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Old 11th March 2004, 11:37 AM   #1
Alien8 is offline Alien8  Belgium
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Default Analog Output Stage

Hi all

I've got some rather theoretical questions....

Below you can see a pic with the schematic of the analog output of my CD950.

I've been reading in different treads a lot and already noticed that removing the mute transistors has a profound impact on sound quality.

I also saw that many removed those resistors (R7 & 8 here) and the last cap (C6).
I just wonder : what is the function of this cap ?
Just a way to change the output impedance ?
What is the consequence of removing it ?

Next problem : the coupling cap (C9, in this case a WIMA? 100 µF 16V)
I've noticed when the player is working, attached to the amp there is about 70 mV DC on them...
A lot of people tend to throw these out as well, again, are there any consequences ?

Last one : what would be the function of R5 ?

I know I'm ranting a lot, but I'd like to understand what's happening, also if I remove them....



Hopefully some good soul would like to explain as I'm not an EE...
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Old 11th March 2004, 11:48 AM   #2
Alien8 is offline Alien8  Belgium
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Hmm seems the image did not attach....
let's try again :
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Old 11th March 2004, 01:14 PM   #3
Alien8 is offline Alien8  Belgium
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Default pic doesn't load

Since I cannot seem to post a pic , here's another way around :

the http://www.ultranalog.com/data/frits/saa7350.pdf
manual contains a similar output stage (see last page)

the parts in question would then translate to C22 (220pF in my case) on the ouput
function ??

coupling cap C15 with the DC voltage across, can I remove it ?

what is the function of resistor R12 (10k to ground) ?
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Old 11th March 2004, 01:54 PM   #4
hjelm is offline hjelm  Sweden
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They increase the stability of the outputstage and somewhat eats hf interferers from the outside.

There is no harm in keeping them provided at least the cap is of decent quality.
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Old 11th March 2004, 03:15 PM   #5
Werner is offline Werner  Europe
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R12 is there to charge the outer side of C15 to ground potential, this to avoid loud bangs when you connect the CD950 to an amplifier. However, I would increase R12's value to 47k or more, because it presents an unnecessary load to the last opamp: signal currents should not run when/where they are not needed.

C22 is indeed an RF filter. It might be better to make a true RCR filter, though, with an additional 100R resistor between top of C22 and output socket.

If you have real DC at the inner side of C15, then please leave it in., but make C15 of the best possible quality. People do the strangest things, including cutting out coupling caps where they are (or might be) needed. Very fine, if you don't care about the occasionally fried loudspeaker or molten down preamp input stage.

If these 100pF caps are cheap ceramics, then you may want to replace them with same-value SMD NPO ceramics, if these fit the board at all.
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Old 12th March 2004, 08:48 AM   #6
Alien8 is offline Alien8  Belgium
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thank you for your replies...

It seems the original pic decided to finally show up.

Concerning that RCR filter, is it really necessary ? I've just removed the muting transistors and C9 (220 pF) so now there's only a 100R left between the coupling cap and the output. C9 was a simple smd ceramic, manual says 220 pF 10% tol. 500V
The player really sounds much more open now...

btw : I've just received my bunch of opamps, time to do some testing...
how long do these usually need to run in ?
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Old 12th March 2004, 09:02 AM   #7
Werner is offline Werner  Europe
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Quote:
Originally posted by Alien8

how long do these usually need to run in ?
Less than 60 minutes if you do not believe all that nonsense and if you accept that fact that one's perception of a repeatedly played-back piece of music never is the same anyway.
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