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Old 5th August 2003, 10:33 AM   #1
Ralph is offline Ralph  Netherlands
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Default LS terminals to earth?

Simple question, do you have to connect the '-' terminal of the loudspeakerconnector to the earthpoint, for security reasons? If your OPTs fail or someting like that?
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Old 5th August 2003, 10:55 AM   #2
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It is safe to leave the loudspeaker terminals floating with respect to earth. If the OPT is an autotransformer, then the connection to earth will have been made inside the amp, and duplicating that connection elsewhere will only cause a ground loop.

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Old 5th August 2003, 03:54 PM   #3
dhaen is offline dhaen  Europe
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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,
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Old 5th August 2003, 06:26 PM   #4
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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.
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Old 5th August 2003, 06:47 PM   #5
SY is offline SY  United States
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Unusual topology: The Audio Research amps where the centertap of the secondary (4 ohm) is grounded. That takes care of the safety issues, a good thing since grounding the "common" lead will not be a healthy thing for the amp.
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Old 5th August 2003, 10:24 PM   #6
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Quote:
Originally posted by dhaen

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.
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.
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Old 5th August 2003, 10:29 PM   #7
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To get around all of these issues, I would just run an extra wire to the driver baskets, bonded to the earthed, (hopefully), amp enclosure. That way there are no potential risks to people or kit from any problems.
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Old 5th August 2003, 10:50 PM   #8
dhaen is offline dhaen  Europe
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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,
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Old 5th August 2003, 11:34 PM   #9
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Angry ???

Hi,

Quote:
Unusual topology: The Audio Research amps where the centertap of the secondary (4 ohm) is grounded. That takes care of the safety issues, a good thing since grounding the "common" lead will not be a healthy thing for the amp.
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?


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Old 6th August 2003, 12:31 AM   #10
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Quote:
Originally posted by dhaen
Hi Jeffreyj,

Sorry if my post seemed impersonal and abrupt. That was not my intention. I merely found myself at odds with your post.
Ah, good. I thought maybe I'd have to send the over to have a little chat with you...

Quote:

... 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.
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?
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