CD740 47uf in signal path!?

wheijke · 27th October 2012, 04:54 PM

Im modding a CD740 and it sounds a lot better after work around the DAC and PS and removing muting transistors, really like this player!
Only it seems different from other Philips players beacuse of the 47uF / 25v in the signal path right after the TDA1549 dac and before the opamp.
So my question, can I remove it, or what shall i do to it?

I've included the piece of the service manual (CD751 which is the same) and marked the spots.

Wouter

Frank Berry · 27th October 2012, 05:26 PM

I wouldn't remove the capacitors. It appears that the DAC is powered by a + supply only. If that is the case, the output of the DAC is sitting at 1/2 the + voltage.
The capacitors are necessary to block the d.c. from the opamps.
Enjoy the player.

wheijke · 27th October 2012, 05:29 PM

Yes only + voltage to the dac, thanks for the explanation!

stephensank · 27th October 2012, 07:23 PM

Actually, you could eliminate the blocking caps if you "bias" the grounded input of each opamp channel to the same dc level as the dac output. You just need to add a trimpot, of I'd say 10K-100K to each channel, connecting CW pot terminal to 5V line, CCW pin to ground, and wiper pin to pin 3 of one channel, and other pot's wiper to pin 5. I would think you should also add a cap to ground from each of those pins, such as the 47u you will be removing from signal path. Once installed, trim each pot to get 0volts on each opamp channel's output pin, which then means you can eliminate the 4.7uf caps on those lines. Only potential problem is the question of whether or not the 2.5VDC offset of the opamp's input stage, versus the +/-10V supply, will cause the input stage to run out of headroom on the positive end of the waveform, but I don't think this can happen, as the peak-to-peak amplitude of a 2VRMS signal is around 6V, and you should have 7.5V positive peak headroom available.

27th October 2012, 05:26 PM	#2
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	I wouldn't remove the capacitors. It appears that the DAC is powered by a + supply only. If that is the case, the output of the DAC is sitting at 1/2 the + voltage. The capacitors are necessary to block the d.c. from the opamps. Enjoy the player. __________________ Frank

27th October 2012, 05:29 PM	#3
wheijke diyAudio Member Join Date: Oct 2005	Yes only + voltage to the dac, thanks for the explanation! __________________ naks.com and vintagecassette.com my websites dedicated to Cassette Decks!

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27th October 2012, 07:23 PM	#4
stephensank diyAudio Member Join Date: Oct 2006	Actually, you could eliminate the blocking caps if you "bias" the grounded input of each opamp channel to the same dc level as the dac output. You just need to add a trimpot, of I'd say 10K-100K to each channel, connecting CW pot terminal to 5V line, CCW pin to ground, and wiper pin to pin 3 of one channel, and other pot's wiper to pin 5. I would think you should also add a cap to ground from each of those pins, such as the 47u you will be removing from signal path. Once installed, trim each pot to get 0volts on each opamp channel's output pin, which then means you can eliminate the 4.7uf caps on those lines. Only potential problem is the question of whether or not the 2.5VDC offset of the opamp's input stage, versus the +/-10V supply, will cause the input stage to run out of headroom on the positive end of the waveform, but I don't think this can happen, as the peak-to-peak amplitude of a 2VRMS signal is around 6V, and you should have 7.5V positive peak headroom available.