Figured I would post here as a last ditch effort to attempt saving this amplifier I have. I purchased this amp used on eBay from a reputable dealer about 3 years ago. Unfortunately for me, all I did when I got the amp was hook it up for a minute to make sure it worked, but I didn't spend much time with it for about the next year or 2 because we were in the process of moving. About a year ago I finally got around to hooking it up, and realized at higher output it does this. As you're watching the video, you can see the woofers "flutter", and hear the song cut very briefly.
B&K Reference 4420 Issue
I sent this amplifier to a local repair guy, he said he couldn't make the amp do what I was describing, which made me wonder if I was going insane. I got the amp back from him and it does exactly the same thing. So I reached out to the best amp guy I personally know... in Portland, OR, and shipped the amp to him. He called me and said the same thing, he couldn't get the amp to react the way it was in my video, and he couldn't at all find anything using a "dummy load". Together, we finally figured out that the amp only rears it's ugly head when it has a real, 4 ohm load, the speakers need to be 4 ohm nominal to make the woofers do this, 8 ohm won't make them do this (and by "real" I mean it has to actually be a speaker connected to the amp).
The guy in Portland was fairly honest, he said he wasn't sure what was making this amp do this, so he was going to replace parts that were potentially problematic, hoping it would fix it. Long story short, he replaced a bunch of parts in the amp- fairly sure the only things that weren't replaced were the main power supply caps, and the mosfets, and replacing those parts didn't help anything at all. He believes this amp has bad mosfets, he thinks this is most likely what is causing this issue, but the mosfets for this amp are NLA. So gobs of money in shipping wasted, plus I paid him for some of his work, though he did give me quite a break because he felt bad he wasn't able to figure it out.
I'm curious if anyone here has any inclination as to what could be causing this issue? FWIW, the amplifier biases just fine, and sounds magical at lower volume. Once the speakers output reaches about 83 to 84 decibels, the amp starts doing what it's doing in that video I posted. And in case you can't hear it in the video, bass is what makes the amp do this, and usually that lower, more reverberant bass. There is no "smoking gun" anywhere, nothing in the amp looks burned and according to both repair techs, it tests fine on a scope. But the 2nd guy was the only one that could make the amp respond the way it was in my video. I really don't want to give up on this amp, it sounds magical up to the point that it has this issue.
B&K Reference 4420 Issue
I sent this amplifier to a local repair guy, he said he couldn't make the amp do what I was describing, which made me wonder if I was going insane. I got the amp back from him and it does exactly the same thing. So I reached out to the best amp guy I personally know... in Portland, OR, and shipped the amp to him. He called me and said the same thing, he couldn't get the amp to react the way it was in my video, and he couldn't at all find anything using a "dummy load". Together, we finally figured out that the amp only rears it's ugly head when it has a real, 4 ohm load, the speakers need to be 4 ohm nominal to make the woofers do this, 8 ohm won't make them do this (and by "real" I mean it has to actually be a speaker connected to the amp).
The guy in Portland was fairly honest, he said he wasn't sure what was making this amp do this, so he was going to replace parts that were potentially problematic, hoping it would fix it. Long story short, he replaced a bunch of parts in the amp- fairly sure the only things that weren't replaced were the main power supply caps, and the mosfets, and replacing those parts didn't help anything at all. He believes this amp has bad mosfets, he thinks this is most likely what is causing this issue, but the mosfets for this amp are NLA. So gobs of money in shipping wasted, plus I paid him for some of his work, though he did give me quite a break because he felt bad he wasn't able to figure it out.
I'm curious if anyone here has any inclination as to what could be causing this issue? FWIW, the amplifier biases just fine, and sounds magical at lower volume. Once the speakers output reaches about 83 to 84 decibels, the amp starts doing what it's doing in that video I posted. And in case you can't hear it in the video, bass is what makes the amp do this, and usually that lower, more reverberant bass. There is no "smoking gun" anywhere, nothing in the amp looks burned and according to both repair techs, it tests fine on a scope. But the 2nd guy was the only one that could make the amp respond the way it was in my video. I really don't want to give up on this amp, it sounds magical up to the point that it has this issue.
Hey Matt, it looks like a "unstable DC problem".
I do not have the schematics; but HiFi Engine has the Reference 4430, I guess they are pretty similar.
I would suggest you to replace the electrolytic caps starting with the one on the feedback loop and the input coupling one.
When you remove the PCB please check the solder side looking for something wrong, you can retouch the ones that don't look good, specially check at the header pins.
I do not have the schematics; but HiFi Engine has the Reference 4430, I guess they are pretty similar.
I would suggest you to replace the electrolytic caps starting with the one on the feedback loop and the input coupling one.
When you remove the PCB please check the solder side looking for something wrong, you can retouch the ones that don't look good, specially check at the header pins.
Thank you, when I get a minute I'll open it up and check this out. For what it's worth, the amp is very quiet, there is no hum, no static, nothing aside from that woofer flutter that would indicate a problem.
I will also add that it doesn't matter if I use XLR or RCA inputs, it behaves the same way with both.