline davbass  Italy
diyAudio Member
Join Date: Jul 2008
tnk mike

the hum is a problem for me in recording session..

so, i tryed:

- disassebly the front panel from the rest of chassis, the hum in peamp section (on post eq DI output ) disappairs. The signal is totally hum free

it doesn't improve:

- I changed the fun AC power supply in DC whit a diode 1n4007 and a 4700uF cap (sm500's mod)
- I rewired with shelded cables the signal patchs in 12ax7 section
- I repalced all op amps
- I tryed, also, to detach and rotate of 90 degree the power supply trafo (3/4 cm 'on air' from chassis)

I have noticed that one of two power amp module have a thread connected to the AC sec of main trasformer (into the AC/DC bridge), so i think this is the reason of the hum in this sections..

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