I have a weird hum (ground loop?) issue with an amp I built using an SDS-254 kit from classdaudio.com.
My source right now is a Wyred4Sound DAC-2, which is connected directly to the amp. However, please note that I had the exact same problem when the HT pass through was used when a Parasound 2100 preamp was connected to the amp, so I doubt the problem lies in the DAC-2. More over, I switched the SDS-254 amp with a Denon integrated amp, and I did not encounter the issue, so I think the problem is definitely with the SDS-254 amp.
The problem is that whenever the HT pass through mode is engaged in the DAC-2, I get a significant hum through my speakers. I also get a loud pop when I change to another mode (i.e. USB). Now, before someone suggests the problem might lay in my Denon surround sound receiver (L & R pre-outs used for HT pass through), this problem exists regardless of whether or not anything is connected to the HT pass through inputs on the DAC-2.
If I am using the DAC-2 as the actual source (i.e. digital audio via USB), then I don't have any problems with the amp (no hum or pops).
I have modified the amp into the following configurations. At all times, the unbalanced RCA input jacks have been insulated from the chassis and not connected to anything other than the amp board inputs. The problem has persisted through all of these configurations.
1) Earth ground (AC ground) connected to chassis, amp board connected to chassis.
2) Earth ground NOT connected to chassis, amp board connected to chassis.
3) Earth ground connected to chassis, amp board isolated.
I have not tried disconnected both the earth ground and amp board from the chassis. Also, as far as I know the power supply is not grounded to anything.
Does anyone have any ideas why this strange problem exists? What is causing it, and how do I fix it?
Thanks.
My source right now is a Wyred4Sound DAC-2, which is connected directly to the amp. However, please note that I had the exact same problem when the HT pass through was used when a Parasound 2100 preamp was connected to the amp, so I doubt the problem lies in the DAC-2. More over, I switched the SDS-254 amp with a Denon integrated amp, and I did not encounter the issue, so I think the problem is definitely with the SDS-254 amp.
The problem is that whenever the HT pass through mode is engaged in the DAC-2, I get a significant hum through my speakers. I also get a loud pop when I change to another mode (i.e. USB). Now, before someone suggests the problem might lay in my Denon surround sound receiver (L & R pre-outs used for HT pass through), this problem exists regardless of whether or not anything is connected to the HT pass through inputs on the DAC-2.
If I am using the DAC-2 as the actual source (i.e. digital audio via USB), then I don't have any problems with the amp (no hum or pops).
I have modified the amp into the following configurations. At all times, the unbalanced RCA input jacks have been insulated from the chassis and not connected to anything other than the amp board inputs. The problem has persisted through all of these configurations.
1) Earth ground (AC ground) connected to chassis, amp board connected to chassis.
2) Earth ground NOT connected to chassis, amp board connected to chassis.
3) Earth ground connected to chassis, amp board isolated.
I have not tried disconnected both the earth ground and amp board from the chassis. Also, as far as I know the power supply is not grounded to anything.
Does anyone have any ideas why this strange problem exists? What is causing it, and how do I fix it?
Thanks.
No takers?
I just tried unplugging all of my other equipment from the wall. The only things left on were the DAC-2 and the amp. Still, no dice. I also tried grounding the unbalanced RCA inputs to the chassis, so that the input ground and amp circuit ground were tied together. It didn't make any difference.
I also tried replacing the amp with my Denon integrated again, just to make sure the Denon didn't exhibit the problem. As I feared, there was no hum when using the Denon.
Now, this may be a dumb, noobish, question, but should the toroidal transformer be grounded to anything? Unfortunately, the transformer leads are not marked, and the only thing I had to go by was a simple diagram which came with the kit. According to the diagram, the transformer is not connected to ground in any fashion.
I just tried unplugging all of my other equipment from the wall. The only things left on were the DAC-2 and the amp. Still, no dice. I also tried grounding the unbalanced RCA inputs to the chassis, so that the input ground and amp circuit ground were tied together. It didn't make any difference.
I also tried replacing the amp with my Denon integrated again, just to make sure the Denon didn't exhibit the problem. As I feared, there was no hum when using the Denon.
Now, this may be a dumb, noobish, question, but should the toroidal transformer be grounded to anything? Unfortunately, the transformer leads are not marked, and the only thing I had to go by was a simple diagram which came with the kit. According to the diagram, the transformer is not connected to ground in any fashion.
No, there is absolutely no hum when the inputs are shorted. Do you think that the cables are picking up some kind of interference?
The only other thing that I can think of then is that the amplifier is picking up stray magnetic field from the transformer. This can happen as soon as you connect leads to the input. Can you move the transformer away from the amplifier. Maybe move the wiring. What you are experiencing is very annoying and very disappointing.
Well, I have good news and bad news.
The good news is that the problem isn't the classdaudio amplifier. The bad news is I'm running out of ideas as to what's causing the problem.
I removed the SDS-254 and hooked up some Wyred4Sound monoblock amps, and I still have the same problem when I use the DAC-2 in HT bypass mode (other modes are fine as before). I have also tried using cheater plugs on the DAC-2 and two amps, all at the same time. No dice.
At this point, I would blame the DAC-2, but I had this exact same problem when using the HT bypass on a Parasound 2100.
After using my Google-Fu to scour the Internet, I have found a few instances where people had similar hum issues when using HT bypass modes (other modes on DACs and preamps were fine). Unfortunately, they never seemed to find a solution (or never bothered to tell anyone).
Here's the only idea I have left. The problem occurs when using HT bypass mode, but it doesn't matter whether the HT bypass inputs are unconnected or connected to my Denon surround sound receiver. This Denon, interestingly enough, only uses a two-prong plug. In other words, the HT bypass inputs are never connected to AC safety ground. Now, if the HT bypass is properly implemented in the DAC-2 and Parasound 2100 preamp, then it is simply passing this ungrounded signal to the amps, which could somehow be creating the hum. I'm going to try to find a way to connect the HT bypass inputs to the safety ground and see if that fixes the issue.
The good news is that the problem isn't the classdaudio amplifier. The bad news is I'm running out of ideas as to what's causing the problem.
I removed the SDS-254 and hooked up some Wyred4Sound monoblock amps, and I still have the same problem when I use the DAC-2 in HT bypass mode (other modes are fine as before). I have also tried using cheater plugs on the DAC-2 and two amps, all at the same time. No dice.
At this point, I would blame the DAC-2, but I had this exact same problem when using the HT bypass on a Parasound 2100.
After using my Google-Fu to scour the Internet, I have found a few instances where people had similar hum issues when using HT bypass modes (other modes on DACs and preamps were fine). Unfortunately, they never seemed to find a solution (or never bothered to tell anyone).
Here's the only idea I have left. The problem occurs when using HT bypass mode, but it doesn't matter whether the HT bypass inputs are unconnected or connected to my Denon surround sound receiver. This Denon, interestingly enough, only uses a two-prong plug. In other words, the HT bypass inputs are never connected to AC safety ground. Now, if the HT bypass is properly implemented in the DAC-2 and Parasound 2100 preamp, then it is simply passing this ungrounded signal to the amps, which could somehow be creating the hum. I'm going to try to find a way to connect the HT bypass inputs to the safety ground and see if that fixes the issue.
I already disconnected both the monoblock amps and the DAC-2 from the safety ground using three cheater plugs, but the problem still existed, even then. I can try reconnecting them one by one, but I don't see how that fix the problem.
Using a cheapo voltmeter, I tested the HT bypass inputs on the DAC-2 and found that they were connected to safety ground. Could this be the culprit?
After a lot of testing and experimenting, I found the culprit(s). First, I have a light in the room with an adjustable light switch. This seems to be causing some of the noise when it is turned on. In the near future, I'm going to replace the adjustable switch with a simple on/off switch.
Secondly, I found that an HDMI cable that I was running from a computer in another room (and on a different electrical circuit) was causing noise when it was plugged into my Denon surround receiver. This had created a ground loop and was causing the majority of the hum. I'm not sure how I'm going to fix this, as moving the computer to the same electrical circuit isn't really feasible, and I don't think anyone makes an HDMI ground isolator. I suppose I could build one. It shouldn't be all that difficult.
Having said that, the hum isn't completely gone, but at least it's low enough that it won't bother me under normal conditions. I cannot hear it at my main listening position, but I can still hear some hum if I put my ear up against the tweeter and mid-range drivers.
Secondly, I found that an HDMI cable that I was running from a computer in another room (and on a different electrical circuit) was causing noise when it was plugged into my Denon surround receiver. This had created a ground loop and was causing the majority of the hum. I'm not sure how I'm going to fix this, as moving the computer to the same electrical circuit isn't really feasible, and I don't think anyone makes an HDMI ground isolator. I suppose I could build one. It shouldn't be all that difficult.
Having said that, the hum isn't completely gone, but at least it's low enough that it won't bother me under normal conditions. I cannot hear it at my main listening position, but I can still hear some hum if I put my ear up against the tweeter and mid-range drivers.
This means that your hum is conducted EMI. It is not uncommon, but few ever think about it. I am inclined to believe that you discovered the problem by accident when you turned off the lights. You may try to place a mains filter on the power-line. The lamp dimmer has a triac that can cause a lot of switching noise on the power line and it will sound like mains hum, but the pitch is different or sharper since a spike is produced at 2 x line frequency.
Well done.
Well done.
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