So I'm trying to figure out this low hum issue. As far as I can tell it's a power issue, seems to be at about 120 hz. The only way I know that is the frequency is that the crossover filters it out at about 120 hz, I have NOT put a oscope on it, only measured it with an app on my iPhone through the microphone and the only frequency that shows up on my multimeter. This is also the output of the rectifiers, double 60 hz.
Bit's of information I have, are that the crossover filters it out, so it must be before that in the circuit. It is there with LFE on or off, and phase switch makes it worse at 0 degrees and less at 180 (it's still there though).
So it seems to be coming from the +/-15VDC PS. When I measure the frequency or voltage with my multimeter on a lot of the +15VDC PS caps the hum goes completely away. It goes away whether or not I have the negative lead attached to ground or not. When I move to the other caps in the +15 VDC PS it is also seriously dimished.
Basically the question is do I replace all the caps in the +15VDC PS or all the caps in both the + and - 15VDC PS? Does anyone have any experience with this that they can shed some light on?
Bit's of information I have, are that the crossover filters it out, so it must be before that in the circuit. It is there with LFE on or off, and phase switch makes it worse at 0 degrees and less at 180 (it's still there though).
So it seems to be coming from the +/-15VDC PS. When I measure the frequency or voltage with my multimeter on a lot of the +15VDC PS caps the hum goes completely away. It goes away whether or not I have the negative lead attached to ground or not. When I move to the other caps in the +15 VDC PS it is also seriously dimished.
Basically the question is do I replace all the caps in the +15VDC PS or all the caps in both the + and - 15VDC PS? Does anyone have any experience with this that they can shed some light on?
I have the exact same issue on my ES250P now... Can you tell me which Caps you replaced? Then I will have to order spare parts and get my unit fixed as well...
Regards
Brian
JBL ES250P hum
Hello @a1970gto, Brian and community members,
It looks like my sub developed the same issue out of the blue. The frequency of the hum in my case also seems to be centered at around 120Hz. I've gone thru the circuit on the service manual and can't figure out where to start.
My observations so far -
Phase____Input_____ Loudness
0 deg____ LFE______Loudest
0 deg____ Normal___ hum reduced
180 deg___Normal___same as above
180 deg___LFE______hum almost negligible
- lowering the cutoff of crossover reduces it too in every case.
- volume control, all the way low or all the way high, doesn't have any effect on loudness of hum. It means the hum is coming from circuit after volume control.
- disconnecting the pre board from amp makes the hum go away completely.
My hunch so far -
- considering the frequency of the hum centered on 120Hz, it could be a faulty capacitor somewhere on power supply
- since the power amp in itself doesn't have hum without the pre, I'm thinking its somewhere on the pre board.
- since volume doesn't have any effect on the loudness of the hum, it seems that it could be coming from U102, TL074 quad op amp.
I've attached circuit diagrams for the 2 stages of Pre and 2 stages of power supply as well.
PreAmp1
PreAmp2
PowerSupply1
PowerSupply2
Any help will be greatly appreciated.
Thank you.
Hello @a1970gto, Brian and community members,
It looks like my sub developed the same issue out of the blue. The frequency of the hum in my case also seems to be centered at around 120Hz. I've gone thru the circuit on the service manual and can't figure out where to start.
My observations so far -
Phase____Input_____ Loudness
0 deg____ LFE______Loudest
0 deg____ Normal___ hum reduced
180 deg___Normal___same as above
180 deg___LFE______hum almost negligible
- lowering the cutoff of crossover reduces it too in every case.
- volume control, all the way low or all the way high, doesn't have any effect on loudness of hum. It means the hum is coming from circuit after volume control.
- disconnecting the pre board from amp makes the hum go away completely.
My hunch so far -
- considering the frequency of the hum centered on 120Hz, it could be a faulty capacitor somewhere on power supply
- since the power amp in itself doesn't have hum without the pre, I'm thinking its somewhere on the pre board.
- since volume doesn't have any effect on the loudness of the hum, it seems that it could be coming from U102, TL074 quad op amp.
I've attached circuit diagrams for the 2 stages of Pre and 2 stages of power supply as well.
PreAmp1
PreAmp2
PowerSupply1
PowerSupply2
Any help will be greatly appreciated.
Thank you.
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