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DAC info on tda1541a ? (Rotel RCD955AX )

restorer-john

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#2
There is plenty to read on that website including the Philips datasheet for the TDA-1541 D/A converter. and a decent explanation of the architecture. Take your time and read it. Basically it contains 10 bit passive and a 6 bit active networks for each channel. The LSB of the MSB top 6 bits feeds the 10 bit passive.

"This principle combines passive division with a dynamic system to improve accuracy in a standard process. The principle requires only some noncritical external filter- ing capacitors and is well suited to fit the demands for the D/A converter design. Moreover, the bit current switches can easily be optimized for fast settling with a low-glitch energy in such a way that both converters can be used without external sample-and-hold or deglitcher circuitry. In the new dual 16-bit D/A converter this dynamic element matching principle is used to obtain the required accuracy for the six most significant bits. In order to minimize the number of pins needed for the external capacitors, a rather unusual approach is used to construct the remaining ten binary weighted bit currents. The principle uses emitter scaling of transistors and is based on the basic statistical rule that the relative accuracy improves with the square root of the numbers involved, assuming an uncorrelated distribution between the transistor matching. "



II. GENERAL CONVERTER DIAGRAM
Fig. 1 shows the block diagram with the most important parts of the dual 16-bit D/A converter and shows clearly the dual-channel approach. The reference current source that is common for the two identical converters is fed to three 2-bit dynamic divider stages which perform the required accuracy for the six most significant bits. One output current of the last dynamic divider is fed to the 10-bit passive divider. The 16-bit currents are switched to the output line of the D/A converter by the bit switches which are controlled by the data latches. To minimize timing errors the converter contains on-chip data latches. In order to obtain a low capacitive feedthrough the data input is in a serial mode which requires only four input pins. The digital inputs are TTL compatible and the circuit accepts two different data input formats. An internal emitter-coupled oscillator supplies the dynamic divider stages with the necessary control signals. In the internal digital part a low-voltage swing unsaturated current-mode logic (CML) is used for speed and low-interference noise.
 
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garbulky

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Nope, not a Philips Crown 1541 ladder DAC.

https://www.stereophile.com/content/rotel-rcd-955ax-and-rcd-965bx-cd-players

Early Philips SAA-7323 Bitstream DAC.
I'm confused. From that link
"
And the 955AX? It wasn't broke, so Rotel has wisely chosen not to fix it; aside from the improved cosmetics, it's still the same lovable 855 on the inside, right down to the Ol' Faithful SAA7220/TDA1541A 16-bit Philips chipset.
"
So it mentions TDA 1541A but also mentions a SAA7220

@restorer-john
I did read some of that. But truth be told I have no idea what any of that means. Is this a multibit R2R DAC or a delta sigma DAC? Or something else entirely?
 

restorer-john

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Nope, not a Philips Crown 1541 ladder DAC.

https://www.stereophile.com/content/rotel-rcd-955ax-and-rcd-965bx-cd-players

Early Philips SAA-7323 Bitstream DAC.
It is a TDA-1541 in the RCD-955AX actually.

Here's the service manual extracts:

rcd-955ax.JPG



rcd-955ax dac.JPG


rcd-955ax sch.JPG


Certainly not a S1 or S2., but a TDA-1541.

Can't trust those stereophile guys with their confusing writeups... :)

"And the 955AX? It wasn't broke, so Rotel has wisely chosen not to fix it; aside from the improved cosmetics, it's still the same lovable 855 on the inside, right down to the Ol' Faithful SAA7220/TDA1541A 16-bit Philips chipset. "
 

restorer-john

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#6
It's a great classic D/A converter and nicely implemented in that machine. You will enjoy it. :)
 

restorer-john

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Is this a multibit R2R DAC
Yes. And no. It is multibit and uses ladder for the bottom 10 bits and active current sources for the top 6 bits. Not remotely delta sigma or one bit.

Just enjoy it.
 

Blumlein 88

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I'm confused. From that link
"
And the 955AX? It wasn't broke, so Rotel has wisely chosen not to fix it; aside from the improved cosmetics, it's still the same lovable 855 on the inside, right down to the Ol' Faithful SAA7220/TDA1541A 16-bit Philips chipset.
"
So it mentions TDA 1541A but also mentions a SAA7220

@restorer-john
I did read some of that. But truth be told I have no idea what any of that means. Is this a multibit R2R DAC or a delta sigma DAC? Or something else entirely?
Yes I guess I had a moment of dyslexia. I read the same thing and got the models crossed. So you do have the 1541 multi-bit DAC chip.

http://www.dutchaudioclassics.nl/Philips-TDA1541.asp#tda1541-grading

This page explains the Dynamic Element Matching of the top 6 bits.
 
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garbulky

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Thread Starter #9
Yes. And no. It is multibit and uses ladder for the bottom 10 bits and active current sources for the top 6 bits. Not remotely delta sigma or one bit.

Just enjoy it.
I appreciate all the info! Wow. So this uses two different techs. I haven't heard of this before. How interesting. So what does active current source for the top 6 bits mean?
EDit: okay I'm kind of gettting it by reading Bilumleins link. It;'s a way of improving linearity.
How does this compare to say the PCM 61/63 dacs? Do people consider it around the same or better? I have a Denon CD player that uses PCM 61 and quite like it.
 

restorer-john

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How does this compare to say the PCM 61/63 dacs? Do people consider it around the same or better?
It is a very highly regarded D/A in the original S1 and the later S1/S2 guises. The base grade D/A TDA-1541 (A) is a classic D/A, but outclassed by the PCM61/63.

Basically, Philips selected (hence the S moniker) the D/As that exhibited the best low level THD and hence the best S/N+D and gave them initially the S1 badging. In later years, (After 1990) they split the Select (S1) graded chips into an S1 and S2 grading replete with one or two little crowns on the DIP package.

Consider Marantz/Philips used the TDA-1541 S1 (specially selected from the S1 stocks) in their statement model CD12/DA12LE transport and standalone D/A converter which in this country (if you could get one- only 500 were made) was AU$6000. That should tell you what level of esteem it was held in at the time (1991).

Things have moved on, but RBCD really can't really be served much better. Also consider however that the D/A itself is only a small part and many would argue with good reason that the implementation of the D/A with all the associated circuitry, power supplies and analogue stages contribute far more to the measured performance than the simple choice of the D/A chip itself.

I know it is fashionable to focus on the choice of D/A converter silicon, but in my experience, it often means absolutely nothing to the final performance of the completed commercial device.
 

restorer-john

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#11
So what does active current source for the top 6 bits mean?
It simply means a more accurate temperature controlled active (transistor constant) current source/s for the most significant bits. Basically, with large currents (MSBs), even really tiny variations would swamp the effects of the least significant bits, rendering them useless and compromising low level resolution and THD.
 

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