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rtate 22nd October 2008 06:03 PM

possible leach problem??
I was doing some routine maintenance on my leach superamps like setting the bias, measuring the offset and cleaning the grounds.

When I plugged it back in and turned it on I started seeing a little smoke coming out of the top!!
I immediately shut it down and when I removed the cover I found that I had left the "transformer centre tap" disconnected from the
star ground!!

I could not see any damage to the pcb or any components.
None of the fuses blew but I don't know what was smoking!!

When I powered it back up it seems ok, the bias is still at 125Ma , however the DC offset is slightly higher at about 60Mv when it used to be around 15Mv.

Should I be concerned about this?
What are the possible consequences of leaving the transformer center tap disconnected?
The amp is for the Sub in my home theater set up.
Should I hook it back up to the speaker and try it??

Bob2 22nd October 2008 06:22 PM


I can't answer your question about what has been damaged.

I don't understand why you would lift the center tap on the transformer. This is a very dangerous thing to do and you should not operate your amp without it. The center tap is your return to ground, operating the amp without it would have the potential for the chassis to become live and could kill someone! Even testing for amp without is dangerous!


rtate 22nd October 2008 07:39 PM

I had lifted all the grounds to clean the connections(lugs)
and the wire from the transformer had sild under the heat sink and went unnoticed when I was hooking it all back up.It was never meant to be powered up with the wire disconnected...

Would powering the amp under this condotion cause the transformer to heat up i.e. smoke, or would it send the amp into oscillation?

ostripper 23rd October 2008 03:25 AM

I hooked up amp before with no ground reference and
heated up resistors in my CCS's and diff. pairs.

Would powering the amp under this condotion cause the transformer to heat up i.e. smoke, or would it send the amp into oscillation
You may have "stressed" a few of your low wattage resistors
since without a ground reference your current sources
and cascodes were not properly biased when you first
turned your amp on.

Since it survived, you "caught it" in time. resistors and
possibly zener/transistors were damaged but not destroyed

P.s. your amp might have tried to run off your input ground
(10 ohm resistor) while lacking real ground ,check it...

rtate 23rd October 2008 10:36 AM

Thanks ostripper, Thats good news, the zener damage and stressed diiff. pairs would explain the higher offset...
I checked the input ground and it seems ok....

AndrewT 23rd October 2008 10:57 AM

as always:-
use a mains bulb tester when ever you have modified or built any new equipment.

The loss of centre tap has nothing to do with mains safety.

The exposed conductive parts must be permanently connected to Safety Earth.
This rule must never be broken.

The centre tap should never share the permanent connection from the third mains wire to chassis Safety Earth.

The return currents from the power amp probably came back through the signal ground and the signal ground isolation resistor (10r). Check it and the signal wiring in case either have been damaged/melted.

rtate 23rd October 2008 03:40 PM

Thanks Andrew

This is a quote from the leach website construction plans,see "wiring the chassis".
If I'm not mistaken the safety ground and the transformer CT share the same connection
"The following wires connect to the central ground point: the green wire on the AC line cord, the ground wires for the filter caps, the transformer center tap, two ground wires from each circuit board, and the two loudspeaker ground wires. "

Please correct me if I do not understand this correctly...

AndrewT 23rd October 2008 04:35 PM

connect the third mains wire (green/yellow in UK or green elsewhere?) directly and permanently to the Safety Earth connection.
Do not share this connection with any other cable.
Do not break this connection by inserting a terminal strip or any other inadvertent break point.
Do not use a soldered joint. It must be a mechanical joint that cannot fail due to overheating.

Keep the Audio Ground (Star Ground) separate from the Safety Earth.
You can interconnect them but remember that the Safety Earth is permanent, add any additional connector above the locked off nut that secures the Safety Earth.

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