Ralph,
I have to disagree with the previous poster. In all cases where the output transformer is an isolation transformer (99%), one terminal of the secondary should be earthed.
The reasoning is that in the unlikely but possible event of a breakdown between primary and secondary, high voltage could appear on the speaker leads, causing a safety hazzard.
The 1% of transformers that are autotransformers (autoformers), should not be used with external speakers at all.
This applies to normal amp topology. There may be exceptions in cases of unusual topology.
I agree that only one connection to ground is necessary, so check if one exists.
Cheers,
I have to disagree with the previous poster. In all cases where the output transformer is an isolation transformer (99%), one terminal of the secondary should be earthed.
The reasoning is that in the unlikely but possible event of a breakdown between primary and secondary, high voltage could appear on the speaker leads, causing a safety hazzard.
The 1% of transformers that are autotransformers (autoformers), should not be used with external speakers at all.
This applies to normal amp topology. There may be exceptions in cases of unusual topology.
I agree that only one connection to ground is necessary, so check if one exists.
Cheers,
Care should be taken however where an amp uses an AC/DC power supply as opposed to the normal mains transformer - designs for which are occasionally discussed on this forum. In these cases there could be a shock hazard if the speaker negative is connected to what might be an earthpoint, but in reality is the chassis and therefore connected to one side of mains. In these circumstances an isolating cap could be used - 0.1uf at 600-1000 volt working should be adequate.
Also, in some designs for both AC and AC/DC amps I've come across in the past the output transformer has had a seperate winding for the feedback circuit and the speaker connections were independent from connections elsewhere in the amp. Connecting one side of the speaker to (chassis) earth in these circumstances could affect the operation of the feedback arrangements.
Also, in some designs for both AC and AC/DC amps I've come across in the past the output transformer has had a seperate winding for the feedback circuit and the speaker connections were independent from connections elsewhere in the amp. Connecting one side of the speaker to (chassis) earth in these circumstances could affect the operation of the feedback arrangements.
dhaen said:
Well, the "previous poster" did some checking around and came to the conclusion that, yes, apparently most OPT couple amps do ground one end of the secondary.
I understand the idea that there is an enhanced level of safety by grounding one end of the secondary, but this practice is in stark contrast to pretty much every other application of transformers with the exception of BALUNs in RF.
A primary to secondary short would be difficult to achieve with modern magnet wire insulation materials, and unless the transformer is bifilar wound (difficult to impossible for greater than 2:1 turns ratio), the windings won't even be in physical proximity. Given that grounding one end of the secondary greatly reduces the transformer's ability to reject common mode noise (ie - RFI/mains hum picked up on the speaker leads), it would seem very disadvantageous from a high-fidelity perspective to do such.
However, I am open to be educated on this particular application because I mainly design switching power supplies, and I suspect what makes a good transformer there is not completely applicable to use in tube amps.
Hi Jeffreyj,
Sorry if my post seemed impersonal and abrupt. That was not my intention. I merely found myself at odds with your post.
Regarding primary to secondary breakdown, it is indeed unlikely, but possible. A typical, quality output transformer is usually wound in a segmented form. The primary windings are wound in each segment in series, then the secondarys are wound immediately on top, but connected in parallel.
Cheers,
Sorry if my post seemed impersonal and abrupt. That was not my intention. I merely found myself at odds with your post.
Regarding primary to secondary breakdown, it is indeed unlikely, but possible. A typical, quality output transformer is usually wound in a segmented form. The primary windings are wound in each segment in series, then the secondarys are wound immediately on top, but connected in parallel.
Cheers,
???
Hi,
What's the difference?
Let's look at the OPTs' secundaries...if we ground the 0 to 8 Ohm tap or the 0-4 Ohm to ground why would that matter?
Once grounded the same laws apply to it...
Oh...what's the point in discussing a safety issue that might not just happen once in lifetime anyway?
Living on top of Dante's peak or what?
Ksshhh,
Hi,
What's the difference?
Let's look at the OPTs' secundaries...if we ground the 0 to 8 Ohm tap or the 0-4 Ohm to ground why would that matter?
Once grounded the same laws apply to it...
Oh...what's the point in discussing a safety issue that might not just happen once in lifetime anyway?
Living on top of Dante's peak or what?
Ksshhh,
dhaen said:
Ah, good. I thought maybe I'd have to send the
over to have a little chat with you... I italicized the above part because I am having a tough time visualizing what you described...
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?
jeffreyj said:
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.
dhaen said:
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
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/voedingstrafo_drie_stuks_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
My powertransformer looks something like this http://www.ae-europe.nl/fotos/voedingstrafo_drie_stuks_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
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...
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...
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,
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,
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:
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:
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:
No such thing on this forum!
EC8010 said:
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:
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:
Ppfff ... and I was thinking this would be the most simple question possible
No such thing on this forum!
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?
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?
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
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
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.
Much more important is the common mode noise rejection that comes from floating the speaker, you can beat that.
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).
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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