Ground lift for Hoffman dual channel preamp

[Rp3703]

 I am trying to build Doug Hoffman's dual channel preamp to run through a Marshall dual 50 watt tube 9005 power amp. As is common with a two amp arrangement, I am getting what I believe to be ground loop noise. It has been suggested that I install a ground lift and Doug has said himself that he built the original version with only a two prong plug. Researching ground lifts, I came across a PDF written by Valvewizard that suggests using what he calls a hum loop block network. Here is how he describes it:

 To combat the above problem of connecting two devices which both have audio inputs and outputs, some appliances will offer a ground lift switch. This disconnects one audio circuit from the chassis, breaking ground loop, as illustrated in fig. 13.16. Ground-lift switches are usually included in low-level preamps and mixers, rather than power amps. Note that the Earth bond between chassis and mains earth must never be broken; the chassis must remain safely earthed at all times.

An alternative to the ground lift switch is a hum-loop block network, as shown in fig. 13.17. The ground to chassis connection is made via a 10Ω resistor in parallel with a 100nF to 470nF poly’ or ceramic capacitor. The resistance should reduce ground-loop currents to negligible levels and must be a power device, say 5W, so that it is resilient to fault currents which might occur. The resistor alone would make the audio ground more susceptible to radio interference, so the capacitor is added to reduce the impedance between ground and earth, at radio frequencies. Sometimes a pair of high-current diodes (shown faint) are also connected in parallel, to act as a bypass path for serious fault currents. A 25A (or more) bridge rectifier package is quite convenient for this.

However, what if an amplifier has more than one channel, each with its own input? Then we must make a decision about where to make the chassis connection. We must not connect all the inputs to chassis since this would create a ground loop between the channels. If one of the channels has very much higher gain than the other(s) then that is the input we should connect to chassis, since the other channels are likely to be less sensitive to ground loop hum.

If the amplifier has more than one high-gain channel then it may be better to make the chassis connection further into the amplifier where the two channels mix and become one. Since this increases the chances of ground loop hum when other pieces of equipment are attached, as fig. 13.15 illustrated, the connection should be made via a hum-loop block network.

Is this overkill?

Another issue I am having trouble with is that the toroidal transformer I am using in order to cram the preamp into a 1u chassis, has what is labeled a static shield that I am not sure where to connect it to. Is anyone familiar with this and is it meant to be used to prevent ground loop noise as well?

[vibrolax]

Another issue I am having trouble with is that the toroidal transformer I am using in order to cram the preamp into a 1u chassis, has what is labeled a static shield that I am not sure where to connect it to. Is anyone familiar with this and is it meant to be used to prevent ground loop noise as well?

The static shield is bonded to the chassis, just like the mains safety ground.   Of course, you're supposed to use a separate bolt from the safety ground. I cram Antek 50VA toroidals into all my 1U projects.

I've never had to resort to using a ground lift, though I often include them for the ground pin on balanced outputs.  I like using transformer balanced outputs to obtain lower output impedance and easy galvanic isolation from the amplifiers, either balanced or unbalanced.

I don't know if there is such a thing as overkill, when it comes to ground loops.  It's just a question of what external gear your equipment needs to function with.  Sometimes, only real galvanic isolation will suffice, other times raised grounds as Marlin (valvewizard) illustrates will be fine.

A transformer isolator box like the Ebtech hum eliminator is a handy tool for instantly fixing any problem you might run into in the wild, and for evaluating the relative effectiveness of alternative measures.  Edcor line transformers are also a pretty easy way to accomplish this. 

[Willabe]

For a test to see if it is a ground loop try this, put a 3 prong to 2 prong adapter on the power cord of the pre amp. Plug in then as usual. If no buzz you do have a ground loop. If there's still buzz it's 1, the other or both pieces of gear.

Millions of people played through gear for decades with a 2 prong plug and like Doug said the the chassis with the lifted 3 wire safety wire will still be grounded through the guitar cord that's in between the 2 pieces of gear.

Just try it long enough to see if the buzz goes away. Then you'll know.     

[Rp3703]

Thanks for the reply Vibrolax. I guess I should stated that this is a dual channel guitar preamp. Sounds like you were referring to a microphone pre. I'm using an Antek transformer as well, I had just never heard of a static shield.
I will try the two prong test, Willabe, and see what happens. Thanks.

[vibrolax]

Sounds like you were referring to a microphone pre. I'm using an Antek transformer as well.

My first project with transformer balanced outs was a mic preamp.  But my dual fender blackface preamp uses tx balanced main outputs, and my ampeg svt preamp has an auxiliary tx balanced line out.

The value of tx balanced output depends on what might come next in the signal chain and how long the cable might be.

[alerich]

A transformer isolator box like the Ebtech hum eliminator is a handy tool for instantly fixing any problem you might run into in the wild, and for evaluating the relative effectiveness of alternative measures.

I love these little devices. I have all three of my amplifiers rigged up with two pedal boards powered by two separate One Spot style power supplies. Lots of ground loop potential. A few of these Ebtech devices inserted judiciously eliminated all of my ground loop hum issues. Not a permanent fix for the OP's situation but I wanted to give a shout out about these. Love them and I cannot discern any audio degradation for having used them.