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First Watt F7 review
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Old Today, 06:36 AM   #1271
Cortez is offline Cortez  Hungary
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Originally Posted by Nelson Pass View Post
I don't have a wire at the back as you depict.

Your simulation shows 1 amp of ground current through the RCA's?

Good thing I don't have that wire.....
Which wire you don't have? I draw a new image to identify the sections precisely.
If I'm right the Signal and Power GNDs are connected together with 0 ohm on the FirstWatt PCBs: theese are the light green wires.
As there is a common PSU with one, common GND, the 2 channels Signal and Power GNDs are also conencted together with 0 ohm: orange wire.
Assuming a standard preamp, outside of the amplifier the GNDs are also tied together: light-blue wire inside the preamp.
Even if the interconnects look separeted: brown wires.

First issue: a pretty big GND loop is created not so far from a big toroid.

The 2nd thing: even if the hum level is acceptable, a pretty amount of current
can travel via the interconnect GND wire between the wires as shown on simulation: <link here>
and so even the original input signal can be modulated by this GND current.
Or am I missing something in my model..?

  1. If the model is right: why this layout does not cause any major problems regarding sound quality?
  2. Why don't FW amplifiers use the conventional 10R hum breaking resistors between the Signal and Power GND on the PCB?
  3. Extra one: the red/blue PS and the green Power GND wires from PSU to Amp PCB aren't twisted together,
    leaving a pretty loop, increasing parasitc inductance. Wouldn't it be better to twist them tightly?

Thanks in advance for your anserws Mr. Pass!
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Old Today, 06:42 AM   #1272
Cortez is offline Cortez  Hungary
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Originally Posted by lhquam View Post
In either case, the nominal resistance from signal ground to chassis ground is high enough that the external ground is not a problem.
As you can see on my last image the issue is not related with the chassis and the safety ground at all.
This GND loop would be formed even without any metal chassis and safety earth conenction.
Half of it is formed by the internal amplifier GND layout having a direct 0 ohm connection as:
Left RCA GND -> Left PCB Signal GND -> Left PCB Power GND -> Common PSU GND -> Right PCB Power GND -> Right PCB Signal GND -> Right RCA GND
Other half at the preamp: the 2 RCA GND is internally connected together of course.
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Old Today, 09:51 AM   #1273
albertNL is online now albertNL  Australia
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First Watt F7 review
Originally Posted by Nelson Pass View Post
Ground loops have an actual closed loop. Assuming that it is not being
induced via multiple AC-outlet grounded chassis, then it would be the
power transformer radiating into the local grounds with the loop closed
by the two grounds going back to the preamp. If you give these grounds
a low impedance connection to each other at the rear of the chassis then
you minimize this.

Were the two small wires added from GND to chassis (heatsink) in the J2 (picture from 6moons) to reduce hum?
How does this work in combination with a CL60 between GND and chassis?
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