I have an Adcom 5500 power amp. When I hook it up to speakers and a pre-amp I get a buzz (sounds a little like an insect – not terribly loud, but audible from listening position, and irritating).
I've completed the following testing. Starting from square one (i.e. everything disconnected from everything):
Connect Adcom to speakers and AC power; power on: silence.
Connect Adcom to pre-amp with interconnects; power on: buzz. (Pre has NO other connections – it's not even plugged in to AC power.)
Disconnect either end of either interconnect: buzz disappears. Reconnect: buzz is back.
Remove interconnects completely; instead, insert shorting plugs into Adcom inputs: silence.
I've been told my ground resistance measurements – between channels and between each channel and chassis – are reasonable (R-gnd to L-gnd 0.2-0.3 ohms, R-gnd to chassis 3.8-3.9 ohms, L-gnd to chassis 3.7-3.8 ohms)
I have tried the following changes (tested one by one, all else being equal); none have had any effect on the buzz:
A kind advisor on fixya.com suggested my next step is adding pi filters to both channels to get rid of RFI. He suggested I insert a 5K resistor into the signal path just inside the chassis, connect 100pF caps on either side of the resistor, and the other ends of the caps to the ground pin of the input socket.
One last observation: There is what I think is an extra wire (maybe after-market?) running between the L and R input socket grounds, directly connecting them. Is that weird?
Any advice on pi filters or other solutions is very much appreciated. Thank you for reading!
I've completed the following testing. Starting from square one (i.e. everything disconnected from everything):
Connect Adcom to speakers and AC power; power on: silence.
Connect Adcom to pre-amp with interconnects; power on: buzz. (Pre has NO other connections – it's not even plugged in to AC power.)
Disconnect either end of either interconnect: buzz disappears. Reconnect: buzz is back.
Remove interconnects completely; instead, insert shorting plugs into Adcom inputs: silence.
I've been told my ground resistance measurements – between channels and between each channel and chassis – are reasonable (R-gnd to L-gnd 0.2-0.3 ohms, R-gnd to chassis 3.8-3.9 ohms, L-gnd to chassis 3.7-3.8 ohms)
I have tried the following changes (tested one by one, all else being equal); none have had any effect on the buzz:
- power conditioner
- different interconnects (even low-capacitance BlueJeans LC-1s)
- different pre-amps
- different speakers
- different locations (2 other residences in totally different neighborhoods, distant from mine)
A kind advisor on fixya.com suggested my next step is adding pi filters to both channels to get rid of RFI. He suggested I insert a 5K resistor into the signal path just inside the chassis, connect 100pF caps on either side of the resistor, and the other ends of the caps to the ground pin of the input socket.
One last observation: There is what I think is an extra wire (maybe after-market?) running between the L and R input socket grounds, directly connecting them. Is that weird?
Any advice on pi filters or other solutions is very much appreciated. Thank you for reading!
Is there a ground wire running from each RCA input socket to the pcb? If so, that probably is an added jumper wire between them.
Also, the main filter capacitors may be weak, which could cause excessive 120Hz buzz. This is not likely to be RFI.
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Thanks! The answer to your question is yes - which I guess means the jumper wire is normal and is supposed to be there.
There is no buzz right up to the point when I connect both inputs up to the pre-amp. If the cause is weak caps, should I expect to hear the buzz when it is just amp + speakers, or amp + speakers + shorting plugs?
Where is the best place to get quality caps of this size (100V 18000uF)? Is there any sense in my swapping out the eight 1000uF caps on the PCB at the same time? These may have been replaced during a previous service.
I neglected to mention this is a multivoltage unit, configurable for 120V or 230V by rearranging the leads from the toroid at the power switch inside the chassis - and that the order in which the transformer leads are connected in my amp do not match either of the diagrams (120V or 230V) shown in the Adcom 5500 service manual I found online.
Thanks
I get a faint buzz in mine from the time I power up the preamp (nad 1155) to when I power up the CD player (sony scd-ce775). As soon as the cd powers up, the buzz is gone. However on power down, it does not happen. I stop and power down the CD player - no buzz. I power down the pre and no buzz.
No idea what that means.
Cool.
Srinath.
No idea what that means.
Cool.
Srinath.
@djoffe,
Thank you for the suggestion! I considered doing exactly that –*but I didn't know if it would blow up the amp and/or my speakers. Have you tried this? Is it safe, and did it diagnose a similar problem for you? What does it mean if the hum stops - or if it is still there?
Thank you for the suggestion! I considered doing exactly that –*but I didn't know if it would blow up the amp and/or my speakers. Have you tried this? Is it safe, and did it diagnose a similar problem for you? What does it mean if the hum stops - or if it is still there?
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OK its quiet with inputs shorted.
That means the hum is in the pre ?
Let me try a different pre.
And all quiet with the Kenwood box system pre I have. Grrrrr Now I have to fix the nad, it was an awesome pre, I guess I gotta look for a bad power supply cap set or maybe it just has a ground issue ????
Thanks.
Srinath.
That means the hum is in the pre ?
Let me try a different pre.
And all quiet with the Kenwood box system pre I have. Grrrrr Now I have to fix the nad, it was an awesome pre, I guess I gotta look for a bad power supply cap set or maybe it just has a ground issue ????
Thanks.
Srinath.
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