I have a Son of Ampzilla that I acquired in 2014. I got great help on this forum back then for a channel drop problem. The amp has been working great until recently.
I now get a pretty severe turn on pulse, followed by a slight hum that is of equal volume in both channels. The hum is more pronounced with the inputs disconnected. The turn on pulse is the same if the inputs are connected or disconnected. The pulse has been getting worse and I am starting to be concerned for my speakers. The amp still appear to work fine as far as the sound goes.
I am thinking something is getting funky in the power supply since both channels are equally affected. Anyone run into this issue before?
I now get a pretty severe turn on pulse, followed by a slight hum that is of equal volume in both channels. The hum is more pronounced with the inputs disconnected. The turn on pulse is the same if the inputs are connected or disconnected. The pulse has been getting worse and I am starting to be concerned for my speakers. The amp still appear to work fine as far as the sound goes.
I am thinking something is getting funky in the power supply since both channels are equally affected. Anyone run into this issue before?
Hi
The usual problem is cold solder connections. Solder is eroded by electron flow so it is best to have bigger connections than small ones. If this amp is old, then all of its connections are suspect.
The thump is from the rails not rising at the same rate and the circuit not settling in a balanced way. This amp should have resistive current sources for the complementary diff amps, which is a good thing and a bad thing. generally, they do rise without thumps, but also, until the circuit reaches a significant portion of its final voltage, the currents in it are low and the gm of the transistors is also low. This can cause little squeelly noises until the final currents are reached or gm is high enough for the frequency response of the circuit to be normal.
The usual problem is cold solder connections. Solder is eroded by electron flow so it is best to have bigger connections than small ones. If this amp is old, then all of its connections are suspect.
The thump is from the rails not rising at the same rate and the circuit not settling in a balanced way. This amp should have resistive current sources for the complementary diff amps, which is a good thing and a bad thing. generally, they do rise without thumps, but also, until the circuit reaches a significant portion of its final voltage, the currents in it are low and the gm of the transistors is also low. This can cause little squeelly noises until the final currents are reached or gm is high enough for the frequency response of the circuit to be normal.
+1 on this being the probable cause for the turn on thump.
I too am in the process of restoring a GAS Son of Ampzilla. In the power supply I replaced the aging 10,000uF/50V capacitors with new 12,000uF/50V ones.
For the hum, make sure the bolt on the diode bridge is tight.
Good luck
Mayank
Two and a half years later and I finally get a chance to work on the GAS Son Of Ampzilla again. All the caps were replaced including the main filter caps. The GAS115 and 117 transistors were replaced on both channels back in 2014 to fix an oscillation issue - that has not come back.
I still get the turn on pulse but not every time. It is always on the same channel. That channel has also developed an intermittent crackling/whooshing/popping noise that is quite random in when it occurs and its exact qualities. Sometimes it will stay in that state and the audio from that channel will be distorted until it suddenly "pops" and becomes clear again. I tried hitting the components with freeze spray while it was occurring but have been unable to pinpoint anything with this method.
Anyone have any idea or recommended steps to troubleshoot? I was using a Kenwood amp in place of the GAS for the last two years and forgot how much better the GAS sounded.
I still get the turn on pulse but not every time. It is always on the same channel. That channel has also developed an intermittent crackling/whooshing/popping noise that is quite random in when it occurs and its exact qualities. Sometimes it will stay in that state and the audio from that channel will be distorted until it suddenly "pops" and becomes clear again. I tried hitting the components with freeze spray while it was occurring but have been unable to pinpoint anything with this method.
Anyone have any idea or recommended steps to troubleshoot? I was using a Kenwood amp in place of the GAS for the last two years and forgot how much better the GAS sounded.
Just came across this old thread - can't believe no one else commented on this gem. ROTFL!
Hal
I forgot that old trick of poking around with a non-conductive stick to try to identify bad solder joints. Thanks for the input - may wind up disassembling and re-soldering.
Are your differential input transistors in sockets? Try reseating them in the sockets. Maybe squirt some Deoxit D5 on them. Check the DC offset when it's acting up, the diff. pairs will cause that. Great amplifier!!!!
Craig
Craig
Yes, they are in sockets. I actually swapped them from one channel to the other when I was trying to diagnose the turn on pulse.
It's zero now, but I'll leave a meter connected so I can watch what happens when the issue occurs.
I have a GAS Thalia preamp in my shop now and left channel phono had a weird problem. I reseated the six transistors and the problem went away. Used D5 too.
The DC offset goes wonky while the issue is occurring. It swung to about 1.5 volts positive when the cracking noise started, then swung about the same negative when a different crackling noise happened, then when back to zero after a popping noise.
I think it stayed on the same channel but not sure. I was going for the turn-on pulse problem though, not the noise. It's not hard to swap them back and see if it switches channels since they are socketed.
Good news! Swapping the 4 socketed differential input transistors from one channel to the other moved the noise to the other channel. With this in mind I can definitely see replacing these transistors. Anyone have a good number for the replacements?