TDA1541 , passive i/v , AD797 for gain ; worth trying??

homemade

Member
2004-07-22 3:42 am
us
Time for me to get into the passive i/v game. Finally getting up the courage. Don't wish to do the tube output thing yet. I have on hand a fist full of AD797 ultra low noise op amps that I would like to use for necessary gain. So I have a few questions. First , is it really practical? I would like to know how much gain is required if I use the commonly suggested 30 ohm resistor value at i out. Should the AD797 be used inverting or non inverting , since it is only being used as a gain stage?? Should the i/v resistor be bypassed at all. , or any passive eq used? I have several Philips based players with TDA1541 that I would like to try this on , and use as much of the existing layout as possible. Can this be done on a Brown Dog dual single to dual op amp board , with , say another 797 or 627 as buffer on the other half of this board?
Thank-you for any suggestions.
 
Interesting idea.

My problem with the concept is that the 1541 needs a compliance voltage less than 25mV. SInce it has a peak of -4mA output , that means the I/V impedance has to be 0.025/0.004 = 6ohms or less for undistorted output. (This applies for active I-V input impedance as well, of course...). You might well get away with 10ohms for an I-V resistor.

So then you need roughly 60dB voltage gain (1000x) to get up to line level (well, 1.7V rms) and to do that with minimal SNR degradation I might look at using two 797s providing 30dB each.

However you slice it, careful design will be needed to realise a noise-floor low enough, and very careful execution; this much gain is equivalent to MC phono stage design, but with a much wider bandwidth of spurious noise on the input!
 
homemade, hi, instead of doing a passive I/V then having very low output and then bringing it back up (with the noise) again with the AD797.
Maybe this should be looked at for the I/V and Gain stage. An AD844 transimpedance opamp, input impedance close to 0ohms just like your passive I/V resistor, yet with gain. A perfect load for the current output TDA1541 to see.

Pedja's design for the use of the AD844 with the TDA1541
http://www.pedjarogic.com/1541a/rev1.htm

Cheers George
 

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homemade

Member
2004-07-22 3:42 am
us
. An AD844 transimpedance opamp, input impedance close to 0ohms just like your passive I/V resistor, yet with gain. A perfect load for the current output TDA1541 to see.

Pedja's design for the use of the AD844 with the TDA1541
http://www.pedjarogic.com/1541a/rev1.htm

Cheers George [/B][/QUOTE]


Thank you all for your replies
I have looked at the AD844 while coming to terms of going this route. It has an input imped of 50 ohms , so I would have thought this to be higher than optimum. Also , I assume this op amp will also need fairly high gain to achieve the 1.7v. The 844 is very fast(2000 slew rate) . Just for fun I tried to use an AD811 in my player. I removed it yesterday and I think it is STILL oscillating.;)
Regarding the i/v resitor , while I do not dispute the values dicussed , I have read here and elsewhere of around 30 ohms to be acceptable , so perhaps less gain with the 797 might be used.
The 797 just appears to be a little bit easier to use for a beginner:) . But keep the suggestion coming. I'm ready
 
Sorry,
but I think that lower value of 22ohm could damage the DAC chip...
So ...
1. You can start with 100 ohm, for the Rl and lowering the value,
2. in the same time You ned to increase the gain of OP amp 797
3. Put the OP in inverting mode (passive R-IV onverts the signal...)
4. Not to forget to apply current compensation (of app. 2mA) for TDA1541A... Again You can find it on PEDJA site...It is simply done by JFET...
5. And You can try to put the DAC in NON-oversampling mode...
Usualy by overriding the chip before the dac SAA7020 one...
Cheers