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Old 12th November 2010, 11:13 PM   #1
tomchr is offline tomchr  United States
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Default To ground or not to ground the OPT secondary, that is the question

Folks,

In my current amp design, the secondary of the output transformer (Edcor CXSE25-8-5k) is floating. This isn't a problem per se, but I'm not a fan of allowing the secondary to charge up to B+ through the inter-winding cap between the primary and secondary.

Should the secondary be grounded at the black speaker terminal or should I leave it floating? If it should be grounded, I take that it should be to the chassis ground?

~Tom
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Old 13th November 2010, 01:47 AM   #2
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Should the secondary be grounded at the black speaker terminal or should I leave it floating? If it should be grounded, I take that it should be to the chassis ground?
There are emotional responses on both sides of this debate, and some have been posted on these forums over the years. Some claim it "clouds the sound".

I always ground one side of the secondary because I found a guitar amp with 300 volts on the speaker leads about 30 years ago. It wound up in my hands when someone tried to make a direct connection between the speaker leads and some SS recording equipment causing smoke to be let out of multiple pieces of equipment.

Modern Edcor OPT's should not short out, but the vintage paper stuff in the south Florida humidity does!

I ground the chassis at one point, usually near (or at) the input connectors. I run a direct wire from that point to the center pin on the power (IEC) connector, and a wire to each negative speaker connector. In the case of the Simple SE and Simple P-P the connection is built into the PC board.
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Old 13th November 2010, 02:00 AM   #3
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Quote:
Originally Posted by tomchr View Post
...

Should the secondary be grounded at the black speaker terminal or should I leave it floating? If it should be grounded, I take that it should be to the chassis ground?

~Tom
two really good articles on grounding, earth and common by regular posters on this site. Shorter one here, longer one here
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Old 13th November 2010, 02:10 AM   #4
jcx is offline jcx  United States
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if the sec has a ct then gnd that but I'd certainly gnd one terminal - output xfmr are seldom built to reinforced/double insulation safety standards and tube supplies can be as lethal as line
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Old 13th November 2010, 02:16 AM   #5
jueic is offline jueic  Taiwan
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Amazon.com: Current-Driving of Loudspeakers: Remedy to the Fundamental Fallacy of Sound Reproduction Technology (9781448695324): Esa Merilšinen: Books: Reviews, Prices & more

Not sure how much Esa's book related to this topic.
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Old 13th November 2010, 04:04 AM   #6
jfitz57 is offline jfitz57  United States
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if the sec has a ct then gnd that but I'd certainly gnd one terminal - output xfmr are seldom built to reinforced/double insulation safety standards and tube supplies can be as lethal as line
I recently got two tube stereo reciever chassis's f or parts. Both from the early/mid 60's. Both of them ground the 4 ohm tap of the OPT's. Could'nt figger out why at first but after thinking about it it's a really neat idea. If you are driving a 8 ohm load (most common) then it makes for a balenced output.
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Old 13th November 2010, 04:23 AM   #7
THD+N is offline THD+N  United States
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I always ground my secondaries as other people have noted. Not only for safety, but some test equipment may not give you accurate results unless the secondary is grounded.

My AP was going crazy when I went to make some measurements (on a Chinese tube amp I bought), and I discovered the secondary of the OPT was not grounded. I tried to compensate by selecting various configuration on the AP, but nothing helped until I grounded the secondary to the ground bus in the amp.
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Old 13th November 2010, 09:19 AM   #8
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in SE transformers, the speakerleads could be coupled capacitively primary to secondary and radiate RF if it can couple back into the inputlead (i had this at a time).
the parasitic capacitance of the total transformer can be smaller if left floating (basically because of a series circuit then, compared to one such capacitance to a low impedance point such as ground), thus the response at higher audio frequencies can be better if not grounded. such can be the case if the secondary is interleaved somewhere in the "middle" of the total primary. (like a PP transformer, but it seesaws around a neutral point usually so usually no issue there)
you may try to find if the secondary has a "cold" end, starting onto the core. you may ground that end without much of that effect. (or the side closest to the cold end of the primary, if that is closer)
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Old 13th November 2010, 12:28 PM   #9
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The McIntosh MC75's OT has the following secondary connections: COM-4-8-16 and a separate winding for the feedback which has one end of the winding internally grounded. There is the option to install/remove a link to ground/float the secondary. Using the link the COM-tap is grounded.
Now iff it works, grounding the 4Ohm tap and using the com/8Ohm would seem like a nice idea. But does it really work that way? Do you get a balanced signal between com and 8Ohm them? What about the 16Ohm tap? Will it really be symmetric to 4OHM=GND? What happens to the primary impedance, does it see what's going on on the secondary in this case?
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Old 13th November 2010, 02:45 PM   #10
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The 4 ohm tap is not the center of an 8 ohm winding. The impedance ratio of a transformer is the square of the turns ratio, so the 4 ohm tap is the center tap of a 16 ohm winding.

Some P-P amps (Audio Research and others) ground the 4 ohm tap and take balanced feedback from the 0 ohm and 16 ohm taps. This can be applied to the output tube cathodes (AR) or the driver tubes. The OPT must be wound with this use in mind (symmetry). A cheaply wound OPT may not achieve a balanced feedback across the entire audio range so attempting a balanced feedback on a cheap OPT may have unexpected results. I tried the AR style cathode feedback connection on some OPT's that I have. It did lower the measured distortion for frequencies below 2 KHz but the HF response was peaky, the distortion was higher, the 10KHz square waves were ugly and the amp sounded nasty.

It works fine on a big Plitron OPT though.
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