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Old June 10, 2008, 02:44 PM
trodas's Avatar
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Location: Czech republic
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Question Opamps stage design questions

Altrough I'm reasonably skilled moder of mainbords and GFX cards and all electronics stuff, I did not play into the audio field much.

So I think I better ask these who know about my ideas how to modify a existing design.

This one:

Complete scheme:

Input swichboard:

As soon as the signal hit the switching part of the amplifier, it is shorted to ground with C99 cap, a 0.1nF one. I think this is completely unnecessary blurring of the sound, as capacitor in general act to prevent voltage changes, so it has to "blur" a little the amplitude to prevent fast and rapid changes of it.
Do I get it right?

Later the signal go thru a 0.9 resistor divider, witch is probably used to put some small load (11k to ground) on the audio source. Is this value optimal for the X-Fi equiped with LM4562 opamps...?

What about keeping just ONE of these dividers and adjust it to the 0.68 as the result of two dividers (0.9 and 0.76) after themselves are in the end.
The aim is less distortion in resistors - or even using a audio grade resistors such as Vishay Audio Resistors:
diyAudio Forums - Vishay Audio Resistors

I also fear that the combination of R and C components can create a slight RC filter that in the end make the "blur" effect of the C99 stronger a little. Right?

After it pass thru the switch, here come another ground-shorting cap, a C1. Now with 0.33nF capacity.

Then come another resistor divider, this time 0.76 and directly after him a first decoupling cap, a C3 - 10uF 25V.
As far as I understand audio, the blocking caps is necessary for the filtering of the DC offset. What if my X-Fi has very low DC offset? Is not no cap better for audio that ANY cap, even quality audio one?

I think the C3 is entirely unnecessary one. I think only one decoupling cap in the whole spekers (or none) is best solution - and placed directly before the output amplifier.

And it get worse. Just after the opamp, there is another decoupling cap! A C9 - again 10uF 25V for all except CENTER and SW channels. First thing I did not like is that the capacity on the output is same as on input - should not be bigger? Maybe is the level of signal not that high still, but... it just did not feel right.
Second thing I did not like at all is the fact that we already removed the DC offset before the opamp, so, why now? Sure, a badly balanced of sucking opamp could produce some DC voltage at the output, but... why not balance it better or remove it and use quality one instead that does not need second decoupling?

I think with the LM4562 or perhaps better AD8599 I can remove these.

And right after the potentiometer we have another decoupling cap - a C13! In fact, he is in serial circuit with the C9, witch bring the ending capacity down to half... not to mention that with the huge resistance between then the impact on the signal can be high.
I hope I'm wrong on this one, but... IIRC the most clean voltage filering is a RC way. Only with the problem that it's output voltage differ with different current - so current has to be always the same and stable...

I think the designer of this speakers just put together the recommended way of the used circuits and then these double-triple decoupling caps are the result.

"It is dangerous to be right in matters on which the established authorities are wrong." - Voltaire
"I believe that all the people who stand to profit by a war and who help provoke it should be shot on the first day it starts..." - Hemingway :) my config - my caps
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Old June 10, 2008, 09:59 PM
vinister's Avatar
Join Date: Feb 2008
Location: Vancouver
Posts: 297

Do you have access to a scope? Without one, I wouldn't suggest changing values in blind faith, you will have no way to verify if it actually does work better. It will 'sound' better to you anyways whether it does anything or not because you have done work, your brain will fudge the results favorably.

The capacitors should be spec'd not to affect anything in the audible range, they are purely for DC protection for badly balanced op-amps as you have mentioned. I think you are worried they are slowing the slew rate of the circuit, right? Lucky for you its easy to measure on a scope. Take a measurement, short the cap you are thinking of removing, and compare the new measurement. Just make sure you have no DC on the output. I think there is also a chance that anything you hook up to it could send DC back in to the circuit, so make sure you test for DC offset with everything connected like you plan to use it.

In any case I just wouldn't do anything without a scope because you have better chances of making it worse than better if you are modding purely from a circuit-theory standpoint.
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