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?
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?
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 !
Keep the suggestions coming please !
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
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
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
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
Pinkmouse - I will give that a try, somehow I doubt problem lies in the pcb (I did try different channel, same thing) . I went with shielded wire instead fine gauge twisted wire thinking it would be quieter
Diode - here is a picture that explains exactly how I have it wired. There is a single toroid with dual secondaries (not center tapped) , two bridge rectifiers and four caps.
Diode - here is a picture that explains exactly how I have it wired. There is a single toroid with dual secondaries (not center tapped) , two bridge rectifiers and four caps.
An externally hosted image should be here but it was not working when we last tested it.
from what I see in the pic is that the speaker ground is star grounded to the power supply and the signal ground is coming off the amplifier board....there is a parisitic interaction here....my guess is the speaker ground but the schematic you supplied is a little vague....you got me thinking now....hope we can help you work this out
Cheers!!The DIRT®
Cheers!!The DIRT®
AH HA,
My opinion....... Junk the 2nd bridge rectifier and tie the 2 middle (exactly according to the drawing) wires together as the 0V refrence and tie the outer wires to the AC points of the single bridge rectifier. You should get the same voltage and the problem should be solved. You don't have much of a refrence and there's current flow between the 2 secondaries somehow and thats the source of your noise.... You just don't have a good refrence point. Give it a try and let us know..... That's just my humble opinion.....
Good luck,
Chris
PS, I have 2 torroids, out of an amp with the exact same non-centertapped secondaries. They used a single bridge and tied 2 of the wires together as a refrence point.
My opinion....... Junk the 2nd bridge rectifier and tie the 2 middle (exactly according to the drawing) wires together as the 0V refrence and tie the outer wires to the AC points of the single bridge rectifier. You should get the same voltage and the problem should be solved. You don't have much of a refrence and there's current flow between the 2 secondaries somehow and thats the source of your noise.... You just don't have a good refrence point. Give it a try and let us know..... That's just my humble opinion.....
Good luck,
Chris
PS, I have 2 torroids, out of an amp with the exact same non-centertapped secondaries. They used a single bridge and tied 2 of the wires together as a refrence point.
Diode said:
Jean, don't do that
I don't see a reason to scrap the better way to wire a PS for a lesser solution. There are entire threads about it on this site.
Instead follow 'the dirt' suggestion. Keep the transformer-rectifier-filter cap gnd separated from the rest of the gnds. This is a stereo amp with a single PS in common, right?
For that I had good luck with the system below. You may want to try variation around the input gnds. Should work for a mono configuration even better.
If this doesn't work it maybe transformer noise or problems up the component chain.
PS I hate gifs, there are a couple of missing connections I hope you can see what I mean
Attachments
Connect the power ground
Jean,
I built the p3a and have a dual power supply as you have indicated in one of your posts.All the ground should return to a single point on the capacitor bank. The wires include ground from PCB, the speaker returns and the power. How ever the power ground passes through 2 6amp diodes, an AC rated capacitor and a 10ohm, 5 watt resistor before terminating at the star ground. I have no hum from this arrangement. You could also check if the wiring is symetrical from the secondry of the transformer to the two rectifier bridges. Hope this help. Don't skip the power earth.... It is there for safety.
Dinesh
Jean,
I built the p3a and have a dual power supply as you have indicated in one of your posts.All the ground should return to a single point on the capacitor bank. The wires include ground from PCB, the speaker returns and the power. How ever the power ground passes through 2 6amp diodes, an AC rated capacitor and a 10ohm, 5 watt resistor before terminating at the star ground. I have no hum from this arrangement. You could also check if the wiring is symetrical from the secondry of the transformer to the two rectifier bridges. Hope this help. Don't skip the power earth.... It is there for safety.
Dinesh
Jean,
the thermistor serves to keep the gnd of the amp isolated from the plug gnd UNLESS a short to gnd is present. It is routinely used by NP.
Anyways, you have quite a few suggestion to try now, although I would never connect the star gnd the way someone is suggesting who knows, maybe it will work.
the thermistor serves to keep the gnd of the amp isolated from the plug gnd UNLESS a short to gnd is present. It is routinely used by NP.
Anyways, you have quite a few suggestion to try now, although I would never connect the star gnd the way someone is suggesting who knows, maybe it will work.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.