tda1541a sound signature

[gaetan8888]

Hello Thorsten

The OPA637 need a minimum gain of 5 but better with a gain of 10 to work good.

How can we use it in a IV amp ?

Thank

Bye

Gaetan

 

[ThorstenL]

Hi,

The OPA637 need a minimum gain of 5 but better with a gain of 10 to work good.

That is not entirely correct. The OPA637 needs a NOISEGAIN of 5 (this is not neccesarily the same as the actual gain) and only at high freqiuencies.

How do you manage to use it in IV amp ?

The usual way?

For a typical TDA1541 CD-Player from Philips/Marantz remove the 15-33pF compensation capacitor between pin 5 & 8 and connect 10nF (low inductance) from the inverting input to ground. The OPA637 will then work without any issues (as the noisegain is 5) and any additional problems caused by the very fast current steps from the DAC are mitigated.

While I derived this principle by my little old self, it was described before by Scott Wurcer in the Application notes for the AD797 somewhere, it was discussed several times here at DIYA, which is how I became of Scott's work on this.

Ciao T

 

[abraxalito]

How can we use it in a IV amp ?

Use what I call a 'gain stealing' network. A series RC to ground from - input raises the loop gain only at high frequencies whilst keeping it at unity for low DC offset. I see T suggests 10nF but does not mention any value for R. You'll want to watch the settling behaviour though on your scope - its easy to get the R and C values wrong.

 

[gaetan8888]

Hello Thorsten and abraxalito

Maby a simulation with LtSpice or Tina-Ti

Btw, is that higher value of cap for compensation will reduce the slew-rate of the 637 ?

You are both living in asia, I think, what time is it there ?

Here it's 3:30 am, I have to go to sleep.

Thank

Bye

Gaetan

 

[abraxalito]

3.30pm where I am.

You can try that LTSpice sim but in my experience the models aren't that close to reality, particularly in terms of dynamics. If you want good results you'll end up developing your own models and that takes longer than building and testing.

Nice job on the CD-77 btw Thorsten. I just wonder about one thing - the little labels on all but one of the ICs (not the TDA1541) in the Six Moons pic. Their purpose is?

 

[gaetan8888]

Hello

And about the slew-rate of the 637 with the higher value of that compensation cap ?

Thank

Bye

Gaetan

 

[abraxalito]

The comp cap Thorsten is talking about is the one needed to make a 5534 stable in unity gain. OPA637 has no such facility to compensate it for unity gain. With the external cap he's suggesting you don't lose any of the 637's better slew rate.

 

[gaetan8888]

Hello abraxalito and Thorsten

Since, in the Thorsten's example for those cd players, the opamp are use in inverting input, that 10nF cap from the inverting input to ground are just an input filtering to filter rf frequency ?

Thank

Bye

Gaetan

 

[abraxalito]

Perhaps its best for Thorsten to answer that one. Myself I shy away from putting naked capacitors on the output of DACs - that's why you see my suggestion has a series resistor too. With the series R, the HF noise gain is clearly defined - so the R can be chosen to be one-quarter the value of the feedback resistor. Then the idea is not RF filtering rather opamp stability.

 

[Marcelinho]

Here you can find a schema for a ad811 i/v. Resister R2 is the key here. (All in dutch though). All inspired on Walt Jung's work.

Toon onderwerp - Philips DAC-960 modificatie • forum.zelfbouwaudio.nl

 

[ThorstenL]

Hi,

Since, in the Thorsten's example for those cd players, the opamp are use in inverting input, that 10nF cap from the inverting input to ground are just an input filtering to filter rf frequency ?

No, it is not. Simply build as instructed and it will work.

If not read up on noisegain, lag-lead compensation and all the related topics. It has been repeatedly discussed at DIYA.

Ciao T

 

[ThorstenL]

Hi,

And about the slew-rate of the 637 with the higher value of that compensation cap ?

I am suggesting to REMOVE the compensation cap. The schematic I suggest does not affect slew rate as such., it primarily assures stability and secondly reduces the problems if the current step from the DAC is so fast that Op-Amp's feedback loop "unlocks".

Ciao T

 

[new2hifi]

Hi,



FWIW, the AMR CD-77 uses special PECL based reclocker/drivers for the TDA1541 which among others also perform attenuation....
....So in many areas it is a tour de force of what can be done.

The sound will not please everyone, nor is it meant to, it is a Player that is firmly "Digital to Analog" in sonic nature. Those who actually LIKE digital sound will be disappointed.

Ciao T

Thorsten, thanks for putting light on CD-77. I tired to build a case on the superiority of CD-77 but no one except you could do that on technical grounds. However, I have since audition many other DACs/Players and CD-77 is in fact closer to vinyl.

 

[ThorstenL]

Hi,

I have since audition many other DACs/Players and CD-77 is in fact closer to vinyl.

The CD-77 was designed and realised by people who like vinyl, single ended triodes and high efficiency speakers. It should not come as surprise that it's sound tends into this direction. Hence also my suggesting "it's sound will not please everyone".

Ciao T

 

[Marcelinho]

I backtracked my steps, and tried to remember when things went wrong. Dem reclocking with a cap, but also the direct clocking of the dac by clock/2 was probably the moment when the sound got worse.

So i restored the connection between the Sa7220 and the dac and shorten the clockline to the 7310 as much as possible by drilling through the pcb, and there was a clear improvement (not perfect yet obviously)

As everybody here is only using NOS, i haven't seen solutions how to sync the divider from the direct clocking of the tda1541a with the now unused output of the saa7220.
So does it have to be in sync? (Data and WS still coming from the digital filter), or should i try the inverse output of the divider?