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Old 8th September 2013, 12:07 PM   #21
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No experience of using 1543 with active I/V, no. I'm firmly converted to the passive I/V camp now though I do sometimes use a common-base transistor if I'm after a tad more output compliance...
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Old 8th September 2013, 04:07 PM   #22
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Does anyone have any measurements of improvements when Vref is left o/c? If the internal current mirror has a flaw then this ought to show up in noise or distortion tests.
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Old 9th September 2013, 12:33 PM   #23
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Quote:
Originally Posted by DF96 View Post
Does anyone have any measurements of improvements when Vref is left o/c? If the internal current mirror has a flaw then this ought to show up in noise or distortion tests.
I tested awith Vref open with a Vdd of 7 volts,
  • with the chip I tried there was no output, that one chip did not fire up,
  • and that chip died, because when I connected to a resistor it did not come above 0,9 volt on the pin.
  • . . could be the chip was dead on arrival, but should I try a second time with another chip?

Could well be that at Vdd of 5V this does work . .
But I tested and there was no output; voltage on pin 7 was 2,25. Tried various output values (pot).
So I'm puzzled at what the mod of leaving it open really is
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Last edited by triode_al; 9th September 2013 at 12:44 PM. Reason: added test
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Old 9th September 2013, 12:52 PM   #24
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If you leave Vref o/c then you must make proper arrangements for Vout such as a potential divider - you can't just put a resistor to ground as you have no balancing current from the internal source. TDA1543 normal output current is a data-controlled sink, not a source. That is why we normally need to use the internal current source to balance it.
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Old 10th September 2013, 12:36 PM   #25
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Quote:
Originally Posted by abraxalito View Post
No experience of using 1543 with active I/V, no. I'm firmly converted to the passive I/V camp now though I do sometimes use a common-base transistor if I'm after a tad more output compliance...
abraxalito, saw your anti-imaging filter. Nice!
What is the approximate output impedance of the TDA1543, dynamically? This is for calculating a filter that would start with R/C then L/C to obtain an 18 dB/oct LPF that is simple;
for instance.
I like you latest filter, looks a bit complicated even though I have an L-meter.
  • [internal resistance+external resistance, aimed at totaling 120 ohms]/27nF = approx 46 kHz
  • 1mH/12nF = approx 45 kHz

So my question is: what series output resistor to add to aim at e.g. 120 ohms?
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Old 10th September 2013, 12:49 PM   #26
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Quote:
Originally Posted by triode_al View Post
abraxalito, saw your anti-imaging filter. Nice!
Thanks, nice to know someone is appreciating my blog

Quote:
What is the approximate output impedance of the TDA1543, dynamically?
I reckon its effectively the I/V resistor's value as its presumably an open-collector whose output impedance will be an order of magnitude or more higher than the I/V resistor.

Quote:
I like you latest filter, looks a bit complicated even though I have an L-meter.
The one I've been playing with most recently is a single inductor value filter, so no L meter is called for as it can work with not-so-very-precise values available off the shelf. The most recent incarnation of this uses Coilcraft 3.3mH SMT coils about 6mm square, 8 per channel. If you'd like the schematic I'll put it up on the blog. Its load impedance is 270R but if you want to work into a different impedance, just scale the inductor values linearly with the chosen load resistance. Say to work into 120R the inductors would become 12/27 * 3.3mH = ~1.5mH. If you don't feel you need the steep slope offered by 8 inductors you can always shorten the length as its using repeated stages which are identical.
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Old 10th September 2013, 01:03 PM   #27
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Default TDA1543 USB dongle!

A dongle being a little interface with cords, that is what I envision now:
one cable to the power supply and output cinch [CAT6 cable], one cable for USB, as short as possible.
Very compact is possible.

This is how it can look with an XMOS I2S card:

in the little box,

Bart aug 2013 TDA1543 dongle L1020218.jpg

where one side will have a little USB cord dangling (dongling?) out - so this little box goes close to the PC;

A closeup of the card/card interface, with possible output capacitors to the right:

Bart aug 2013 TDA1543 dongle L1020213.jpg

Resistors not attached in this concept.
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Old 10th September 2013, 01:56 PM   #28
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Quote:
Originally Posted by abraxalito View Post
I reckon its effectively the I/V resistor's value as its presumably an open-collector whose output impedance will be an order of magnitude or more higher than the I/V resistor.
I have 4 chips now parallel each with 1700 Rout; so it is effectively 425 ohms.
Then I tap each chip with a 200 ohms resistor to a common point with a capacitor of about 6,8 nF. This gives 49 kHz and is intended to be the first pole. I use this summing resistor to keep the outputs somewhat independent in DC level. The four chips differ several tens of mV on the output.
The problem is I can't measure in anyway what the LPF frequency is . . .

Quote:
The one I've been playing with most recently is a single inductor value filter, so no L meter is called for as it can work with not-so-very-precise values available off the shelf. The most recent incarnation of this uses Coilcraft 3.3mH SMT coils about 6mm square, 8 per channel. If you'd like the schematic I'll put it up on the blog. Its load impedance is 270R but if you want to work into a different impedance, just scale the inductor values linearly with the chosen load resistance. Say to work into 120R the inductors would become 12/27 * 3.3mH = ~1.5mH. If you don't feel you need the steep slope offered by 8 inductors you can always shorten the length as its using repeated stages which are identical.
For me the 8 steps steepness is a bit overdone but it depends on using NOS or (some) OS - and on pre/amplifier and speakers.
I want to go at least 2-times Interpolation and Oversampling in the PC, so my case is more like the one on the right than on the left:

Attachment 370420

The diagram: The shapes on the right are the images we want to reduce.
For me I think and LFF or 45 kHz of 18 dB in total is rather good as the images would be at 160 kHz if I understand it correctly. And I have some stock 1 mH chokes at hand
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Old 10th September 2013, 10:10 PM   #29
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You're right that going for 2X OS you don't need nearly so steep a filter as for NOS. I got an 'invalid attachment' message so can't see your piccy but with 2X OS your images will start around 2*fs-20k = 68kHz. At a guess a 2 inductor filter should be sufficient. I could have a stab at designing it for you if you tell me what the DCR of your 1mH inductors is.
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Old 11th September 2013, 09:16 AM   #30
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Originally Posted by abraxalito View Post
You're right that going for 2X OS you don't need nearly so steep a filter as for NOS. With 2X OS your images will start around 2*fs-20k = 68kHz. At a guess a 2 inductor filter should be sufficient.
I have tried by trial and error but just 1mH/15nF has too low a Fc, even with 10 nF it is (sweep). The attenuation at 20 kHz is already 1 dB, this is not a wanted outcome.

Quote:
I could have a stab at designing it for you if you tell me what the DCR of your 1mH inductors is.
Great. My 1 mH chokes have a DCR of 12 ohm. I also have 27 mH with a DCR of 170 ohm. The output impedance after the DACs is 507 ohm.

Picture:
Nyquist output and oversampling filters kopie.jpg
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