Almost a year ago, a friend and I made the original design together. I had reduced the size of NFB cap and input filter cap, because the quality of the bass was so awful on TDA7294. Its known for this problem, plus gorgeous mids and treble. Today, just an hour ago, the problem is gone.
The question for people smarter than I am, is why is there more lower bass notes and a warmer heatsink, without changing the cap value to bigger? I really don't need an answer, but rather just very glad that it plays right. This is so nice, with features gained, none lost.
The question for people smarter than I am, is why is there more lower bass notes and a warmer heatsink, without changing the cap value to bigger? I really don't need an answer, but rather just very glad that it plays right. This is so nice, with features gained, none lost.
if it's oscillating it may not live so long. But with a 560r resistors I don't feel as if there would be problems with that.
You may try turning the diode around, and seeing what that does to the sound.
I just can't comprehend how a chip could have bad bass... Usually the hardest part is working around parasitics to get low distortion for frequencies over 1KHz. Thermal distortion...?
- keantoken
You may try turning the diode around, and seeing what that does to the sound.
I just can't comprehend how a chip could have bad bass... Usually the hardest part is working around parasitics to get low distortion for frequencies over 1KHz. Thermal distortion...?
- keantoken
keantoken said:
Turned diode around = same as uninstalled.
It runs very cool, and still does; but, its getting more work done, so slightly warmer. On a little heatsink, its far under max spec still.
TDA7294 bass = tubesound bass. That's good for treble, bad for bass.
If one had options, not locked away in a little chip, the odd harmonics make great clear bass, while the even harmonics make great smooth treble. Why is this?EDIT: Sorry for making a terrible choice of TDA7294 chipamp for the experiment, but its the one that needed it.
It would appear that your diode is either canceling out harmonics or it is creating 3rd harmonics.
Also, check for DC offset! This may be why it's getting warmer, as that cap might be isolating DC that the diode is actually passing. The fact that it goes back to normal when you reverse the diode shows that in in the non-reverse position, there is current flowing through it. And that cap is probably there to prevent DC current flow.
If there's not DC offset, it looks like you're good.
What about the original project and topic of this thread?
- keantoken
Also, check for DC offset! This may be why it's getting warmer, as that cap might be isolating DC that the diode is actually passing. The fact that it goes back to normal when you reverse the diode shows that in in the non-reverse position, there is current flowing through it. And that cap is probably there to prevent DC current flow.
If there's not DC offset, it looks like you're good.
What about the original project and topic of this thread?
- keantoken
keantoken said:It would appear that your diode is either canceling out harmonics or it is creating 3rd harmonics.
Also, check for DC offset! This may be why it's getting warmer, as that cap might be isolating DC that the diode is actually passing. The fact that it goes back to normal when you reverse the diode shows that in in the non-reverse position, there is current flowing through it. And that cap is probably there to prevent DC current flow.
If there's not DC offset, it looks like you're good.
What about the original project and topic of this thread?
- keantoken
If the NFB cap is an Achilles Heel, the little mod is a crutch. All that I can really say about the exploration is that I got less ringing from the NFB cap due to bypassing it with stuff. There's no DC offset.
The original project is an amp with an NFB cap.
danielwritesbac said:
When you think of it like this, it becomes so in your mind and you convince yourself that your "mods" make an improvement.
Instead, learn exactly what this capacitor does.
Try a simple switch that shorts it or includes your "mod" and have a friend switch it in or out without your knowing which is which - aka blind test.
Sometimes we hear improvements that aren't there.
Also, troubleshoot the rest of your circuit to ensure it is performing well. Adding or subtracting components on intuition may not be the best way to design an amp.
It seems to me that you are haunted by the presumption that caps "ring". Caps only ring if they are in parallel or series with an inductor. When we put this in the feedback path of an amplifier we call it a bandpass amplifier or filter if it has no gain.
Caps only have a very very low amount of inductance because of the way they are made, with spiraling plates with a dielectric between them. It would only ring if your amp was goofing up and sending spikes through it, and even then it would probably not be any frequency you could hear.
What you may be hearing is high-frequency harmonics as a result of distortion, or perhaps your amp starts oscillating with a certain input.
I don't think the cap is ringing. Do you have a schematic?
- keantoken
Caps only have a very very low amount of inductance because of the way they are made, with spiraling plates with a dielectric between them. It would only ring if your amp was goofing up and sending spikes through it, and even then it would probably not be any frequency you could hear.
What you may be hearing is high-frequency harmonics as a result of distortion, or perhaps your amp starts oscillating with a certain input.
I don't think the cap is ringing. Do you have a schematic?
- keantoken
Dan has already told us that his NFB cap is too small.
That informs us that it has an Audio signal across it when low frequencies are present in the signal.
That in turn develops distortion that can be avoided.
Adding a bypass diode across this cap will limit the AC voltage across the cap. This is likely to be heard because of the error in building the amp. If the cap has no AC voltage across it in normal operation then the diode is never asked to pass any current.
The diode should have no audible effect.
If it has then that confirms the error in the build.
Correct the build and make the amp perform as it should.
That informs us that it has an Audio signal across it when low frequencies are present in the signal.
That in turn develops distortion that can be avoided.
Adding a bypass diode across this cap will limit the AC voltage across the cap. This is likely to be heard because of the error in building the amp. If the cap has no AC voltage across it in normal operation then the diode is never asked to pass any current.
The diode should have no audible effect.
If it has then that confirms the error in the build.
Correct the build and make the amp perform as it should.
But, that schematic has repeated the same mistake.
Audio AC voltage across the NFB capacitor (C2) because it is too low in value.
C2>=R2 / R3 * C1 * sqrt(2) >=36uF.
i.e. for 100k, 2.7k, 680nF use 39uF or 47uF.
I would use 56uF or 68uF and increase the input capacitor to ~ 1uF.
However, the rest of the circuit is flawed.
If Zin=100k then R4=100k and R3 becomes 2k2, this forces C2>=64uF.
If Zin=47k, then all the component values must be scaled accordingly.
Finally, add RF attenuation to the input.
That schematic sucks. 2 missing components and 5 out of 11 components incorrectly selected.
Audio AC voltage across the NFB capacitor (C2) because it is too low in value.
C2>=R2 / R3 * C1 * sqrt(2) >=36uF.
i.e. for 100k, 2.7k, 680nF use 39uF or 47uF.
I would use 56uF or 68uF and increase the input capacitor to ~ 1uF.
However, the rest of the circuit is flawed.
If Zin=100k then R4=100k and R3 becomes 2k2, this forces C2>=64uF.
If Zin=47k, then all the component values must be scaled accordingly.
Finally, add RF attenuation to the input.
That schematic sucks. 2 missing components and 5 out of 11 components incorrectly selected.
keantoken said:
Do you remember the first sentence of post #7? You ain't seen nuthin' yet.
danielwritesbac said:
No, it is not. The point is everybody is always only looking at the very simplified Typical Applications, when they should be looking at the reference designs, e. g. on page 21 of LM4780's datasheet.
Now this has gone way off topic.
Hi,
has it gone off topic?
Dan wants to try building a discrete amplifier.
Here he is learning that each component must be in the correct place and of the correct value.
Shortly he will learn what semiconductors to use in the appropriate locations since his own choice were rubbished.
Then he'll learn about tolerances and how to select appropriate components.
Who's going to teach him how to read datasheets?
And I suppose the difficult one is to recognise all the mistakes/myths/lies that exist on the net and filter out all the nonsense.
This is going to be a long journey.
has it gone off topic?
Dan wants to try building a discrete amplifier.
Here he is learning that each component must be in the correct place and of the correct value.
Shortly he will learn what semiconductors to use in the appropriate locations since his own choice were rubbished.
Then he'll learn about tolerances and how to select appropriate components.
Who's going to teach him how to read datasheets?
And I suppose the difficult one is to recognise all the mistakes/myths/lies that exist on the net and filter out all the nonsense.
This is going to be a long journey.
pacificblue said:Do you remember the first sentence of post #7? You ain't seen nuthin' yet.
No, it is not. The point is everybody is always only looking at the very simplified Typical Applications, when they should be looking at the reference designs, e. g. on page 21 of LM4780's datasheet.
Now this has gone way off topic.
Wow, even worse. Is it a reference for a home theater amp?
AndrewT said:Hi,
has it gone off topic?
Dan wants to try building a discrete amplifier.
Here he is learning that each component must be in the correct place and of the correct value.
Shortly he will learn what semiconductors to use in the appropriate locations since his own choice were rubbished.
Then he'll learn about tolerances and how to select appropriate components.
Who's going to teach him how to read datasheets?
And I suppose the difficult one is to recognise all the mistakes/myths/lies that exist on the net and filter out all the nonsense.
This is going to be a long journey.
Thanks man! I was just thinking that it was going to be a long journey indeed. As far as myths, those are especially troublesome when they're in the datasheet. That really mixes me up.
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