Center tap offset 12vac

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I bought 2 1.5kva toroids from a guy on the forum a few years back... I had an ground hum issue with one of the toroids...

Here is the issue. The center (gnd) tap of Toroid A is showing a 12 volt difference compared to Toroid B.

Both are installed in different power amps. Amp with toroid B, plays well in my system, but addition of amp with Toroid A, introduces noticeable Hum.

The Toroids have dual primaries and dual secondaries.

The voltage between the secondaries of Toroid A, is a rock solid 43.3Vac on both secondary's. (Same as Toroid B)... however with both Toroids hooked to mains, there is a 12vac across one ground to another.

This potential difference increases to 27vac when I reverse the polarity of the mains primaries feeding the bad Toroid.

What do you think could be the issue when the primaries of both Toroids show +/- 56vdc when hooked to amplifiers... ? just 12vac across the center taps from one Toroid to the other?


PS: I have measured a 0v (gnd) with the good Toroid B and other toroids and amps... while Toroid A always shows some voltage. It pretty much confirms why the amp introduces hum when hooked to this Toroid A, although without a pre-amp connection and by itself, the amp shows no hum... only when hooked to other power amps in the system (presumably via RCA grounding).

How can I troubleshoot this Toroid assuming I have began unraveling the shielding of Toroid A...
 
I think we need a diagram. Remember that the potential of the secondary CT depends on what you connect it to. A secondary by itself is floating. You may have a wiring error in the amp which uses the B transformer.

Added:

I have removed the toroids and am measuring raw AC off the secondaries. I think there is a problem with Toroid A.

Test Point 1 (TP1) to TP2 measures 12 vac.
TP2 to TP3 measures zero volts
TP1 to TP3 is also 12volts ac.

Does this explain it some more?

Oh yes, I can measure the ampereage across TP1 and TP2 if you want, but I know it will be high... it is not as floating as we think.

Lastly, reversing the polarity of the primaries (not shown) of Toroid A, the voltage difference jumps to 27 vac. I think this is important info for anyone who winds toroids.... I think toroid A secondaries are phased out by a certain degree that it not zero or 180 degrees... maybe 30 or 40 degrees... I have never wound a toroid, so I have no clue about what I just said...

One more comment... while I cannot see this, is it possible that a short in the windings could cause this issue?
 

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Something unusual about the equipment which uses A. As I said, secondaries are floating - they only gain potential with respect to ground when they are connected to something. One exception: the transformer insulation has broken down, but this is unlikely.

It is very unlikely that a signficant phase shift is taking place. Transformer secondaries are either 0 or 180, unless fed through an unusual mains filter which adds lots of phase shift. Unlikely - unless there is something you haven't told us.

Can you put up a diagram of the PSU in A? Your measurements are consistent with a ground point somewhere in the middle of a secondary.
 
I have removed it out of the amplifier, but both were wired the same, even after taking out the Toroid, it exhibits the same 12v difference.

I first noticed the problem when the amplifier was working fine on its own, but as soon as I connected the RCA cables (thereby making connection to the gnd of the rest of the system) I suddenly heard a lot of hum, then measuring the gnd of this toroid vs rest of system which has 6 power amps, this guy had 12v difference.

I have used other toroids in same amp with no issues... by the way the amp is a Pass Forte Model 3, so that is not the issue.... it is this one Toroid.

I am beginning to suspect that one of the windings has shorted out. The Toroid had shielding that seemed to have been tampered with, i.e. fixed with scotch tape... so perhaps the original owner had an issue with them and off loaded them to me... cant say for sure.

In a system with only 1 power amp, this toroid may not have an issue since it will drown out the smaller components and bring them to it's ground potential... it is only in my situation where the system has 6 power amps and this guy cannot influence the others that causes this issue.

I even took a 10 gauge wire to short it's gnd to the other amps, it made the hum go down, but I still measured a 2vac difference in gnd and some hum still creeping in.

I would have been sure of a internal short, but the fact that both secondaries of it are 43.3vac make me suspect this theory.
 
Something unusual about the equipment which uses A. As I said, secondaries are floating - they only gain potential with respect to ground when they are connected to something. One exception: the transformer insulation has broken down, but this is unlikely.

It is very unlikely that a signficant phase shift is taking place. Transformer secondaries are either 0 or 180, unless fed through an unusual mains filter which adds lots of phase shift. Unlikely - unless there is something you haven't told us.

Can you put up a diagram of the PSU in A? Your measurements are consistent with a ground point somewhere in the middle of a secondary.

I think we are ignoring the elephant in the room :p

Why does it jump from 12v to 27v when I invert the polarity of the AC mains connected to the primaries? I think this is the key clue. In normal toroids, inversion of primaries does not alter ground potential does it?
 
I bought 2 1.5kva toroids from a guy on the forum a few years back... I had an ground hum issue with one of the toroids...

Here is the issue. The center (gnd) tap of Toroid A is showing a 12 volt difference compared to Toroid B.

Both are installed in different power amps. Amp with toroid B, plays well in my system, but addition of amp with Toroid A, introduces noticeable Hum.

The Toroids have dual primaries and dual secondaries.

The voltage between the secondaries of Toroid A, is a rock solid 43.3Vac on both secondary's. (Same as Toroid B)... however with both Toroids hooked to mains, there is a 12vac across one ground to another.

This potential difference increases to 27vac when I reverse the polarity of the mains primaries feeding the bad Toroid.

What do you think could be the issue when the primaries of both Toroids show +/- 56vdc when hooked to amplifiers... ? just 12vac across the center taps from one Toroid to the other?


PS: I have measured a 0v (gnd) with the good Toroid B and other toroids and amps... while Toroid A always shows some voltage. It pretty much confirms why the amp introduces hum when hooked to this Toroid A, although without a pre-amp connection and by itself, the amp shows no hum... only when hooked to other power amps in the system (presumably via RCA grounding).

How can I troubleshoot this Toroid assuming I have began unraveling the shielding of Toroid A...

Have you tried switching the toroids between the amps ? could be bad filter caps in amp A .
Is the mounting of the toroids ok ? no short winding with the mounting , use brass or nylon bolts

Cheers ,
Rens
 
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The results are consistent with an insulation breakdown partway through a winding, shorting to the transfomer frame/core. Unlikely in a toroid, as usually the primary is next to the core? However, if it has been 'got at' by a previous owner then anything is possible.

Changing the polarity of the supply to one transformer will swap from any adding to subtracting of AC (or vice versa). I am still puzzled, as you could have two strange transformers!

A secondary with a single short to ground will still gives its usual secondary voltage, but absolute measurements will show that the winding is no longer floating. A shorted turn is quite different, and will cause voltage reduction and severe heating.
 
I have reinstalled toroid A back and the DC rails are a solid +/-56.1 (both rails symmetrical) after unwiding the electrostatic screen and separating the gnd coils a couple of mm one way (just in case they were shorting) then put heat shrink tubing on them to further insulate.

I tried the toroid in the amp installed but NO BOLT.. and it seems to run fine without offset issues... this is perhaps good news.

Next I will try with the bolt installed and tighten it.... I fear tightening might bring back the issue... but hoping it does not.
 
K-amps said:
Next I will try with the bolt installed and tighten it.... I fear tightening might bring back the issue... but hoping it does not.
Yes, of course - I should have thought of that. The clamp washer is cutting through the outer layer of windings and so shorting to ground partway along the winding. I said that could be happening, but I couldn't imagine how. I had forgotten about the huge metal washer often used with toroids.

Assuming the winding is basically intact, just add some extra insulation - usually there should be a rubber washer under the metal washer. Check the washer for burrs left from when it was stamped out, and file them away if necessary - watch your fingers as they could be sharp. Don't tighten the bolt more than is necessary to securely hold the transformer.
 
Yes, of course - I should have thought of that. The clamp washer is cutting through the outer layer of windings and so shorting to ground partway along the winding. I said that could be happening, but I couldn't imagine how. I had forgotten about the huge metal washer often used with toroids.

Assuming the winding is basically intact, just add some extra insulation - usually there should be a rubber washer under the metal washer. Check the washer for burrs left from when it was stamped out, and file them away if necessary - watch your fingers as they could be sharp. Don't tighten the bolt more than is necessary to securely hold the transformer.

I think it is ok for now... by now I mean not connected to the rest of the system, but just comparing to Toroid B/Amp B, the ground shows 24vac but shorting the 2 grounds together brings the potential to 0 volts... even on an mV scale it is like 0.21mVac... this did not happen before.

Final test to come tomorrow... I am done with this today. Thank you for your support.

PS: It already had a rubber washer which I cleaned and reinstalled. No burrs on the washer... but the toroid itself had windings crossing over each other... significant pressure (like if the amp is stored upside down during shipping) could fuse the 2 windings...

Will update once it is introduced to the family :eek:
 
K-Amps,
I use the resin that is being used to join electrical AC cables. It is a small bag with two different chemicals that is mixed without getting your hands dirty, I think every electrical shop should have them. They dry quite quickly. Don't get it on your hands !
 
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