E. UnruhThe Second X-Dac

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Taking theX-DAC-3.0One Step Further

by

Erland [email protected]

09/16/97


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Taking the X-DAC-3.0 One Step Further by Erland Unruh

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Taking the X-DAC-3.0 one step further

Since I bought the very first X-DAC kit I have been working on the design, trying many modifications. Some of

these mods involved pretty advanced active devices such as the Walt Jung wide band super regulators and op-amp

combos (AD797+AD811) in the output stage. As documented in the Audio Electronics issue 4/97 Showcase article

Taking X-DAC into the Stratosphere. The latest set of mods I have tried were of such a nature that they called fora fresh start with a new X-DAC pcb. The theme this time was a "low-tech" implementation inspired by Pete

Goudreau's postings to the internet news group RAHE (Rec.Audio.High-End) about his modifications of a Parts

Connection Assemblage DAC-1. Low-tech here means keeping things very simple and avoiding active solutions

in favor of passive ones. While trying and adopting Pete's mods I also carried out a number of other mods. The

focus of most of these mods are to get the overall noise level down on the pcb and keeping RFI and EMI at a

minimum. This in many cases means using SMT*

parts. In short, almost all small parts like resistors, caps and

inductors are SMT. This paper tries to guide the reader through the different mods as well as describe some of

Pete's mods. Anyone trying to duplicate these mods should expect a couple of hundred working hours for putting

the "kit" together as well as a couple of hundred hours getting all the parts and thinking it through.

PCB layout mods

One of the earliest ideas I wanted try on this DAC, that I really couldn't try on the previous one, was altering the

ground plane split on the pcb. Recent work done by DAC-chip makers shows that the chip shouldn't be located

across a ground plane split, rather the delicate chip including e.g. the CS4328's digital side should all be on the

analog ground plane. Some of the reasons for this are to not let the die in the chip end up over a split (performs

better over copper ground) and that power feed to e.g. the CS4328 is complex enough without having two grounds

around the chip. Not forgetting that if we can manage to get data and clock signals to the chip with a precision of

single ps jitter or so, it would be quite useless if the receiving circuitry was power fed and set up in such a way that

the surrounding noise level was a threat in preserving the quality also on the digital side. Thus I altered the pcb in

such a way that the split instead of turning up and going underneath the CS4328, it continued a bit further towards

the ECL clock receiver and went through it up to the edge of the pcb (for those who know the X-DAC pcb layout).

This of course ruins the ECL clock area, but not to worry, it's solved later on. The old split was filled with copper

tape. At this stage I also removed the ground plane island intended for the PLL part of the CS8412. It was instead

made an integral part of the digital ground plane.

Ground/chassis bypassing

Another mod that also means lots of hacking on the pcb is bypassing signal grounds to chassis. The basic idea here

is very simple, signal grounds have to be bypassed to chassis at every possible point so that no RF voltages can exist

between points across the board or between board and chassis. I used 22nF/100V 1210 X7R caps, but lower voltage

caps could be used as well. Five or so in parallel at most of the pcb standoffs. I even added an extra standoff close to

the original center standoff, but on the analog plane. This was done so that the analog plane would not be bypassed

at only one end. Each signal ground should also at some point connect to chassis via a 10R carbon composite

resistor. (The use of carbon comps is primarily for their very low inductance and high pulse power handling.)

Among other things this resistor makes sure that the voltage across the caps never gets too high.

* SMT is short for Surface Mount Technologies. I have been thinking of trying this in projects before, but never got around to it. Partly because it at

first glance seems hard to handle for a hobbyist. But when a project is aiming for noise levels at single mVpp and really low inductance bypassing etc.,

there really are no alternatives. Its a good investment to get proper tools though. A really tiny soldering iron tip (or a special SMT iron), 30swg

(0.3mm) solder and tooth picks of wood and a good sharp pair of tweezers. The toothpick is used to hold the part when soldering. About everything else

than wood are bound to attract the part and accidentally move it out of position. Wood is also soft and prevents harming the part. Tweezers are good

for handling parts before soldering. SMT parts like caps and resistors have a physical size. Common sizes are 0603, 0805, 1206 etc. The two first digits

is the length, the two last the width. The unit is 1/100 of an inch. A size of 0612 means that the parts is wider than long, thus a very low inductance part

since the soldering areas are located at each end of the two first digits measurement.


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Fig 1 showing the pcb alterations on the component side of the pcb. The DAC chip is located a

bit up in center. Chassis bypassing is seen on some of the standoffs.

Terminating HF traces

Even more pcb hacking, all traces carrying HF signals are series terminated at the source end with a 50R 1206resistor. This will reduce rail noise considerable. This is simply done by cutting the trace and bridging the gap with

the resistor.

New canned oscillator

While into pcb hacking I also laid out a new oscillator on the expansion area, completely replacing the ECL clock.

The corner stone here is an SMT oscillator from Valpey Fisher. V-F makes extremely low jitter oscillators for all

kinds of equipment, not normally for digital audio though. Jitter is specified at less than 10ps rms, but according to

V-F the value is as low as 1ps rms. When using the AD1891 the frequency of the oscillator isn't very important, so I

used a 12.35MHz oscillator kindly supplied by V-F. The problem when using such a high quality oscillator like this

is to make sure that the low jitter signal is distributed without significant quality losses. Not a simple task at all. I

used a 74F244 as a buffer to avoid loading the oscillator unnecessarily and to provide different buffers for CS4328and clock divider feeds. The F-series adds little jitter if bypassed properly. For optimal jitter performance the

oscillator needs to be isolated from vibrations. For that reason four standoffs were added close around the oscillator

to stabilize the pcb. In the center of these standoffs, just under the oscillator, a short standoff was fastened to the

chassis, not penetrating the pcb. The top part of the standoff was covered with Sorbothane, mounted in a way for

the Sorbothane to be compressed 10%. These standoffs together makes for a very well vibration isolated oscillator.

It's very important to keep vibrations to a minimum because just the kind of vibrations caused by putting the

equipment in a listening room couples into the oscillator causing sound correlated jitter.


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Fig 2 showing the oscillator area with standoffs.

Ground splitting

Next pcb change was splitting the digital and analog ground planes where they join at the DIN power connector.

The external ELPAC power supply ground was split so that there is no common ground point. Also made use of the

spare wire in the outgoing power cable using two separate wires for respective ground. Earlier mods to this

ELPAC included adding a mains filter, upgrading all caps and rectifier diodes and removing regulation, replacing

each regulator with a 1R resistor. Thus the ELPAC now puts out unregulated raw feeds for digital +5V and analog

+12V and -12V. While doing this mod I also removed all signal traces going between digital and analog sides on

the X-DAC pcb, all but the reset trace. It's actually not many traces, it's data and clock signals to the CS4328.

These traces were replaced by miniature 50 ohm coaxes, shields connected at each end at respective chips ground

pin. Thus the only connection between digital and analog ground is the coax shields. This is good because it's thelowest impedance return for respective signal. The deemphasis trace is skipped altogether including the complete

deemphasis network. This is partly because it's difficult to run the signal from digital to analog side feeding relays

but the major reason is that so few CDs are using emphasis and that all CDs not using it must suffer from the

network. Removal of the network means removing the series resistor between the CS4328 and the output buffers,

this is the main benefit. I kept the emphasis indication on the front panel though, to indicate "erroneous behavior".


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Fig 3 showing the modified ELPAC.

Fig 4 showing the rework of the solder side layout including the miniature coaxes bridging the

ground split.

Bypass networks

The last thing calling for larger pcb alterations is new bypass networks for all chip power feeds. Also room for a

+5V digital regulator that used to be in the ELPAC. I located the new regulator below the DSP connector where

there is lots of free space. Two kinds of bypass networks are used. Simple 10uF tantalums for less sensitive feeds

and Pete's special triplets (a bypassing philosophy discussed below) on the rest. The original triplets consists of one

10nF 0612 AVX cap, one 270nF 0612 AVX cap and one 120uF Panasonic HFQ cap. Since the 0612 caps are

extremely hard to get, I used a 10nF 0805 and replaced the 270nF with four parallel 68nF 0805s. These four caps

have very much the same properties as the original 0612's. They are much harder to fit though. For optimal results

these five 0805 caps must be located as close to the feed pin as possible. For starters all chips must be soldered in

directly without sockets to reduce distances. Then I first glued (fast super glue, avoid the possible fumes when

heating first time) then soldered the fifthlets together. Finally soldering the package directly on to the feed pin, the

ground side directly soldered onto the ground plane. I stacked the fifthlets 2+2+1 on top of each other. To be able to

fit such a package there is need to trim the areas around each feed pin in such a way that no shorts will occur. The

120uF caps are mounted on either pcb side whatever is most convenient.


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Fig 5 shows a fifthlet on the pcb.

The special thing with these triplets is impedance properties. Each cap is chosen in such a way that there will be no

tank effects causing oscillations. (A tank is a resonant circuit of capacitance and inductance.) This is true for a very

high band width. Don't get tempted to replace any of these caps with something having different properties.

Although these are ceramic caps, they are used on both analog and digital feeds. There is no such thing as specialuse of film caps or the like on the analog side. (What seems to be causing audible effects with different bypass caps

are tank effects, not primarily material choice. Avoid these effects and it doesnt affect the sound.) This bypass

network is also used on the CS4328 filter pin. Since this is only the bypass network, there is a need for matching

regulators.

3-pin regulators

The main property of the regulator is that they shouldn't be wide band regulators. Simplistic 3-pin 7805's etc. are

used together with pre and post caps. The regulators band width is about1kHz. (Check Petes posts for more details.

In short the pre cap delivers a purely resistive feed to the regulator while the post cap delivers a purely resistive feed

to the bypass caps.) This upper regulation frequency thus holds for both load changes and raw feed changes. As

complete units this kind of regulation is very passive, active regulation only takes place below 1kHz. For all this towork it's important to neither get tempted to use other regulators than simple 3-pins. Others would only change the

properties of the regulation for the worse. It's important to maintain high dropout for the regulators for good

operation. 5V is a good value, can cause heat sink problems though. (The regulators need that much overhead for

biasing of the NPN pass element otherwise their closed loop performance suffers and noise goes up and regulation

speed goes down.) Pre regulator caps are Panasonic FA, 5600uF/25V for 5V regulators and 3300uF/35V for 12V

regulators. Post regulator caps are Panasonic HFQ, 12000uF/6.3V for 5V regulators and 6800uF/16V for 12V and

15V regulators. These regulator caps are very physically big, and it takes very careful planning to fit them all on the

X-DAC pcb. With big caps like this around regulators, and relatively high voltages, its important to have a

protection diode like e.g. 1N4002 across the regulators Vin and Vout. Further protection is recommended by using

e.g. a 1N5818 across the post cap to prevent rail inversion.

Only seven regulators used

While on regulators I can mention I only used a total of seven, that's less than the original X-DAC design. One for

the whole digital side including the CS8412 PLL. Two for the CS4328 and four for the output stage. I derived the

CS8412 PLL feed from its digital feed via a 1uH inductor. In the same way the CS4328 digital feed was derived

from its analog feed via a 1uH inductor. I used 1uH inductors in many other places as well. It's suitable both for

blocking outgoing noise and for blocking ingoing noise. In the whole picture the AD1891 is the worst noise maker.

It really pollutes the rail and inductors are needed. It's also very important to keep the oscillator component feeds

noise free.

S/PDIF input and noise problems

To keep complexity down I also decided to make do with a single BNC input for digital S/PDIF input. It's very

complex to do a good multiple input implementation so I took the easy way out here. With a single input it's much

easier to focus attention to the important RFI problems and trying to reduce coupled noise from the transport. It's

"small" things like this that make different transports sound different even though the DAC "architecture" should

prevent just that. E.g. the case of using the AD1891 for removing source jitter, it wont do anything about RFI and

other noise problems. Thus different transports will still sound different unless the AD1891 is accompanied by a

good fire wall against noise coming in with the S/PDIF signal.

The BNC connector used for the digital input is a special kind made by Trompeter. It's built up in a way so that the

signal ground is insulated from chassis, but at the same time there is a built in cap that couples signal ground to


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chassis ground. An implementation like this shunts noise on the coax shield directly to chassis without ever

entering the enclosure. On the inside there is a short coax pigtail that connects directly to a Scientific Conversion

SC944-05 2:1 transformer. The coax pigtail has three TDK beads on it, optimized to suppress noise around and

above 11MHz. On the secondary side of the transformer there are two 150R 1206 resistors. One resistor from each

output pin is run to a common signal ground point. (A 2:1 transformer has a 4:1 impedance relation, thus the need

for a 300R termination instead of a 75R one.) Each of these resistors are also paralleled with a 22pF cap, stacked on

top of the resistor, to further shunt noise to ground. That common point is directly coupled to chassis ground with22nF SMT caps. All this forms a balanced digital signal that is run directly to the CS8412's balanced digital input

via series 10nF 1206 caps and then 150R Z-skew cable to maintain proper impedance matching.

Fig 6 showing the back of the DAC.

CS8412 and jitter

Continuing with the CS8412 it's important to get at least a G revision of the chip. From that revision the die is

made in a different manufacturing process making it smaller and also providing it with better jitter specs. Although

jitter on the recovered clock might seem irrelevant because of using the AD1891, it's important to keep all sources

of noise coupling into the circuitry at a minimum. This is also related to another of Pete's mods, placing a 1nF cap

between the PLL filter pin and ground, improving jitter performance even more. All this fits quite nicely if all threefilter parts are made SMT.

The power connector

Moving along the back of the panel it's time for the power input connector. Here the original DIN connector was

replaced by a 9 pin D-sub connector. This was done for several reasons. One is that this kind of connector is

available in versions with built in caps just as with the Trompeter BNC connector. Thus all nine pins has built in

caps shunting noise directly to chassis. Another reason for the change is that the connector is available in shielded

versions and with screws for improved fit and safety.

The analog output stage

Finally at the back comes the two analog output RCA connectors. I used chassis mounted RCA's. Unfortunately no

RCA connectors are available with built in caps for chassis coupling of ground. So a pair of good connectors were

equipped with coupling caps directly inside chassis. Suggested cap for this is a Roderstein Y ceramic 4.7nF/400V.

Connectors are then connected to the pcb via GS-4 coax, with one TDK bead on each coax. To make it easier to

take the pcb in and out of the enclosure, I mounted a copper plate at the end of the pcb and mounted all connectors

to that. This plate the connects to the back of the enclosure which in turn has big enough holes to let the connectors

run through it.


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Fig 7 showing the copper plate with mounted connectors.

The GS-4 coax is connected to the output BUF04 op-amps via a series 40R Caddock resistor and a 100pF film

bypass cap to ground. Using BUF04s are another of Pete's ideas. It's very simple to use with built in feedback

resistor. But one of the main reasons for using it is that the chip doesn't include circuitry for current limiting the

output. This makes the BUF04 very fast and dynamic. For it to perform well it takes good bypassing. As described

earlier, use the triplets here as well. Some may dislike the use of ceramics here, but it works real well and is the best

choice here.

Front panel touch up

A last little touch up was rearranging the logic behind the push buttons on the front panel to be both buttons in out

position for normal operation (inverting the function of one button). Makes it a bit easier to determine how theDAC is setup for operation.

Noise level results

All the mods above led to much lower rail noise than in my previous super regulator X-DAC. In fact the noise level

is about as low as it's possible go with a two layer pcb. Below is a table showing some of the interesting

measurement points. All measurements are performed with a special probe with a very short ground connection and

all values are in mV and peak-to-peak. During all measurements the DAC is in normal operation making music

and loaded with a normal pre-amp. Some of the chips makes lots less noise when in idle or playing digital zeros.

Since this is close to the resolution limit of my scope, absolute values can be a bit off, but the interesting result here

is really the comparison between the two DACs. Below SR X-DAC refers to the Jungian super regulator equipped

X-DAC as published in the Audio Electronics article.


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This X-DAC SR X-DAC

Osc feed 1,5 10

Buffer feed 2 20

744040 feed 8 20

7486 feed 4 25

AD1891:7 feed 80 200

AD1891:22 feed 80 200

CS8412:7 feed 2 na

CS8412:22 feed pll 1 na

CS4328:3 feed +5V analog 2 15

CS4328:5 feed -5V analog 2 30

CS4328:28 VREF 0 4

CS4328:16 feed +5V digital 14 120

Output stage feed 0 8

So how does it all sound?

While I write this the DAC is away for test listening in Stockholm. So hopefully there will soon be an update here.

In the mean time my own impression is that this DAC is much better than e.g. the Jungian super regulator DAC

that is my own best reference. It has an extreme pin-pointing of voices and instruments and an enormous level of

detail. I fancy female jazz vocalists very much, with this DAC they sound great. The same goes for saxophones and

pianos.

Another result of the improved signal/noise ratio is that I now appear to have even less CDs that are well recorded

and well mastered. CDs that I used to believe was well recorded now show up with never before heard tape hiss.

Still, good CDs like e.g. Cheskys later Sara K recordings performs very good and leaves little left to wish for.

E. UnruhThe Second X-Dac

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