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What is this characteristic that leads to this sound in a cheap audio amplifier?

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What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby DC-Choppah » Wed Nov 06, 2019 3:00 am

Listening to reference material on various playback systems, I am noticing a feature that I can now hear almost right away on cheap systems, but I don't hear on my studio reference system, or on hi-quality audio systems.

When a deep bass note rings loudly, the overall music starts to slowly pulsate in level.

So say a piano hits a chord and it rings. At the same time the bass not hits and holds a strong note. The piano inherits a slow (maybe 2-4 Hz?) change in level that goes up and down, like a slow tremolo.

Hit the same piano note without the strong bass note and the piano level is smooth and natural. It decays, but does not slowly pulsate.

Move the same music to the high quality system, and you just get the bass and the piano as they are, but they do not interact.

There are many specs associated with audio gear, but I was wondering what this spec would be? Some form of inter modulation? It seems to be one of the the first things that I can pick up now when listening to a sub-par playback system now that I am aware of it. But it only works if you have a track that you KNOW doesn't actually pulsate like that.
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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby awjoe » Wed Nov 06, 2019 4:08 am

Okay, cool - now you can reliably ID sub-par systems. (That's the silver lining of having associated with so many sub-par systems. :bouncy: )

I'd like to hear what you're hearing. But I'm not going to be in England until April, so I guess it's out of the question. :mrgreen:

How many cheapo systems have you heard it on?
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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby ef37a » Wed Nov 06, 2019 8:32 am

Are the cheap amplifiers always driving the same set of high quality speakers? Or are the cheap speakers being driven by the high quality amplifiers? If not you have not eliminated the variables.

Over the 50yrs or so that I have been interested in audio there have been many "fashions" regarding forms of distortion...TID,SID, Dopler, slew rate induced and more. None seem to have lasted the test of time as serious problems.

Perhaps the biggest common penny pinch with cheap amps is in the PSU and that is a possible source of signal modulation, except the time scales are wrong. On 50Hz, mains caps refresh twice a cycle not at 2Hz. There will of course be "inter-string" beats from a piano.

Still, if you can hear it you must be able to record it.

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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby Wonks » Wed Nov 06, 2019 8:47 am

I'd guess it would be down to the porting of the speakers. Poor cabinet design tends to produce ported enclosures that resonate in the bass when a note near the resonant frequency sounds.

Especially true when the port is used to achieve an excessive bass extension for the size of speaker and enclosure.

So you get the speaker reproducing one note and the port producing a very slightly different frequency, so that you get the beating effect (amplitude modulation at the difference between the two frequencies). Like tuning a guitar using harmonics.

I'd also guess that it's volume driven, two similar bass notes will be louder than one, and so set the port resonating due to the higher air velocity in the port itself. If so, turning the volume down should stop the resonance, and turning it up should get some beating going on a single bass note of the right frequency.

If the speakers are of a sealed box design, then the above won't apply.
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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby Hugh Robjohns » Wed Nov 06, 2019 10:57 am

DC-Choppah wrote:When a deep bass note rings loudly, the overall music starts to slowly pulsate in level.

So say a piano hits a chord and it rings. At the same time the bass not hits and holds a strong note. The piano inherits a slow (maybe 2-4 Hz?) change in level that goes up and down, like a slow tremolo.

Hit the same piano note without the strong bass note and the piano level is smooth and natural. It decays, but does not slowly pulsate.

I'd say the most likely cause would be a problem with the power supply not being able to sustain the rail voltage when under heavy demand. So basically, either a faulty or under-specified power supply....

...but the modulation rate you describe also suggests the possibility of a beat frequency going on somewhere. The beat-frequency peaks would stress the power supply at that rate, so that's still my favourite culprit, but Wonk's resonant port thing is also possibility -- smaller, cheaper speakers do tend to have very under-damped ports to try to get far too much bass out of far too small a box.

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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby James Perrett » Wed Nov 06, 2019 3:03 pm

Are you listening to these systems in the same room?
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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby Martin Walker » Wed Nov 06, 2019 5:55 pm

Hugh Robjohns wrote:I'd say the most likely cause would be a problem with the power supply not being able to sustain the rail voltage when under heavy demand. So basically, either a faulty or under-specified power supply....

That was my first thought as well - a case of sagging PSU due to under specced capacitors or similar.


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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby DC-Choppah » Thu Nov 07, 2019 5:00 am

James Perrett wrote:Are you listening to these systems in the same room?

Yes. Same room. Same speakers. Same reference track. Same source (iphone).

The only thing that changes is the integrated amplifier/receiver.
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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby Tim Gillett » Thu Nov 07, 2019 5:19 am

There's not necessarily anything wrong with the cheaper amp. You're probably just trying to push it to match the sound of the more high specced unit. In doing so, the cheaper unit probably overloads and distorts.

Maybe too the cheaper amp is not designed for the speakers it's connected to and would be better suited to ones of different sensitivity or impedance.

Either way, no matter what the gear, its best to understand its strengths and limitations and not expect it to perform at a level it was never designed for.
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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby DC-Choppah » Thu Nov 07, 2019 5:38 am

I am helping some musicians and music teachers and other folks without much money set up some playback systems to work with their students and practice with, learn songs etc.


Here is a inexpensive amp that works great and sounds very good and does NOT have the pulsating piano effect.

http://audio.teac.com/product/ag-790/specifications/

These are all over and I can get them for $25 - $40.


But here is an inexpensive amp that does have the pulsating piano:
https://www.pyleaudio.com/sku/PDA6BU. about $100.

So the amp that is more expensive sounds much worse.

The TEAC is much much heavier. But this is no problem. These go on a music teacher's bookshelf with a nice pair of old 'Realistic' speakers for $40 and with an aux cable we are good to go.

If it is the power supply I am hearing as you say, perhaps that is why all of the cheap systems that sound good to me that I have tried for folks, are also heavy.

The systems with the pulsating piano effect are very light.

That is the only spec I can find that seems to matter - the weight. When it comes to cheap amps, always get the heavier one?!?
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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby ef37a » Thu Nov 07, 2019 7:36 am

The weight difference of around 2 to 1 is almost certainly due to the Pyle amplifier uses Switch Mode Supply and the Teac a conventional 50Hz transformer and therein maybe lies the problem?

SMPSUs often have rather high impedance feedback circuits and thus the pulsing effect could well be due to that. I do however agree with the suggestion made that the cheaper amps are perhaps being pushed beyond their limits. You tend to get what you pay for!

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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby Sam Spoons » Thu Nov 07, 2019 11:11 am

And that TEAC looks old school and 'proper' so may well be much higher quality than the Pyle unit.
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Re: What is this characteristic that leads to this sound in a cheap audio amplifier?

Postby Tim Gillett » Thu Nov 07, 2019 12:03 pm

DC-Choppah wrote:...That is the only spec I can find that seems to matter - the weight. When it comes to cheap amps, always get the heavier one?!?

The spec of RMS power in Watts and at what distortion (THD) is the normal one of interest.

But notice that while the two amps quote about the same RMS power ratings, only the Teac quotes the distortion at that power. The Pyle does quote a distortion figure of less than 0.1% but omits to say at what power this applies, so it's kind of meaningless. The Pyle may put out the quoted power but at what distortion? Probably a lot more distortion than 0.1%, if your listening experience is a guide.

Specs really are important if they're stated truthfully and meaningfully, but they also need to be understood to translate them into everyday usefulness. Some people tend to disregard specs in favour of "real listening" as if the two are totally independent of each other but they're not. Pretty much everything about an amplifier's, a speaker's, a mic's, a recorder's etc performance can be measured and described in such terms, and it all comes out at some point in the listening.
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