M2X strange DC offset and transformer noise problem

[MEPER]

This is direct from the wall (what goes into the filter and isolation transformer).
I can also see that the negative DC rail voltage on both amps are less negative than usual. Rail voltages are like -22.7 VDC and +23.3 VDC. This morning they were almost symmetrical. So this is the reason why DC offset is a bit off now........
So probably amps are fine......but transformers does not like the sine.
How can I "reshape" a bad sine?

 

[MEPER]

The sine out of the isolation transformer looks identical (have just checked). I see the same difference between AC and DC coupling. The difference is probably just the input cap in AC mode that filters the signal.

 

[MEPER]

Here a better view of the shape out of the isolation transformer after I have aligned the line in Gnd mode exactly at the center line. Sine is not 100% symmetrical around the Gnd center line.

 

[MEPER]

This old analog 2225 scope is so much better to work with than TDS 210......but 210 more portable but seems to have much more internal noise. For some applications it is ok.

 

[Extreme_Boky]

I can also see that the negative DC rail voltage on both amps are less negative than usual. Rail voltages are like -22.7 VDC and +23.3 VDC. This morning they were almost symmetrical. So this is the reason why DC offset is a bit off now........
So probably amps are fine......but transformers does not like the sine.
How can I "reshape" a bad sine?

You can not reshape the AC mains... unless you generate your own by using inverters and batteries.

Do you live in a house that may have reactive loads connected to mains supply? Do you live in an industrial area?

You could also try this: you have two monoblocks... so, try swapping the primary wires around on only one monoblock transformer. That may do something

Or, plug the monoblocks straight into the mains wall outlets... do not use power boards, if possible. Maybe that will help...

Also -> unplug everything in your household from the mains supply, and then see if the mains waveform changes🙂

 

[MEPER]

I live close to central Copenhagen in a flat.
I can ask the distributor of electricity if I can get my money back. I pay for a sine wave not for something like this......
What I need to check is how the waveform looks like when transformers are silent and DC offset stable within 1 mV.
It is easy to swap primary as I can just turn the mains connector 180 degree. Even that male power connector is Schuko the other end is symmetrical (not Schuko but works).
The good thing is that even in this condition amps plays nice music......
Would something like this work?
https://www.audioquest.com/Niagara

 

[Zen Mod]

Here a better view of the shape out of the isolation transformer after I have aligned the line in Gnd mode exactly at the center line. Sine is not 100% symmetrical around the Gnd center line.

those flats on top and bottom of sine are DC

make DC blocker then observe your sine

 

[MEPER]

The last image was from secondary of isolation transformer.
I looked inside the box with isolation transformer and there are diodes and two capacitors that usually forms a "DC trap"?
Now only one diode is visible in image. In fact there are four. Two between the two caps and one on the outside of the caps which I think are those that are involved in "DC trap".

 

[Extreme_Boky]

The DC trap will trap the residual DC (put your oscilloscope on DC when you are checking the mains waveform, and then see if there is any residual DC... of let's say 20-30V..???); however, it can not correct the flat peak-to-peak mains waveform.

Try unplugging all your appliances (fridge / deep freezer in particular!!!), and then see if the waveform has a round top end.

If the sound is okay, maybe you can learn to live with it...?

 

[Zen Mod]

well, it is DC blocker , though I prefer something more substantial ( regular 25A metal cased diode bridge, wired appropriately) ...... and even two of them wired in series, so DC blocking is extended to sorta double level

is there possibility that these caps are old and tired?

as you already implied in post #11 ?

as I see it - easiest and fastest is that you scramble some parts from drawers and make one quickie DC blocker , with fresh parts

as always with Rod Elliot - no frills : Mains DC and Transformers

 

[MEPER]

The last scope image is in DC mode and in Gnd mode the trace was adjusted to the Zero line. There was not jump (as I could see) when shifting between AC and DC mode. Only the flat top was more like a "down hill" in AC mode.
The freezer is off in the moment. Then I have only the fridge and maybe it is not the same "phase". I can check waveforms in different outlets.
I think now I know what it is.....I can probably live with it. But I want to see when no noise and DC offset stable that waveform looks more like a sinus......

 

[MEPER]

The caps are old (+20 years)

I have seen that there are many variations of DC blockers.....

There is also the Sjöström Audio version:
Sjostrom Audio - DCT01 The DC trap, high-end style

But I have two 25A bridges as far as I remember......then I can remove more than 0.7V DC......?

I can look at Elliot......

 

[Extreme_Boky]

Ok, so there's no DC.
Why don't you remove the DC blocker then...
Check with the mains electricity supplier/authority, send them the photo of that waveform and see what they have to say!

 

[Zen Mod]

The last scope image is in DC mode and in Gnd mode the trace was adjusted to the Zero line. There was not jump (as I could see) when shifting between AC and DC mode. Only the flat top was more like a "down hill" in AC mode.
The freezer is off in the moment. Then I have only the fridge and maybe it is not the same "phase". I can check waveforms in different outlets.
I think now I know what it is.....I can probably live with it. But I want to see when no noise and DC offset stable that waveform looks more like a sinus......

flat crests of sine are not exactly what we are talking about when we are mentioning DC in mains ( which is more easily explained as asymmetry of sine - mid line being moved up or down ref. to neutral) ......... but flat tops are exactly the same in nature as clipped sine at output of your amplifier ........ and we know that's exactly the thing which is killing tweeters

xformer will complain wherever it sense steady value of current , be it from one or another reason

DC blockers are capable to solve slight sine asymmetry while those flat crests are hardly solvable without resorting to your own Niagara

proper isolating transformer , with differential secondary and with non-toroidal core ( so EI) usually is making problem lesser, (slightly softening flats) but with price of own humming, so simply taking away some (humming) burden from your equipment to own trouble

I'm sure there are guys with serious and proper insight in power correction problems, maybe some of them will chime in ....... as in many other cases , I have doubts that complain to your local power company can get you any good

if you have proper voltage , they're always sure that everything is proper

 

[grataku]

MEPER,
that's some fugly AC you got. Sorry about that. That can't be good for other appliances either. Are you friend with your neighbors or have a friend in the area? I would suggest you test the AC elsewhere in the building or in homes nearby. The power is not supposed to be like that. Honestly I have no clue what would cause clipping in the AC like that.

 

[MEPER]

I will check other outlets in my home to see if it looks better elsewhere.
We have lifts but they do not run all the time. Then we have in the Parking cellar a lot of the "light tubes" and some air flow cleaning which maybe can cause something to the AC line. Maybe we have too many windmills.....or it could be the waterfalls in Sweden causing this 🙂

 

[MEPER]

Even with a bad looking sine the DC offset stability is sometimes better then when it is worst. I think it has something to do with the symmetry of the sine and how stable it is. Even that I have an isolation transformer and a weak DC blocker a heavy duty version could make the symmetry better.......maybe......it can never make it worse.
The bad sine seems to be all places in the outlets around. I will buy a heavy duty filter and spend DIY on something more fun.......
Think I will get this one ready made:
DCT03 DC filter for toroidal transformers - heavy duty version

It will be about double up in price compared to a rough calculation if I should buy the components on my own. The 68000/16 caps in good quality is not for free.

 

[MEPER]

Maybe I will skip the isolation transformer and the old line filter and use the box for the DC blocker only......

 

[Extreme_Boky]

As I stated before, I do not believe the DC blocker will help in your case. You might as well remove it and gain in sound quality improvement.

Contact the electricity supplier and ask them to check the root cause for mains not being within the specifications in your area.

 

[MEPER]

I can contact them.
If the DC blocker can ensure a symmetric waveform it could help the DC offset instability problem. DC blocker should prevent the sine wave (or what we call it) jumps slowly up and down. We will see.....now I try it.....