Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smackoj on August 16, 2015, 02:56:08 pm
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I have been trying lots of different combinations of amps (both tube and SS), effects preamp boxes, and several different types of preamps. I try SS preamps with tube power sections and SS power sections with tube preamps. Basically trying all the older gear I have acquired with other pieces, some of which I have built and some pieces I have bought to see if there is a combination that excels in tone, power and 'vintage cool'.
What keeps rearing it's ugly head is many of these combinations cause buzzing and humming problems. I know a lot of newer gear has a "ground lift" switch to combat this problem, but none of my home made or older stuff has such a switch in it. How would I try to lift the ground on a power amp or a preamp? How do you know which piece of gear to lift the ground on? Is there a simple rule of thumb for ground loop problems or is it more like trial and error tweaking?
thanks much amigos. :icon_biggrin:
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There's lots O reading here on that problem. Have tried bolting a beefy ground wire chassis to chassis, try rotating one chassis 90deg to the other, isolate one ground with a cheater *might be a shock hazard*, plug them both into a computer grade isolation buss strip, probably 20 other ideas but my body and brain are fried!
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... combinations of amps (both tube and SS), effects preamp boxes, and several different types of preamps. ...
What keeps rearing it's ugly head is many of these combinations cause buzzing and humming problems. I know a lot of newer gear has a "ground lift" switch to combat this problem ... Is there a simple rule of thumb for ground loop problems or is it more like trial and error tweaking?
Most grounds loops which result in hum/noise are created when you have cables connecting multiple pieces of gear (not so much an issue with a loop inside any one item). Often, these have their own power cords/supplies, and also have a cable connecting the items which has both a hot and a ground. In this case, the loop is from one piece of gear, through the connecting cable ground, to the other piece of gear, to its power cord ground wire/prong, through the wall, to the first piece of gear's power cord ground wire.
The resulting large "loop area" gives a better chance that one ground is not at exactly the same voltage as all other grounds, which results in ground current. Long lengths of wire of non-zero resistance provide the possibility of setting up a hum voltage when the ground current flows through the non-zero resistance. And the large loop looks like a big antenna for pulling buzz out of the air.
To solve the problem, you break a ground connection somewhere along the loop. Anywhere works in theory, as all other points are still connected together, same as if you undo the clasp of a necklace. Where to break the loop?
The old-school expedient was to break the ground prong off the power cord of one item, then the hum stopped. But that ground prong is ultimately connected to your chassis and is there is trip a circuit breaker if high voltage connects to the chassis (so you don't get zapped). So breaking that is a bad idea.
I never tried breaking the ground connection within a guitar cable connecting pieces of gear, though in theory it might work. I'm not comfortable suggesting to try that, though someone here might have done it before.
If you had a small transformer (1:1 ratio, 2x leads for the primary, 2x leads for the secondary) you could place it in-line with the connecting cables between pieces of gear. Signal is relayed from one side to the other, but there is not direct ground connection. Loop broken. In fact, Whirlwind makes such a device for XLR cables (http://whirlwindusa.com/catalog/black-boxes-effects-and-dis/transformers-isolation-devices/isoxl) but I bet there's a version somewhere for unbalanced guitar cables (aside from plugs, you could just use Whirlwind's device and connect the pin 1 shield to the nominal ground wire of your guitar cable).
How would I try to lift the ground on a power amp or a preamp?
Unless you choose to use an isolated input jack and add a 1:1 input transformer (instead of in-line as described above), you probably don't want to attempt ground lift inside an existing piece of gear. Reason is that you need to think about isolating the entire circuit ground from the chassis throughout the amp, which it connecting at only one point (next to where the power cord ground wire connects to the chassis). That's not trivial and is best incorporated when the amp layout is designed (I did just such a thing with my last amp). The ground lift switch also usually has some other components in series from circuit ground to chassis, which are shorted by the switch in the "Ground" position. In the "Lift" position, those components usually break the connection at d.c. and low a.c., but retain a connection at radio frequencies. The Lift position is actually really noisy if you are playing through only that one amp.
So the added in-line transformer is probably the easiest solution. Whirlwind isn't the only manufacturer, just the first link I found...
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The signal ground elevation in Fig 15.16 at the end of this article works
http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf)
I've used this on a couple of stand-alone reverb units and a stand-alone vibrato unit already. Totally works
In fact the whole article on grounding is useful in general
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OK; breaking the ground loop in the signal cable, NOT the amp or preamp chassis. That is a much safer idea. Thanks gents
:icon_biggrin: