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| Amplifier got higher noise floor, after hooked up to bad preamp? |
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| kris2ff3r:
I have a 100w AB stereo amplifier which suddenly got a higher noise floor. By that I mean audiable noise in the background, without or with low signal. I had a little accident a while back where I hooked it up to a crappy DIY tube preamp, that wasn't even tested. I don't know what happened, but it did send some sort of high level hum (like a ground issue) through it. I turned it off pretty fast, but pretty sure this now present background noise wasn't there before, at least not this noticable. The amp works just fine, but this no signal noise is a bit irritating. It stays the same level pretty much the whole time, and there are no audiable distortion during playback. No components inside show any damage, and the heatsinks have perfect temperature. So.. Could any of the transistors have been damaged with that shitty preamp? Not short circuit, but send just a tiny bit of current in the wrong direction? Would the symptoms add up? |
| JS:
How is your noise? Humm or buzz from main filtering? Or static, hiss, crackle? For the first one some PS issue, follow it and find it. For the second one... Low noise BJT transistors tend to loose that property when the BE junture is polarized backwards and enters the zenner region. Depending on time and current on the failure it gets worse. The first stage of your amp might suffer from that, so changing the input stage migt do. To protect low noise BJT input opamps from this you can hook up a pair of diodes shunting the non inverting and inverting input toghether. While operating in the linear region shouldn't be any voltage across the diodes, when it saturates inputs cant get fuether than 0.7V, minus the BE juncture of the input transistor polirazed in the right direcrion you are left with maybe 0.2V reverse EB junctute polarization. Not even close to the zenner brakedown at about 6V. Electolytic caps also can show a problem in your situation, they can get noisier if abbused. I'm talking about input decoupling caps. JS Enviado desde mi LG-M250 mediante Tapatalk |
| kris2ff3r:
The noise is 50Hz, but it is more audiable than before. The amp had some modifications done earlier, where the 2x10000µF per channel reservoir was replaced wih 2x15000µF's but did not experience this issue before the preamp was connected. The amp is also DC coupled, so no coupling caps what so ever. |
| JS:
--- Quote from: kris2ff3r on June 08, 2018, 09:06:48 pm ---The noise is 50Hz, but it is more audiable than before. The amp had some modifications done earlier, where the 2x10000µF per channel reservoir was replaced wih 2x15000µF's --- End quote --- Follow your PS rails to see the noise levels, a scope if you have or an audio amplifoer\speaker. You have to conect the probe ac coupled, maybe attenuated and clamped with some diodes to be safe, you don't want another blown amp. Use a small one! I use an old guitar cab for this. JS Enviado desde mi LG-M250 mediante Tapatalk |
| kris2ff3r:
--- Quote from: JS on June 08, 2018, 09:10:27 pm --- --- Quote from: kris2ff3r on June 08, 2018, 09:06:48 pm ---The noise is 50Hz, but it is more audiable than before. The amp had some modifications done earlier, where the 2x10000µF per channel reservoir was replaced wih 2x15000µF's --- End quote --- Follow your PS rails to see the noise levels, a scope if you have or an audio amplifoer\speaker. You have to conect the probe ac coupled, maybe attenuated and clamped with some diodes to be safe, you don't want another blown amp. Use a small one! I use an old guitar cab for this. JS Enviado desde mi LG-M250 mediante Tapatalk --- End quote --- Ahh, yes. just another time where I wich I had a scope, as always. This is a power amplifier and I do have it hooked up to the speakers, but I don't see how I can troubleshot this way. The amp never blew up by the way. Maybe you misunderstood? I do not want to fix the preamp, I'm not using it. The 50Hz noise is still audiable with a different (fully functional) preamp. it's the power amp that is the problem, with the now ''gained'' 50Hz noise. |
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