About a discrete I/V preamp for a no-os TDA1541 Dac - diyAudio
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Old 10th September 2007, 09:59 PM   #1
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Default About a discrete I/V preamp for a no-os TDA1541 Dac

Hello

I want to made a discrete I/V preamp for a no-os TDA1541 Dac, here is a circuit. The 51R resistor are for the I/V conversion.

Do it need a nfb ?

What is the formula to calculate the value of L1 L2 C1 for a bessel 20khz low pass filter ?

Thank

Gaetan
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File Type: jpg i-v fet preamp-with l.jpg (22.5 KB, 861 views)
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Old 10th September 2007, 10:37 PM   #2
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Hello

For the bessel filter, I was mean a second order type.

Thank

Gaetan
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Old 11th September 2007, 01:15 PM   #3
oshifis is offline oshifis  Hungary
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Hi Gaetan,

Just a side note: the 51R will be too much for the TDA1541A, because the full output current of 4 mA will drop 200 mV on it, while the data sheet permits 25 mV only. The result is distortion. Also the idle current is 2 mA that is equivalent to 0 V analog output. This is likely to create a click when the CD starts or stops playing.

Laszlo
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Old 11th September 2007, 02:49 PM   #4
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what about HF imgaes coming out of NOS dac arent those shouting in some way (Into the I/V) (...)
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Old 11th September 2007, 06:03 PM   #5
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Hello

If 51R are too much, which value you suggest ?

For the HF from the 44khz, I can filter much.

My circuit seem not very good.

I was think to use op-amps like THS4032, LM6172 or AD625, but op-amp may not sound as good as a good discrete circuit.

But with op-amp I would use with a 3rd order modified Bessel filter as illustrated in the following image.

Thank

Gaetan
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File Type: jpg 19khz low pass 3nd order filter-6-b.jpg (77.9 KB, 631 views)
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Old 12th September 2007, 09:36 PM   #6
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Hi,
I just use an LM6172 as recommended in the TDA1541A datasheet, followed by a 3rd order passive filter, but for NOS, I don't bother with a filter.
I also want to go discrete, but I am actually quite happy with opamp IV for now. I've tried various opamps, and I've returned to the LM6172. Others are also good (I also like the LT1361), but I prefer the '6172 personally.

Opamp IV is a good stop gap on the way to going discrete, and shouldn't provide an awkward load - like the humble resistor.

Cheers,
Phil
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Old 12th September 2007, 10:28 PM   #7
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If you want to use a resistor for I/V with the TDA1541, you must keep the compliance voltage below 25mV as noted. So the biggest resistor you can use is just 0.025/0.004mA = 6ohms.

Which isn't very big.

The next problem is simply that your maximum audio signal output is 24mV peak-peak, so you need a gain of 100x to bring this up to line level. Doing this and preserving a decent signal-to-noise ratio will be fun...

I've stayed with more traditional active circuits.
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Old 13th September 2007, 01:54 PM   #8
oshifis is offline oshifis  Hungary
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What about eliminating the I/V resistor and feeding the DAC current output into the source of the FET (or emmitter of the transistor)? It needs proper biasing, though.
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Old 13th September 2007, 05:13 PM   #9
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Hello

Since the discrete fet circuit need to much mods to work good, I will use an op-amps. But I don't know for now if I will use passive or active I/V conversion, some say that passive sound better.

Thank

Gaetan
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Old 14th September 2007, 12:38 PM   #10
oshifis is offline oshifis  Hungary
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Active opamp I/V can be very good. I used an OPA627 with 1.5 kohm feedback and an 1 uF PP output capacitor, it sounded very good. If you want a "better" I/V stage, consider a transformer/tube circuit.
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