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Old 7th March 2013, 08:38 PM   #1
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Join Date: Sep 2006
Location: West USA
Default Missing decoupling caps

I have bought and repaired a Denon DVD-5000. I bought it originally for the 4- PCM1704 and 100% silmic decoupling dac section. I have the repair manual and have been looking over the schematic . Could I get some comments on the following items?

First, all digital signals pass through TC74AC157s (1ea-internal signal, 2ea-external) before entering the CS8414, then through the final one before passing on.

The CS8414 +Vd and +Va share the same supply and decoupling(1000pf and .01uf ceramics). The data sheet indicated that the Va on the CS8413 must be as quiet as possible for clock recovery, but not on the 8414? How about a low value inline resistor and local decoupling as a minimum?

The PCM1704s has NO high speed +Vdd or -Vdd decoupling(?) with a shared +Vdd/Vcc and -Vdd/Vcc feed. The schematic shows two caps locations for each, but they have been left off(open on the schematic). These caps would be feeding the chips throught vias as they are located on the underside. The data sheet says there is no advantage in separate + and - Vdd and Vcc supplies. Vcc is decoupled with 47/50v silmics, so I guess it is also decoupling Vdd.

The + and - supplies are feed from blog 7805/7905 reg, which of course could be improve upon. I don't want to experiment extensively with different PS/decoupling configuration. I have learned that taking boards in and out many times thought other cd player modding practice puts heavy wear on items. I have modded many to destruction in the past. Not opposed to cutting circuit board traces, or maybe even SMD pin to pin caps, but like to keep the modding down to get 90/95% of the dac capability. Comments!

Did Denon choose not to put in the decoupling to keep the noise off the grounds?

Analog output noise is 2mvpp..............
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Old 2nd April 2013, 04:52 PM   #2
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Join Date: Sep 2006
Location: West USA
Question added some decouple caps

After studying the layout, which is quite good compared to other manufactured players I have modded, I proceeded with the first of the "improvements". First I must say that I thought the dac in this player was fairly good. Not as good as the modded Adcom I had been using. I am using an Arcam Alpha 5(modded with clock, PS, no dac chip) as a transport.

I first bypass all the internal switching that routed the SPDIF signal so that it when directly to the CS8414. I at this time also changed to 75 ohm BNCs in both units and scoped the signal all the way to ensure a nice signal. Nice improvement overall. I did adjust the terminating resistor in the transport to 72 ohms. Also, am using a 5ft Belden 1694a cable.

With this encouraging improvement, I decided to decouple the dac digital side PS. Only wanting to make changes slowly(learned from the past not to do everything at once) I added very large ceramic decouple caps(10uf) to the -Vdd only. That is the largest power user(20ma) on these chips. WOW, I didn't know what I had been missing all these years. That digital edge had all but disappeared. The smoothness that has been talked about has to be experience! Why the factory left these off is a mystery to me. There are two position pads for each of the PS on the chip, but they were empty. I scoped the PS and there is a marked decrease in Noise. I was happy to see no increase in noise on the ground, especially on the analog system. The noise on the output didn't change.

What addition caps should be added to complete the decoupling? Any other suggestions?
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