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SCD-1/777ES Power Supply Mods: Part I

12.247.42.46

Posted by jhietbrink on November 13, 2002 at 17:08:14

Power Supply Mods: Part I

Since I’ve finished modding the analog signal path, I’ve moved on to the analog power supplies. The power supplies are very good – but there are a few little design faux pas that keep them from realizing anything close to their potential. The power supplies are so good that the cost of the mods was just $13 – and a few hours of my time. But first, a little about the design.

The design philosophy for the “analog” power supply could be summarized as “big transformer, big rectifier and small but high quality caps”. Sony describes the two transformers as “analog” and “digital”, but “DAC” and “transport” would be more accurate descriptions. The “analog” transformer feeds everything on the Analog Board, including the digital filter and DACs. There are three sets of secondary windings on the “analog” transformer feeding three separate power supplies on the Power Supply board. A +5 volt supply feeds the VC24 chip that contains the filter and noise shaper for CDs and the APC system for SACDs. The +/- 9 volt supply feeds the oscillator, S-TACT chip (voltage-output DAC) and current pulse DACs. The +/-9V supply is further regulated on the Analog Board by no less than seven 3-pin regulators. The +/-18V supply feeds the analog portion of the Analog Board and the rectifiers for this supply feature 10A Nihon ultra-fast recovery diodes.

Regulation for +/-18v supply (to +/- 16V) is located on the Audio Board and is implemented with all discrete parts. Power is distributed to the op amps and transistors by two sets of three-part copper rails each one consisting of separate +16V, -16V and ground rails. There are two 1000mf caps just after the series regulators at the beginning of the rails bypassed by .1mf film caps – and that’s it – no decoupling caps at active devices. This is an unusual design, but the rails have low enough impedence to keep the notoriously finicky LM6172s more or less happy.

I should say a word about the capacitors in the +/-18V supply since they are a frequent target for replacement. The caps immediately after the rectifiers are 4700mf, 63V Nichicon Gold Tunes. All of the other caps in the 18V supply are 63V Elna Silmics. These are great capacitors for analog applications and they have a very high voltage rating to minimize impedance. Blackgates (Type FK or N only) are the only caps that could be considered to have an alternative sound of comparable quality or to be better. I know I’ll get hate mail for saying this, but some audiophiles prefer the sound of Elna Silmics to Backgates.

So what is there to do? Well, at least five things:

1) Get the inductor out of the analog side of the AC filter on the AC Board. This inductor with a high DCR literally chokes off the good stuff after it. See the link at the end of this post for instructions on how to do it.

2) Check the AC polarity and change it if it measures better and sounds better reversed. Check this link http://www.boundforsound.com/tweak.htm for instructions on how to check AC polarity. Many SCD-1/777ES owners have reported reversed AC polarity (including me) – but others have reported correct polarity. If you find that your AC polarity is reversed, you can change it permanently on the AC board. Just unsolder the connectors that lead to the transformer primaries, turn them 180 degrees and resolder them to the board. This keeps the fuses the right leg of the AC supply and eliminates the need for cheater plugs.

The changes I experienced from reversing the AC polarity included better dynamics, a larger soundstage and increased body. And for best results, make sure that you have the optimal AC polarity for all of the components in your system.

3) Get rid of the earth ground/chassis ground connection to the Audio Board ground. This is a bit tricky since the connection is made by a massive copper bus that also serves as the audio ground. Instructions for dealing with the bus can be found in the link at the end of this post for the output grounding mod. But don’t follow my instructions to reestablish the connection of the audio ground and earth ground!

It should be noted that using a cheater plug to break the earth ground connection doesn’t accomplish the same thing as this mod because the connection to the chassis ground remains. The chassis ground links the main board ground and the audio board ground – which to my ears isn’t a good thing. And this mod allows the chassis to be connected to the earth ground – good for both electrical safety and shielding the innards from RF nasties.

The changes here are quite dramatic with improved dynamics and body topping the list. There is also improved soundstaging – especially with SACDs and less treble harshness.

4) Establish a single ground point for the Audio Board power supplies. The grounds of the three Audio Board power supplies are not connected at a single point on the Power Supply board. The grounds for each power supply run separately to three different places along the length of the Audio Board and are interconnected by the Audio Board ground plane. Really – I’m not making this up. This has got to make for a noisy ground plane as different ground potentials of the three power supplies seek equilibrium.

Fixing this is relatively easy. Just clip the ground wires for the 5V and 9V supplies at the Audio Board, twist them together and crimp on a ring connector. Remove the screw holding the ground for the 18V supply in place on the Power Supply board, slip the new connector under the existing one and tighten. Just make really sure you cut the right wires. On my SCD-777ES, the 5V ground is the white wire on connector CN301 and the 9V ground is the center black wire on connector CN104. However, there could be production run variations and your unit may be different!

This is probably the most significant of the five changes. There is a profound increase in transparency and the highs become smoother and more extended. Tape hiss on old analog recordings is more apparent. And at the other end of the frequency spectrum, the bass is firmer and more energetic. TOC reads are consistent and sure. The changes are far more significant than would be expected from cleaning up an analog ground – so the oscillator and digital circuits must be benefiting too.

5) Bypass the 1000mf caps at the outputs of the custom series regulator with something more substantial than the stock .1mf film caps. The .1mf film caps might be useful for cleaning up RF noise, but aren’t going to do squat for lowering the inductance of the power supply at higher frequencies or for lowering overall inductance. I replaced the .1mf film caps with 47mf, 63V Elna Silmics – ordered as replacement parts from Sony.

This mod requires break-in – although it never sounds bad. After awhile, a better separation of voices and individual instruments emerges. The sound is fuller – especially on dynamic peaks like horn blats. And the music just breathes more like the real thing – becoming more engaging.

In many ways, the results of the five mods are additive – resulting in a bigger, fuller presentation that’s more dynamic, more extended and more articulate. Although much progress has been made, I have a feeling I’ll be doing more with the digital supplies on the Analog Board that I will cover in Part II.

P.S. Those of you who have Blackgated your power supplies can benefit from all five mods – but use 47mf Blackgate type Ns in place of the Elnas.

Other SCD-1/777ES Mods and Information

AC Board:

Removed choke on analog power supply
http://www.audioasylum.com/forums/hirez/messages/32357.html

Analog Signal Path

Replaced RCA jacks with Vampire OFC jacks
Replaced output wire with Kimber AGSS
Revised grounding creating a star ground for audio board
http://www.audioasylum.com/forums/hirez/messages/41691.html

Replaced AD712s in the output buffer stage with hi-spec OPA2132s
http://www.audioasylum.com/forums/hirez/messages/78259.html

Replaced resistors and capacitors in I/V conversion stage
http://www.audioasylum.com/forums/hirez/messages/93363.html

Set filters to “custom”

Power Supplies

This post is Part I

General Description

Analog Signal Path
http://www.audioasylum.com/forums/hirez/messages/33130.html

Accessories

Electraglide Reference Glide power cord
Chang Lightspeed HT1000 power conditioner (w/upgraded capacitors)
AudioDesk Systeme CD Lathe
Aurio Pro Media Isolation Bearings w/optional tungsten carbide balls

This post is made possible by the generous support of people like you and our sponsors:

Schiit Audio

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