Amplifier got higher noise floor, after hooked up to bad preamp? - Page 1
Forum section: electronics / beginners

author: kris2ff3r
author_rank: Contributor
timestamp: June 08, 2018, 07:40:17 pm
content: 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?

author: JS
author_rank: Frequent Contributor
timestamp: June 08, 2018, 08:53:15 pm
content: 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.JSEnviado desde mi LG-M250 mediante Tapatalk

author: kris2ff3r
author_rank: Contributor
timestamp: June 08, 2018, 09:06:48 pm
content: 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.

author: JS
author_rank: Frequent Contributor
timestamp: June 08, 2018, 09:10:27 pm
content: Quote from: kris2ff3r on June 08, 2018, 09:06:48 pmFollow 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.JSEnviado desde mi LG-M250 mediante Tapatalk

author: kris2ff3r
author_rank: Contributor
timestamp: June 08, 2018, 09:15:53 pm
content: Quote from: JS on June 08, 2018, 09:10:27 pmAhh, 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.

author: kris2ff3r
author_rank: Contributor
timestamp: June 08, 2018, 09:47:46 pm
content: Okay, so wierdest thing happened..I coincidentally noticed that the noise is gaining as I reach my hand towards the input wires (the lid is off).. three single leads twisted together - no shielding (amp is balanced). and if I touch them it's even worse. And if I snap my fingers onto it I can hear it in the speakers. The issue is actually microphonics??This was all with the preamp disconnected. And now that I connected the preamp, it doesn't react when I snap the wires at all?? How is this possible?

author: JS
author_rank: Frequent Contributor
timestamp: June 08, 2018, 09:50:46 pm
content: Ceramic caps might get microphonic, so check them tapping them with a plastic screwdriver so you dont induce other antenna, capacitive effects.I'm saying to use a different amp to troubleshoot the bad one, you alreay hear the problematic noise in those speakers.JSEnviado desde mi LG-M250 mediante Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: June 09, 2018, 12:44:25 am
content: Quote from: kris2ff3r on June 08, 2018, 09:06:48 pm50Hz or 100Hz? If ripple from the power supply was getting into the amplifier, I would expect it to be the 100Hz full wave rectified signal and not the 50Hz AC signal.I like JS' suggestion about base-emitter breakdown damaging the input transistors. This would degrade the hfe yielding higher bias current and poorer common mode rejection.

author: C
author_rank: Super Contributor
timestamp: June 09, 2018, 04:52:58 pm
content: let me state what JS said a different way.A scope lets you see the waveform.A sound scope lets you hear the waveform. A lot of normal scope use is out of your audio range, but here it is not.A sound scope is just an audio amplifier with a good attenuator on input. The larger the signal the more attenuation you need.Now you do not need accurate measurements.A great amplifier is made up of positive half processing & negative half processing. Your output is a NPN & PNP for example.So the sound you get from the positive supply should be close to sound from negative supply.Sound from NPN collector almost = PNP collector.So much of your testing is just doing comparisons of sound at a test point.This is not a new testing method.Some old test equipment would do this with output to a meter, but with the loss of detail your ear can pickup.C