Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: archaos on December 07, 2010, 04:57:01 pm
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Playing several amps simultaneously using several audio routers/switchers I'm getting both foreseen & :cussing: ground loops...
As far as I'm fond of Merlin's tips through that forum, his website, & his books (ordered for X-Mas...), & since I've read his "Grounding and Ground Schemes", I've discovered & enjoyed that the hum-loop block network -attached- totally eradicated ground loops between amps or other gear, such as stereo FX.
Indeed I've plugged each amp with 2-prong cords (w/o earth) + a 50R 12W resistor connected to the earth with one test probe, & to the amp ground with another test probe. Actually, I tried to be close to the ground scheme attached.
Yet I have now some questions in so far as it has worked for me :
Was I totally wrong if the resistor wasn't connected between the power output stage star & the chassis ?
Would it be safe to connect the resistor between the earth bond & the chassis ?
Which resistor values/ratings are conceivable ?
Thanks for your answers.
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Indeed I've plugged each amp with 2-prong cords (w/o earth) + a 50R 12W resistor connected to the earth with one test probe, & to the amp ground with another test probe.
Your statement is not clear. Did you: a) ground the amp circuitry internally; then b) connect the chassis externally to Earth through the 50R 12W? If so, then it appears that there is no hum block circuit inside the amp where (Merlin says) it belongs. Also, if your hum-block circuit is properly located, your resistor seems to lack the required bypass cap; and it's value is too large -- should be 10R not 50R. I see your point that, theoretically, an external resistor in the ground line should isolate an amp chassis from ground loop connections through the house wiring. But this is not Merlin's circuit and does not give hum block protection inside the amp. An external resistor in the ground line may also violate the Electric Code, and be just plain dangerous.
Otherwise, please post a LAYOUT diagram (not a schematic), maybe photo's too, of the actual placement of your wiring and components.
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Your statement is not clear.
Sorry.
Did you: a) ground the amp circuitry internally; then b) connect the chassis externally to Earth through the 50R 12W?
Yes, that is very well summed up.
But this is not Merlin's circuit and does not give hum block protection inside the amp. An external resistor in the ground line may also violate the Electric Code, and be just plain dangerous.
That's what I thought ; actually I just wanted to do a test to check if the ground loops went away or not. Yet I indeed assumed the resistor connecting the chassis to earth, prevented the ground line to act as it should.
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Would it be safe to connect the resistor between the earth bond & the chassis ?
Absolutely NOT. The chassis must be firmly connected to mains earth (dirt) with wire. Think of the chassis as a physical extension of planet Earth.
Hum-loop block components can then be used to link the chassis to the rest of the audio circuit.
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Another approach:
Use the modified star grounding scheme as follows: Each block of the amp has its own star (preamp, power amp, supply). These sub stars are connected together at the chassis to the main star with the mains ground wire. Make sure you isolate the input jacks from the chassis using insulating washers, and the jack ground is connected to the preamp star ground. This has worked well for me.
There's a lot of good info on grounding and shielding available from the AES (Audio Engineering Society). Bill Whitlock gives frequent presentations on the subject at conventions, and probably has some stuff on line. I also briefly cover this in "Design and Construction of Tube Guitar Amplifiers."
Bob
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Another approach:
Use the modified star grounding scheme as follows: Each block of the amp has its own star (preamp, power amp, supply). These sub stars are connected together at the chassis to the main star with the mains ground wire. Make sure you isolate the input jacks from the chassis using insulating washers, and the jack ground is connected to the preamp star ground. This has worked well for me.
Actually my DIY tube amps are exactly built like this, & they are dead quiet when used in a mono configuration with just a guitar plugged in & eventually one or more mono FX(s).
Things are just becoming critical when the amps are used in stereo, when the signal is splitted.
Anyway, thank you for your answer Merlin !
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Have tried the hum-loop block circuit inside the amp & properly connected : it works perfectly too ! :toothy12: