This power amp makes a strange noise!

Yesterday I reviewed the Aiyima T9 Pro on KZbin. Today, stupidly, I messed about with it some more. And it made a strange noise. Watch my video to hear it. Warning: Includes shameless self-promotion. Ft. Betty and Debbie.
'Hyde Park Corner' on Spotify - open.spotify.com/album/3rimqS...
'Hyde Park Corner' on Bandcamp - davidmellor.bandcamp.com/trac...
Previous review video - • Does this amp distort?...
The product illustrated was given to me by the manufacturer in return for one review video. This condition was satisfied by my previous video on this product and therefore this video does not contain a paid promotion.
DAVID MELLOR'S MUSIC
David Mellor's music on Bandcamp - davidmellor.bandcamp.com/
David Mellor's music on Spotify - open.spotify.com/artist/6OkaD...
Available on all good streaming services
EQUIPMENT USED TO MAKE AUDIO MASTERCLASS VIDEOS
CAMERA - Sony Alpha A6600 amzn.to/3uj7Dtq
COMPUTER - Apple Mac Mini M2 Pro amzn.to/47tW6Wg
MICROPHONE - Sennheiser MKH 416 amzn.to/3QWWvuO
ALTERNATIVE MICROPHONE - AKG C414 amzn.to/3SLqCad
BUDGET MICROPHONE CHOICE - Rode shotgun amzn.to/3G0ni36
MICROPHONE PREAMPLIFIER - Comica LinkFlex AD2 amzn.to/40SMHFT
TELEPROMPTER - Padcaster Parrot alternatives amzn.to/40A55TJ
TELEPROMPTER SOFTWARE - PromptSmart Pro promptsmart.com/
AUDIO INTERFACE - Behringer UMC202HD amzn.to/3ugj6db
HEADPHONES - Sony wired amzn.to/3sEjlht
SPEAKER CABLE - KabelDirekt amzn.to/3ucgQ6T
POWER AMPLIFIER - Fosi V3 amzn.to/3vRumNH
KEYLIGHT- Amaran 100d amzn.to/40A5Gor
FILL LIGHTS - Switti RGB amzn.to/3ufhOPu
SOFTBOX Phot-R alternatives amzn.to/3QGoTjJ
LIGHTING STANDS Phot-R alternatives amzn.to/3QWXOKe
MONITOR - Samsung UJ59 amzn.to/3MEoBsk
MONITOR ARM - Invision alternatives amzn.to/40SLqi5
MIC ARM - Neewer amzn.to/49Ob0sD
TRIPOD - Velbon alternatives amzn.to/46gu2oa
GREEN SCREEN - Elgato alternatives amzn.to/3SG2F3L
DESK - FlexiSpot E7 Pro amzn.to/3QVIWvS
VIDEO EDITING - Apple Final Cut Pro www.apple.com/final-cut-pro/
IMAGE EDITING - Adobe Photoshop amzn.to/3FZ7Ww0
AUDIO EDITING - Avid Pro Tools amzn.to/3QA5Wz7
NOTEBOOK - Leuchturm 1917 hardback amzn.to/47zonKX
PEN - Sakura Pigma archival pen Sakura Pigma archival pen amzn.to/47zz3cP
OSCILLOSCOPE - Hameg HM 203 alternatives amzn.to/47tWN1O
GROOMING - Philips 3000 nose hair trimmer amzn.to/40BdLJo
GROOMING - Nose plier amzn.to/41IOZaR
HAIR COLOUR - Just For Men amzn.to/475OgS1
HAIR CONDITIONER - TRESemmé Silky & Smooth amzn.to/41AdWFd
Disclaimer: Videos on this channel may include affiliate links and may feature software that has been received free of charge with an NFR (not for resale) licence, equipment that has been loaned or donated by a manufacturer or distributer, or equipment that has been bought for less than full retail price.

[@deanrantala]

In general, many class D amps should not be run without a speaker load connected. While i am not an EE, i have done many projects utilizing various Class D chips and the source of that sound is (as previously mentioned), the internal inductor coil. When a class D amp is outputting signal while unloaded, the voltages sent out by those integrated chips evidently spikes higher than usual and can even damage the internal components or even the chip itself. The speaker (when connected) is actually part of the RC filter network needed by the chip to damp the output. Without the speaker connected, excessive voltages can be developed within the choke and ultimately get fed back into the chip. This is all dependent on the chip and implementation of coarse. Some class D amps are fine without load, others may not be. When dealing with these china mini amps, i would suggest you not attempt any high volume operation without speakers connected just as a precaution.

[@nazarsliunchenko9869]

The tubes are not the source of that sound. It the coils on the amp's output. It is very common with class D amps. Almost all of them have output LC filter and if they are made cheaply, you will hear music coming out of it.

[@AudioMasterclass]

Thank you for calling my track music. The mic is by the tube, by the way, because that’s where the sound seemed to be coming out, from the hole.

[@johnsutton6797]

Depends, if the amp outputs are connected to a load. Also, inductors have no internal magnetic field to act as a voice coil.

[@paullongtailpair3812]

You hit the nail on the head !

[@paullongtailpair3812]

@@johnsutton6797 The magnetic field in the ferrite cores is generated by the coils.

[@Douglas_Blake_579]

@@johnsutton6797 The sound comes from the wire windings of the coil vibrating. Cheap inductors!

[@rienpost3145]

The cymbals that come in at 1:45 in Hyde Park Corner. Absolutely brilliant. (And thanks for making it available on Bandcamp! Bought it.)

[@Douglas_Blake_579]

Sounds like output coil noise ... this can often happen with Class D amplifiers when no speakers are connected. The speaker impedance is actually part of the output low-pass filter and with no speakers connected that filter becomes inadequately damped causing back EMF in the coils that can, in some cases, damage the amplifier chip. Hook up to a dummy load, see if the noise is still present. If it is, get another amplifier.

[@AudioMasterclass]

Your answer is floating to the top of my belief system at present. I do have a dummy load handy and I'll give it a go. I'll wait a few days for more speculation from my commenters.

[@Douglas_Blake_579]

@@AudioMasterclass FWIW ... I've been running a lot of these little mini-amps through my test bench lately and some did surprisingly well... others not so much. The one common theme in the bad ones is noisy coils. It's a saturation effect and the source of the sound is actual vibration of the wires in the coil. Some coil noise is acceptable as the amplifier approaches clipping and largely results from an increase in harmonic content. But when you start hearing it at lower powers you know the coil is underrated, saturating early and likely to overheat and fail.

[@user-mu2dp9hg4o]

The class D amplifier chip requires an output LC filter. The LC filter introduces a phase shift that can influence the stability of the circuit. The load impedance acts to damp the Q of the LC filter and increases stability by diminishing the phase shift. Normally the circuit design would include a Zobel Network that provides a load at high frequencies to help circumvent this problem.

[@Flarptube]

Well, now that confirms what I think is the second best use for this amplifier: a static display of a living room at IKEA and a fine compliment to the non-functional television. The best is the cause célebrè that gave rise to this entertaining offering on KZbin.

[@jppj1231]

Good to hear you man

[@humblestever24]

. "Hyde Park Corner" What is that long lost theme With the sound of a long lost band. Bobbies two by two In the green and pleasant land.

[@moe5809]

Shocking. Don't worry the Aiyima T9 Pro Ultra plus will be out soon. It will be twice as good.

[@spacemissing]

Yeah... Energy has to go somewhere. Amplifiers with output transformers will "sing", too, sometimes.

[@Roosville1]

A long-long time ago I would use a old electric bar fire as a load for high power amplifier test, you could clearly hear Dark Side of the Moon from the electric bar fire, I guess the reflector helped with the audio.

[@AudioMasterclass]

One of the many crazy ways to be an audiophile.

[@andymouse]

I would say as it's off load perhaps thats inductors rattling along with the tune or some other coil like structures digging the beat....cheers!

[@callumbrowns]

Nice content man, Can you do a video on what sound's better, a 7 inch record or a 12 inch

[@AudioMasterclass]

I might.

[@ac81017]

Just listened to Hyde Park Corner 3 times. Love it. Where can I get a copy of the lyrics. Yes, I'm a bloody northerner.

[@AudioMasterclass]

Another bloody Northerner says Bandcamp. There's a handy explanation of what they mean also.

[@Douglas_Blake_579]

Add a 0.1uf capacitor across the meter movement at the rear connections... if it clears up you got off easy.

[@joewlodynski3521]

David, do you have an option on the T9 to bypass the tubes and go direct solid state. I have a T10 which is a pre amp/DAC and I can bypass the tubes. Your site is GREAT. Joe Wlodynski Wesley Chapel, Florida USA

[@AudioMasterclass]

Being of a sneaky disposition I did try it with the tubes out. It works, but quieter. It would seem therefore that the tubes are in parallel with the signal path.

[@robertgwatts1858]

Sounds like you were actually broadcasting on short wave. The unloaded cables acting like antennas to the amps switching frequency. Not enough RF power for me to tune into you in Spain, unfortunately. 😂

[@johnsutton6797]

😅Hi David I'll bet the noise is coming from the meter movement, acting like a speaker. Enjoy your channel. Jack

[@Douglas_Blake_579]

Most likely output coils but this is also a valid possibility.

[@paulstubbs7678]

Are you sure it's not coming from the meter movement? Yes you have to be a bit brutal to make a meter movement sing like that, but it can.

[@middleearthltd]

Thankfully we have the chat to get this explained Shout out to the chat

[@ac81017]

It's cheap non-audiophile crap that's the problem. Will check out your new release. 👍

[@jeremymorrison3494]

Bandcamp 2 ? Never watched the first movie. Was it any good? 😉

[@AudioMasterclass]

It was fabulous.

[@matthewhilty4209]

That is probably coming from the coils vibrating. Hot glue them and see if that helps.

[@analoghardwaretops3976]

Your. amp really does like the song & wants to dance to it's beat ..The meter does put up a nice dance show...and is not shy of HYDING in the Corner..😅😅

[@RUfromthe40s]

in older decks thatrecord still with perfect sound, once i noticed i could record to plus 3 or 0 but in a certain position the needles seem to dance more and the recordings started to have more dynamic sound

[@michaeldeloatch7461]

I wonder if it's the VU meter.

[@user-ol6kh8od3k]

Powering off preamp makes a popping sound, is it bad for the speaker? My preamp is 7.5W class A. If I turn the volume all the way down, it will minimize the popping sound. If I wait for 5 minutes to let the class D amp go into standby mode, no popping sound at all. Please advise, thanks!

[@Douglas_Blake_579]

Simple solution ... turn the amp off first... turn the pre-amp on first. No it's not harmful... just very annoying.

[@AudioMasterclass]

Professionally, power amp goes on last, goes off first. If it makes any noise on switch-on, get rid of it.

[@user-ol6kh8od3k]

Thanks a bunch!

[@user-ol6kh8od3k]

@@Douglas_Blake_579Thank!

[@frankd.b.9233]

1 question why do you use USB if you have other options, I always try to avoid USB, why ... USB 2 crap

[@jimbohnenkamp5082]

Hyde Park Corner...not the worst thing I've ever heard.

[@AudioMasterclass]

Come on, it would be an honour to be worst.

[@Roddy451]

Oh... Huh?.. eh... hmm... Riiiiiiiiiiight... Well, the song is great. No comment on the amplifier.

[@jimdavis5230]

Hi David, I understand you have a degree in audio and technical stuff. I don't have a degree just an HND in electronic engineering. Far lower than your degree. Now I have designed and built my own pre and power amplifiers that sound superb. Therefore why don't you do the same rather than waste time with these cheap modules? I have never heard any power amplifier that sounds better than my own designs.

[@AudioMasterclass]

I have a Tonmeister degree with a couple of electronics modules, and quite a lot of professional practical experience in electronic design and construction. But I've been there and done that to my satisfaction so I'll leave high-end amp building to others.

[@earthoid]

Just another example of tubes adding distortion to the sound.

[@adotopp1865]

Well if you mess about this is what you get

[@Audiodreamer192-24]

Hyde park corner…😂😂😂

[@marxman00]

By plugging in the valves you have passed the test ....you now have a low level interocitor...use it wisely !

[@AudioMasterclass]

This Island Earth I believe.

[@fogbullit1000]

Filter coils can sing and transformers can do the same 😃

[@AudioMasterclass]

Yes it's clever how it can turn my music into actual singing.

[@idtubenod]

🤣🤣🤣

[@gjnbouwmeester5860]

Hahahahahaha, Man cave... Green screen u mean

[@AudioMasterclass]

I’ll be replacing it with a holodeck later this year.