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Old 11th February 2007, 11:55 PM   #1
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Default Hum problems with my MOS amp

Hello everyone,

New on this forum.

I would to submit you the following problem: I have a boring hum at output of my MOSFET stereo power amplifier, when it is in stand by with a signal source connected;

The amplifier is composed in this way:

- Pre-amplifier stage with its own power supply unit
- Output stage power supply unit
- MOSFET right-channel amplifier
- MOSFET left-channel amplifier

All units are powered from the same thoroidal multi-voltage transformer.

I tested the units singularly and I found that they work properly;
Output stage dual voltage (50+50V)power supply provides 30,000 uF capacitors on each "branch".
Mosfet output stages tested with short-circuited inputs give no hum.
Pre-amp unit is also equipped with its own dual voltage (15+15V)power supply that provides 5,000 uF capacitors on each branch.
Pre-amp signal is pure.

The problem seems to be some form of ground loop,since the hum comes out when I connect the units to get the amp operating.

Initially that was the general grounding configuration:

AC power cables from transformer to the output stage power supply unit and to the pre-amp unit (with its own built-in power supply unit).

DC power cables (with their ground cable)from otput stage power supply unit in parallel for both MOSFET output power amplifiers.

Pre amp outputs connected to amp inputs by signal cable shield.

All units insulated from the metal case.

I already reviewed pre-amp PCB grounds and power input points making some modifies but nothing happened.
I changed signal cables with higher quality ones, but nothing happened.
I tried to move away the transformer , but nothing happened; I even replaced the transformer,but nothing happened.

After some tests I changed the configuration of the DC power cables: Now they start from the power supply unit connecting only one amplifier unit(the nearest); then, from this one, cables connect the second unit.

This manoeuvre seems reduced in some way the hum but still not at an acceptable value;
I noticed that hum comes out inthe exact moment I connect the ground shield from the signal cable on the second power amplifier.
Also I noticed that hum increases if I move the ground cable that connects both amplifiers closer the transformer!...

Now I don't know wath to do anymore...

So,please,ifanyone has an useful tip for me I'll be very grateful for the precious help.

Sincerely
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Old 12th February 2007, 02:06 AM   #2
Leolabs is offline Leolabs  Malaysia
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Sounds like a ground loop problem between channels.
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Old 12th February 2007, 02:27 AM   #3
quasi is offline quasi  Australia
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Are you able to post a simple picture showing the exact layout and wiring?

Cheers
Quasi
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Old 12th February 2007, 10:44 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
have I read this correctly?

Disconnect one signal from one power amp and hum drops.
Reconnect two signals to two power amps and hum rises.

Which speaker does the hum come from?
Which metal cases are connected to safety earth?
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Old 12th February 2007, 11:12 AM   #5
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Well,

- Left channel connected
- Right channel not connected

No hum.

-Left channel connected
-Right channel ground shield connected

Hum in both amplifiers.

Only the whole amplifier metal case is connected to safety earth, but all circuits are insulated from it.

Also,I'll try to send you a simple schematic.
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Old 12th February 2007, 11:25 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
we await the grounding schematic with the input wiring and safety earth connections.

There is certainly an earth loop in there.
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Old 12th February 2007, 06:16 PM   #7
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Here is the ground schematic:

I wait your news...

Thanks a lot!
Attached Images
File Type: jpg schematic.jpg (84.1 KB, 297 views)
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Old 12th February 2007, 08:19 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
your input RCA grounds are commoned after they enter the amp.
That's your earth loop.

You also have an internal loop between the power amps link and the two signal inputs.
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Old 12th February 2007, 11:05 PM   #9
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Well,

I previously connected output power supply grounds in parallel (both starting from the power supply unit) to the power amplifiers just to avoid the loops you're talking about but hum was the same...
so the loop would be elsewhere...
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Old 13th February 2007, 01:23 AM   #10
quasi is offline quasi  Australia
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Hello Mastermosfet,

I am having trouble determining the actual wiring layout from your schematic. I need to see the actual wiring and the actual PCB and chassis connections. I'm wondering whether a photo or handrawn picture would be better.

Thanks & Cheers
Quasi
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