Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: antieatingactivist on April 05, 2010, 12:11:50 am
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I want to build one of these. I frequently find myself playing through 2 amps at once. 2 options are living with the awful 60hz buzzing, or breaking the earth ground, which is unsafe. I know that radio shack makes these devices with a 1:1 transformer inside that get rid of the hum when plugging your iPod into your car stereo. I want to use this same concept for an A/B switch. I would rather get just the transformer from mouser.com, but I have no idea what to look for. I guess I could just buy one from radio shack and remove the transformer from it. Any advice? Is this a bad idea?
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I just read that lifting the ground on one of the signal outputs might be the best option. Does anyone have any suggestions on designing the best passive AB switch?
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Radial engineering makes an A/B/Both switch that has a ground lift. Worth a try.
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I just read that lifting the ground on one of the signal outputs might be the best option. Does anyone have any suggestions on designing the best passive AB switch?
For what you are trying to do, the "best" option is to have a 1:1 transformer that goes between your guitar and 1 of the amps. That breaks the ground loop without undermining the safety ground on each amp. You would need to check phasing to make sure that the isolated signal isn't getting phase-reversed and cancelling your straight signal; fixing that is as easy as flipping either the primary or secondary wires.
If you didn't need the A/B switch part of things, you could mount the transformer in a small box with a pair of 1/4" jacks and be done. At least 1 jack has to be isolated from the box unless you use a plastic box (so that you don't couple the jack grounds through the box's metal).
I'm not sure which transformer is the best choice, but power handling is no concern, just the impedance of the primary and secondary. I think 10k:10k will work for you, but it might drop your signal strength.
If your box also had a pair of stomp switches (1 of A/B, 1 for Y), then you have your pedal done. But pedals are cheap when you're not paying for a transformer, so you can probably buy one for as-much or less than you can make it, unless you already had the parts lying around.
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I found this.
http://www.jensen-transformers.com/as/as013.pdf
these transformers are pricey, but the other option is using a buffer with some cheaper 1:1 transformers (geofex), which I don't really want to do. I already have all the parts to make a simple A/B switch, so the only thing I would need to get would be the transformer
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Ok.What noise are you referring to?The noise when you have a guitar cable unplugged on the guitar end?
If that's it,then simply build a no-noise ABY switcher.It simply grounds the cable that isn't being used.Same as if you pulled the cord out of the amp and the input jack grounds the hot side.
Or if it's the power cord,disregard this.
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Ok.What noise are you referring to?The noise when you have a guitar cable unplugged on the guitar end?
If that's it,then simply build a no-noise ABY switcher.It simply grounds the cable that isn't being used.Same as if you pulled the cord out of the amp and the input jack grounds the hot side.
Or if it's the power cord,disregard this.
The kind that only happens when you play out of 2 amps at the same time. Eliminating the 3rd prong on the power cords makes it go away.
I'm basically trying to build my own Lehle AB switch. Not just to same money, but just to do it. Is the mixing knob on these things just a simple signal mixer? or does it do something sophisticated?
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If your'e playing through 2 amps, Plug a 3 prong to 2 prong adapter on one of the amplifier's cords but leave the ground on the other amp. This way you still have ground protection and remove the ground loop.
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If your'e playing through 2 amps, Plug a 3 prong to 2 prong adapter on one of the amplifier's cords but leave the ground on the other amp. This way you still have ground protection and remove the ground loop.
Works good!
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If your'e playing through 2 amps, Plug a 3 prong to 2 prong adapter on one of the amplifier's cords but leave the ground on the other amp. This way you still have ground protection and remove the ground loop.
I figure if you are going to remove a ground, why not lift the ground on the signal to one of the amps? I don't really want one of my amps grounded through my guitar.
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A/B, just break ground along with signal.
> playing through 2 amps at once
That needs an A/B/Y.
> radio shack makes these devices with a 1:1 transformer
These work quite well from <<100 ohm sources such as dash-heads and iPods.
Guitar impedance runs far over 100K. With these transformers the signal will be weak, gutless, and dull.
> these transformers are pricey
It is possible to wind trannies to work good at higher impedances. It aint cheap.
The infamous R.G. has studied the issue and gives this low-cost solution:
http://www.geofex.com/FX_images/humfree2.gif
Yes, it is not passive, and not even a simple one-9V-battery job. However people say it works.