Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on December 27, 2011, 12:19:32 pm

Title: Ground Loop Breaker - don't remember about it
Post by: kagliostro on December 27, 2011, 12:19:32 pm

Time ago there was a discussion explaining the reason for this circuit

(http://www.pmillett.com/images/DCPP.h8.gif)

I've lost part of my archive so, if someone can give help explaining the reason for which in some commercial amps is present that "Breaker" it will be nice to know (thanks)

the attached image is from an amp build by Pete Millet and he give this explanation

Quote

I isolated the AC safety ground, which is connected to the AC plug ground and the chassis, from signal ground using a "ground breaker" circuit.  This is an arrangement of series / anti-parallel diodes that presents a high impedance unless there is a voltage of more than two diode drops (about 1.4V) across them.  These diodes are large - they must be sized such that enough current can flow through them to cause the fuses to blow if there were to be a short between signal ground and the AC line.

as told a fuse blow if there is a short between signal ground and AC line

But usually the signal ground isn't directly connected to the chassis and the fuse will blow if there is a short ?

is the purpose of this circuit to avoid ground loop with other equipment ?

I really don't remember what was previously explained

Thanks

Kagliostro

Title: Re: Ground Loop Breaker - don't remember about it
Post by: kagliostro on January 27, 2012, 01:37:05 pm

Quote

is the purpose of this circuit to avoid ground loop with other equipment ?

Today by chance I found the answer

see page  13 and 14 of this pdf that can be downloaded at Merlin's website

http://valvewizard2.webs.com/Grounding.pdf (http://valvewizard2.webs.com/Grounding.pdf)

Kagliostro

Title: Re: Ground Loop Breaker - don't remember about it
Post by: jeff on January 28, 2012, 02:37:22 am

 That is some great info. This should be a must read for all future amp builders. Changed my life. Haven't made a noisy amp since.

The two most important things I learned from that:

-Always connect your plate resistors directly it's the supply cap. Never hitch a ride on the wire connecting the dropping resistor to the supply cap because it's convienent. Use a seperate wire if you need to.

-Always connect each gain stages ground to it's supply caps ground point. The grid and cathode of one gain stage can share a wire to it's supply cap's ground point but don't daisy chain all the grounds or share a wire from different stage to ground.

Some of my amps are built on "trees" and to look at the you'd say that's got to be noisy but they're pindrop quiet. I've built other amps which have "pretty" broards with everything laid out nice and straight with the caps on one side of the board and all the grounds daisychaind that were noisy as hell.

 What I've found in my experience is that layout keeps the oscillation away, but good grounding keeps the noise away.