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Old 22nd September 2007, 01:19 AM   #1
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Default Explanation for this problem?

Hi,

I have a problem which I have solved (so I guess it is not really a problem anymore) but I was hoping for an explanation.

When my amp has no input leads plugged in, it is dead silent even at full volume. When the input leads are plugged in (no signal) there is a very soft 50hz hum through the speakers at full volume.

Now, the input jacks are connected to earth via a 10 R resistor as per the circuit diagram but when I bypass this resistor and connect the earth side of the input jack directly to the signal earth bus bar the hum goes completely.

What's the explanation for this? Why put in a resistor when it causes audible hum?

Appreciate any thoughts.

Cheers,

Rob
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Old 22nd September 2007, 03:36 AM   #2
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First, since the amplifier is dead quiet without the leads connected, I think this clears it of any wrong doing other then having 10 ohm resistors where they shouldn't be. Having them there sounds like some strange new-age design philosophy that doesn't work.

You didn't say if the other end of the leads were connected to another componet or just floating. But I think the 10 ohm resistors are elevating the input low side just enough to let the leads act as an antenna picking up a tiny amount of ambient power field hum which permeats all around us.

If the leads are connected to other componets, this elevation may also be causing minor little ground loop issues. Since things work well without the resistors, then simply get rid of them. They sound like bad design to begin with. A good example of too many parts.

My 2
Victor
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Old 22nd September 2007, 04:06 AM   #3
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Hi Victor,

Thanks for the explanation - I realized that the input cables were acting as an antenna but not why. The hum occurred with or without anything attached to the other end.

The only thing that concerned me a bit is that the design that I used was the Dynaco ST 70 - I would have thought this was a tried and true circuit. That's what really surprised me. If I had used some wacky schematic I found on the internet it wouldn't have really concerned me but I would have expected that such an established design as the Dynaco would not have these issues! Maybe it was just the way I built it.

Cheers,

Rob
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Old 22nd September 2007, 05:04 AM   #4
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Yes Rob,
I went and looked at the ST-70 diagram in the transformer catalog and you are correct. The 10 ohm resistors are shown alright. It's screwy. They did have a power take-off scheme for a preamplifier they sold. The ground return for that also shows a 10 ohm resistor going to the same point on the buss. I really don't know what they were thinking when they did that. Possibily some form of decoupling between the two units for some crazy reason. Or perhaps they were intended as sacrifical resistors incase one external componet had a "hot" chassis. But I'm just guessing.

Victor
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Old 22nd September 2007, 05:39 AM   #5
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Default Re: Explanation for this problem?

Quote:
Originally posted by Rob11966

What's the explanation for this? Why put in a resistor when it causes audible hum?

Hi,
From what I understand (and have done) is that the resistor is there to stop contamination from the star (chassis) ground to the signal ground. The idea is that electricity will take the path of least resistance, letting all of the "dirty" power ground go straight to earth and not to the signal ground.
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