Jeez Andrew, the OP asked a very fair question. You are overreacting.
In regards to the popular LM3875 and LM3886 chips, there will be 40-120mV DC at the output regardless of filtering capacitors at the input or having the feedback filtering capacitor. This is a bit high compared to most amps and may affect the sound of the speaker, but it will not damage the speaker.
Last edited:
This is the typical rubbish posted by ill-informed Members.
Take your version of physics to another Forum.
R is guessing by swapping in and out components.
If R would just do some numbers, he could home in on solving the missing bass without having to guess.
Eg, if we/he knows that the input filter is not rolling off the bass early AND if we/he knows that the NFB capacitor has virtually no AC voltage across it (by being sufficiently higher RC) then we/he knows it is not the filter frequency that is at fault. We/he knows to look elsewhere.
At the moment neither he nor us have a clue where to look because R won't state what the RCs are in the amplifier.
If R would just do some numbers, he could home in on solving the missing bass without having to guess.
Eg, if we/he knows that the input filter is not rolling off the bass early AND if we/he knows that the NFB capacitor has virtually no AC voltage across it (by being sufficiently higher RC) then we/he knows it is not the filter frequency that is at fault. We/he knows to look elsewhere.
At the moment neither he nor us have a clue where to look because R won't state what the RCs are in the amplifier.
My LM3875: no caps at the input and no feedback caps....I have about 30mV DC at the output.....6 years and still running and no damage to the loudspeakers. My DAC has output caps (TDA1543/CS8412) so no need for another caps at the amp input.
The sound is definitely better without caps no matter how expensive they are.
The sound is definitely better without caps no matter how expensive they are.
That is not the issue at question.
Guessers, and the wise, can be both rude and polite.
I never understand this. Isn't your DAC output cap just the same as an input cap? You do have caps in the signal path, just not ones you put there in that piece of equipment. You just didn't add another cap.
Yes, Jr.
He has a DC blocking cap and the signal is passing through it.
To state he has no coupling cap is a nonsense.
I did not respond initially since I was not sure if he was just trying to "wind me up".
But, now I see that at least one other is aware of his fallacious argument.
He has a DC blocking cap and the signal is passing through it.
To state he has no coupling cap is a nonsense.
I did not respond initially since I was not sure if he was just trying to "wind me up".
But, now I see that at least one other is aware of his fallacious argument.
Last edited:
Sorry, I didn't want to wind anyone up.
About input caps:
We are speaking about input caps in our gainclones or chip amps. So, I would say there is no need for input caps in the chip amp because all CD players and other commercial available sources have output caps to block DC or they block DC some other way.
When I removed input caps from my gainclone I never looked back.
And about caps in negative feedback to ground:
I don't have caps in the feedback loop to ground because I have discovered, that those caps have a large influence on the sound. Yes, without those caps you will have some amount of DC voltage at the output of the amp. Still, I never had DC offset larger then about 50mV on all my LM1875 and LM3875 and OPA549 amps.
Regards
About input caps:
We are speaking about input caps in our gainclones or chip amps. So, I would say there is no need for input caps in the chip amp because all CD players and other commercial available sources have output caps to block DC or they block DC some other way.
When I removed input caps from my gainclone I never looked back.
And about caps in negative feedback to ground:
I don't have caps in the feedback loop to ground because I have discovered, that those caps have a large influence on the sound. Yes, without those caps you will have some amount of DC voltage at the output of the amp. Still, I never had DC offset larger then about 50mV on all my LM1875 and LM3875 and OPA549 amps.
Regards
Andrew, you are being very emotional and need to chill out. I can easily post data on LM3875 and LM3886 chips (as well as the OPA541s which have near-zero DC offset) to show how either with or without DC blocking capacitors the change in DC offset is small.
You are just embarrassing yourself, Andrew, so I will do you a favor and not embarrass you further.
You are just embarrassing yourself, Andrew, so I will do you a favor and not embarrass you further.
You've been telling him that for 3 pages, Andrew. Calculating poles is the basic of basics, but he would rather listen to woo.
30 seconds of quality time with a calculator would answer his question.
I guess I wasted all that time and money on engineering school. I could have just talked to a couple of kids with soldering irons and learned everything I need to know.
Near zero offset with a test signal. Real world signals are not perfectly symmetrical. You could do some measurements and find that out for yourelf.
Using a mathematical model, the integral of a sine wave (or two sine waves for IM measurement) is zero. (Not the integral of the absolute value!) The integral of a real world musical signal is not zero.
What does the integral of the signal give us? The theoretical DC offset of a "perfect" amplifier with zero quiescent DC offset.
i tired adding two 1uf/50v silmic back to back to make bipolar cap so the resultant is .5uf in parallel with 1uf styroflex. I got the bass properly as expected and have all the mids and low mids and high bass are very well rounded and still the highs are very well extended and even the vocals are not overtoned...
I removed the styroflex from the loop and tested only with the silmics there was complete loss of the upper mids and highs... or I just got all the bass freq added..
then added the styroflex on top of it gave the mids and upper freq extension and open sound...
strange thing is that why I couldnt get this even after adding two 1 uf styroflex in parallel... which now got with just .5uf of silmics in signal cap... i was just a try but was giving decent results....
Andrew what can be done with RC time constant? like i have posted in the above reply...
can i get back the bass with the same styroflex caps? I have total 26 of them with me if you can tell me what value to be used it would be great... as I have replied in the earlier post reg the time constant...
I removed the styroflex from the loop and tested only with the silmics there was complete loss of the upper mids and highs... or I just got all the bass freq added..
then added the styroflex on top of it gave the mids and upper freq extension and open sound...
strange thing is that why I couldnt get this even after adding two 1 uf styroflex in parallel... which now got with just .5uf of silmics in signal cap... i was just a try but was giving decent results....
Andrew what can be done with RC time constant? like i have posted in the above reply...
can i get back the bass with the same styroflex caps? I have total 26 of them with me if you can tell me what value to be used it would be great... as I have replied in the earlier post reg the time constant...
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.