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Old 16th January 2007, 05:39 AM   #11
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phn

Passive I/V with a resistor, in theory, is not a proper way to convert current to voltage. In fact this works with most DAC chip. However, in most cases, except for tda1543 and may be tda1545, you will not get a voltage large enough to drive an amp.

Loading the DAC chip with a resistor to ground will cause linearity error because these DACs are designed for output to be connected to virtual ground. If the resistor is small enough linearity error may be acceptable and the DAC may even sound good subjectively. If you increase the passive i/v resistor to certain value depending on DAC chip, it will cause heavy distortion and may not even work.

Practice show that tda1543 work with large i/v resistor and most other chips don't. That is because the tda1543 is internally different from other chips. This allows it to tolerate large i/v resistor. This is unusual. The designers of tda1543 or may be some one here would know why.

However it's still a good idea to keep the passive i/v resistor as small as possible. That's why people stack up 8x or 16x tda1543. By this way the i/v resistor can be reduced by 8 or 16 times and still give the same output voltage.

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Old 8th January 2009, 06:05 PM   #12
Gabdx1 is offline Gabdx1  Canada
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Hi all, how about making a discrete kind of op amp ???

TDA1541 --- 5uf cap --- 2K --- (filters // with ground : 100pf cap + 100K) ---- (+) of discrete op amp)


TDA1541 GROUND ---- (-) of discret op amp
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Old 8th January 2009, 09:54 PM   #13
jonners is offline jonners  United Kingdom
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For what it's worth, my TDA1543 dac -with 8V supply and 2.7K I/V res - gives a measured 1.9V RMS out with a 0dB sinewave input.

John
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Old 9th January 2009, 09:30 PM   #14
Gabdx1 is offline Gabdx1  Canada
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The point is not using the 2k like a i/v resistor...
It is to buffer the i/v discrete op_amp

The (+) is 2x BC546C in pp
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Old 10th January 2009, 03:21 PM   #15
oshifis is offline oshifis  Hungary
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It must be DC coupled. The output of the TDA1541 is sinking 2mA at zero analog output. This current varies between 0 and 4mA from negative to positive analog output peak. The other restriction is that the voltage at the current output pin must not go beyond 25mV. Therefore a passive I/V resistor of as low as 6 ohms or so should be used. In order to get full 2V output, about 160x voltage gain will be necessary (assuming 6 ohms I/V resistor), and noise problems may arise. The virtual GND opamp I/V stage is much simpler.
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Old 10th January 2009, 04:32 PM   #16
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Hi Oshifis,

To check I have understood this right: About 1541, 25/2=12,5ohm is the highest allowable resistor? Output with 6 ohm will be 6*2=12mV?

A parentesis: About the CD 2V out std I find this unnecessary as not even a poweramp needs this much. Wouldn´t 0dBu=0,775V be enough? Or why not the semiprostd 300mV/10k? If we go for 6ohm load and 0,5V out we then only need 40x gain.
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Old 10th January 2009, 08:04 PM   #17
Gabdx1 is offline Gabdx1  Canada
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The higher the i/v resistor value the higher the output.

I have checked and most people say around 35R is what gives the best sound. The tda1541 should have a minimum output power and the minimum resistor value should be around 15R but I need to confirm that!

What I propose is not that thing at all, it is to replace the opamp by a discrete ( non chip component) stage.

What my question is : what resistor value should I use to buffer the transistor in a push-pull class A stage.

I will use a capacitor and filters at the output of the tda to filter noise and DC so it is not polluting the discrete amplifier.

Thank you!
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Old 11th January 2009, 10:58 AM   #18
OzMikeH is offline OzMikeH  Australia
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Unless you really want to reinvent the wheel..
Look at Rudolf Bourtjes's (spelling?) SSIV gain stage.
As used in the USB Monica DAC.
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Old 11th January 2009, 08:18 PM   #19
Gabdx1 is offline Gabdx1  Canada
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HI ozmikeh ! I looked at it and I don't think ill use that. I prefer tweek my own i/v and set proper resistance and filters values.

Also I wonder why they use the tda1545A ??????


The Rudolf i/v comes with a filter and noise canceller and DC blocking cap at the end of the i/v, I wonder why he chose to put it at the end and not at the beginning , any idea ???
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Old 12th January 2009, 12:03 AM   #20
OzMikeH is offline OzMikeH  Australia
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they use TDA1545A to avoid putting the signal through SPDIF which has some issues.

I think it might be because putting it at the end means you're blocking DC on a bigger signal. Thus the capacitor has less effect on the micro-dynamics?

I'm sure Rudolf is/was a member on here, you may be able to find him and ask him.
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