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Guido
10-01-2005, 03:41 PM
I managed to get a GFA 555-II. This is a rally beefy amp. Amazing headroom.

BUT I have a slight hum when there is no music signal. Is this one of this ground probs?
What can I do?

Thanks for help.

johnaec
10-01-2005, 04:15 PM
Do you get hum with nothing at all, (no cables or anything), connected to the inputs? Have you tried changing the location to see if it's interacting with something else?

John

Guido
10-01-2005, 04:22 PM
Do you get hum with nothing at all, (no cables or anything), connected to the inputs? Have you tried changing the location to see if it's interacting with something else?

John

Nope! With disconnected input cables no hum.
BUT When I connect my preamp to my harman amp I do not have any hum.

For clearifiction I'm talking about a slight slight hum.

johnaec
10-01-2005, 05:00 PM
What about with just cables and no pre-amp? Bad cables? Does changing the polarity, (power plug), of one unit or the other make a difference?

John

Guido
10-01-2005, 05:08 PM
Thanks for help John.

It's 2AM here and I'll check this tmorrow. I'll come back to you with this.

johnaec
10-01-2005, 05:13 PM
It's 2AM here and I'll check this tmorrow.Aw, c'mon Guido - it Saturday! :cheers: :dancin: :rockon1:

John

LE15-Thumper
10-01-2005, 10:23 PM
From my previous work as an ADCOM warranty guy here in Winnipeg, I remember some flameproof ground float resistors opening on a few occasions. If you feel handy and have a multimeter. I would check R104 and R154 (one in each channel) They are 100 ohms-1/4 watt and don't show any signs of heat when they open. They also cause a DC offset which your woofers will not like !!:biting: They are located on the small input board at the rear.

Make sure those fat main filters are at zero volts as well. :shock: Use a 22 or 33 ohm-10 watt resistor to discharge them after the amp is shut off. Although they already have a 3.9k across them according to my schematic, so your fingers should be safe. Always have the amp unplugged and your speakers disconnected as well when doing work like this. I know that may sound obvious, but it is not obvious to all. Good luck, let me know if you need more help

Guido
10-02-2005, 02:27 PM
Interesting, seems to be some kind of ground loop. The adcom amp continues playing music for some 20 seconds after switching of. When I pull the main plug the hum stops even though the music still plays.

LE 15 thumper. With your help I'm sure I can fix it. A schematic will help I PM'd you.

Zilch
10-02-2005, 02:29 PM
When I pull the main plug the hum stops even though the music still plays.Guido has discovered cold fusion again? :p

[It's the residual charge in the main power supply capacitors, actually....]

Guido
10-02-2005, 02:31 PM
Guido has discovered cold fusion again? :p

Unfortunately it's only the old capacitor principle :biting:

But the hum stops so it's an ground issue as LE15 thumper supposed.

P.Warner
10-02-2005, 05:58 PM
I'm grabbing at straws here by assuming you are using this in Germany (230v-50 hz.) If you know that you are working from the same power source and a common (neutral) and ground for all equipment then it's time to get curious about the Adcom's transformer primary. If this 555II was originally 120v then cross check any conversion work with an Adcom schematic.

LE15-Thumper
10-02-2005, 08:37 PM
I'm grabbing at straws here by assuming you are using this in Germany (230v-50 hz.) If you know that you are working from the same power source and a common (neutral) and ground for all equipment then it's time to get curious about the Adcom's transformer primary. If this 555II was originally 120v then cross check any conversion work with an Adcom schematic.


That's a good idea too !!

Guido
10-03-2005, 11:49 AM
I'm grabbing at straws here by assuming you are using this in Germany (230v-50 hz.) If you know that you are working from the same power source and a common (neutral) and ground for all equipment then it's time to get curious about the Adcom's transformer primary. If this 555II was originally 120v then cross check any conversion work with an Adcom schematic.

I think it is an original 230V model but will check this. Waiting for the schematic ;)

boputnam
10-03-2005, 12:18 PM
BUT When I connect my preamp to my harman amp I do not have any hum.Since you're going through all of it, tell us just what are these, and how were they interconnected?

Oh, and do you still get the GL hum from the preamp Adcom 555-II combo when there is nothing plugged into the preamp?

Guido
10-04-2005, 03:04 PM
Since you're going through all of it, tell us just what are these, and how were they interconnected?

Oh, and do you still get the GL hum from the preamp Adcom 555-II combo when there is nothing plugged into the preamp?

Bo you where right!

It was the russound A-Bus Module connected to my Rotel RC-972 that caused the hum. With disconnected A-Bus Module everything is fine.

But now I have another Problem. What to do with the A-Bus? Anyone here with experience in this Field?

Guido
10-04-2005, 03:06 PM
From my previous work as an ADCOM warranty guy here in Winnipeg, I remember some flameproof ground float resistors opening on a few occasions. If you feel handy and have a multimeter. I would check R104 and R154 (one in each channel) They are 100 ohms-1/4 watt and don't show any signs of heat when they open. They also cause a DC offset which your woofers will not like !!:biting: They are located on the small input board at the rear.

The resistors are ok. Funny thing is that the are 50 ohm types in my Adcom.

Thanks for help and the schematic.

Do you have an idea regarding the hum(ing) A-Bus module?

boputnam
10-04-2005, 06:05 PM
It was the russound A-Bus Module connected to my Rotel RC-972 that caused the hum. With disconnected A-Bus Module everything is fine.Nice one, Guido!! :applaud:

With GL's it's always best to start nekked, with everything unplugged from the preamp and then work into it, one component at a time...

Soory - I'm ignorant on the Rotel line... :o:

Guido
10-05-2005, 03:19 AM
What to do with the A-Bus?

Connect it with chinch cables that have connected shield only on one side?

boputnam
10-05-2005, 12:31 PM
What to do with the A-Bus?

Connect it with chinch cables that have connected shield only on one side?No. I think you should try an induction coil - we need to isolate that A-Bus AC/DC power supply converter (that is the problem) from the signal ground.

Here in the States, Radio Shack makes a great in-line GL Isolator.

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-054

I've used these between the pre-amp and the offending peripheral device, when needed. Work great with little / no degredation of signal strength.

Guido
10-06-2005, 03:42 AM
Thanks for the Help Guys.

This Adcom really is a killer. Power without limits on my 250ti.
Can't wait to test it on my 4435 (not finished yet:o: )