Troubleshooting Audio Amplifier Noise - Page 1
Forum section: electronics / beginners

author: savagemadman
author_rank: Newbie
timestamp: June 21, 2017, 01:46:40 am
content: Forgive me if this is something obvious, but my knowledge behind the theory of operation of electronics is minimal. I'm from an industry where we just replace whole modular boards if something goes wrong.I purchased a pair of relatively old two channel Sony power amplifiers and one of the two has an issue with what seems *almost* like an unusually high noise floor. The distortion is white noise-like, and its audible even with the attenuator set to infinity (no output). The attenuator has *some* impact on the noise level, though no where near as much as it should with the normal noise floor. The noise is on both channels, at the same volume, and it's independent on both (it's noise in stereo!). I've fed it a square wave source and checked the output on an oscilloscope that I have on hand and it looks no different than the usual random noise, just exaggerated. It does not seem to be an issue with the switches or potentiometers, though I've hit them with contact cleaner just in case. I've adjusted the DC-bias, no audible change.Here are the important bits of the service manual.Am I missing something? Any suggestions as to where to start? Do I just need to start desoldering components and test individually? I have a Fluke 187 multimeter, Fluke 99B-II scopemeter, and a cheap "Blue ESR" tester on hand.

author: Audioguru
author_rank: Super Contributor
timestamp: June 21, 2017, 04:25:05 am
content: The only thing that is common on both channels of a stereo power amplifier is its power supply. Maybe a filter capacitor is defective and causing both channels to oscillate at a high frequency.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 21, 2017, 08:05:51 am
content: The fact that it is present on both channels tells you that something is going on that is shared by both - this could be supply noise, external injection, ...Second, the fact that it happens irrespective of volume tells you that it is likely happening after the preamplifiers - as far as I know volume control happens in the preamplifier, with the poweramplifier just boosting the current levels.This means that you have to look at something like defective capacitors that have started to cause issues, somewhere where they are shared, such as Audioguru's suggestion of the filter capacitors.

author: savagemadman
author_rank: Newbie
timestamp: June 21, 2017, 09:02:27 am
content: Hopefully it is something in the power supply common to both channels before splitting off as it doesn't look like it's composed of very much at that point. It also probably rules outthe four gigantic and expensive looking capacitors sitting in a red velvet-like thing in the middle of the amp.Fortunately the power section of the PCB is accessible on both sides without removing the board.The attenuator looks like it just cuts the input level if I'm reading the diagram correctly.

author: The Soulman
author_rank: Super Contributor
timestamp: June 21, 2017, 09:06:27 am
content: Quote from: savagemadman on June 21, 2017, 01:46:40 amIf you plan on keeping this amplifier replace all electrolytic capacitors and see what that does.However these STK based amplifiers aren't famous for their low noise...

author: The Soulman
author_rank: Super Contributor
timestamp: June 21, 2017, 09:24:06 am
content: You can leave those large Elna's for now and only swap electrolytic's on the poweramp boards.However these Elna's aren't famous for their long life...

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2017, 03:23:29 pm
content: I will assume that you have the power amplifier configured for stereo instead of bridged mono.There are what look like some low value aluminum electrolytic bypass and coupling capacitors in the signal paths of the two amplifiers. C302, C309, C352, C359, C304, and C354 should be replaced and there is nothing wrong with also replacing the small decoupling capacitors like C306 and C307 also. My guess is that measurement will show that they have dried out.If these capacitors are solid tantalums, then I would look for other causes. It would be fun to measure the output noise *between* the outputs to see if it is common mode.Is it my imagination or are the details for IC301 wrong? Shouldn't that NPN output transistor be a PNP with the emitter and collector reversed to make a cascode?This datasheetindeed shows that the schematic is wrong.

author: savagemadman
author_rank: Newbie
timestamp: June 21, 2017, 04:28:39 pm
content: I've managed to test a few of the electrolytic capacitors. They generally measure within spec on my Fluke. However the ESR meter shows what I believe to be too high a value on every one I've checked - at least going by the chart in the instructions I have. This is my first practical attempt at using the ESR meter so I could be completely wrong, and yes I did zero out the probes.I'll order a new set of capacitors and see how it goes.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 21, 2017, 05:04:34 pm
content: Unless the nature of the "noise" is actually mains-frequency HUM (+ harmonics) it seems doubtful that replacing power filter capacitors (or ANY capacitors, for that matter) will mitigate the noise.As @The Soulman said, those STK modules are not noted for high performance. They are jelly-bean, short-cut commodity stuff used in lower-end gear. You may be seeking performance beyond what the gear was designed for.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 21, 2017, 06:14:27 pm
content: Some amplifiers have a relatively high noise level that is audible if a sensitive loudspeaker is used. I have a pair of speakers that is too sensitive and thus make noise audible on an amplifier that is ok and even rather low noise. So it might be a good idea to get a real noise measurement so one could compare with specs or expected values.Bad capacitors have a slight chance to cause the circuit get unstable. This might lead to higher noise, but more common problem would be overheating.One could check the standing current (e.g. TP301 / TP351).It might be also worth checking (can be done in circuit) R313/R314 as they would likely blow if the amplifier would start oscillating.

author: retrolefty
author_rank: Super Contributor
timestamp: June 21, 2017, 06:32:34 pm
content: Of course using a scope can help troubleshoot the symptom quicker and more efficiently.

author: savagemadman
author_rank: Newbie
timestamp: June 21, 2017, 09:02:53 pm
content: QuoteHow would a scope help here? Remember I'm nearly tabula rasa when it comes to troubleshooting like this.QuoteThey measure 8.21 and 8.14 ohms for me, I'm assuming they are good even though they are rated at 15 ohms.QuoteI do have a 2nd one of these that is much quieter, no audible hiss unless you're right near the speakers, and as I said the noise is not quite normal - it's choppier. It also has random spikes every once in a while. I've tried to record a sample of it below, the effect is far more pronounced on my floor standing speakers but it would take a while to move my recording equipment. Please forgive the ambient street and fan noise:https://youtu.be/cnd682PkG8A

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 21, 2017, 10:17:28 pm
content: I can't tell what we are listening to here? I heard a lot of noise, then a click, and more noise. What was that supposed to be? I really have no clue here.If you have an identical unit that is working "properly" then you have an ideal situation to measure voltages at various points in the circuit and compare "good" vs "bad". As @retrolefty said, using an oscilloscope would allow you to probe various points along the signal path and see what/where is the origin of the noise. Without at least that kind of test gear, you are rather in over your head with a problem like this. If it were a case of simple mains hum, then replacing power supply filter capacitors would be a pretty good bet. But your symptoms of noise indicate something more like IC, transistor or even resistor.It may just be a coincidence that both channels are exhibiting the same symptoms. It is hard to conceive that the problem could be originating in the only shared circuitry (the power supply). OTOH, there is that shared circuit right at the input for the stereo-mono switch. I would try just removing all that circuit (along with the IC op-amp) Assuming you aren't planning on using it as a monaural bridged amplifier.

author: mzacharias
author_rank: Frequent Contributor
timestamp: June 21, 2017, 10:23:16 pm
content: I cannot say if this applies to your model but during this time period some Sony models were NOTORIOUS for a buzzy type of noise (not specifically mains related) caused by grounding problems. Any screws which secure heat sinks and metal chassis parts to any circuit board ground pads should be tightened and preferably star-washers added.

author: med6753
author_rank: Super Contributor
timestamp: June 22, 2017, 12:45:38 am
content: Quote from: mzacharias on June 21, 2017, 10:23:16 pmGood advice. I had a Sony STR-AV210 Receiver with this exact issue. Constant background buzz caused by high resistance between circuit board ground and chassis. Once the screws were tightened the buzz disappeared.

author: David Hess
author_rank: Super Contributor
timestamp: June 24, 2017, 04:49:01 pm
content: Quote from: savagemadman on June 21, 2017, 09:02:53 pmWith a square wave source, an oscilloscope will show the transient response which should be clean. The transient response will also reveal the low and high pass frequency response. A sine or triangle wave source will reveal high levels of zero crossing distortion if present; this could be the result of an improper idle current setting. A triangle wave source will reveal high levels of non-linearity if present. If there is any spurious oscillation, then it will be visible on an oscilloscope.Tests should be done with different realistic loads.If your oscilloscope can perform an FFT, then the noise spectrum could be measured which might indicate something. In the old days one might sweep a bandpass filter to make spot noise measurements.Quote from: The Soulman on June 21, 2017, 09:06:27 amThe hybrid's bipolar input stage should pretty quiet operating at a collector current of 2 milliamps but the input resistance at the non-inverting input and probably the inverting input as well is too high for good noise performance. The application example for the STK3102 from Sanyo shows a quieter design which assumes low impedance drive so I get the feeling that the Sony designers did not quite know what they were doing.Counterintuitively, lowest noise will be with the attenuator set to either minimum or maximum so you might try that. With the input shorted, see how the noise changes with attenuator setting.Quote from: savagemadman on June 21, 2017, 09:02:53 pmIt sounds like it could be aluminum electrolytic capacitor noise to me.Quote from: savagemadman on June 21, 2017, 04:28:39 pmI am not a fan of ESR measurements because they are so often equivocal. Dissipation measurements are much more useful because they are consistent across a range of capacitance values.QuoteAluminum electrolytic capacitors are cheap, have a limited life, and are easy to change so it makes sense to replace them early when the cause of a problem is not readily apparent.

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2017, 05:39:36 pm
content: Quote from: savagemadman on June 21, 2017, 01:46:40 amIs this a recent purchase?What's more important to you: learning something or just getting it resolved?If so, then it's faulty. Return it for a refund or replacement. Whilst trying to fix it may be a good learning experience, it will almost certainly void any warranty and won't do any other poor mug who's bought the same faulty product any favours.

author: onlooker
author_rank: Frequent Contributor
timestamp: June 24, 2017, 07:47:54 pm
content: The OP mentioned that he has 2 units of the same model and he is used to replace boards. I would think an easier thing to try is just swapping the boards to isolate the issue a little before changing anything.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 24, 2017, 08:43:06 pm
content: Quote from: onlooker on June 24, 2017, 07:47:54 pmIndeed. But measuring the DC voltage (referenced to chassis ground) at various spots in the circuit and comparing the bad vs. good unit would be easier before unsoldering all those joints and swapping boards.Swapping boards also carries the risk of getting everything connected properly, and then the possibility that something in the bad unit will cause failure of the good board. Especially in a DC-coupled power amplifier circuit like this.

author: onlooker
author_rank: Frequent Contributor
timestamp: June 24, 2017, 09:57:10 pm
content: From google pictures, the inter-board connections are not soldered. It seems they all use connectors/plugs. Though, comparing voltages are easy too. Maybe, the point is to try non-invasive methods before invasive methods.

author: savagemadman
author_rank: Newbie
timestamp: June 25, 2017, 07:43:01 am
content: QuoteMost definitely not a grounding buzz, I'm quite familiar with that phenomenon.QuoteMy only clean waveform source is the rather limited fixed function generator on my oscilloscope and I believe it's output is too high to safely feed into an audio amplifier. I'll need to get my hands on a proper signal generator at some point.QuoteNo FFT here.Closest capability I currently have is the function in Audacity, but the process of capturing to a computer would add too much noise to be useful.QuoteI've been sitting on these amps, along with a few other projects, for months. I'm primarily doing it for the learning experience. Having just quit my job (a long story) I finally have some free time to do something with them.QuoteThat would be too easy. I need to do this the hard way, and I'm in no hurry.I'm also planning to recap another even older Sony amplifier with a PWM (uncommon?) power supply - which unfortunately I do not have a component list for. The insides of this other amplifier closely resemble a bowl of spaghetti which is making the cataloging a bit of a tedious process. Hopefully I'll have my Digikey order together for tomorrow.

author: BMack
author_rank: Regular Contributor
timestamp: June 25, 2017, 08:56:04 pm
content: The easiest way to troubleshoot this is with an old piece of equipment, a signal tracer. I made one by using a 3.5mm plug, connected ground to an alligator clip and the left and right channels to a probe. Then I plug that 3.5mm plug into a small amplified speaker. You clip the alligator clip to ground and trace the output probing at the top of caps and chip legs until you don't have any distortion...the same way you'd use a scope in this scenario. I like this more than a scope in this scenario because you can look at the unit you're trying to fix and listen, you have no screen to watch.

author: savagemadman
author_rank: Newbie
timestamp: July 05, 2017, 03:41:08 am
content: I've replaced most of the electrolytic caps and it didn't help at all. I skipped the selector board, four others I figured didn't matter (C408-C411, C319) and the giant four C320-C323 - I couldn't even source immediate replacements for these. Unfortunately it didn't help at all - on the up side at least it was soldering practice.What is an acceptable tolerance range for the test point voltages? I've checked the silkscreened points and they seem to be off by 10%or more.

author: David Hess
author_rank: Super Contributor
timestamp: July 05, 2017, 05:30:16 pm
content: Quote from: savagemadman on July 05, 2017, 03:41:08 amIt depends on which voltages. The power supply voltages are not closely regulated.