Coil whine/dampening in a cheap hifi amp - Page 1
Forum section: electronics / beginners

author: klausman
author_rank: Regular Contributor
timestamp: August 20, 2025, 09:51:16 pm
content: I recently repaired a Hitachi HA-3 amplifier. It's very simple unit, which was a delight to repair. However, after said repair, I noticed a whine which I suspect is coming from the coupling coils on the output. It might also be the transformer, but a) I doubt that and b) there wouldn't be much I can do about it. If it is the output coils, would putting a on a glob of epoxy help? I've seen hotsnot and cellastic used by manufacturers, but I don't have easy access to either.Thoughts/Alternatives?

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2025, 10:07:34 pm
content: Any guess on the frequency of the audible whine?Unless this is a class-D switching amplifier, I don't know how any coils at the output would cause this.Is it perhaps a problem with the output protection circuitry?I didn't locate a full schematic online.

author: Benta
author_rank: Super Contributor
timestamp: August 20, 2025, 10:29:35 pm
content: If it's the HA-3 that I know, it's a mid-1980s linear power amp, no class-D there.The whine could be local oscillation from a broken compensation cap/resistor, or a wrongly selected replacement part.Dunno what's meant by "coupling coils".

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2025, 10:40:39 pm
content: Are the "coupling coils" actually relays in the protection circuit?

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2025, 05:59:58 am
content: If they are in the output circuit, then I would expect them to be part of a Zobel network, but then I would not expect a whine caused by them.

author: klausman
author_rank: Regular Contributor
timestamp: August 21, 2025, 07:43:58 am
content: Ok, I should have included more info, sorry 'bout that.It is indeed an 80s class AB amp. Super simple design. I cleaned the whole thing and wanted to test it, so I ran 1kHz sine from a siggen into it and put two 100W 8 Ohm resistors on the speaker outputs. The amp is rated at 80W so that should be fine. If I turn the volume to about 6~7/10, a high-pitched whine is audible from the amp, roughly where the two coils in the output stage sit (see below). I have a primitive spectrum analyzer on my phone that shows a peak at 2khz, another smaller one at 3kHz and so on. If I alter the signal input frequency, the whine also changes pitch. I am familiar with coil whine from GPUs and other digital equipment, so that's what I thought it was.Here's a picture of the insides of the amp(before I cleaned out the dust), the coils are just to the bottom-left of the main heatsink:and a closeup:Part of the schematic, (one of) coils highlighted:A screenshot of the spec analyzer on my phone. The lines at 2kHz and beyond are fading in and out as I raise & lower the volume. The noise becomes clearly audible at about 6~7/10 on the scale. The consistent noise at and below 1kHz is just background and fan noise.

author: klausman
author_rank: Regular Contributor
timestamp: August 21, 2025, 07:45:06 am
content: And the whole Service manual:

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 21, 2025, 08:16:59 am
content: IDK, but if it is just slighty heard with almost full power than I guess its okay.Many amps may make some slight noises with high load.

author: tggzzz
author_rank: Super Contributor
timestamp: August 21, 2025, 08:18:37 am
content: Quote from: klausman on August 21, 2025, 07:43:58 amThat's a helpful characterisation.That sounds like some form of harmonic distortion.Have a look at the PSU voltages and ripple. Since it is power related, consider the "1988 week 45" PSU smoothing capacitors may need replacing.Use a scope on the output to see if the waveforms are clipped or slew-rate limited. Be cautious of where you put the probe's shield. Since the problem occurs at high power, you may be able to spot the problem without connecting the shield!If it is only at high power, isn't that whine drowned out by whatever you are listening to?

author: klausman
author_rank: Regular Contributor
timestamp: August 21, 2025, 08:59:38 am
content: Quote from: tggzzz on August 21, 2025, 08:18:37 amThe main filter caps I have already replaced, they both had really high ESR (>10Ohms) and were at very low end of nominal capacity ±20% (should be 10'000µF, but was at or below 8'000µF). I also replaced some others that looked like they were (on the verge of) bulging or that tested out of spec. They're all 37+ years old, so I might replace the whole lot. Some are very low-capacity (like 0.33µF), so I might put in C0Gs instead. They're all in the tone circuits, so I don't think having elec-cap levels of ESR is functionally important.On the scope, the waveform doesn't clip, but might be slightly distorted (hard to tell visually). I may run it again with only one channel through the amp, and then siggen output directly into the second channel, side-by-side to check the shape. The probe shield I didn't connect since my Siggen is part of the scope (Rigol MOS5074), so I have ground reference via the BNC-to-RCA cables. I usually try to minimize ground connections, reduces the chance of shorts and reduces the cable mess.Quote from: tggzzz on August 21, 2025, 08:18:37 amYeah, I figure it's not ofpracticalimportance, but I wasn't sure if it needed addressing, since my other amps don't show this behavior. I only noticed it since I ran the amp at high power into dummy loads. My neighbors prefer I do it that wayThanks for your insights and advice!

author: tggzzz
author_rank: Super Contributor
timestamp: August 21, 2025, 09:26:09 am
content: Quote from: klausman on August 21, 2025, 08:59:38 amClipping is easiest to see with a triangle wave, since the peak becomes less sharp.Slew rate limitation easiest to see with a square wave, since it becomes trapezoidal

author: Grandchuck
author_rank: Super Contributor
timestamp: August 21, 2025, 12:24:23 pm
content: Could be 2nd and 3rd harmonics due to crossover distortion. Not a problem unless it does not sound good in normal use.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2025, 02:04:00 pm
content: Quote from: klausman on August 21, 2025, 08:59:38 amIt is likely that the small aluminum electrolytic capacitors just dried up. Electrolyte leakage is proportional to the perimeter of the rubber seal, while the amount of available electrolyte is proportional to the volume, so small capacitors dry up faster than large ones.I see some used for decoupling around the power amplifier like C910, C704, and C713.

author: klausman
author_rank: Regular Contributor
timestamp: August 21, 2025, 04:17:02 pm
content: Quote from: David Hess on August 21, 2025, 02:04:00 pmOh, that's a thing: I saw C910 in the service manual. But it's just not populated in this unit. Probably a cost cutting measure. Maybe I should pop in the cap that spec'd and see what happens....

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2025, 05:33:48 pm
content: Quote from: klausman on August 21, 2025, 04:17:02 pmThat could be, and there are probably others that I missed. I would at least try adding and replacing the low value aluminum electrolytic capacitors which are used for decoupling.The symptom is not a common one so diagnosis is not trivial.

author: CaptDon
author_rank: Super Contributor
timestamp: August 23, 2025, 01:31:04 am
content: If you sweep the amplifier at high power into a dummy load you may find those coils have a mechanical resonance where they will sing loudly at certain frequencies. You seem to show 2nd and 4th even harmonic energy. Even harmonics are musically pleasing and an artifact of most push pull amplifiers when driven into their non-linear regions. Once you hit clipping the odd harmonics will grow like a forest. Bottom line, I think your amp is working about as good as the original design allowed.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 23, 2025, 10:14:21 pm
content: Quote from: klausman on August 20, 2025, 09:51:16 pmSmall 2-3 μH air-core inductors and a large ferromagnetic core power transformer nearby. From those two 90 % possibility that the noise is coming from a power transformer as a more obvious source.Actually, I guess most old power amplifiers (with 50/60 Hz power transformers) have this sound in some parts (mostly the transformer, but other different parts are not silent either). It is obviously heard only if the PA is loaded onto a dummy load. And large ceramic wire resistors of dummy load often make this wine too.In our usual life, we listen to music through an acoustic system (and we don't listen to dummy loads), so it doesn't bother anybody. This parasitic sound is 1000 or 10000 times quieter than the actual sound from the acoustic system.