14-bit DAC
 

I recently discovered four TDA1540D’s in my box of tricks when tidying my shed. This coincided with me seeing for sale an eBay a circuit board from a chap in Italy which took the i2S signal and converted it to 14-bit Left and Right Data plus Clocks.
''What the heck’’ I thought,......''I need another DAC (I really don’t but can’t seem to stop making them, I do enjoy it!) and am in need of a little project’’ so set about designing.
I am fortunate that at work I have access to PCB CAD design software so was able to draw the circuit and create a PCB layout. The O/P files are then sent to a Chinese company and two weeks later a PCB arrives at a surprisingly reasonable cost!!!!!

I presume it was a marketing decision for Philips, 'who would want to buy 14 -bit CD Players when the Discs were 16 bit and Sony’s Players were also'? More bits had to be better from a marketing viewpoint and is technically true from a resolution point of view.
The early 14- bit Players from Philips always sound good to me, I own a few CD-104's, but this all made me wonder what ‘straight’ 14 bits would sound like with no over-sampling and noise shaping, which Philips engineers came up with to make their first players be technically as good, on paper as Sony’s first CDP-101.
This then is what I came up with. Buying the circuit board from the Italian fellow and incorporating into my design I have a DAC comprising of four TDA1540D’s. They are fed from either i2S from SPDIF sources converted by a Wolfson WM8805 or from a USB to i2S Chinese PCB.
From the O/P of the Italian PCB I then generated Left and Right Data Streams plus their complementary inverted Signals which I fed to the four TDA1540D’s respectively. The L & L- and R & R- Analogue current O/Ps from the DAC I.C.’s then feed a Transformer. This acts to ‘sum’ the Signal but also acts as the I/V conversion for the DAC chips.
This signal then feeds an ECC82 Anode follower gain stage and then a cathode follower O/P stage to be able to ‘drive’ loads down to about 20kOHM without much loss of signal.
A Chinese ‘knock-off’ Goldmund Hi-Fi case was purchased and consequently filled!
I don’t really know what I was expecting but I have to say I am very pleased with the results. I am not great at describing sound in Audiophile terms. Suffice to say I am enjoying it very much indeed.

The TDA1540D’s are the ‘original’ Ceramic encased devices which seem to fetch silly money now on eBay, £120 plus each!!! I have no idea if there is any sonic difference. The Plastic Packaged TDA1540P are relatively cheap at around £14 each so as these chips I have made socketed I may buy four to see what, if any, the differences are when funds permit.

Re: 14-bit DAC
 

I like the nice clean look of the complete unit, very professional finish. Did you make the case yourself or find something suitable to reuse?

Re: 14-bit DAC
 

This looks a bit familiar: https://www.whathifi.com/reviews/gol...590-nextgen-ii

Re: 14-bit DAC
 

Chinese knock off Goldmund case. Not cheap but worth it for the professional look.

Re: 14-bit DAC
 

Exactly

Re: 14-bit DAC
 

Ceramic packages are probably better than plastic if the ambient temperature inside the case is high, otherwise there is probably no difference in performace for the plastic parts.

Re: 14-bit DAC
 

Using those DACs with an oversampler ahead of them does TWO things for you, if you do 4x oversampling as Philips sedigned them for, you not only get approximately 2 bits more resolution (the oversampler effectively controls the average of quads of samples to insert those extra 2 bits of resolution into the analogue signal) but it also pushes the sampling frequency at the DAC up by a factor of 4. This makes the anti-alias reconstruction filter that is needed after the DAC an awful lot easier. The 44.1kHz sample rate inherent in CDs forces the use of a very tight analogue filter. This creates droop, ripples in the frequency response, and large phase shifts towards the edge of the passband.

So you may not feel the need for the resolution improvement from oversampling, but the easing of the requirements on the AA filter is most worthwhile. This one factor is ample justification for even the one-bit oversampling DACs.

David

Re: 14-bit DAC
 

Before digital audio was standardised to 16 bit the BBC did a lot of investigation and concluded that 14 bit was entirely adequate for their purposes.

Re: 14-bit DAC
 

Whilst I agree with you, technically and don't really want to get into a debate about the merits of filtered over filter-less, I doubt as a Mod you'd agree to that, for me personally, the less in the signal path before and after the DAC chip makes for a much more musical and fun listen.

NOS and Filter-less for me always now. And Multi-Bit too.

You are more than welcome to come and have a listen !!! I have a CD-104 or two we can use as a reference !!

James.

Re: 14-bit DAC
 

That is very interesting indeed.
Do you have any information about the DAC's they were using?

James

Re: 14-bit DAC
 

Filterless merely means that other things that weren't necessarily intentional wind up doing the filtering. Maybe power amps, maybe ears...

I've mucked about with all sorts of analogue-digital and vice versa conversions, pushing them to limits somewhat beyond audio. There's a patent I did on using multiple noise shaping systems operating in parallel, but with the pseudo-random noise de-correlated between the rival paths. At the time I cast it in terms of frequency synthesisers, but it'll work for audio DACs and ADCs. US pat 6509800. The listing of who's cited it makes interesting reading. Fun!

And I don't let being a moderator get in the way of a good technical discussion. We've probably both made our minds up on what viewpoints we take, and they're probably different, but that's no reason not to be friendly and respect each others right to their view. Any form of bulldozing would be quite improper.

David

Re: 14-bit DAC
 

Definitely. Cables, Connections and Equipment all perform some form of filtering so by the time it gets to your ears........who knows what the signal is.


I couldn't agree more.

James.

Re: 14-bit DAC
 

To be clear, the mods have no special status here when it comes to normal forum interaction. It's normally clear from the context if we are posting in 'moderator mode' - at other times we are just ordinary forum members.

Re: 14-bit DAC
 

No but given it was so long ago (late sixties?) they were probably discrete.

Even in 1985 the company I was working at were still using discrete 4 bit DACs for RGB graphics.

The number of components required for a simple DAC is more or less proportional to the number of bits, but of course the required resistor accuracy increases as the resolution increases.

Re: 14-bit DAC
 

Nicam is a 14 bit system, though that isn't directly comparable.

Re: 14-bit DAC
 

Wow, I would love to have seen a schematic for those....''BBC 14 bit discrete DAC'' 8-)!!!

James

Re: 14-bit DAC
 

I remember the joke going around at the time it first came out....

''Would sir like to try the new (Insert Brand name here) Video recorder? It has NICAM Stereo sound''
''What does NICAM mean?''
''Near Instantaneous Companded Audio Multiplex''
''Near Instantaneous? I'll have one when it is Instantaneous thanks very much''

Always made me chortle anyway !!!

I didn't know it was 14 bits Paul.
I suppose the DAC was embedded inside the LSI NICAM chip?

James.

Re: 14-bit DAC
 

Most of the BBC Research Deparment NICAM design documents from the 70s are available online, e.g. https://www.bbc.co.uk/rd/publications/rdreport_1973_41