Ground loop problems with ZEN4

[Skorpio]

Hi,

I have one old ZEN4 that has been working nicely for some years and is a dayly pleasure for me
It is build with a trafo with seperate windings for each channel and is completely silent.

Now to the problem...

Recently I hav made another ZEN4 for a friend, build around a trafo with common winding for both channels. And now I have problems with hum!

When analysing the problem it is obvious that connecting the source RCAs are making a ground loop (due to the common GND on the power supply cap). On my older version this is no common point here as there are seperate windings/bridges/caps.


The question is, is it impossible to make this amp work with a common supply for both channels? My guts says no....but after trying several grounding layouts (both "accepted" and "no-go versions") I am beginning to wonder if I can make it...

Is the only way to rebuild the unit to a dual supply (this will be major work on an almost finished amp ) or is there some nice solution for making it work?

One possible solution, that I don't like, is to disconnect the GND on one of the input RCA, opening the loop. But what is someone connects only RCA cable in this RCA socket? Then you have input signal without GND...


Also, reading the original ZEN4 article, Mr. Pass has build a test unit from common PSU....so I have not given up yet

 

[AudioSan]

there is only one way. seperate the signal ground from the power ground.
or build it as a dual mono and separete the 2 power grounds to safety earth with a NTC for each ch.
is it build on a board or P to P?

 

[Zen Mod]

pics

 

[Skorpio]

there is only one way. seperate the signal ground from the power ground.
or build it as a dual mono and separete the 2 power grounds to safety earth with a NTC for each ch.
is it build on a board or P to P?

Hi,

It is build on original ZEN4 boards where all GNDs are made on PCBs. There are only 6 wires on each boards:

-RCA input: two wires (wire + shield) directly from isolated RCA socket
-Speaker output: Two wires (twisted) to speaker terminals
-PSU: Two wires (twisted) from common cap

http://www.firstwatt.com/pdf/art_zv4.pdf, wired like pic on page 4


If i should seperate power and input ground, should i cut the copper on the pcb? What is input GND and what is power GND?

 

[Zen Mod]

connect central grounds of two pcbs with fat wire bus

 

[Skorpio]

connect central grounds of two pcbs with fat wire bus

Ok?

You mean eg the two speaker ground terminals on each pcb joined with a short fat wire...amnd still also going to speaker terminals?

I will try this on sunday

 

[Zen Mod]

central gnd point on channel pcb where gnd wire from PSU and speaker gnd wire are met .

connect that point to same point on other pcb , with faaaaat and shoooort wire

 

[Skorpio]

ZMs suggestion had me wonder and the more thinking, the more I liked the idea...so I could not wait until sunday for at test.
Today I have made the upgrade, placing a 2.5mm2 wire 6cm long between the two pcb grounds.
The result is just plain amazing! The unit were dead silent with no input cables before, but now it is both without and with input source connected...yes!

Measuring with my AC- voltmeter, normally I consider values below 1.5mV as ok. Before mod I measured 0.8mV without input and 5.8mV with input.
After mod the measures say 0.8mV and 1.0mV....

ZM you have made me very happy, warms thanks to you from Denmark!