CD304MKii output stage question

wheijke · 16th November 2012, 10:16 AM

I'm modding a Philips CD304 MKII and am looking at the output stage.
Having seen many TDA1541 based player schema's I haven't seen this before (see picture). There is even a special regulator on the decoder board for this. To me being in the signal path it looks like a bad thing.

Can someone explain/advise?

Wouter

Mooly · 16th November 2012, 10:35 AM

Perhaps it is an attempt to reduce offset errors in the NE5532 and following circuitry by injecting a current into the virtual ground point.

How does that resistor and cap (the 2K7 and the 10uf cap) affect the "noise gain" of the opamp by it "throwing away" some open loop gain ? Sonically that can be beneficial in the right circumstances but I'm not sure how it works out in this configuration as an I'V convertor.

wheijke · 16th November 2012, 10:57 AM

If I compare the CD650 schema it looks nearly the same with this part missing.. so maybe it was fixing something that was not a problem..

Mooly · 16th November 2012, 11:11 AM

You would have to do some tests. Why not remove the 2K7 and see if the offset changes.

aparatusonitus · 16th November 2012, 11:58 AM

Quote:

Originally Posted by wheijke

Can someone explain/advise?

TDA1541 has -2mA current offset on L/R outputs...Those resistors and cap are for nulling that current offset (8VDC/4k9=1,63mA) so output could be DC coupled (with dc servo)

16th November 2012, 10:35 AM	#2
Mooly diyAudio Moderator Join Date: Sep 2007	Perhaps it is an attempt to reduce offset errors in the NE5532 and following circuitry by injecting a current into the virtual ground point. How does that resistor and cap (the 2K7 and the 10uf cap) affect the "noise gain" of the opamp by it "throwing away" some open loop gain ? Sonically that can be beneficial in the right circumstances but I'm not sure how it works out in this configuration as an I'V convertor. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

16th November 2012, 10:57 AM	#3
wheijke diyAudio Member Join Date: Oct 2005	If I compare the CD650 schema it looks nearly the same with this part missing.. so maybe it was fixing something that was not a problem.. __________________ naks.com and vintagecassette.com my websites dedicated to Cassette Decks!

16th November 2012, 11:11 AM	#4
Mooly diyAudio Moderator Join Date: Sep 2007	You would have to do some tests. Why not remove the 2K7 and see if the offset changes. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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