Noob needs help..Adcom GFA-545 DC blocking caps

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need4speed

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Joined 2004
#1
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
 
#2
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
 
#3
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?
 
#5
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
 
#6
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.
 
#8
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
 
#9
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
 
#11
Chris....

I'm not much on patience once I have a solution so I dug through my drawer of caps and found a 10uF 100V that I figured would lend itself to at least test the theory.

I didn't have any resistors laying around, so I patched the cap in series on the positive side of the RCA cable....the pop is gone. I did notice that the hum when the amp was on with no input signal was a bit louder than before the mod. I'm sure that was due to the less than stellar quality cap and the fact that I patched it in using aligator clips, so there were alot of uninsulated connections. But nonetheless it worked. I will make myslef a little black box of the filter you laid out for me tomorrow.

Now....for my final question. The adcom supposedly goes 10-20khz. Assuming those numbers are real i'd like to stay as close to the 10hz as possible, especially considering this amp is driving a pair of 15" IB Atlas'. a 1uF should do the trick? or should I get something a little bigger just to make sure?

Thanks again for all your help and feedback. This was driving me nuts and I was sure it had something to do with the lack of filters on the input, but I just couldnt be sure.

Can you recommend a place to get some nice high quality caps? and what material should I be looking for?
 
#12
Hi Patrick,
The 100K resistor provides a DC path to ground. This will make sure the capacitor is discharged and reduce "pops". We want to keep the input DC resistance of the Adcom constant. Otherwise the DC conditions around the diff pair will change whenever a lower resistance is presented across the input terminal, resulting in a "pop". The cap isolates the Adcom input from the Pioneer output as far as direct current is concerned.

I haven't figured out the frequency response, but it's a simple R-C filter. Look at the input resistance in parallel with the 100K resistor and you can figure it out. 10 uF should do it. Use a plastic film type capacitor if you can find it. Avoid electrolytics and tantalums. Unbiased, they have non-linear curves. Not what you want.

BTW, you may not get anywhere near 30 Hz depending on the box size and tuning. Then there is the room response. An 8" may actually go lower in the same size box, but will not have the impact.

-Chris
 
#13
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