Ground loop/hum on Inca Designs ID1P/ID75 - diyAudio
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Old 8th September 2008, 03:31 PM   #1
quickie is offline quickie  United Kingdom
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Default Ground loop/hum on Inca Designs ID1P/ID75

I have a low level hum/buzz with the above amps that I am trying to get rid of.

With both inputs on the power amp shorted or one channel connected to my CD player (other shorted) though the ID1P pre,it is near silent.Its only when both channels are connected that I get the hum which sounds to me like there is a ground loop somewhere.

Lifting the ground on the right channel of the interconnect to the power amp made the left channel slightly better but not by much,so I tried disconnecting the shield to each channel on the pcb and running a wire direct from 0v between the caps to the RCA's but this made it much worse.A 10 ohm resistor in ground of one or both channels interconnects made no difference

All the electrolytic caps have recently been replaced except the two on the soft start board/new bridge rectifier(I would guess the amps are around 20 years old)

I was thinking that there could be a fault with the pre-amp but my other ID power amp is fine with very little noise at all !

Any suggestions ?

Cheers............

ID1P

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ID75

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Other power amp

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Paul.
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Old 9th September 2008, 06:42 AM   #2
Mooly is offline Mooly  United Kingdom
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Hi,
See no one has replied
With the usual warning of just trying this for purposes of elimination have you tried isolating mains lead grounds. Perhaps try first the CD player and then the preamp, possibly both together.
Also read,
Hafler 9130 Hum
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Old 9th September 2008, 04:35 PM   #3
quickie is offline quickie  United Kingdom
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Hi Mooly,

I've just read through the thread on the link you provided and it doesn't really sound like a deep pure mains frequency hum,there is definitely harmonics/buzzing as well.

Lifting the mains earth on either the cd/pre or both at the same time doesn't change it.

If I turn off the cd player it says the same.Switching off the pre/or connecting just the power amp with inputs open results in more off a hum sound (buzz the same)

Touching just the grounds of the interconnects to the power amp with system connected up as normal=no change.

Disconnecting the power amp from the wall socket as well when the caps are discharging (it runs for a couple of mins after turning off) and now the hum is slightly louder again......don't know if this means anything.

I hope the above makes some sense !

Paul.
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Old 9th September 2008, 05:21 PM   #4
Mooly is offline Mooly  United Kingdom
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Is this a commercial amp or a kit built or DIY etc ?
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Old 9th September 2008, 05:23 PM   #5
quickie is offline quickie  United Kingdom
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Commercial amp.

Paul.
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Old 9th September 2008, 05:27 PM   #6
Mooly is offline Mooly  United Kingdom
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O.K. I was asking because "hum with harmonics" sounds more like either an internal wiring problem or fault.
You say it's silent with inputs to the power amp shorted -- your sure ?
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Old 9th September 2008, 05:36 PM   #7
Mooly is offline Mooly  United Kingdom
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You will know from the other thread how difficult hum problems can be to eradicate, particularly without actually being able to see how it's all wired up. Harmonics usually are introduced through wiring problems within the item. A buzz at 100 hz (twice mains frequency) points to the ripple on the PSU somehow getting into the signal line, either directly or through the grounds within the amp.
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Old 9th September 2008, 05:37 PM   #8
quickie is offline quickie  United Kingdom
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This is what I don't understand.

I've just plugged it in again with both inputs shorted......near silent.You have to get your ear right up to the speaker to hear a tiny hum.

Either channel on its own is the same.

Paul.
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Old 9th September 2008, 05:39 PM   #9
Mooly is offline Mooly  United Kingdom
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Now plug it into the preamp with your interconnects, with NO inputs (signal sources) connected to the preamp. Switch it all on and see what happens.
Is there any buzz ?
Does the buzz vary with the volume control ?
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Old 9th September 2008, 05:44 PM   #10
quickie is offline quickie  United Kingdom
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With both channels connected to the pre-amp= buzz.

It doesn't vary with the volume control.I've tried it with a passive-pre as well and its the same.

Paul.
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