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Ground wire adds hum to Harmony H-400A AC/DC amp

tonebrulee

Member
2008-03-31 12:08 pm
Following on here from my earlier questions about an isolation trafo for my Harmony H-400A AC/DC amp.

Tonight I replaced the power filter caps, yanked the isolation trafo and left the amp setup with just a polarized plug. It is dead quiet now with greatly improved performance!! When I add the 3-prong ground wire though, it introduces significant hum, and the amp also puts out some nasty noise as it shuts down after turning off the power switch. Without the ground wire - no hum and silent shutdown. Is there something about adding a ground wire to the chassis of an AC/DC device that I need to address?

I anyone can provide any insight from looking at the schematic I would be grateful.
 

korey

Member
2008-03-04 2:00 pm
That is one of those transformerless amplifiers where one side of the AC line is live. Those amps can be a little tricky if you don't know what your doing, but since it wasn't designed with a ground wire, then don't ground it. Just don't be standing in a puddle of water when working on it powered up. DUH!:bigeyes:
Some people will try to tell you that these are dangerous little amplifiers, but these types of amplifiers came out of those portable record players where they were built into the box with the plastic knobs so there was no chance of coming into contact with the chassis.

Just build youself a nice little box (if you haven't already) and you should have no problems as long as you don't expose yourself to the chassis on purpose!:D
 

tonebrulee

Member
2008-03-31 12:08 pm
Thanks for the advice korey. I'd like to keep the amp in it's original box and try to address the ground wire issue, if it's possible. Since it has a polarized plug now and I have done some re-wiring to improve safety, I'm partway to making it safe. But the ground wire would address mis-wired outlets so I'd like to take that last step.
 

tonebrulee

Member
2008-03-31 12:08 pm
Hmmmm - looking at the schematic some more, I'm wondering if there isn't an intrinsic problem adding a ground to the chassis for an AC/DC circuit like this? Wouldn't this effectively bypass SW on the neutral AC mains input? Perhaps the "nasty noise" I'm hearing when SW is opened is due to the circuit taking neutral though the circuit in the reverse direction (note C6) near R1 (after the input jacks) rather than at AC mains neutral?

Reading a bit on star grounding, I'm wondering if I could move SW to AC mains hot and then disconnect (-) for C5A and C5B and tie them directly to ground?
 

korey

Member
2008-03-04 2:00 pm
Personally ,I still would not worry about adding a ground wire. On the SW side of the line is actually considered HOT. Your AC outlet is wired for HOT, NEUTRAL, and GROUND.

If you add a ground to the SW side with C5A and C5B, your actually grounding to the HOT side of the line. Plug it in, and 'POOF'!

ON your schematic, it actually shows no ground. The two pitchfork looking symbals are actually negative (-) tie points in the circuit.

If you look at capacitor C2, if it were to short, it would short circuit the negative side to the rectified positive side coming from your 35W4 tube, and most likely smoke R9, R8 and R5 and probably destroy C5.

Like I said, this is a transformerless circuit and one side is tied directly to HOT. Where-as in a transformer powered amplifier, there is no direct connection tied to the HOT side of the line. Check out this schematic of a transformer powered amplifier.
 
You could try disconnecting the ground of this amp from the powerline neutral (white wire) to the safety ground (green) wire of a 3 prong power cord. That should eliminate the house wiring ground loops and load induced house wire neutral AC voltages that are causing the hum. You're not supposed to, though, from standard practices in house wiring. Issue being, that if any breaks in the safety ground from that outlet back to the circuit breaker panel exist or happen, the ground becomes hot. But the amp stops working if that happens. If you connect the white wire to the green wire, that should minimize that problem, but if the outlet is miswired, you could get a dead short to ground. But that's pretty obvious if that condition exists. Again, the amp wont be working. Always something....

One thing to do, move the power switch electrically from switching the ground connection, to switching the incoming hot wire. That way, the ground stays at ground. And this should solve the nasty noise at shutdown problem. But be sure to dress the hot wires away from low level input signal wiring, to avoid hum pickup.
 

tonebrulee

Member
2008-03-31 12:08 pm
wa2ise - I think I'm missing something here: with the chassis grounded to the 3-prong green wire and powerline neutral/white disconnected from the chassis how would a break in ground cause it to become hot? Maybe you mean a break in AC neutral would cause ground to become hot relative to powerline black?

Connecting back green to white is the original scenario that is causing the hum.
 
tonebrulee said:
wa2ise - I think I'm missing something here: with the chassis grounded to the 3-prong green wire and powerline neutral/white disconnected from the chassis how would a break in ground cause it to become hot? Maybe you mean a break in AC neutral would cause ground to become hot relative to powerline black?

Connecting back green to white is the original scenario that is causing the hum.

Hi, if the green wire were to have a break, the chassis, via the heater string, would become elevated to 120VAC. The heater string resistance is something like 800 ohms, more than low enough to pass enough current to zap you.