trying to move around "P3", the quest for reducing the 2nd harmonic...
All the things you see on the table is Jet-Propulsion-Lab certified
I was able to reach at 1W/8ohm 0.004 THD and 0.056 THD+N. with P3 centered.
From there i was able to lower to ~0.002 THD adjusting P3. Start listening...
I have a cycling hum from the transformer... maybe the windings are a little loose...
If i have this behaviour also with the transformer behind a variac... i think i can exclude a DC problem on the mains... what do you think ?
All the things you see on the table is Jet-Propulsion-Lab certified
I was able to reach at 1W/8ohm 0.004 THD and 0.056 THD+N. with P3 centered.
From there i was able to lower to ~0.002 THD adjusting P3. Start listening...
I have a cycling hum from the transformer... maybe the windings are a little loose...
If i have this behaviour also with the transformer behind a variac... i think i can exclude a DC problem on the mains... what do you think ?
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as soon as you said the word "autoformer" i felt stupid...
Another hint in the "DC direction" is that the trafo is a 1400VA one, the amp is biased with 3a total @ 55v so 165VA is drawn... nonetheless is getting warm, i think it must be stone cold with that kind loading.
Another thing is that i am using a bridge made of ultrafast diodes (mur3020wtg)... i had read around that they can couse some humming on the transformer too... always used in other projects, and never had an issue, but never on such a big transformer.
the filtering at the moment, per rail, is : 1400VA -> ultrafast bridge -> 44mF -> 160mOhm -> 44mF -> LOAD.
I will try asap the DC blocking circuit... do you think the RodElliot iteration is good enough ? Mains DC and Transformers
Another hint in the "DC direction" is that the trafo is a 1400VA one, the amp is biased with 3a total @ 55v so 165VA is drawn... nonetheless is getting warm, i think it must be stone cold with that kind loading.
Another thing is that i am using a bridge made of ultrafast diodes (mur3020wtg)... i had read around that they can couse some humming on the transformer too... always used in other projects, and never had an issue, but never on such a big transformer.
the filtering at the moment, per rail, is : 1400VA -> ultrafast bridge -> 44mF -> 160mOhm -> 44mF -> LOAD.
I will try asap the DC blocking circuit... do you think the RodElliot iteration is good enough ? Mains DC and Transformers
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feeling stupid and advancing from that ...... is great thing
I'm here to learn... is the process of learning itself that most drive my obsession.
Passing the mains through a Variac will not improve the intolerance to DC of your transformer.
Reducing the supply voltage slightly will reduce the flux level in the transformer and this may give a slightly lower mechanical hum.
I'm here to learn... is the process of learning itself that most drive my obsession.
I was just looking at your supply.
It looks like there is a cable running from the positive terminal of the negative rail to the positive terminal of the positive rail on one of the caps.
There's something definitely related to a dc component because with the dc blocking circuit I've got the transformer much quieter Indeed.
Definitely there's more on the saturation because with lower input voltage the transformers became almoat silent. I think I will redo the Transformer with the company and maybe this is the good occasion to redo the shielding also in a Proper Way.
... I think was a false alarm, i was pretty sure i've solved the winding uhum for good, but yesterday night, when i have fired up again the test bench, the transformer came back in all its humming glory.
So... i think the rect bridge was not the issue.
I will bring the transformers to the factory again and I will ask to re-wind the primary with a 20% more of the nominal flux. I think is a saturation issue.
Attached the bridge i have designed for the discrete mur3020.
C1,2,4 and R1 are not populated
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Fast forward almost 2 weeks.
The transformer has been redone, different core, +30% of the nominal primary winding turns.
Now they are dead quiet. just a very little mechanical noise, that i think is quite normal at this load.
Biased at 320mA per device, 55 volt rails filtered 44mF->3.5mH->44mF.
Nulled the second harmonic by distortion analysis. I enjoy a lot the resolution and details coming from this amp. On my speakers, seems a lot sharp but not hard...
Is time to machine some chassis ...
keep you posted !
The transformer has been redone, different core, +30% of the nominal primary winding turns.
Now they are dead quiet. just a very little mechanical noise, that i think is quite normal at this load.
Biased at 320mA per device, 55 volt rails filtered 44mF->3.5mH->44mF.
Nulled the second harmonic by distortion analysis. I enjoy a lot the resolution and details coming from this amp. On my speakers, seems a lot sharp but not hard...
Is time to machine some chassis ...
keep you posted !
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