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Old 13th May 2009, 10:23 AM   #11
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I reckon Olof is 100% correct.

To drive speakers from an ungrounded source in which there is any possibility at all of B+ or the mains coming into contact with the speaker leads would be foolhardy and potentially dangerous.

The fact that the seondary of the OPT is insulated under normal conditions means nothing if the amp should ever be subjected to severe accidental physical damage due to impact or fire.
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Old 15th May 2009, 02:06 AM   #12
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OK, the inportant stuff has been said.

Just a further small point: Grounding the secondary gives a certain reference to the interwinding primary-secondary capacitance. Ungrounded the secondary floats somewhere between B+ and earth; even that can 'bite' you when touching loudspeaker wires.

Bottom line: While there is no technical 'demand' to ground the secondary in your case, there is nothing against it and every reason from several points of view to do it. (Direct, no resistor.)
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Old 15th May 2009, 02:29 AM   #13
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When 50W was a huge power amps had floating 100V output and a separate feedback winding.
Why?
Because they used to drive symmetrical lines through the whole building so grounding could cause undesired feedback on input lines.
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Old 16th May 2009, 02:32 AM   #14
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OK Wavebourn, admittedly a good reason!

I was thinking more of the domestic case, or at most a stage situation with relatively short cables. Yes, don't remind me. Running sound through a building can be a real nightmare!
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Old 16th May 2009, 03:44 PM   #15
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Thanks again guys. I can see the safety point of it clearly now.

I was concerned that the back EMF of the speaker might somehow influence the smaller driver tubes by modulating the ground, forming a sort of undesired feedback circuit.

My thinking was that if I had the OPT floating (with regard to ground) , the OPT itself would act as a buffer/isolator between the driver and the rest of the circuit.

IIRC, sometimes industrial/commercial AC power transformers are case grounded, but windings are not .




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Old 17th May 2009, 01:20 AM   #16
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Quote:
Originally posted by Nihilist
I was concerned that the back EMF of the speaker might somehow influence the smaller driver tubes by modulating the ground, forming a sort of undesired feedback circuit.
Do recognise that grounding one side of a winding will not cause signals to be necessarily set up in the 'ground'. That will only happen when part of that output circuit is common to the rest of an amplifier. Thus it is important to ground at one point only so that no part of the OPT-loudspeaker circuit (carrying relatively high signal current) is common to the rest of the amplifier to any appreciable degree.
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Old 2nd June 2009, 03:06 AM   #17
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The first amp I built from a diagram I found on the net did not show the secondaries of the opt's grounded So that was how I built it.
Then I built a slightly more powerful amp. I wanted to use the smaller one to drive rear speakers only in a Hafler matrix. Of course it didn't work because there was no complete circuit. I was confused at first, but I remembered seeing diagrams with one side of the secondaries grounded, made the change, and it worked. I realize now I could just have connected the black terminals on my amp together with the same result. After reading this thread and the safety aspect. I sure am glad I grounded those suckers!

Rolf.
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