Is it possible to have DC on the secondary of an output transformer. It is DC between secondary to ground, NOT across two secondary taps. This happens on a push-pull no-feedback amp so secondary is not grounded. Primary has 360VDC and I measured 130VDC from secondary to ground! I even desoldered all the wires just to make sure I am seeing this right. Is it because the insulation material inside the transformer at fault? This is the first time I see this happens. Before discovering the DC, I was able to play music through my speakers and luckily nothing touches ground!
I searched online and could only find one case like this in another forum.
I searched online and could only find one case like this in another forum.
schematic
It's my friend's diy amp with potted unknown transformers based on the Western Electric 86 circuit. It's a zero feedback design. WE is known for not using feedback from the secondary so even in their amps with feedback, it's taken from the primary. Anyway, all their secondaries are not grounded.
It's my friend's diy amp with potted unknown transformers based on the Western Electric 86 circuit. It's a zero feedback design. WE is known for not using feedback from the secondary so even in their amps with feedback, it's taken from the primary. Anyway, all their secondaries are not grounded.
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I would suggest that there is a fault in the circuit. No one designs amplifiers that have high voltage on the speaker terminals, including WE. Anyone injured by such an amplifier would have an easy case in court.
An ungrounded secondary is dangerous, and perhaps it is just leakage that would go away with a resistor
connected from the output to ground. If so, the resistor should be installed permanently.
> unknown transformers ... Western Electric 86 circuit..... their secondaries are not grounded.
W.E. knew their transformers were well insulated. We do not know the insulation in unknown transformers.
Put 10k from output to ground. (10k is a fair approximation for dry skin.) If the voltage goes WAY down it is safe (for now). While 40V in 10k is 'legally safe", coming out of unknown potting it would scare ME. If the insulation is really good, it should be far below 1V DC. (There will be some stray AC voltage due to inevitable capacitance.)
W.E. knew their transformers were well insulated. We do not know the insulation in unknown transformers.
Put 10k from output to ground. (10k is a fair approximation for dry skin.) If the voltage goes WAY down it is safe (for now). While 40V in 10k is 'legally safe", coming out of unknown potting it would scare ME. If the insulation is really good, it should be far below 1V DC. (There will be some stray AC voltage due to inevitable capacitance.)
It's dangerous and foolish to not ground the secondary of the output transformer. Nobody cares about your sorry butt, but children, pets, other civilians matter. Don't be that guy.
All good fortune,
Chris
ps:: It doesn't matter what some idiot on the internet says. You're responsible - be responsible.
All good fortune,
Chris
ps:: It doesn't matter what some idiot on the internet says. You're responsible - be responsible.
Who says that an OT's speaker winding needs to be grounded? So you please should have asked Mr Mile Nestorovic of McIntosh why he didn't ground the secondaries in his well known and well respected MC-3500/MI-350 power amplifiers. Sadly, he has passed since many years 
.
Back to topic: I'd also regard the OT as faulty if it presents any DC leakage.
Best regards!
.Back to topic: I'd also regard the OT as faulty if it presents any DC leakage.
Best regards!
Who says that an OT's speaker winding needs to be grounded? So you please should have asked Mr Mile Nestorovic of McIntosh why he didn't ground the secondaries in his well known and well respected MC-3500/MI-350 power amplifiers. Sadly, he has passed since many years
Back to topic: I'd also regard the OT as faulty if it presents any DC leakage.
Best regards!
You say there's no grounding with the MC3500?
Here's the schematic of the OPT....... hmmmm.
Attachments
zero DC after grounding secondary
Thank you for all the comments. I grounded one side of the secondary and the DC is gone. After almost an hour of playing, double checked the secondary, zero DC.
You're probably on the money.
Here's the schematic.
Thank you for all the comments. I grounded one side of the secondary and the DC is gone. After almost an hour of playing, double checked the secondary, zero DC.
You're probably on the money.
Here's the schematic.
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All the transformers i have tested sound better (to me) when the secondary is not grounded. Still waiting for my amp to kill me
Indeed - don't want shocks with lethal voltage from amp systems... If all your speaker connections are isolated properly (say using SpeakON connectors), this is less of a risk (but then you have to use speaker grills to prevent contact with the drivers themselves). Definitely should ground somewhere on the secondary side to guard against internal faults.
All the transformers i have tested sound better (to me) when the secondary is not grounded. Still waiting for my amp to kill me
Over the years I have run across more than one output transformer with a low resistance primary to secondary short. I would not tempt fate.. FWIW I have noted that grounding the secondary sometimes degrades HF response a bit due the add capacitance between primary and ground - even so I still do it.
Some of my amps have over 1kV on the primary of the output transformer, even a small leakage current could be lethal under these conditions so I grounded the secondary without further thought.
I once got a brand new output transformer that had the primary shorted to the laminations and end bells!
Check your transformers.
Then, in case there is a short as time goes by . . .
Scrape off the insulation where your mounting screws and lock-washers contact the end bell mounting holes when you build the amp.
Check your transformers.
Then, in case there is a short as time goes by . . .
Scrape off the insulation where your mounting screws and lock-washers contact the end bell mounting holes when you build the amp.
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