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Old 4th March 2004, 12:07 PM   #1
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Join Date: May 2002
Location: UK
Default TDA1543 IV conversion help!

Hi Folks
I have been reading all kinds of books etc on matters electronic,
Trying my best to learn this complex subject.
But the more I read the more I realize that I have to learn.

Anyway the thing that has me somewhat confused at the moment is the output of the TDA 1543 DAC!

I am intending to put an OP IV converter on this.
But I hate not quite understanding what is actually going on here.

For example on the data sheet, the op configuration.
[This is a trans-resistance arrangement eh?]
Shows a 1.2K feedback resister with a parallel 3.3nf cap.
Voltage ref appears to go to the non inverting OP input?

I am unsure as to what is going on here.
What is the V output of the data sheet implementation?
What sort of gain is this arrangement providing?
How do I calculate it?
Is the cap for stability/HF roll off, Both?
And the vref setting?


I have obtained some AD 8066 op samples to use for the above
and would like to know what I was doing [vaguely]


I would greatly appreciate some help here to put my head straight on the above matters.


Pretty please...



Setmenu
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Old 5th March 2004, 08:09 AM   #2
mikelm is offline mikelm  England
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Default Re: TDA1543 IV conversion help!

Quote:
Originally posted by setmenu
I am intending to put an OP IV converter on this.
But I hate not quite understanding what is actually going on here.

For example on the data sheet, the op configuration.
[This is a trans-resistance arrangement eh?]
Shows a 1.2K feedback resister with a parallel 3.3nf cap.
Voltage ref appears to go to the non inverting OP input?

I am unsure as to what is going on here.
What is the V output of the data sheet implementation?
What sort of gain is this arrangement providing?
How do I calculate it?
Is the cap for stability/HF roll off, Both?
And the vref setting?


I have obtained some AD 8066 op samples to use for the above
and would like to know what I was doing
this is a fairly standard inverting op amp arrangment where the non negating i/p is held at a set voltage ( Vref ) The signal comes into negating ip in the form of current. The op amp o/p will move to keep the two i/p's at same voltage. The 'gain' is not really gain because this is I/V coversion But the o/p can be calculated by using ohms law. The DAC o/p current swing x the resistance of Rf = Voltage o/p swing.

cap is for "both"

Vref from op amp point of view could be any value some where mid point between the rails but the DAC needs it to be 2/3 of it's rail voltage = about 3.3. The DAC needs to "see" this voltage on it's o/p for best dynamic range performance. It makes a big difference to the sound if this is not set correctly.

I don't know AD8066, but I do know if a separate IC is used for each channel each with it's own Very quiet supply and star earthing from DAC it makes a BIG difference.

hope this helps

mike
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Old 8th March 2004, 12:11 PM   #3
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Location: UK
Hi Mike
Thanks for the info.
Sorry I have not replied earlier, as I resisted doing so to prevent
myself bombarding you with further questions over a whole range
of related subjects.
As I am new to electronics, reading about the subject is both a fascinating and confusing activity!

I had considered using a dual OP device for the IV conversion[for
compactness as this is for portable use] but in light of your
comments I will probably go for a pair of single devices.

My initial starting point for this project has been the circuits of
Doede and Fedde.
A further question I have [amoungst many] is regarding the 22 Ohm resisters in series with the three signal lines entering the
1543 in Fedde's schematic,.
What purpose do they serve? As I note their use is not universal
with this popular NOS design.

Cheers

Setmenu
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Old 8th March 2004, 06:21 PM   #4
mikelm is offline mikelm  England
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My guess is that they are there for damping.

If the values are too high they increase jitter - apparently.

good luck with your cct's

I just posted a I/V conversion cct that I have just built that beats any op amp that I tried. So if you are feeling adventurus this is another option.

here it is

"super pair" I/V new 12.7V version

mike
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Old 8th March 2004, 06:57 PM   #5
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Hi Mike
Thanks.
I am already in haze of uncertainty about various topics related to this project [A little knowledge is a bad thing etc.. ] so My feeling was to get this first one
done the best I can and see what happens!!
Once I have built one that actually works I can then set about
trying to improve it.
Hopefully I will have learned enough to better understand schematics such as yours.

I really have no Idea what to expect with this NOS economy DAC
stuff .
My primary aim with this at the moment is to use it with my
portable audio setup feeding it from a Sony Portable CD player.
I hope it sounds better than the stock player.
I will of course be facinated to listen to the dac with my Stax phones.

Finally another question..
I read on Doede's site that when there is no signal lock there will
be HF noise, due to lack of mute.
I have not read any comments here about this.
My understanding of mute is that it is applied in between music
tracks etc.
Any ideas of what I can expect here?
I do not like thought of a sudden attack of HF noise at the end of play

cheers

Setmenu
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Old 8th March 2004, 07:02 PM   #6
mikelm is offline mikelm  England
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I have not used mute on a cd player for at least 5 years

I have not noticed a problem

but I am talking about analogue mute, I wonder if that is what he meant.

mike.
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Old 8th March 2004, 07:12 PM   #7
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Oops !
It was not Doede but Fedde who says:

"Put the volume of your amplifier down if no digital source is connected to the DAC (or the source is not powered on), because the Nonoz III DAC makes HF noise in that case. The Shigaraki DAC has the same problem. Muting has to be added to the circuit to solve this problem, but this is in my opinion not worth the extra complexity and cost. "

[I hope Fedde does not not mind me quoting from his site]

The above was the 'muting' I was referring to.


Cheers

Setmenu
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Old 10th March 2004, 05:40 PM   #8
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Hmm,still not sure about something.

The V ref pin.
The data sheet shows this connected to the non inverting op
connection and grounded at the other end.
I presume this is the 1543 ground, not the OP v ground?

What is the function of the Vref ?
I recollect someone [Elso?] saying that they did not even connect
it to the OPs but grounded it through a small cap, and that the sound quality was better this way .

Cheers

Setmenu
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Old 10th March 2004, 11:21 PM   #9
mikelm is offline mikelm  England
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V ref is the voltage that the DAC o/p likes to look at for lowest distortion and Max dynamic range. On the 1545 it is 2/3 the dac rail voltage. probably the same on 1543

you can generate this by other means on the I/V cct, ( two resistors and a cap will do it ) It may be less prone to digital hash this way but to make this method reliable you will have to have regulated supplies so that drift in rail voltage does not change Vref with regards to the DAC supply. It is important to get this accuate.

I am thinking to get a test CD to check that mine optimised for lowest distortion

mike
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Old 10th March 2004, 11:33 PM   #10
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Quote:
Put the volume of your amplifier down if no digital source is connected to the DAC (or the source is not powered on), because the Nonoz III DAC makes HF noise in that case.
Im on experimenting with those DACs, some findings here:
TDA1541A/S1 Arrangements

There is too much noise with non optimal values for Rref in relation to the load of AOL an AOR. There was a lot of HF noise with the values i found in schematics around. I used pots for the Rref and load, played a sine from test CD, connected PC with FFT software, and tuned for best SNR and THD. All desrcibed in that thread.

My DAC will be balanced and i will compare passive I/V and D1 stage.
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