Genius SW-5.1 HT mod - are my ideas good? - diyAudio
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Old 8th June 2008, 07:06 PM   #1
trodas is offline trodas  Czech Republic
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Question Genius SW-5.1 HT mod - are my ideas good?

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First the good news. They send me schematics:
http://rapidshare.com/files/11550467...me_Theater.pdf

...so, here we go:

Click the image to open in full size. Click the image to open in full size. Click the image to open in full size.


Genius SW-5.1 HT capacitors modifications - proposal

Some things are real simple. Known bad caps, such as CapXons and Su'scons has to go off. All of them. Replacing caps for voltage filtering purposes are Samxon GC/GD and Panasonic FM/FC. For the big input caps a Samxon KM/Nichicon VR. Panasonic KG as one of the super small ones. For audio transfer/DC offset filtering a Elna RFS or Rubycon Black Gate C series.

What is worth deeper consideration is the entire design of these speakers. To show easily what I mean, let's just look at the audio path signal for the FL channel. It is on the top on the scheme. 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 LM4562 opamps...?
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 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.

Of course I do understand that commercial thing has to be ready for HUGE DC offset on the input, so... it has to have the input decoupling cap. But that it it. On input, and maybe one just before the amplifiers. Nothing more.

Right?

The most pathetic is the subwoofer, tough. After the ground-shorting C104 on the input it come to another ground shorting C54 one. 0.1 and 0.33nF as before, and then a decoupling C56 and then C62 (both 10uF 25V) after first opamp stage.
The subwoofer channel use 3 opamp stages! One together with the CENTER, one (IC9) entierly for himself.
But now the irony (or WTF is this?!) come. After the potentiometer it is there another decoupling cap, a C52 with is - surprisingly for the SW channel full of bass - only 1uF 50V one!!! WTF?!
And on the output from the first amplifier part of IC9 it is C64 - only 0.1uF 50V capacitor. Then it go to the second amplifier inside IC9 and on the output is C67 - a 0.47uF between IC9 and TDA7296.

I cannot help myself to think that 5 (five) decoupling caps in the SW line is way too much.

It is also worth mention that the CENTER channel utilize a 0.22uF (!!!) cap after the potentiometer and before the TDA7360 output amplifier. Again decoupled to the ground with C116 - a 0.47nF one.



What I think should be done?

Removing all ground-shorting ceramic caps (including C116 - nowhere near to be found in recommended TDA7360 schematics).

Removing and shorting all decoupling caps except the last ones just before the amplifiers:
C13, C14, C20, C21, C5 and C67.
(L, R, RL, RR, CENTER, SW respectively)

Using a quality decoupling capacitors there - suggest some good ones, please. Elna RFS? Rubycon Black Gate C series? Kimber? They should be - I think - around the 10uF in capacity for all the channels - even the C5 is 0.22uF and the C67 is 0.47uF ...

Add to each opamp on their + and -12V lines (pin 8 and 4 for IC6, 7 and 8 - only for IC9 it is only pin8 and 15,8V on it) pins filtering ceramic caps 10uF 16V (Murata X7R, 10uF 16V, 1206, 490-3911-1-ND) so the voltage will be stable and filtered in any case.

Between these there opamps (IC6, 7 and 8) solder 2x 1000uF 16V Samxon GC d8 caps to support the main voltage/power spikes in them. So these opamps will not "steal" the power between them, depending on the music played.

To stabilize the 15.8V for the IC9 opamp I would add a 470uF 16V Samxon GC d8 cap near to it's pin 8.

Powering voltage for the aplifiers need support close to these amps too. So, for the L, R and RL, RR channels amps (2x TDA7269A) in both cases powered from + and -19V lines I add to their legs (IC3 and IC4) 470uF 25V Panasonic FM caps - 2x for each TDA7269A (P12389-ND) - on pin 3 is the + voltage, while pins 1 and 6 are connected both to the - voltage.

Also CENTER channel amp TDA7360 need (as recommended application diagram show) voltage filtering cap near itself. So, a nice 470uF 25V Panny FM cap (P12389-ND). It is 16V and on pin 9. Pinc 3 and 6 are gonnected to the ground.

Amplifier for the SW channel is powered from + and -29V (using a TDA7296) and positive voltage is on pins 7 and 13, negative on pins 8 and 15. So, caps are has to be two and two pieces of Panny FM 470uF 35V (P12414-ND) should do the job.

I'm right, or wrong and why?
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