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Superclocking a cd 3.5

I'm working on it myself Mat, you may have seen my recent post on the reclocking thread. I am fitting the Tent clock but, as I bought the basic kit [clock and inductor], I will be making a power supply as well.

Good luck, I shall watch with interest.

I take it you have already installed 825s and 8610s in the output side of your 3.5?
 
If your going to try op-amps like AD8065 remember to use small non inductive decoupling caps as close to the suppy pins as you can.
AD8610 in the first stage and OPA627 on the last stage sounds nice in some cdp's. I tried AD8610 for the I/V and AD8065 for the filter /buffer in my Alpha 5+ but it sounded too thin, just get some free samples and see which you prefer, Ad825 sounds good aswell in some players
 
one wire goes to ground, the other goes to pin 11 of the SAA7220

I take it you have removed the crystal, and the two capacitors that make up the original clock circuit.

While you are at it, try AD744's in the output buffer and filter stages.

Regards,

Colin
 
one wire goes to ground, the other goes to pin 11 of the SAA7220
Click to expand...

Ignoring for a moment the CD3.5 doesn't use the same chipset as the CD3 (therefore no SAA7220), the clock feed should NEVER, repeat NEVER go to the filter chip first.

It should always go to the DAC, directly to the DAC and not pass go or collect 500ps of jitter on the way ;)

No doubt an improvement can be gained from feeding a better clock elsewhere, but the leads from clock to DAC should be as short as possible, shorter if you can!

In the case of both the CD3 and 3.5 there will be a 74HCU04 unbuffered inverter that formed part of the Pierce oscillator, whether this can be bypassed depends upon whether it is used for further clock distribution, but as a basic guide you need to remove the crystal, and the two load caps connected each end.

See figure 1 in the data sheet link below, RF will be external in the Naim circuit.

http://www.microchip.com/download/appnote/articles/fact001.pdf

In the Naim circuit, IIRC, it's a well-implemented Pierce so it will have a series resistor between the '04 output and the crystal input (this forms the first 90 deg. phase shift for the feedback loop and limits crystal drive level) and probably a large value resistor between input and output.

Interestingly the Pierce oscillator is one of the better types for jitter, but can be improved upon readily with discrete implementation.

Your new clock has three connections, in effect, +Vin, 0V and clock out.

The 0V next to the clock output goes to one of the vacated 0V holes where the caps were removed, the clock output goes to either the input of the '04, or maybe directly to the DAC, should you be prepared to draw the circuit around that area. These wires should be as short as practical.

You then simply need an approximately +12V feed from a convenient point, if it were me I'd seriously consider adding a pre-reg, using an LM317, from one of the internal raw supplies. I use a modified super-reg to provide a v. low noise feed. PSU's are critical for low jitter.

Colin,

The big advantage of the AD744 was the ability to bypass it's very poor output stage to gain improved performance, by taking the output from the compensation pin (pin 5).

The performance of the 744 has been surpassed by a whole ranch of complementary and extra-fast complementary bipolar opamps these days, I'd never recommend a '744 as a good opamp without additional buffering and circuit modification.

Andy.
 
Hi Andy,

Sorry, my mistake, I should have read CD3.5, the CD3 however does not use a 7404 but instead uses 2 inverters built into the 7220.

I agree that the 744 sounds far better when the output is taken from the comp. pin and externally buffered, but I wouldn't describe it's own output stage as poor. Certainly not compared to the OPA627 that was mentioned earlier in the thread.

Best regards,

Colin
 
Hi Colin,

The AD744 pre-dates the introduction of the complementary bipolar processes that the semiconductor mfr's now use and as a consequence the PNP transistors were far from the equal of the NPN devices.

The o/p stage of the AD744 is a 'complementary' pair, and it's performance, compared to many modern equivalents, is pretty poor when in most real-world situations. Most op-amps from this era are pretty poor in terms of linearity, by modern standards. The specs look great, but THD is often degraded considerably by reactive and real world loads.

I don't have experience of the OPA627, so cannot comment on that, but the AD825, AD8610 and best of all, AD8065 are much better in this regard, the primarily limitation of the latter two being supply voltage restrictions.

They are all orders of magnitude better, sonically than the AD744, in my view.

I have built a good amp though, using an AD744 with an AD811 CFB buffer, but that bypassed the o/p stage and used some clever feedback topology.

Andy.
 
Hi Andy,

You wrote: The AD744 pre-dates the introduction of the complementary bipolar processes that the semiconductor mfr's now use and as a consequence the PNP transistors were far from the equal of the NPN devices.

Yes, this is true, but if you put a 10k from the output pin to the -ve rail, or better still a current source, it will force the output to bias into class A and eliminate crossover distortion at normal signal levels.

Colin
 
"should you be prepared to draw the circuit"

I have been working from my dig-photos this afternoon, my wife thinks I'm mad. I also have red eyes;)
 
it will force the output to bias into class A and eliminate crossover distortion at normal signal levels.
Click to expand...

Whilst this is partly true, it improves the crossover discontinuity a lot, it doesn't eliminate it, even over the class A range. It's one of those myths that the inherent non-linearities in the stage magically disappear, because both devices are now conducting.

The same trick also works for the newer op-amps too, and can indeed work well and improve the sound, providing there are no negative side effects.

You have to watch, for example, the capacitive load a current source presents, a cascoded isource can work well here.

Anyway, we're veering off-topic here ;)

Andy.
 
Andy

I know very little about electronics, but through reading yours and others posts on this site I have come to realise that power supplies do not come under the description Garbage In Garbage Out. I understand that variable voltage regulator (LM317 and LT1086) are better than fixed (L7805). Plus putting the inferior LM317 as a pre regulator before the superior LT1086 is beneficial.
I installed an audiocom clock (bought on ebay) in my Cd player a couple of weeks ago. I have been thinking about building or purchasing a power supply, however I came to the conclusion that it was pointless using variable regulators in the PSU as the clocks on board regulators are L7805’s. My logic (probably flawed) being, the quality of the power fed to the crystal will ultimately only be as good as the last regulator. Is my logic flawed.
 
Mat

Things don’t appear very straightforward with your clock installation. If I where you I would initially connect the clock wire with a 1K Ohm resistor or see if anybody is willing to trade the AD825’s for a couple of mogadon pills for use prior to power up.
:)
 
Fatcat

I've just replaced the 78L05 feeding the clock circuit in my alpha5+ with a basic LM317 board with its own resistors and caps, and the improvement is very noticeable. The 78L05 had about 13v going into it so there's plenty of headroom for the LM317 (hmmm just realised there's enough headroom for back to back 317's)

anyway I reckon it's worth a try - I just desoldered the 78L05 and connected the in, out and ground of the 317 board in its place

Tim
 
Tim

I am thinking of doing the same thing in my Rotel 965. I will probably be posting a question soon regarding the decoupling capacitos (I think that’s what there called) already on the board associated with the existing fixed voltage regulators.
The point I am trying to clarify is this. Is there any benefit in providing pre regulation with a LM317 or LT1086 for a L7805? The audicom clock is fitted with three L7805CV’s. Please nobody suggest I swap them for LM317’s.
I have just looked at the Audiocom Clock PSU on their wedsite. I cannot see any resistors near what looks like a voltage regulator (although that isn’t to say there isn’t one out of view).
 
the 78L05's are a fixed voltage regulator giving 5v out so don't need resistors to set the output like the 317 etc's which are adjustable regulators
 
The benefit of adding another, better regulator in front of even a realtively poor fixed one, is you give it an easier time, in many ways.

One of the figures of merit, for example, in a regulator is line rejection, i.e. how well it attenuates noise at the input, compared to the input.

The advantage of this figure is it cascades, so if your cheap reg has 50dB of LR at low frequencies, but the pre-reg has 80dB, you now have 130dB total.

In reality things are bit different but this is one reason it can be beneficial. In the clock for a CDP a significant proportion of the jitter modulation mechanism is PSU-induced. A pre-reg can help lower any noise already on the rail you pick to feed it from, lowering jitter in the final output.

It also helps to overcome earthing issues too.

At some point noise becomes limited by the final regulator performance, but it's surprising how this is so often not the limiting factor. Those reg's look great at 100Hz, but at 20kHz or more it's a whole different ball game.

So it is a sort of GIGO situation, it's just that in this case PSU's are often so very poor the GI is a bigger problem than the self-generated GO!

Also there can be problems with reg's interacting, it can get complicated, if care isn't taken. I'm also less clear about the ideals for power feeds to digital circuits, compared to my experience with analogue.

And yes, the Audiocom clock will, ultimately be limited by those on-board reg's, but you won't need to worry about that yet ;)
Andy.
 
I'm trying some Blackgate N types in my Alpha5+ clock plus Tims Idea of using adjustable regs(I was using a diy Elso Kwack clock 7 before) once the caps have had time to burn in, I'll let you know how it compares to the Kwack clock.

Andy, Any idea's when you will be getting some of your super regs back in stock, I'm really missing that magic I got with the Arcam when I borrowed John's, sorry to be a pest:D
 
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