Hello everyone. I'm posting for two reasons. 1: trying to get some help with a minor issue on my Aleph J and 2: to hopefully learn something in the process.
I have built an F5 and Aleph J. Both amps have the exact same power supply parts, case and PS board from the diyaudiostore.com. The Aleph J has a 120Hz buzz in both channels when no inputs are hooked up. The F5 does not. Both amps work and sound great. Enough about the F5. Observations about the Aleph J:
1: The buzz is 120Hz measured by a frequency monitor and the very inexpensive scope I have. Approximately equal in both channels.
2: When a preamp is hooked up, the buzz quiets to the point you have to get very very very close to the speaker to hear it. This means normal listening volume you really are not affected by it.
3: Touching the shielded wires on the inputs increases the buzz.
4: Disconnecting the inputs from the PCB board has no effect on the buzz.
5: Shorting the inputs (hook + to -) eliminates the buzz.
6: Safety earth and the only connection to ground (through a cl60 thermister) from the power supply are to the same bolt to the chassis.
I'm assuming this situation is likely from a ground loop. Here is what I have tried to eliminate the buzz ( I realize some of this is not specific ground loop related):
1: Disconnecting input wires from PCB, checking for no continuity with the chassis on the inputs. Changed from a twisted solid pair to Cat 5. No effect.
2: Changing orientation of the transformer both by rotating and tipping up on its side. No effect.
3: Checking and rechecking continuity from the MOSFETs to the chassis. No continuity from any MOSFET to chassis when the power supply is disconnected.
4: Mains are run though the base mounting plate under the plate to the euro block shown in the picture. They are twisted the whole way. No effect.
5: Connecting the input grounds together has no effect.
What I find most curious about this is that the amp quiets when a preamp is connected. I'd like to understand this as much as fix the problem. The amp really does work very well and I know I'm letting my obsessive nature take over here. Any input or direction would be appreciated.
I have built an F5 and Aleph J. Both amps have the exact same power supply parts, case and PS board from the diyaudiostore.com. The Aleph J has a 120Hz buzz in both channels when no inputs are hooked up. The F5 does not. Both amps work and sound great. Enough about the F5. Observations about the Aleph J:
1: The buzz is 120Hz measured by a frequency monitor and the very inexpensive scope I have. Approximately equal in both channels.
2: When a preamp is hooked up, the buzz quiets to the point you have to get very very very close to the speaker to hear it. This means normal listening volume you really are not affected by it.
3: Touching the shielded wires on the inputs increases the buzz.
4: Disconnecting the inputs from the PCB board has no effect on the buzz.
5: Shorting the inputs (hook + to -) eliminates the buzz.
6: Safety earth and the only connection to ground (through a cl60 thermister) from the power supply are to the same bolt to the chassis.
I'm assuming this situation is likely from a ground loop. Here is what I have tried to eliminate the buzz ( I realize some of this is not specific ground loop related):
1: Disconnecting input wires from PCB, checking for no continuity with the chassis on the inputs. Changed from a twisted solid pair to Cat 5. No effect.
2: Changing orientation of the transformer both by rotating and tipping up on its side. No effect.
3: Checking and rechecking continuity from the MOSFETs to the chassis. No continuity from any MOSFET to chassis when the power supply is disconnected.
4: Mains are run though the base mounting plate under the plate to the euro block shown in the picture. They are twisted the whole way. No effect.
5: Connecting the input grounds together has no effect.
What I find most curious about this is that the amp quiets when a preamp is connected. I'd like to understand this as much as fix the problem. The amp really does work very well and I know I'm letting my obsessive nature take over here. Any input or direction would be appreciated.
Attachments
Thanks ZM. I definitely see how a ground loop can be set up in many amp designs including mine. What I still don't understand is both examples in that post need a source connected to complete the loop. In my amp, once the source is connected it actually quiets the amp. The opposite of what is shown here. Am I wrong to think I have some other problem?
120hz sounds like poor rectifier ground, or PSU cap problem. But, in your case you are getting a better ground via your preamp, so, it's a PSU ground issue.
Cheers,
Greg
Cheers,
Greg
You have the mains capacitor connected to just one of the primaries of the transformer. Try moving the cap to across the mains side of your connector block.
Also check your CL60 connections to the primaries, it could be my eyes, the photo or nothing.
Also check your CL60 connections to the primaries, it could be my eyes, the photo or nothing.
If I understand correctly you have a hum when you have nothing connected on the input and the hum goes away when you short the input to ground? It’s normal to pick up some noise when you have the input air patched. The difference in hum between the two amps when you have nothing connected may be due by the difference in input impedance / configuration between both design.
Hubert
Hubert
This explanation makes the most sense to me. The amp is getting a better ground from the preamp. I’ve checked the rectifiers with the diode test function on my multimeter and they seem to be flowing the correct direction only. I have a good ground connection when my chassis ground is connected, when disconnected there is no continuity with the chassis in any of the rectifiers or grounds on the psu. This leaves a bad cap as the most likely source by process of elimination. Is my thinking correct prior to starting to pull those big suckers off of there to test?
As a quick reply to moving the small cap on the euroblock to the mains side: wouldn’t I be getting a 60Hz hum if that were the problem? I checked the cl60s they are connected well.
don't you have main audio GND connected to chassis , in any possible way?
either plain short, or via 10-15R NTC, or though last fashion - specially connected diode bridge and NTC and cap (all in parallel)
I mean , I see CL60 between PSU pcb and IEC , so confirm that connection with ohmmeter ......
either plain short, or via 10-15R NTC, or though last fashion - specially connected diode bridge and NTC and cap (all in parallel)
I mean , I see CL60 between PSU pcb and IEC , so confirm that connection with ohmmeter ......
Did not mean to be confusing. The main audio ground from the amp boards are connected to the power supply ground. The power supply is connected to the chassis in only one place, through a cl60 thermistor. The point that the cl60 is bolted to the chassis is the same place the safety earth is connected. The power supply ground (as well as audio main ground) to the chassis reads 15ohms with an ohm meter.
I was simply stating that when I disconnect this cl60 from the psu ground I have no other continuity to the chassis that I can find. I am not running the amp with the audio main ground disconnected.