I got a good deal on a couple of Sonance Sonamp 2120s. There is quite a bit of 60 Hz noise and related harmonics with no signal and also increasing in magnitude with a signal. There is no noise when the amplifier is pluged in and on standby (speaker output relay is open, power supply to amplifier section relay is open, but power supply is fully operational). I have made some differential leads (per a post on this forum) and that helped some, but the noise still dominates THD+N for low signal levels. I changed out the 10000uF power supply capacitors with 22000uF ones and that did reduce 120 Hz noise levels and its harmonics by 7 dB, but did not budge the 60Hz levels. I then added a RC filter to the VAS and IPS stage and that basically did nothing for 60 and 180 Hz levels, but drove the 120 & 240 Hz levels to the noise floor.
I have tried using differential leads with both input and/or output signals and found out that differential probe across the 8 ohm load gives me the lowest 60 Hz levels. I have tried grounding in various places and found out that grounding the USB shield that does into the QA403 gives a huge noise reduction. I have changed USB cables, tested in the garage and testing with a laptop running on a battery. All give the same 60 Hz noise as shown below within 1 dB. What could be causing all of this, or is this just normal?
To give you a quick but partial initial answer, what you are seeing is your mains frequency, and related harmonics. I can tell that you’re are at least not in Europe, because my spike is at 50Hz + harmonics Mains electricity by country - Wikipedia.
How best to deal with it from an amplifier measurement perspective is something that I’ll leave to someone with more experience to answer.
The spikes at multiples of 60Hz are harmonics, not ripple, I would say. But yes, it’s to do with grounding and the fact that the QA403 has an isolated USB input/power supply.
If there are multiple devices connected together particularly from different power sources, then grounding and Pin-1 issues may be in play. Pin-1 issue is when the ground -Sleeve of a 1/4" or RCA connector or pin 1 of an XLR is sensitive to current flow, ie. current on the shield becomes audio inside the device.
Otherwise look for magnetically induced interference. The device’s own power transformer may be inducing voltage into the circuit. Try moving or reorienting it if lead length allows. Or there may be external magnetic fields in your service bench area. If moving or re-positioning the device you’re testing changes the 60 Hz noise, then the source is external. Move it in the direction that makes it worse to find the source.
For magnetically induced noise, the solutions are distance, orientation since the fields are directional, and twisted or coaxial conductors so that the same voltage is induced on both sides of the circuit and will cancel out.
I pinpointed the issue down to magnetically induced interference. This amplifier has a toroid transformer and the leads are just the magnet wire of the windings, so the secondary leads are fairly stiff. I managed to take the transformer out of the case and put it to the side. With about an inch of distance from the case to the transformer, the ripple was at the noise floor, about -125dB. Placing the transformer right next to the case did reveal 60 Hz components and their harmonics.
With the transformer still out of the case, but touching it, I tried some aluminum plates between the transformer and case to buck the stray flux out of the case, but this has only a minor effect. I tried some mild steel between the transformer and the case also, but the piece was too small to have much effect. I also tried the aluminum plates under the transformer when it was installed in the case, but it had no effect. However, I do have 0.014" thick electrical steel (core steel) and wrapped the toroid transformer with about 5 layers and installed the transformer back into the case. The core steel is a continuous piece and extends the height of the transformer. The results were very promising:
(Results are for left & right channels in dB with no signal, both before the core steel addition and after installing it)
60 Hz: Before: -107, -98 After: -117, -109
120 Hz: Before: -121, -118 After: -120, -118
180 Hz: Before: -110, -101 After: -119, -113
240 Hz: Before: -119, -119 After: -120, -117
300 Hz: Before: -102, -97 After: -113, -110
360 Hz: Before: -121, -121 After: -121, -120
420 Hz: Before: -102, -96 After: -113, -110
Peak uV: Before: 8.7, 15.1 After: 2.3, 3.2
As you can see, the 60Hz components and the harmonics went down by over 10 dB, and the 120 Hz components and their harmonics were not effected.
I still need to do testing under power, but I assume the results will be similar reductions in 60Hz.
I will also put some core steel under the transformer to better shield it from the case.
