why would changing transformers...

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karatestu
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#1 why would changing transformers...

Post by karatestu »

result in hum?

I swapped a 500VA toroid for a 100VA EI in a pre amp power supply. It is a seperate box to the preamp itself. All i did was swap twin secondary, centre tapped toroid for an EI.

To rule out emi from the ei being the problem i made up a 2.5 metre cable and placed the power supply as far away as i could from the preamp (2.5 metres). Turned everything back on and the hum is still there, at the same volume. When the volume is turned up i cant hear it and the music sounds awesome.

I am stumped as to what is causing this. Any ideas ?

Thanks, Stu
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Nick
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#2 Re: why would changing transformers...

Post by Nick »

I assume that the 100va transformer is enough to supply the pre?

The best solution is to start with measuring the hum on the power supply rails.

Have you tried going back to the toroid to check the hum i not caused by something else that happened when you made the change>
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Paul Barker
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#3 Re: why would changing transformers...

Post by Paul Barker »

1/ torroids have less magnetostriction, especially when that size doing an easy job. The ei modern commercial spec ei's are underwound for cheapness which increases magnetostriction,

2/ torrid magnetic path is self contained. ei it sprays out.

most likely botho of the above.

reason to keep the ei. Some ability to handle dc as even those whose assembly is alternating interleaf direction (so strictly speaking no gap) there is effectively a small gap whihc handles small dc offset better than torroid which has no tolerance for dc ofset.

To eliminate 1 and 2 a/ specify overwinding not underwinding b/pot
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Nick
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#4 Re: why would changing transformers...

Post by Nick »

Confused now, what have you gone from and too?
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karatestu
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#5 Re: why would changing transformers...

Post by karatestu »

Thanks for the advice.

The 100VA is more than big eough for the load which is 65mA in total. I dont have a scope or know how to use one, sadly. The EI was purchased from Avondale audio and they use them extensively with no problems.

I will switch back to the toroid and check the hum disappears. It is a shame as i prefer the sound with the ei (when the volume is turned up to drown out the hum).

Stu
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karatestu
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#6 Re: why would changing transformers...

Post by karatestu »

All i did was change transformers, nothing else was changed. The toroid was a standard Naim issue nuvotem in a highly modified hicap. The 100VA ei is what Les at Avondale audio uses to modify hicaps and build his own power supplies.
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karatestu
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#7 Re: why would changing transformers...

Post by karatestu »

Wired the toroid back in and no hum . What the feck is going on. Even with the EI transformer and psu 2 metres from the rest of my gear there was still hum of the very same volume as when the EI was 40cm from the preamp.

Puzzled of North Yorkshire.
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Nick
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#8 Re: why would changing transformers...

Post by Nick »

Whats the test of the circuit like? Are the toroid and EI the same output voltage? If there is a regulator after them, if the EI voltage is a bit lower the regulator may not have enough headroom.
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karatestu
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#9 Re: why would changing transformers...

Post by karatestu »

Hi Nick,

Toroid after rectification gives 39V, EI gives 36V. There is a reg in the preamp enclosure. It is called a tpr4 and is a gyrator followed by a lm317 based tracking pre regulator. I did lower its output voltage to 30V so it has 6V to work with the EI. Maybe i will lower the regs output some more and see if it makes a difference - trim pot makes it easy to do.

Thanks, stu
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#10 Re: why would changing transformers...

Post by karatestu »

Lowered regulator output voltage to 24V so it has 12V of headroom - still hum.
Connected frame of EI to mains safety earth and still hums.
Tried a different, identical EI transformer and it still hums.

Going to try a different rectifier and cap bank with 30.000uf and see what happens.

Stu
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#11 Re: why would changing transformers...

Post by karatestu »

Tried different cap banks & rectifiers and the hum persists.

Am now thinking further down stream i.e the wiring in the preamp , but I am not sure how that would highlight this difference between hum toroid and EI.

Transformers are both centre tapped twin secondary but only toroid has interwinding electrostatic screen
I have ruled out EMI from proximity of the EI
Different rectifier & cap banks show no difference
Two identical EI show the same level of hum
Giving the regulator lots more headroom to work with did not change the level of hum

My preamp uses the Avondale 821A single ended, zero feedback transconductance modules. Regulator is right next to the preamp boards.
I use a star earth in the pre. The 0V connection from the cap bank goes straight to the regulator and from there to the preamp board 0V connection and from there to the star 0V which connects to everything else I.e vol pot, sources, 4 monoblock power amps. There is only one link from signal 0V to mains safety earth and that is in the CD player. Wiring from cap bank to regulator is twisted pairs but wiring in the pre itself is not twisted as the runs are so short. Cases are all off at the moment.

I was thinking about shared impedances with the way I have 0V daisy chained from cap bank to regulator to preamp board to star ground but I remember when I changed to this arrangement way back and I thought it was a slight improvement over what I had before where the reg OV was wired to the cap bank and from there went to the star ground - preamp board 0V reference wire went straight to the star ground. But it does not explain why it is fine with the toroid.

Looks like I might have to stick with toroids for some reason, although I have never heard anyone have this issue before and it is really bugging me. I would really like to know what property of EI design is giving me this problem.

Stu (sorry for the essay).
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Nick
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#12 Re: why would changing transformers...

Post by Nick »

Its chaos here at the moment, but as you are in Leeds/York and I am in Halifax, maybe bring it over one weekend and we could point a scope at it to see if that shows anything.
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karatestu
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#13 Re: why would changing transformers...

Post by karatestu »

Thanks for the extremely kind offer Nick. I may take you up on that but I am going to play around some more first and Les at Avondale audio (Chesterfield) has also offered to scope it for me. I might be in touch but I am going to try and sort it out blindly by myself.

Hope I don't drive myself mad in the process. They say tracking down the source of a hum can often do that.

Stu
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Nick
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#14 Re: why would changing transformers...

Post by Nick »

Yep, Les is a good guy, say hi from me.
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#15 Re: why would changing transformers...

Post by Mike H »

Image

Watching.

.
 
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