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Old 18th March 2003, 04:30 PM   #1
tktran is offline tktran  Australia
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Default Rod Elliot P3A amp noises

Hello all,

I have built my first amp- the Rod Elliot P3A.

However there exists a small amount of hum. I
have yet to build a preamp so at the moment I have a
pot to control volume, and whilst the hum is soft it
seems to be loudest at min volume and softest at ~ 1/3
max volume. Culd it be the pot that's causing the 50Hz hum?

Is there a way to test the source of the hum? I have
access to multimeters and a CRO.

In other designs eg. commercial, the transformer is
usually situated at the far left/right of the case,
the PCB on the other side and an internal heatsink in
the middle, which also acts to shield the PCB/input
from the supply.

I made the mistake of putting the transformer at the
front centre of the case, and PCB at centre rear,
connected to an external heatsink. I did this because
I wanted the amplifier weight to be evenly distributed
(for carrying safety) however now there is nothing
shielding the transformer from the pcb.

Is it worth shielding the transformer? If so, how?
Or should I relocate the transformer?

Thanks.
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Old 18th March 2003, 06:47 PM   #2
paulb is offline paulb  Canada
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Hum can be induced into your input wires. Try shorting the input right at the PCB to see if that helps. Although if it's loudest at minimum volume, it's doubtful this would help.
I used an external power supply for my P3A for exactly this reason; I had problems with a previous amp. With the previoius amp, I was able to move the input wires around to minimize the hum. Maybe you can do some wiring (input, power, or output) rearrangement to reduce your hum.
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Old 18th March 2003, 07:04 PM   #3
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Hi!

But most of the time you got hum in an amp like P3A because of improper grounding.

Use a real star ground, with no connection to chassis (all connections to ground go to that star ground)

Earth the chassis via the earth of the mains cable, and then make a connection from star ground to chassis via a resistor (about 100 Ohms), bypassed by a capacitor (with mains voltage rating, better more), for example Wima MKP10 0.47 uF 400V AC.

Today I fired up the testing version of my P3A (both channels), in somewhat the same layout you used (but not in metal chassis, have yet to gather enough money for chassis... ), and no hum was audible!

Actually the first amp I build that worked from the start on (and seems to be real nice...)

You can also try using shielded cables from input connectors to amp.

Bye,

Arndt
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Old 20th July 2016, 06:18 PM   #4
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hi people, I just built my p3a and it works great but I have an huum issue...
if I short the input rca it disappear completely. the same if I connect only one channel.
This is the schematic of my layout, except for the star point

Click the image to open in full size.

I used two bridge diodes; one DC protection for each channel, each protection is supplied in AC by the same secondary winding of the transformer (the same 0-24VAC supllies both the protections).
Speaker GND is taken directly from GND of PS, and also each protection GND and AMP GND. Input GND goes from RCA socket to AMP PCB.
any help will be appreciated
regards
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Old 20th July 2016, 07:38 PM   #5
Bigun is offline Bigun  Canada
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Can't see the image - you should upload it again.
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Old 20th July 2016, 07:41 PM   #6
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Click the image to open in full size.
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Old 20th July 2016, 08:13 PM   #7
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Hello cucicu

In principle it is not a good idea to connect the earth and the ground star sets.
Here are the rules I respect and never any noise problems.

- The earth connected to the case (and only that)
- The ground star (isolated chassis) for the gnd power and gnd loudspeaker
- Entries on the amplifier cards (hot and ground)
- Insulate the input and out plugs of the case.

regard's
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Old 20th July 2016, 08:13 PM   #8
Bigun is offline Bigun  Canada
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Doesn't work for me. I can see images in other threads but not this one.

Anyhow, based on what you posted... the input is where the hum is getting in since when you ground it there is no hum.

Can't remember what your circuit details were - you made your own pcb if I remember. Most likely the issue is

a) grounding problems - do you have a ground-loop breaker circuit i.e. back-to-back diodes between amplifer/signal ground and the safety earth (mains earth + chassis) ?
or
b) pick-up from stray electric or magnetic field in the chassis to the input wiring
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Old 20th July 2016, 08:19 PM   #9
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Quote:
Originally Posted by Bigun View Post
Doesn't work for me. I can see images in other threads but not this one.
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File Type: jpg MP_XP_Mech_1405.jpg (86.5 KB, 184 views)
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Old 20th July 2016, 08:25 PM   #10
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Quote:
Originally Posted by Project16 View Post
Hello cucicu

In principle it is not a good idea to connect the earth and the ground star sets.
Here are the rules I respect and never any noise problems.

- The earth connected to the case (and only that)
- The ground star (isolated chassis) for the gnd power and gnd loudspeaker
- Entries on the amplifier cards (hot and ground)
- Insulate the input and out plugs of the case.

regard's
HI Project16

I must say the earth is not connected.
The ground is not perfectly a "star" cause the GND connections go from bridges to PS pcb's and from there I hace 3 ground cables to:
-Speaker
-Protection GND
-Amp GND
-input and output are isolated from the chassis.


BIgun:
I used ALex mm layout for pcb. No gnd loop breaker on pcb
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