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Old 6th September 2013, 08:58 AM   #11
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Quote:
Originally Posted by DF96 View Post
There is bound to be a bit of variation. With a supply voltage at the upper end of the valid range you should have enough leeway to use a fixed resistor. At lower supply voltages it might need adjustment for each chip.

Have you seen my web page on passive I/V for the TDA1543?
Yes, and it was a great inspiration.

The diagrams are clear. And where you enter the area to the right of the crosspoint at a specific voltage, then there is indeed some 'leeway', or latitude, for the component values. This allows the selection of a point with a nice amount of 2nd harmonic, such as 0,5%.
Too little is a bit bland.

Just a build on your page, - You might also mention that the voltage on the "Vref pin" is about 2,15 Volt independent of the Iref. [i.e. 2,15 V at 7 Volt; haven't measured now at other Vdd's]. A diagram would taker away the myth of a voltage sensing pin immediately! In the Philips implementations the Vref is the reference for the to the output follower, the I/V converter. This is the base voltage to force the AOL, AOR to with the opamp, if I say it correct.
In the past I monitored the voltage on the Vref pin and indeed couln't get a clue what I was doing but still at the end after a lot of experiments it sounded good. Sometimes the latitude was so small that it could just runaway and the DAC would stop working
I had the advantage of having some Philips CDP's with the TDA1543. It was great to see the improvement when adding a super low jitter Tent-clock. The effect was here more pronounced than in other players.

The TDA1543 is great, but a bit wild at times. I remember a tweaked machine where we just came from a concert, bought the CD and could recreate the whole live stage and dynamics at fortissimo with this chip. Other chips in comparison are much more subdued...
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Old 6th September 2013, 11:37 AM   #12
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Quote:
This allows the selection of a point with a nice amount of 2nd harmonic, such as 0,5%.
Too little is a bit bland.
I prefer bland. Note that you need to be careful that your 0.5% 2nd does not come mainly from asymmetric clipping as this will give you lots of higher order distortion too - not nice!

Quote:
You might also mention that the voltage on the "Vref pin" is about 2,15 Volt independent of the Iref.
It never occured to me that people might think otherwise. The data sheet makes it clear that this is a voltage output from the chip.
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Old 6th September 2013, 01:43 PM   #13
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Dave,
what post-I/V filter do you suggest?
Some prefer no filter at all; that is a bit demanding I think.
Target: 88.2 or 176.4 kHz stream.
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Old 6th September 2013, 05:15 PM   #14
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I'm still thinking about that. Some form of diplexer, perhaps?

For high-rate streams the filter can be quite simple.
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Old 7th September 2013, 04:02 PM   #15
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ECDesigns bias tweak can be found here.

Building the ultimate NOS DAC using TDA1541A

Pin 7 is left floating and +3v from a couple of 1.5v batteries is fed to the other two resistors (Pins 6 & 8).

Best 50p I ever spent!
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Old 7th September 2013, 05:11 PM   #16
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As the chip is designed to have a current drawn from Vref I can't see what problem there is in actually doing this. If a resistor to ground will inject too much noise, as alleged, then ground is too noisy anyway so we are stuffed. Sending the output to an elevated supply rather than ground is a good idea, and brings even passive I/V a little closer to the datasheet circuit.
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Old 8th September 2013, 09:12 AM   #17
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Quote:
Originally Posted by Flied Egg View Post
ECDesigns bias tweak can be found here.

Building the ultimate NOS DAC using TDA1541A

Pin 7 is left floating and +3v from a couple of 1.5v batteries is fed to the other two resistors (Pins 6 & 8).

Best 50p I ever spent!
Quote:
ECdesigns: Same applies for Vref (pin7) if this pin is connected to anything, sound quality degrades dramatically. This is caused by ground or power supply noise polluting both bias current source and DAC output current, then it's over. So never ever connect this pin!
It of course highly surprises me that the pin could be left floating at all and the bias is still correct.
Then by inference the Vb must be 5 volts I guess. Only variable is then Rout; and reducing the voltage to out with a 2,9-3,1 volt bias is interesting too. ECdesigns uses a rather low 680 ohms (low for one chip); the current though is not too high because of the bias of 3V.
I had been thinking myself that the output - that tries to dump a certain current (specs: -0,6 - +5 mA) could also be connected to a current source, the rest then being high impedance for the AC current; and the Aout resistor then being adjustable for level + distortion.
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Old 8th September 2013, 09:43 AM   #18
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Quote:
Originally Posted by DF96 View Post
I'm still thinking about that. Some form of diplexer, perhaps?

For high-rate streams the filter can be quite simple.
I like the filter that Philips implemented in all their players.
The stock implementation is bad bad with the opamps.

But I have used the following filter electronics in my TDA1541 setting:

TDA1541 “I-V filter”.jpg

here is the basis of my filter, it is 40 kHz LP; at 20 kHz it is less than 0,2 dB down from flat:

cd160-5png-small.PNG

It is like the filter in the later CD950 - note slightly other values:

cd950-output-stage.gif

The buffers are based on the article by Jean Hiraga on the Kaneda electronic 3-way filter, as published in L'Audiophile (version 1985). All passive.
The result is a very flat response and while I can't measure the phase, it is claimed to have nice characteristic.
The filter does not have a NOS shelving, because the SAA7220 seems to have (is said to have) the shelving built into its digital filter.

Anyway the combo is rather natural and though not in far like my vinyl in depth it is quite good (not being sarcastic). I refurbished the components from a CD160; the unit I found on the flea market had a TDA1541 S1

Yes, a higher LP point could be used. Though Philips says one goal is to get rid of the 44.1 residues that would still appear with oversampling and a gentle slope - even though Accuphase seems to have made quite a nice sounding implementation of a 88KHz 6 dB LP filter.
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Old 8th September 2013, 12:14 PM   #19
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Its interesting to read here that ecdesigns has found the same as I have - that TDA1543s sound best with nothing connected to pin7. I have just posted on my blog a mod to a cheap 1543 SDcard player where I created a 3.75V reference from a TL431, filtered it passively and then used this quiet voltage source as the AC ground for the I/V resistors. It obviates the need for the internal bias current sources. The soundstage improves quite a bit for such a cheap and simple mod.

I also have some passive filters on the blog - I started out with elliptical designs but now reckon the easiest kind to build is a high-order chebyshev. Thanks to ecdesigns pointing out a very handy passive filter design widget online
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Last edited by abraxalito; 8th September 2013 at 12:17 PM.
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Old 8th September 2013, 12:42 PM   #20
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abraxalito, what about when using active i/v such as ad844? can the pin referenced to anything besides the i/v resistor?

i've also found the chip to sound much better with pin 7 floating. it's even possible to connect vref to the center tap of the line transformer if you're using 1:1:1:1 or such. i still haven't been successful running two 1543 in such manner with differential output, however...
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