Adcom GFA 5300 pop n' ZZZZZZ (help)

[asilker]

Finally got my 5300 back to the apartment after a weekend back homeward. It's plugged in and works ok, both channels working etc.

However there are two irritating problems. First, when it turns on, there's a loud PUP noise. Second, even with no signal from the preamp, I'm getting a low- level, quiet "ZZZZZZZZZZZ" from both tweeters. What's that all about?

I was just using an NAD 2150 in the same configuration, and it was a quiet operator and didn't ZZZ at me, so if anyone can tell me what the issue is, I'd be much obliged. Maybe a recap is needed? Idk!

If it's pertinent, I'm running a pair of infinity reference five speakers, as in my current signature.

 

[JURB]

They got no relay or anything on that it seems. A slight turnon transient is probably normal.

The noise you are hearing, is it there with the volume all the way down on whatever preamp you are using ? If so it might just be shitty cables, or it could be house wiring. Try reversing the power cords, like we used to all the time on turntables. Also, if you have a light dimmer you might never get rid of the ZZZZZZ. I had three one hundred watt bulbs on the ceiling and always kept them down. It was cool, I could have LIGHT if I wanted it, but usually kept it turned way down. It made noise w2hen I had a turntable, not so much on other sources. When I ripped vinyl, I turned it all off including the furnace.

If the ZZZZZ is there with the volume all the way down try a new set of cables to the amp. If that doesn't fix it we go from there.

We'll burn that bridge when we come to it.

 

[asilker]

Thanks for jumping in, jurb. I've tried it with 2 preamps, 3 interconnect cables, and a number of different inputs in 2 houses with 2 different speakers. While listening las night at my apartment, the only light was a 40 watt bulb in a lamp.

In all of these conditions, the noise is there.

 

[audiojones]

Yeah, those were the two things I was thinking too - either bad input cables or a dimmer switch nearby. Turn the amp off and wait for it to fully power down, yank the input cables from the back of the amp and turn it back on. If it's quiet you either have bad cables or they might be routed too close to the power cords, if it's still noisy something else is going on like a dimmer nearby, house wiring or possibly an internal ptoblem with the amp.

 

[asilker]

Bump!

on the DIYaudio forum, I'm told it's likely a poor connection from a smoothing cap to ground. I plan on taking a look this weekend when I have some time. my multimeter came in yesterday so I plan on checking cap leads to whatever point is next to see what kind of continuity I get. It's a start...

I have a few questions that have gone unanswered over at the other forum. If I reflow the smoothing cap connections, would that solve a poor connection?

Also if I'm soldering a smoothing cap, do I need to be aware of my solder iron forming a grounding circuit and discharging the cap? Not sure if this is possible or what.

Also, the hifiengine PDF for the Adcom GFA 5300 does not include page 7 which is the PCB layout. I have the schematic, but if anyone can send me the layout or link me to it, I would really appreciate it.

 

[RossW]

In my experience you'll almost never get rid of the quiet hiss/buzz. There's just too much EMI floating around these days. Some amps are more sensitive (my Marantz Model 30 is unfortunately quite bad despite having a lot of work done to it) while others you can barely hear.

 

[asilker]

Hey Ross, thanks for jumping in. My standards are set a bit high as the last power amp I was using (NAD 2150) was dead quiet at any input level. I haven't heard noise complaints regarding Adcom gear, and I've played with a GFA 545 which was totally silent, so I'm thinking either you're right and the mosfet ones are a bit hissy, or there's just a grounding issue.

I'm just gonna go ahead and reflow things and see how that does. On DIYaudio, a few people seemed pretty confident that a poor connection was a likely culprit.

When I was a bit younger and playing guitar a lot more, I remember jumping through a lot of hoops to quiet that 60 cycle pickup hum. But no matter how much shielding and what kind of light bulbs you played under, it was always there. HMMMMMMMMMMMMMMM

 

[asilker]

I've been reading around and it looks like since my 5300 is DC coupled, it just is going to pop when turned on. Ok, no biggie.

But that ZZZ sound is still driving me nuts. Was trying to play some Miles while writing a paper this morning and the persistent ZZZZZZZZZZ felt like it was stuck in my ear. I recently read the DC offset thread started by EchoWars (here: http://www.audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/). So following the instructions, I tested the DC from speaker outs. I got these readings: R/56.1 mV, L/56.1 mV

Wondering if/how this relates to my noise problem.

EDIT: Read the manual and went to adjust the DC Offset. Adjusted it a bit, and when I went to test it, there was a bit spark. So I returned the trimpot to the position where I found it. Now the DC offset is measure 112 mV on each channel.

Sometimes this hobby really kicks you between the legs. I guess I'll have to take it in to a tech and hope the cost is less than $150.

 

[asilker]

At this point, this thread has become kind of an embarrassing journal. But I'm alright with that, figure I'll just keep going and try to learn more.

I replaced a blown fuse last night. This morning things seemed to be running fine. I checked the bias settings, and the amp was running at 20 mV above the manual suggested bias, on each channel. I rebiased each channel to 20mV (+/-2 mV). After that I set the DC offset to under 1mV on each channel. She's working fine today, with a small change in my buzz problem.

With no inputs attached, the 5300 is significantly quieter, with a minimal fuzzy sound that reminds me of radio interference in guitar amps. With no inputs attached, when powered off each tweeter makes a squeak sound like a worn brake pad.

With inputs plugged in, the buzz is quieter but still present like it was.

I tried to get an eye on the underside of the power supply board, but there was no way I could tilt it. I would really like to reflow those joints, but I'll have to figure out how to flip that pcb