To ground or not to ground the OPT secondary, that is the question

[tomchr]

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

 

[Tubelab_com]

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.

 

[aardvarkash10]

two really good articles on grounding, earth and common by regular posters on this site. Shorter one here, longer one here

 

[jcx]

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

 

[jueic]

Amazon.com: Current-Driving of Loudspeakers: Remedy to the Fundamental Fallacy of Sound Reproduction Technology (9781448695324): Esa Meriläinen: Books: Reviews, Prices & more

Not sure how much Esa's book related to this topic.

 

[jfitz57]

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.

 

[THD+N]

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.

 

[jechentau]

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)

 

[Rodeodave]

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?

 

[Tubelab_com]

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.

 

[boywonder]

My speakers require it....

I have a pair of vintage Polk SDA 1's (with the umbilical interconnect between speakers) and they require that the speaker common terminals be grounded (or at least connected together in a stereo amp), especially with monoblocks, so all of my amps have grounded speaker common leads. The alternative is to put an isolation transformer in the umbilical between the speakers.

 

[DualTriode]

The green wire is safety ground!

Hello All,
I use a pair of Electra-Print 8000 : 300 headphone transformers with a grounded secondary center tap. If the secondary was not grounded the headphones would not go on my head.
DT
All just for Fun!

 

[tomchr]

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.

[...]

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.

I'm all about sound quality, but this is a safety issue. I shall ground my secondaries. Thanks for confirming my suspicion.

two really good articles on grounding, earth and common by regular posters on this site. Shorter one here, longer one here

I openly admit that I didn't look at the "long one" in detail. But the "short one" seems like a good primer on signal grounding. I.e. what ground loops do and how to avoid them. For that kind of material, I actually find Ralph Morrison's book, Grounding and Shielding.... to be the authoritative guide on the topic.

In my case, it's more of a safety/earth grounding issue. I'm trying not to kill the cat (or myself) here... I wasn't exactly clear on that in my original question.

Thanks for bringing those links out in the limelight, though. That Valve Wizard site has a lot of easy-to-approach information on it.

~Tom

 

[tomchr]

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.

I've seen that on my HP8903A as well. In my case, easily mendable by flipping the GND switch on the front panel to ground the (-) input, thereby, the secondary.

Note that having both the input and output ground referenced may also give you inaccurate readings on your test gear as it can set up a ground loop. I've seen THD+N figures drop by a couple of dB when the ground loop was broken.

~Tom

 

[rsdio]

Safety grounding requires a metal chassis with the green ground wire attached to it. The chassis serves as a protective barrier between you and any hazardous potential in the circuit, e.g., loose wires. If something bad happens, the safety ground conducts back to the circuit breaker. It really is the circuit breaker that saves your life, and the safety ground that makes the circuit breaker trigger as quickly as possible when there is a fault (well, except for a safety ground fault). You really should not need to ground anything but the chassis.

Headphones might be an exception to this, since they conduct voltages from inside the chassis to outside. I have not heard any opinion on whether grounding the headphone cable shield would be enough to protect against electrocution.

In other words, it's unclear to me why grounding the secondary would be absolutely necessary, unless the headphones could specifically conduct serious current away from the amplifier.

 

[trobbins]

Amps with a reasonable wattage output, especially those with 16 ohm outputs, can have speaker voltage levels that exceed certain safety limits (eg. 32Vrms). This situation is seen as much worse in the eyes of some safety standards if the voltage is floating, compared to referenced to PE (protective earth). This is a simplistic comment - as anyone who has had to navigate product compliance through a variety of safety standards can attest.

If safety is an aim, then I suggest reviewing whether you can ground a ct or tapping on the secondary, which has the benefit of minimising the voltage level on the speaker wires wrt ground.

Ciao, Tim

 

[rsdio]

Excellent point. I recently learned at an AES meeting that speaker wires are treated just like 120 VAC power wiring when it comes to codes for conduit or other built-in wiring runs. Makes sense. I've certainly been shocked by the speaker wires on a guitar amp.

 

[trobbins]

Which I suspect is why most manufacturers are/have transfered to Speakon connectors. Yet another change to make for those who have an aim for safety first - especially at the speaker end.

 

[rsdio]

My impression has always been that speakON connectors make a higher quality connection anyway. Seems like safety, reliability, and performance are all improved by using speakON.

I've certainly modified some of my speaker cabinets to take speakON instead of any of the other standard connectors, partly for the biamping capability. The manufacturer even provided a blank panel since there really was no room left on the standard panel.

 

[trobbins]

Checking the Neutrik Speakon spec shows a 250VAC rating (to IEC 61984), and although no un-mated IP rating was given, it probably meets IP3x. A titch better than a 6.5mm phono jack ;-)