Adcom GFA-555 + Onkyo TX-NR709 Mute Thump? - diyAudio
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Old 12th September 2011, 08:28 PM   #1
diyAudio Member
Join Date: Feb 2010
Location: Fairfax, VA
Default Adcom GFA-555 + Onkyo TX-NR709 Mute Thump?

I recently changed out my receiver and main amp for an Onkyo TX-NR709 receiver and an Adcom GFA-555. After much tweaking, I am still left with a medium level thump when I do various things such as changing inputs on the receiver, muting, change channels on the cable box, and sometimes during commercials on TV (likely a switch from national to local programming). I have tweaked the receiver so that it is not changing surround modes during any of these switches, but I am still getting the thump.

I suspect that the reciever is muting during these operations by clamping the input and that is causing the issue. I also have turn on/off thump from the Adcom so it may have some DC offset.

My question is how should I go about diagnosing and correcting the problem?

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Old 12th September 2011, 09:15 PM   #2
diyAudio Member
Join Date: Feb 2010
Location: Fairfax, VA
I took some measurements (no audio) while not muted and muted and here is what I found. These are DC offset measurements in MV at the speakers.

Adcom GFA-555: [mains]
--Not Muted
L=45.4, R=16.8
L=40.3, R=10.7

Adcom GFA-5400: [surrounds]
-- Not Muted
L=14.0, R=95.1
L=9.7, R=90.0

Since I am using Klipsch speakers all around, this thump is very annoying. Should I just live with this or is there a way to tweak the DC offset in the Adcoms? Or, am I on the wrong track here?

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Old 13th September 2011, 12:48 AM   #3
diyAudio Member
Join Date: May 2010
Location: Florida
Do not change the DC offset , it will have no affect on the thump. The "thump" will not damage you speakers. You can try a 10 ohm resistor with a .1uf cap across the speaker term and see if that helps. resistor to pos and cap to neg. Use a high quality film cap, this has helped some of the 555's I have had, depends on the preamp/processor your using also. Some of them short the line out when in mute, some just cut the signal down which is better to me.
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