Adcom GFA-555 turn-on thump

[kman]

I have a GFA-555 that has an audible thump upon turn on-- not too loud but something I'd like to avoid with my vintage Infinity loudspeakers. I measured the DC offset in both channels and it quickly settles in on single-digit or low double-digit millivolts in about 10 seconds. I have replaced the electrolytics in the feedback loop and across the bias pot but the thump remains.

I have a second GFA-555 of slighty newer vintage which has similar offset characteristics and no turn-on thump.

My question: is this turn-on thump normal and if not, what else may be causing it?

I have searched the archives but could not find a specific reference to this problem with this particular amp.

[dave_gerecke]

Hello Kman, I'm don't remember if the 555 has any sort of protection relay in it. If it doesn't, then a slight turn on thump isn't uncommon. I have a pair of AB International amps that does the same thing. And if I remember correctly, my GFA6000 thumped a little also. As long as it isn't too large of a voltage/current surge, and it doesn't last for too long, it shouldn't be a problem.

Peace,

Dave

[kman]

Originally posted by dave_gerecke
Hello Kman, I'm don't remember if the 555 has any sort of protection relay in it. If it doesn't, then a slight turn on thump isn't uncommon. I have a pair of AB International amps that does the same thing. And if I remember correctly, my GFA6000 thumped a little also. As long as it isn't too large of a voltage/current surge, and it doesn't last for too long, it shouldn't be a problem.

Peace,

Dave

Hi Dave,

There is no output relay on the GFA-555. The thump is audible on my Infinity RS-4.5 woofers when I biamp using this amp for the bass. It is not a loud thump and I suppose I would not be too concerned about it except that my other GFA-555 produces no thump when turned-on.

I really don't have the right kind of test equipment to accurately measure this transient output upon turn-on It does quickly settle down to 10-20 mv DC offset in each channel, though.

Regards,
Keith

[dave_gerecke]

Hello Keith, Do you have access to either an analog voltmeter, or an O'scope. With either of these, you could monitor the output of your amp on turn on and see how large and how long the turn on spike is. I doubt it's going to be a problem, being that you are driving the Infinity's with bass notes that will have similar behavior to the turn on thump, at least as far as the woofer drivers motion. You just want to make sure that you don't have any REALLY Large spikes, or any long term DC voltage. DC voltage on the voice coil for long periods will cause heat, due to the resistance of the voice coil. This will eventually burn out the voice coils.
How old are the Infinity's. If they are over 10 years old, I would recommend taking a look at the foam surrounds on the woofers. They rot over time.

Peace,

Dave

[kman]

Originally posted by dave_gerecke
Hello Keith, Do you have access to either an analog voltmeter, or an O'scope. With either of these, you could monitor the output of your amp on turn on and see how large and how long the turn on spike is. I doubt it's going to be a problem, being that you are driving the Infinity's with bass notes that will have similar behavior to the turn on thump, at least as far as the woofer drivers motion. You just want to make sure that you don't have any REALLY Large spikes, or any long term DC voltage. DC voltage on the voice coil for long periods will cause heat, due to the resistance of the voice coil. This will eventually burn out the voice coils.
How old are the Infinity's. If they are over 10 years old, I would recommend taking a look at the foam surrounds on the woofers. They rot over time.

Peace,

Dave

Hi dave:

I do have both but I was using my Fluke DVM to make the DC offset measurements. I suppose I could set up the scope to give enough persistence to show the extent of the transient on turn on. Might be easier just to observe the output using my old RCA 'Volt Ohmist' VTVM!

I noticed the mechanical action of the on-off switch is a little soft and this appears to be a problem with these GFA-555s. Adcom recommended a replacement switch from Part Express which I have ordered and will put in. I wonder if there is internal arcing in the switch that might cause a turn-on transient?

BTW, the Infinity's are from 1980 and the woofer foam does indeed rot on these babies. It was replaced by a previous owner, sparing me that job!