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Ground loop question? Answered

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Hello , im making a little guitar amp with ROG tonemender as preamp and tda2003 power amp.This is my schematic , can you check it for me please!. Will it get ground loop problem ?? My English is bad so i can't understand clearly some documents about ground loop.

i have 2 seperate board (a power amp based tda2003 and a ROG tonemender as preamp).They will use the same power supply .some thing like this picture http://i247.photobucket.com/albums/gg132/chipmapple/noobiedsn.png

and this is schematic 


6 Replies

gmoonBest Answer (author)2012-02-08

You might find a ground loop on a schematic, but generally they are an issue with implementation--actual wiring, rather than the concept.

For instance, you don't have the power supply for the tonemender in your drawing. If it's the same power supply for the LM386, then you might have a ground loop, if you actually include the ground bus (at the bottom) on your board.

Both Steve and QA give good advice. A star ground is a great idea. However, even star grounds require care--some points early in a power supply shouldn't be attached to the "star." But it's not a problem if the power supplies are wallwarts or batteries.

Here's a few pointers:

-- Traditional (conductive) jacks can cause loops. The jack has a tab for the ground, but the conductive outside also contacts the chassis, which is usually grounded.

While metal body jacks are still used successfully all the time, some people prefer isolated (plastic) jacks. Most modern high-gain amps use the isolated jacks.

-- Done right, the chassis (enclosure) should be a shield only, and not a ground path. This is currently the norm, but traditionally it was not.

-- Shielded wire in certain places is great. But if there's already a ground path between the two points--like between the two sides of your project, then the shield will be a second ground path.

One trick--if you want to use shielded wire, and if there's already an existing ground path, attach the shield only at one end. That way you get all the benefits of the shield, and no loops.

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login721 (author)gmoon2012-02-08

thanks . I just update the schematic : http://i247.photobucket.com/albums/gg132/chipmapple/combineta.jpg

in the real amp, i will have 2 seperate board (a power amp based tda2003 and a ROG tonemender as preamp).They will use the same power supply .some thing like this picture http://i247.photobucket.com/albums/gg132/chipmapple/noobiedsn.png

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gmoon (author)login7212012-02-08

OK, cool.

The power supply diagram looks good to me.

RE: the overall schematic--

Other than the opamp GND pin, I'd have VREF as the ground connection for everything left of the LM386. VREF is your virtual ground.

You can drop the 100K resistor after the tonemender output, and replace both that and the 10K volume POT with a 100K audio taper POT. The POT will act as both the volume cont and the resistor in the RC filter after the opamp (I would still use VREF as the signal ground at that point).

You may not need the 10uF input cap on the 386--it's already capacitively coupled by the 470nF cap after the opamp.

I'm not sure you need a 2200 uF cap on the 386 output. With an 8 ohm speaker, that puts the corner frequency for the cap/speaker combination about 9 Hz--well below human hearing, or the bass response of any speaker... 470uF / 8 ohms is about 42 Hz, still below the response of most speakers...

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johnyradio (author)2012-06-08

Howdy, I'm running the tda2003 audio amp with this kit: http://www.hobbykit.eu/hobbye/h1002.html It's a battery system, no high-voltage AC house current involved. Getting a lot of RF. I'm planning to install in a metal chassis. Should I connect the chassis to the battery negative? also, would the "connect just one end of the cable" tip be a good idea? That means the audio cable, right? Which end? The audio input is a stereo cable connected to a stereo mp3 player, combined into the mono amplifier input, with 5K or 10k resistors on the Left and Right conductors just before they are combined at the amp input, to protect the mp3 outputs from driving each other.

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steveastrouk (author)2012-02-07

Take the grounds from all the blocks to a SINGLE point, and connect that to your supply 0V. Its called a star ground and is the most effective way I know to avoid "hum loops."


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Quercus austrina (author)2012-02-07

Ground loops are the bane of existence to all audio people, be it professional, consumer, or musician. This small board should have no real problems (keyword: small). Just try to route the signal at right angles (90 degrees) to power lines when they cross. For the off-board wiring (potentiometers, switches), make sure you turn the loose wires into "twisted pair" (or "triples" as it might be) to turn those connections into shielded runs.

Don't forget to go back to ROG and add in the VRef voltage divider (lower left corner) and connect the VRef out to all the unconnected Vref points on the Tonemender. And since this is just a cut-n-paste schematic with appropriate connections added, it should work as long as the schematics are good to begin with (which they seem to be).


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