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Old 13th September 2005, 10:45 AM   #1
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Default Noob needs help..Adcom GFA-545 DC blocking caps

Hey guys...I'm new to the amp forum, but I was hoping someone here could shed some light onto my situation.

I have a Pioneer 1014TX being used as a pre-amp for the LFE channel, which is in turn powered by an ADCOM GFA-545.

The amp appears to be working fine, but anytime I change sources (CD->DVD->TV) or the pioneer changes audio signal format (5.1->stereo, etc) the Adcom pops. As I understand this is due to the lack of any DC blocking caps in the adcom.

So I have a couple questions. Why is the Pioneer sending DC transients across the pre-outs (this doesnt happen on the pioneers internal amps) when there is any signal switching? Is this normal? I would have thought there would be some sort of DC block prior to the pre-outs.

What can I do to make it go away? The pop doesnt bother me most of the time, but it can get annoying after a while so I'd like to see what options there are. If at all possible I'd like not to affect the frequency response of the adcom too much.

Thanks all.
patrick
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Old 13th September 2005, 04:18 PM   #2
anatech is offline anatech  Canada
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Hi Patrick,
This could be the impedance changing on the output of the Pioneer, or it's not muting the output during source changes as it does internally. Check this by installing a cap in series with bleeds to ground between the Pioneer and the Adcom.

The best fix is to get a higher quality source. -Sorry Your amp is wayyyy better than the Pioneer.

-Chris
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Old 13th September 2005, 05:02 PM   #3
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can you be more specifc about how the impedance change would cause the amp to pop?

as a side note, whe I manually mute the pioneer using the mute button on the remote, the problem goes away and adcom does not pop at all. Would the GFA-545 MKII be an improvement?
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Old 13th September 2005, 05:07 PM   #4
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also wanted to mention that I was using a Parts Express plate amp (250w) previous to the adcom and it did not exhibit the pop. That makes me believe that I could do something to remedy the situation.

-patrick
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Old 13th September 2005, 07:03 PM   #5
anatech is offline anatech  Canada
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Hi Patrick,
I suspect the muting transistors are changing the output impedance. This would cause a level shift at the input of the Adcom since there is no isolation. This may cause a small noise when it occurs.
Put a 100K resistor from the center of the rca to ground on both the Adcom and Pioneer side. Insert a 1uF film type cap between the units (center rca to center rca). You can make an adapter to test this and plug the rca's in each side so there is not modification in the equipment to test.
How loud is it?

-Chris
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Old 13th September 2005, 07:15 PM   #6
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First off..Thank you

It's not very loud at all, it sounds just like the pop that occurs when the amp powers on/off. I could live with it if it only occured when switching sources, but it also happens when the pioneer swithes the audio signal.

For example, the transition between a dvd menu and the main movie. The receiver is dropping out of stereo and into 5.1.

But the biggest annoyance is during a layer change on dvd, when the audio signal drops temporarily into stereo or PL and then back to 5.1. It's bad enough the have the layer change, but ten fold as annoying when the pop occurs...it is very distracting.

It also happens when changing channels on the cable box. The cable box is hooked to the pioneer via optical digital cable.

It's almost as if something is discharing on the amp when the audio signal is lost temporarily. But I can't explain why it doesn't happen when the pioneer is muted.
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Old 13th September 2005, 08:35 PM   #7
anatech is offline anatech  Canada
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Hi Patrick,
Well, try the cap / resistor thing and let me know.

-Chris
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Old 13th September 2005, 09:38 PM   #8
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will do...just to make sure i dont blow anything up...

"Put a 100K resistor from the center of the rca to ground on both the Adcom and Pioneer side"

are you saying to tie the + and the - of one of the channels together with the 100k resistor?

"Insert a 1uF film type cap between the units (center rca to center rca)"

Then place a cap in series between the center pin of the pioneer and the adcom

so one channel would look like this

pioneeer output / adcom input

pos ------------------1uF--- pos
|
100kohm
|
neg ------------------------- neg

thanks

i will make an adapter cable for one channel tomorrow at work, since I dont have a cap that large handy here at home
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Old 13th September 2005, 09:55 PM   #9
anatech is offline anatech  Canada
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Hi Patrick,
You are missing a 100K resistor for the other side. We are dealing with the signal leads feeding the Adcom. The goal is to provide a constant DC impedance to the input of the Adcom. I picked 1uF out of the air. You can try a smaller value. We aren't looking for full frequency response, just trying to see if the DC resistance is changing on the input of the Adcom.
You may find the resistor on the Pioneer side is not required, but it might be ...

-Chris
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Old 13th September 2005, 10:22 PM   #10
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its crude...better safe than sorry though :

http://cyberfrogs.net/files/adapter.jpg

I'll have to see if i got any caps laying around...otherwise i'll take care of this first thing at work tomorrow and post back with the results about this time in the eve.

can you explain to me what the 100k resistors are for? will the value of these depend on the input resistance of the adcom?
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