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 Post subject: Power Supply Harmonics
PostPosted: 21 Mar 2020, 14:24 
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Hi Everyone, All you who are smarter than me....What would cause 180 and 300 HZ harmonics in a power supply. 60 and 120 are easy. But my education has not apparently covered the higher ones. I have a tube amp with SS power supply well filtered etc. Even regulated, but still the 180 and 300 are pretty significant. With a zero dbv reference they show up in the output in the range of -55 and -60 dbv respectively. Far to high for for a phono preamp. The design is one I use all the time and the in the others they are about 20 db lower. So what might cause this as the components are quite similar only the physical layout is different. It is nearly identical to the attachment. Ignore the title of the file, it was not used that way.

Good listening
Bruce
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Corrected Valve Phono Preamp as Built for article April 2017b.jpg


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PostPosted: 21 Mar 2020, 14:58 
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The first thing that comes to mind is the regulators.
But that's not a smart comment my first thought would be SMPS but you don't have one :D
Second thought its already on the incoming mains supply.

Next thought..

Instability?
Does it do it if you run one channel or the other?
Disconnect each of the B+ Supplies and see if it stops.
Or is it intrinsic in the PSU with no load?
Try a 0.1uf across the 1uf on the B+ rail.

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M. Gregg

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PostPosted: 21 Mar 2020, 20:54 
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Hi Thanks, Both channels do it and each does it separately. On the AC line...I had not thought of that one and will have to check. Possibly as there is a voltage equalizer on the shop outlets (a Triplet 1500 watter).

No indication of instability. I'll have to try the 0.1uf.

Only thing I could think of is some sort of resonance in the power transformer, possibly because of the loading. Or some interaction between the heater circuit and B+ circuit.

Really strange that the two harmonics are much bigger than any of the possible fundamentals. I don't believe I have ever seen that before.

Anyone with other thoughts?

Good listening
Bruce

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PostPosted: 21 Mar 2020, 21:01 
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Is likley to be a grounding prolbem in the system.
Or you done something wrong with the voltage reggulator.

If it works previously but not anymore then its gonna be the grounding gone wrong.
Get the grounding wrong and 180-300HZ of noise is by all means no supprise.
You can expect frequenciy noise even in above 20KHZ to cause audtiable noise if you get ground wrong, normally only applicable to SMPS


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PostPosted: 21 Mar 2020, 23:20 
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gofar99 wrote:
What would cause 180 and 300 HZ harmonics in a power supply.
These are 3rd and 5th harmonics of the 60Hz fundamental. Odd order harmonics are indicative of square waves or waveforms with pseudo flat tops. The Fourier series for square waves consists of the odd order harmonics.

After a quick look at your schematic I suspect you may have too short a conduction angle. The hard switching is leading to the odd order harmonics. My first recommendation would be to reduce the first filter capacitor after the rectifiers to about 33µf and remeasure the higher harmonics. It may be simply that you're switching the rectifiers too hard. Since your overall load is small (i.e. low current) this can drive you to very small conduction angles that can give rise to the higher order harmonics.

Just a thought.

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PostPosted: 22 Mar 2020, 03:51 
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Voltage regulators normally have quite nice filtering at below 1KHZ so if there is any it should filter it out to like at least few mV to tens of mV, unless something gone wrong like the ground wirering or layout.


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PostPosted: 22 Mar 2020, 06:00 
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Hi,

Just another thought, you don't have DC riding on the mains supply?

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M. Gregg

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PostPosted: 22 Mar 2020, 08:47 
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Hi, Good thought on the filter cap. It could just do that. I have hunted for DC on the mains and last check there was none to speak of. Plus other gear doesn't have this problem. And yes they are 3rd and 5th of the 60 HZ. The quirk is that in this project only they are bigger than the fundamental. In all other builds they do show up a bit, but way down in level. Good thinking everyone. More ideas are always welcome. I'll post what the result is.

Good listening
Bruce

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