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Old 6th August 2003, 03:18 AM   #11
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Quote:
Originally posted by jeffreyj
Are you saying that each winding is composed of many "mini-windings" of, e.g., 10 turns each, and that the mini-windings for the secondary are connected in parallel while the ones for the primary are connected in series?
It depends on the manufacturer. My Lundahls have 4 seperate primary and 4 secondary windings. The primaries are connected in series and the secondaries can be configured in a number of different ways depending upon the turns ratio required.

A guy I had some correspondence with a while back told me that the primaries on some of the Partridge OPTs I have consist of 19 seperate windings.

I don't see any practical need to earth one of the secondaries, though I do understand the potential safety issue. Getting struck by lightning is probably more likely.
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Old 6th August 2003, 03:31 AM   #12
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Quote:
Originally posted by dhaen
Hi Jeffreyj,

Regarding primary to secondary breakdown, it is indeed unlikely, but possible.
Cheers,
and its very interesting to note that if you ground the sec, its much more possible since there is now a reference and a place for the shorted current to go.

leave the secondary floating and it will do just that... it won't present a reason for the current to "break through" i'm not sure what happens when a floating sec "floats" up to B+ and then sees a 50K load to ground (sweaty finger kneeling on wet concrete)... i suppose the small charge would drain and if that causes full pri-sec breakdown its lights out...

i see a cathc 22 here and am not sure of the answer... any thoughts??

and fwiw i do have and listen to an autoformer output amp... i just use a negative supply in the cahtode and ground the plate... placing an autoformer output at HV is indeed insane.... now where did i put those autoformer ESL stepups :-)

dave
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Old 6th August 2003, 08:35 AM   #13
Ralph is offline Ralph  Netherlands
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Ppfff ... and I was thinking this would be the most simple question possible

My powertransformer looks something like this http://www.ae-europe.nl/fotos/voedin...vogelpersp.jpg with on the input 0-230V-SHIELD.

My 2.5K:8 outputtransformer looks like http://www.ae-europe.nl/fotos/ae_5k_se_ugt.jpg with connections labeled VA (from PSU) - 0 (to 300B) - 8 - 4 - 0
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Old 6th August 2003, 08:52 AM   #14
EC8010 is offline EC8010  United Kingdom
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There are two safety schemes in operation worldwide. Both rely on two independent barriers each capable of withstanding the enclosed voltage between the you and that voltage. Class I uses insulation and a conductive barrier bonded to earth. Class II uses two independent layers of insulation. Adding extra layers of insulation to an O/P transformer changes its HF response, but bonding the secondary to earth effectively adds a conductive barrier to the interwinding insulation, making the O/P transformer Class I.

Of course, you could argue that the loudspeaker leads and connectors themselves should be completely insulated, and that it should never be possible to come into contact with them.

Frank: Agreed, it's a "once in a lifteime" hazard. Trouble is, it could enforce the end of a lifetime. DC shocks (from the HT supply) are more dangerous than AC.

At the risk of going off-thread, I've seen two electrostatic headphone leads (carrying 500Vpk-pk) that appear to be single-insulated...
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Old 6th August 2003, 10:12 AM   #15
dhaen is offline dhaen  Europe
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Hi,

Few output transformers could conform to Class 2. Twin bobbins would make for poor performance.

Bournvilles point about old "AC/DC" equipment is relivant. Gear of that age was built when safety standards were more lax. Bear in mind that the chassis may be "live" on this stuff, and insulation poor.

Regarding "insulated" speaker cables. Nice idea but no thank you!
Unless you live alone, it's a trap waiting for someone

Cheers,
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Old 6th August 2003, 01:10 PM   #16
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Ok, how about this for a compromise: connect a neon lamp/current limiting resistor from secondary to ground. If a primary to secondary fault occurs, the lamp will light up. An asute builder/user of this amplifier will then note that something is awry - of course, the smoking from the windings that typically accompanies insulation punchthrough will probably be a dead giveaway, as well as the cherry red anode(s) because of the drastically lower impedance a carbon track will present to them, but consider these "supplementary indicators" that trouble is afoot.


EC8010 said:
Quote:
Adding extra layers of insulation to an O/P transformer changes its HF response...


Understatement, indeed. It also greatly increases the amount of leakage inductance. I don't know what effect this will have in audio output circuits, but it wreaks all sorts of havoc in switching power supplies.

dave slagle said:
Quote:
and its very interesting to note that if you ground the sec, its much more possible since there is now a reference and a place for the shorted current to go.


Good point, dave. If one grounds the secondary, then high frequency voltages - from parasitic oscillation and/or ringing, for example - would have a capacitive path to ground. This could cause local heating of the insulation, hastening its demise.

I think the "ground the secondary" rule is a bit atavistic. After all, insulation materials have come a long way from the primitive enamels used when these classic tube circuits were developed. The highest temperature a transformer could reliably withstand back in the old days was probably 60-75C (surface) whereas today it is easy to achieve a 150C operating temperature - which implies a central core temperature close to 175-200C. Vacuum impregnation of the windings also goes a long way towards making the transformer the most reliable part of the amp - even moreso than the chassis!

Ralph said:
Quote:
Ppfff ... and I was thinking this would be the most simple question possible


No such thing on this forum!

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Old 12th August 2003, 04:27 AM   #17
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Default Several points

I am sorry but I just have to respectably disagree.

1) Try telling that to the insurance company and the lawsuit if someone dies or even gets shocked.

2) It only takes once to die. The odds may be long, but when in college it was said of me "it could only happen to you, steve". I can tell at least one story, where a professor told me the odds were in the tens of millions or hundreds of millions to one, concerning an incident that happened between me and a sweetheart at the time. I have had too many weird things happen, so I would rather be safe.

What is the breakdown rating of some OPTs? I have heard around 1750 volts peak to what, say 3000. I can't believe that at, say 800 volts peak, a breakdown couldn't occur. If isolated from ground, it only takes a person touching ground and a connector to form the connection. Why take the chance?
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Old 22nd August 2003, 04:51 AM   #18
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Reserructing this thread again since I'm re-building my amp with new OPTs...

So let's say that I decide to ground the OPT secondary. The only real safety argument against it seems to be that it'll increase the chance of an insulation breakdown finding a path to ground through the secondary. However, since the secondary is grounded, that'll probably hurt the transformer but not a human. So, that's not too bad. The main sonic reason against grounding the secondary seems to be RFI and noise - with one side grounded, common mode noise on both speaker leads would no longer be common mode? (I'm not entirely sure how or why this happens).

* If I ground the secondary, where should it be grounded? The closest audio earth (which would be the cathode bias resistor/cap on the output tube), or straight to chassis/safety earth?

* If I have a 4 ohm tap (which I assume would be midway between 0 and 8 ohms), does it make any difference if I ground the 0 tap, or the 4 ohm tap? Would grounding the 4 ohm tap help with common mode noise in any way?

Thanks,
Saurav
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Old 22nd August 2003, 05:20 AM   #19
grataku is offline grataku  United States
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I will align myself with the NO gnding the secondary side. It is much more likely to get hit by a car than that for the transformer dielectric to be perforated and to get HV volts on the speaker. I'll buy dave and jeffery arguments any day.
Much more important is the common mode noise rejection that comes from floating the speaker, you can beat that.
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Old 22nd August 2003, 11:05 AM   #20
EC8010 is offline EC8010  United Kingdom
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Default Feedback is the decider as to position...

If you need global feedback, you ground the secondary to the amplifier's 0V line at the point where you want to apply the feedback, usually somewhere near the cathode of the input valve. In doing so, you also take care of the safety issue.

I cannot understand why this thread has meandered for so long about whether (or not) the secondary should be grounded. The fact of the matter is that the primary has a high enough voltage to kill, and we must be protected from it. One layer of insulation is not enough. Grounding the secondary converts it into a second safety barrier, and ensures safety. If the amplifier's stability can't cope with a grounded secondary, then it is marginal anyway, and deserves to die.

The secondary is connected to a balanced feeder (the loudspeaker leads), which are themselves connected to a loop aerial (the loudspeaker voice coil). As such, connecting a loudspeaker to the amplifier introduces a path for noise to be injected into the amplifier. If the amplifier has global feedback, the RF noise, which is on both wires, and therefore common-mode, is converted into differential-mode, and injected directly into the most sensitive part of the amplifier. Since most amplifiers roll off their HF to optimise stability, the RF is injected unattenuated into a sensitive point. Possible solutions are:

(1). Ground the chassis of the loudspeaker, and take this back to the chassis of the amplifier. (Tannoy now provide the appropriate terminal.)
(2). Screen the loudspeaker leads. All feeders leak, and by reciprocity, they are all aerials, so screening will reduce the amount of RF picked up. (Incidentally, cheap aerial lead from your TV aerial picks up as much signal as the aerial - except that it picks up broadband noise, whereas the aerial is tuned to the desired band of frequencies.)
(3). Add an LC filter at the output of the amplifier to attenuate RF coming back into the amplifier.

If you don't have global feedback, life is much easier. You could centre-tap the secondary so that there are equal voltages either side. The centre-tap of an 8R winding would be at 2R (impedances are transformed by "n" squared). Of course, this assumes that the transformer has equal capacitances from each end of the secondary to sensitive points. In practice, this is highly unlikely. Output transformers are sectioned to produce best coupling from primary to secondary, not to balance coupling capacitances. In short, from an RF point of view, it won't matter which point of the secondary you choose to ground.

From a safety point of view, the safest place to ground the secondary to is the chassis.

It is just conceivable that picking a different point on the secondary could slightly change the HF response of the amplifier. If you are worried about that, you could apply a square wave to the amplifier and test for which point produces the cleanest leading edge on the square wave. Frankly, I would expect very good test equipment and technique to be required to see any change.

Finally, dead people are deaf. Knowing that glorious sound is washing over my dead body is not an incentive for leaving the secondary floating. Output transformer insulation can break down (I have had it happen).
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