Please help fix an Adcom hum?

tom t

i just dont want to get the amp back and it works good here. i would be paying for shipping both ways for nothing

danger boy

tom t wrote: »

danger boy, how far are you from him. it wouldent hurt if you took the map home with you to hook it up to see if the hum goes away. then we would know if there is a problem with the amp or not

he and I did talk about that.. since he's tried just about everything else that was talked about without spending $$ on a power conditioner.. i'll see if he has time this weekend to bring the amp over here or I could always pick it up too

i only live a few miles away. (like less than 3 miles)

danger boy

and it's a beautiful, powerful sounding amp.. it's a beast of an amp. but the hum would bother me too.. it's loud enough that on soft parts of a song you can hear it in the back ground.. and my hearing isn't all that sensitive either.. so i'll continue to work on a solution with Brent and see where it goes from there.

tom t

thanks, that sounds good. with a problem like this, having it hooked up in different houses and systems would decide if the amp really has a problem or not. i have no problem taking it back if something happened to it in transit. thanks for all the help trying to fix the problem. tom t

tom t

o,no. that means if you get it to your house and the hum goes away, will he get it back???

danger boy

tom t wrote: »

o,no. that means if you get it to your house and the hum goes away, will he get it back???

well i'm sure my SDA's would love the extra power from the GFA-5500


oh yeah, i guess I could give it back.

tom t

well, if its powerfull sounding, then we know one thing. its working!!. i think if there was a real problem it would shut down or something. the fuses on the back are not blowing,right?. the amp is not going into protection. if its putting out good powerfull sound, the hum has to be a side affect from something in the house.

tom t

i have to gi now, let me know how you make out. thanks

BottomFeeder

Hi guys,

Been slammed at work for the past few days & trying to get some things taken care of as I'll be heading out of town for a while next week.

Will be working on the amp this weekend, intending to git r done, one way or another!

Latest: connected the negative speaker posts on the Adcom w/a larger strand of speaker wire and, though I only played it very momentarily through 1 speaker, the hum was reduced still more. So that's positive anyway.

Will listen more & through both speakers tonight or Saturday and, if necessary, take the big fella over to DangerBoy's joint to see what happens over there. (Thanks DB!)

Will also disconnect the cable tv & see what happens w/that.

Question: Why would the Adcom have a, I think it's being called, "line in voltage" problem when my NAD never has? And if it's just cuz it's a different kind of amp (common vs. non common ground?), would I be better off w/an NAD amp? (Fear not, Tom, the answer to this question will in no way impact my decision to keep / return the Adcom! Just askin'! I love the sound of the Adcom & want to keep it sans the hum! If we can get rid of the hum &/or find out it's just a problem w/mi casa, the amp is mine. Fret not.

tom t

no problem brent. i hope you solve the problem because its a nice amp and will bring better proformance to your system. just keep plugging away at it. keep in touch. tt

tom t

try some .0 gauge ---about a half inch round. that aught to do it!!!

BottomFeeder

Ok!

Thanks to everyone for all of your excellent and caring advice!

& Props to DarqueKnight for telling me about connecting the two negative speaker posts on the Adcom together. Having done so, there is now a barely discernible hum - I can only hear it when my ear is smack dab on the speaker itself.

So, we're cool & now I can sit back & listen to this baby power my RTA12C's!

It sounds terrific.

Thanks very, very much everyone!:D:D:D

tom t

FREE AT LAST, FREE AT LAST. GOD O MIGHTY,WE ARE FREE AT LAST.

and a special thanks to darqueknight and dangerboy for all the help.
and the rest of you guys. MR T

bigyank

danger boy wrote: »

anyone know what kind of screws might be on his Adcom amp? they should be some kind of standard ones right, unless someone replaced them somewhere along the way.

SAE Allen wrench is what is needed with my GFA-5800.

BottomFeeder

bigyank wrote: »

SAE Allen wrench is what is needed with my GFA-5800.

Yeah, took a closer look & figured that out all by my big boy self!

Learning. Slowly.

BottomFeeder

tom t wrote: »

FREE AT LAST, FREE AT LAST. GOD O MIGHTY,WE ARE FREE AT LAST.

and a special thanks to darqueknight and dangerboy for all the help.
and the rest of you guys. MR T

That's really good!

danger boy

that's great news.. now you can enjoy that beefy amp

ChromeDomeAudio

I know I'm new here, but I'm going to jump in and help us all out at the same time.

Obviously I found this thread because I have the very amp on my bench and it's driving me nuts. Much like you guys.

But I can help you out....

1st off, this problem is a result of a component level failure.

2nd. Try running the amp with only one RCA patch cable plugged in.

No hum...

There is a 10 ohm DCR reading between the two channels negative output terminals (The black ones)

However, there shouldn't be.

This amp is a dual mono block setup, sharing a common ground (Chassis) and a power transformer. (Dual secondary)

If you look inside you will notice that the negative speaker jack is wired to the ground of the RCA jack, which is isolated from the amp chassis.

If you break that connection. Black speaker jack, ground on RCA jack, the amp gets REALLY pissed.

This is because of the insane level of negative (Or positive depends on your ground reference) feedback.

This feedback is vital due to the nature of the N channel FETs that have really high gain. Without the feedback it runs away and has lots and lots of distortion.

So all of this being said, by plugging in two RCA cables you are in effect bringing the outputs of the amp together. ( The black jacks)

Also, real quick. This has absolutely nothing to do with cable TV wires, fans, dimmers or anything else for that matter up to but not limited to Santa Clause at the North Pole.

As for the fix.

I'm not there yet.

I had to put it down for the day or I was going to shoot it and the customer wouldn't like that.

The bottom line here is, it's an unbelievable problem and as of now, I give ADCOM a capitalized F for their ignorant design.

I have Polk speakers and did have an ADCOM amp, and this repair makes me happy I sold the POS on ebay.

Not to hurt anyone's feelings. But this problem should not even be a possibility.

I will know for sure what the deal is with it by tomorrow.

And will let anyone who desires the answer know.



Try this folks...

Take two iPods, use a 1/8th inch to female RCA adapter for both. Play one song on one and another on the second.

Hook #1 iPod up to the right channel and # 2 iPod up to the left.

The amp will play marvelously.

It just won;t be the same music coming out of both channels.

The reason for the iPod is to ensure you do not share grounds.

Adam
CDA

ben62670

I have had many Adcom's that have been great. Did you get the service manual from them?

heiney9

This is NOT a known problem and must be a pretty isolated incident. Good luck with your repair.

Systems

ChromeDomeAudio wrote: »

There is a 10 ohm DCR reading between the two channels negative output terminals (The black ones)

However, there shouldn't be.

Are the grounds of the 2 power suppllies tied together?I have seen instances with some dual mono designs where small values of resistance have been used to isolate the grounds of each channel from the other and the chassis.

heiney9

GV#27 wrote: »

I have seen instances with some dual mono designs where small values of resistance have been used to isolate the grounds of each channel from the other and the chassis.

I believe this is the case with the 5400; 5500; 5800 and 5802.

ChromeDomeAudio

Ok,

What we're all dealing with here is an A/C supply ground reference problem.

Just like in a tube amp that uses 6 volt heater taps.

You have to use two reference resistors.

I,e. 100 ohm from each leg of the secondary to ground.

Except, 100 ohms won't work for the voltage we're talking about.

This works fine until one of the resistors swings out of value vs. the other.

Because when this happens, you are now loading the A/C to ground unequally, which creates potential, which creates noise which creates...

Business!

No just kidding.

The only thing is, I already buttoned up the amp and it was picked up so I can't go look at the number to tell people.

Oops.


The bottom line is, this is circuit failure in the amplifier. Not a wiring problem in your house or fans, or cable tv wires....

ChromeDomeAudio

I don't mean to come off like a smart mouth.

This amp just really really irritated me. But I guess, I and everyone for that matter needs to be challenged from time to time.

I've just gotten comfortable in my ability and this one threw me for a loop.

They really aren't bad amplifiers. The problem was still ridiculous if you ask me, but it was more of a "Who supplies the part" issue then anything else.

The part shouldn't have swung out of tolerance as bad as it did.

But cold weather and humidity can do these things.

If someone can post a picture of the internals of the amp, I can point out which parts are suspect.


These amps should be very quiet when everything is hunky dory.

And I have to admit, that my GFA 5500 drove the hell out of my RTA-15TL's.

It was a good combo, but I'm a tube guy.

I figure if you can build your own tube gear, why mess with SS.

That and why buy someone else's design.

Actually, when I was a kid, I couldn't afford other peoples stuff, so I learned how to make it myself.

Systems

ChromeDomeAudio wrote: »

I,e. 100 ohm from each leg of the secondary to ground.

Huh,are you saying Adcom have installed 100 ohm resistors from the secondaries to chassis ground?:eek:consider me sceptical as that makes no sense,it would serve no purpose.

heiney9

Here's a thread about the 5800 (which is the same design) that I posted at the DIY forum about common ground issues and SDA's.

Anatech especially, along with the other guys are former factory authorized techs who did warranty work. Perhaps some of their posts could be helpful with this issue. These guys are not hacks and in many instances are professional repair people.

See post #4 especially

http://www.diyaudio.com/forums/showthread.php?s=&threadid=65855&highlight=

H9

ChromeDomeAudio

Ok, I read what he said, and that's what I said... In so many words.

Still a sill **** design if you ask me, but no ones asking so...

megasat16

I read the link H9 provided too but my head started to feel hurt so I stopped reading back and forth between the posts.

I am not familiar with the Adcom in discussion but 200ohms in series between the secondary windings the transformer is not really necessary. I think they deliberately trying to make a false common ground at the speaker output.

I really need to see the Schema of such half **** design and try to understand why Adcom had done it. Could any of you please do me a favor by posting a service manual or schematics here?

Systems

OK the link confirms what I posted previously, that the purpose of the resistors are to isolate each channels ground from one another and chassis ground.They are not used to connect the secondaries to the chassis ground.

megasat16

Thanks GV! Saves me from the head spinning!