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Old 13th May 2003, 03:38 AM   #1
Jean is offline Jean  United States
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Question Trying to figure out how to get rid of noise, help is welcome!

Hello!

Thinking that I could get rid of all slight hum that is present in my p3a amplifier I decided to rewire signal grounds. That was the only thing left that’s not grounded to the same "star" ground point in the power supply capacitor bank.

I used fairly short magnet wire and connected all 4 rca jack grounds to it. Picture 4 solder points on the strip of wire, two on the left side and two on the right, wire is aligned physically between the rear panel of the amplifier where all 4 rca jacks are mounted. I then run a single run of wire from the "star" ground in the power supply section, to the middle point of the magnet wire. It basically formed a "T" section, with two rca jacks on each side of the wire.

My reasoning for doing this is the following. I was getting hum when TWO or more rca cables were connected to the amplifier, completing a circuit in a sense via cables and whatever source unit. I discovered that if I lift one ground off the RCA cables, noise would be decreased significantly. I decided to go ahead and connect all 4 rca grounds as close to each other as possible, hence the magnet wire alignment.

Result? Slight noise still present when nothing is connected to the rca jacks. Once I connected more than one cable, I have now more noise than I had before!

My previous setup had individual RCA grounds going directly back to each channel circuit board. Since I have four channels, I have four boards and 4 rca jacks. I used 4 short runs of shielded rca cable wire on the inside of the amp, from the boards to the rca jacks. Each board power wires are grounded to the capacitor bank in the power supply, located in the center of the amplifier.

Finally, my AC ground is connected to the chassis. It is not in any way connected to DC or signal ground.

Any suggestions?
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Old 13th May 2003, 03:49 AM   #2
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try removing the AC ground...almost sounds like your source to the amp is causing problems
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Old 14th May 2003, 10:46 PM   #3
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Removing AC ground does not solve it , so to the top it goes
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Old 14th May 2003, 11:01 PM   #4
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Did you leave the grounds on the sheilded signal cable from the inputs to the pcbs connected?, If so, you have created a ground loop where none was present before.
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Old 15th May 2003, 03:10 AM   #5
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No, only one end of shilded rca grounds from each board is connected. The other end only has the signal wire connected to the RCA jacks, not the ground shield.
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Old 16th May 2003, 03:21 AM   #6
Diode is offline Diode  United States
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I had same problem with kit amps I built. I don't use earth ground and it is much better. I haven't broken them open and attached a 100 Ohm resistor from cap ground to earth, to see what happens though. I need to someday though.

Chris
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Old 16th May 2003, 04:43 AM   #7
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Well I just took the whole amp apart. Going to get different power supply caps this time (with screw mounts) so I can rewire that section in more orderly fasion. I am also thinking about connecting all 4 (-) banana jack together and then run ONE single wire to the capacitor bank. I don't really see how that would prevent a ground loop, since speakers are not really physically connected electrically, unlike the source unit .

Keep the suggestions coming please !
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Old 18th May 2003, 05:12 AM   #8
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Well here is what I did.

I disconnected everything from the power supply board (4 power supply caps are on a small rectangular board, and this is where everything was connected to).

Next I connected the + and - from two power supply bridges to the capacitor board. I have two bridges on one transformer, one bridge per secondary . I then connected one amplifier channel to the board , and connected shielded signal wire from the amplifier board directly to one RCA jack. I then fired it up, very very slight humm still present in the speaker. This can't be a ground loop problem between the amplifier boards or rca jacks since there is only ONE amplifier board connected at this time.

Next I disconnected that amplifier board, and tried another one to see if it makes any difference. Here I am thinking maybe amplifier board 1 has internal humm problem and I could eliminate that possibilityby trying another board. I did get the same slight humm this time around with the second amplifier channel.

Next, I disconnected ground contact from the shielded rca wire that connect the rca jack to the amplifier board. I then used a jumper wire and connected RCA ground directly to the center point (ground) on the capacitor board. It made no difference at all.

Whats next ? Could a bridge / power supply caps/ transformer be the problem here ? Its clearly not a ground loop between the multiple amplifier boards that I have in the case.

Diode - I tried running a 10 ohm resistor from Earth ground to DC ground , it made no difference.

Help
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Old 18th May 2003, 08:43 AM   #9
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Ok,

Try removing the input link cable completely from the amp PCB.

If the hum goes away, then you need to get better screened cable. If not, then you may have a problem on your PCB, or inducted noise from your transformer.
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Old 18th May 2003, 03:00 PM   #10
Diode is offline Diode  United States
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Well, it appears that you have a good one here, by golly.
Do you have a schematic of your amp that you can post? Did you lay out and build the amp yourself? I am building my amp from a magazine article and they talk about PSRR, which I had never heard of before. CMRR, I have. PSRR (power supply rejection ratio) I need to read closer or find info on this aspect as it seems very important. I'll also ask another tech about different ways they ground tube amps as hum is a big problem with them.
Also, are your secondaries identical voltages and you parallel them to get 2X the current? If so, try taking one of them off and see what happens. I'm considering doing the same thing as I have a Peavey transformer from a PV .750 and I need the current for a 500W amp. Maybe you are getting some weird thing going on there. Make sure that your secondaries are in phase with each other as well. If they aren't, just switch the AC going into one of bridge rectifiers only, not both. This may be an issue if you didn't think of it.... Just trying to think of something..................

Chris
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