Fake Components

[Mark Hennessy]

You bet that was for running the reduced load with the headphone resistors in circuit.

Why?

[Refugee]

A full load begins to add effects from the speaker current driving the transistor elements into there non linear range.
The distortion figures are measured at about 10% of full power in order to get a low figure.
Full power is normally measured when the distortion reaches about 1%.

[Mark Hennessy]

None of which supports your assertion that the quoted distortion figures are being massaged by being quoted into a higher impedance loads.

Of course, an amplifier has lower distortion into higher impedance loads, to a point. LSN distortion is well known. However, the datasheet has several graphs of distortion vs frequency or output power level, and in all cases, they specify the load impedance (4 or 8 ohms). There is no indication that they are lying in the name of specsmanship; there is no reason to assume that this IC won't meet the published specification when properly implemented. So please, let's not casually throw accusations about without the data to back them up.

[Refugee]

I have had a look at the data sheet again. About 2 or 3 screens down from the top of the data sheet it shows the distortion figures over the power range from 5W to 60W with the distortion varying from 0.005% at 5W to 0.5% at 60W.
The headphone socket in the set has a dropper resistor to drive 30 ohm phones. The power needed is small to drive phones.
Have a look at the data sheet in the link. There is nothing dishonest as all the distortion figures are clear to be seen.
If you have any more Qs I can open the PDF manual for the set and trace the wiring to the headphone socket.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwjBw5jAwKHkAhUB6qQKHV6oC9cQFjAAegQIAhAC&url=https%3A%2F%2Fwww.st.com%2Fresource%2Fen%2Fdatasheet%2Ftda7294.pdf&usg=AOvVaw0xLiTP17BCgVKKPLF_2SOt

[Mark Hennessy]

Exactly - the datasheet shows that it can do 0.005% into 8 ohms at 1kHz. All the way up to 55 watts (figure 4). Therefore, there are no grounds for this statement:

I looked up the audio amp chip and the distortion figure was more like that of a preamp. You bet that was for running the reduced load with the headphone resistors in circuit.

You say that the figures in the datasheet were into a higher than typical impedance to reduce LSN in order to get an artificially low distortion value. That simply isn't true.

[Refugee]

If you look at figure 4 and 6 you will be able to see that the distortion lower with an 8 ohm load compared to a 4 ohm load.
It could be projected down to a higher impedance that the amp will see with a headphone load on it.
For what I bought the set for the distortion figure at 5W is easily good enough.
The headphone resister is quite a bit more than 8 ohms so the figure is a guess with the headphone output being used.
I just made a 3.5mm stereo plug up with 1/4 inch jacks on the other ends with 33 ohm resistors hidden in them.
The amp inputs are 1 meg.
If the amp had been of a lower spec I would have gone in and tapped the audio beck up the audio chain so a look inside can wait for the cassette belts to stretch.

[Mark Hennessy]

If you look at figure 4 and 6 you will be able to see that the distortion lower with an 8 ohm load compared to a 4 ohm load.
It could be projected down to a higher impedance that the amp will see with a headphone load on it.

No, you simply can't make that assumption - you'd have to run some tests to prove it. There are many other sources of distortion in a power amplifier. With a blameless amplifier, going to 16 ohms rarely causes a reduction in distortion.

You might also note that the LM3886 is pretty much free from LSN - when you get into the fine detail, it's a much better chip than the TDA7294.

For what I bought the set for the distortion figure at 5W is easily good enough.

Of course. Needless to say, 0.005% is orders of magnitude better than anyone can hear.

In fairness, ST did quite well to get the distortion down to that level, given that they used a MOSFET output stage - presumably for marketing reasons, as there's no rational technical reason to build an audio output stage using them. I wonder if that explains the peculiar slow rise in THD as the power rises from 0.005% to 1% over the power output range of 55 to 70 watts - compare that with the LM3886, where the distortion stays low until clipping - the rise from <0.005% to 1% takes place over just a couple of watts, which is exactly as you'd expect for well designed solid state amplifier

But that is the raw distortion of the IC in an ideally-implemented test setup. Whether JVC managed to get the same results is another question.

When I build a design with the LM3886, I have no problem getting 0.003% at 1kHz. A friend of mine designed a PCB for a pair of them, and got nearly 1% on one channel and around 0.1% of the other. He didn't believe that the PCB layout could be a problem, but to humour me, he implemented some of the changes I recommended. The new PCB produced much lower distortion - around 0.07% IIRC (the notebook with the results in is not in this office), but still with differences between channels, which is a pretty big clue that the PCB layout is still not optimal. Finally, I cut tracks and added jumper wires to implement the last of the changes I suggested, and the distortion fell down to very close to the expected 0.003%.

All of this was purely PCB layout - none of the component values were changed during this. There are many ways to mess up even a simple circuit. And even if there is almost certainly no subjective difference between the first PCB and the final version, it still shows the value of measuring THD to prove that you haven't screwed something up.

[ppppenguin]

Mark, what sort of PCB problems caused this excess distortion? My first guess would be poor power and ground distribution causing high currents to flow through tracks that supply low power stages.

[Mark Hennessy]

Entirely grounding.

[Mike Watterson]

Entirely grounding.

Always been an issue. Audio, Instrumentation, Logic, PSUs.
One solution for non-PCB is the star approach, even if an earthed chassis or a PCB. Another is the heavy aluminium chassis, layout still matters. I had a Guitar amp here the other day with terrific hum. Painted steel chassis and the paint under some earth tags! Also earth return for an ECC85 grid via screened cable (reverb springline). A 470 K Ohm at the valve base cured that.
A third approach, common on USA radio sets, (isolated or transformerless) and some amps is buss-bar. But layout and connections in the right order are important.
A PCB doesn't magically make earth path and layout issues go away, even if it has a massive ground plane. The pulse currents between transformer / bridge / main smoothing cap must not flow anywhere else!