Tda1545a vref capacitor

[ash_dac]

Hi,

TDA1545A is in my Philips cd723.

Any recommendations for the vref cap size or make?

Also the vref calc as attached picture:-

10000/(10000+15000) * 5V = 2V vref

I want to try the capacitor route before trying JohnW's external vref generator(adjusted) here :- http://www.diyaudio.com/forums/showt...396#post300396


Regards,


Ashley.

[eVITAERC]

Any low-ESR cap with >10uF and >6.3v should work well

Caps are caps, so as long as you use them there's no need to get super big or super designer ones. If you insist, OSCONs or Cerafine should work well

[jean-paul]

Originally posted by eVITAERC
Caps are caps, so as long as you use them there's no need to get super big or super designer ones. If you insist, OSCONs or Cerafine should work well

Oh no, not again.

In this case with 10 kOhm in series almost any cap will do. Panasonic FC ?

[eVITAERC]

I was going to say that, but then I know fully well that if he's asking this quesiton in the first place then he's bound to be wanting to use "designer" caps

That was basically just saying "OMG don't use BG or Silmic"

[ThorstenL]

Konnichiwa,

Originally posted by ash_dac
Any recommendations for the vref cap size or make?

The Vref input fundamentally drives both full scale current and offset current in the TDA1545, when using the original circuit it also becomes an input signal for the output stage, where it is at least passed through at unity gain.

You have de-factor around 7.5KOhm Resistance in the circuit to form a lowpass for supply noise.

You need a capacitor with VERY LOW ESR (as ESR limits the maximum attenuation) and good high frequency behaviour, which because of the crappy supply in this player also needs to be VERY LARGE in value to make sure you knock out at least 60db @ 100Hz, in other words the RC filter needs a turnover of 0.1Hz.

From that we get 220uF as minimum, I would say make it a Sanyo Os-Con, 6.3V while you are at it. When using traditional electrolytic capacitors something closer to 2,200uF/6.3V will likely be needed.

Finally, if you really want to get decent sound quality from the CD-723 consider re-building the entire powersupply as follows:

1) New well build regulated supply for analogue stage (+/-12...15V if using Op-Amp's).

2) Completely galvanically seperate supply for the DAC, given that the DAC has crappy PSRR on both Vref and Vdd pin you cannot do "too much" here.

3) Completely galvanically seperate supply for the Clock, the CD-723 definitly needs a seperate clock, Guido Tent's is afforable and works fine.

4) Leave the originally supply for the drive section in place, but beef it up.

5) Clean up the digital supply section (+5V) and use it to drive the transport logic only.

I found that LM6182 Op-Amp's make a superlative output stage, but whatch the peculiarities of that Op-Amp and give it well decoupled (RF!!!) supply and remove the reactive components from the feedback loop, 100 Ohm build out resistor in the output and a suitable filter capacitor to replace the filtering in the NFB loop complete this.

Sayonara

[eVITAERC]

Very informative post Kuei!

I have no idea that the PSRR of TDA1545 is THAT crappy

[ThorstenL]

Konnichiwa,

Originally posted by eVITAERC
Very informative post Kuei!

I have no idea that the PSRR of TDA1545 is THAT crappy

Well, the full scale current of the TDA1545 AND the bias (offset) current are both directly proportinal to Vref, with around 10db gain IIRC, so in effect -10db PSRR on Vref.

The PSRR on the supply line of the TDA1545 is listed as 30db.

In other words, unless both Vref and Vdd are squeaky clean, Vref to 18Bit levels and Vdd to 12bit levels the TDA1545, referenced to the output the chips performance will degrade, potentially very seriously.

Sayonara

[eVITAERC]

Jebus, 10db gain on Vref!? I know much less about TDA154x's than I thought

EDIT: Looked at the datasheets, and noticed the feedback cap and resistor values on the output.... Jebus

Is this typical for all DAC's, or just for the "continuous calibration" type DAC's?

[ThorstenL]

Konnichiwa,

Originally posted by eVITAERC
Is this typical for all DAC's, or just for the "continuous calibration" type DAC's?

Well, the only DAC's that APPEAR to have have really good PSRR are differential, you can work out how good they REALLY are, as of course the noise only becomes common mode and is not actually removed untill you mix the two polarities together!

Generally "fancy" capacitors (assuming that fancy means low ESR, low ESL and low Distortion) and carefully designed supplies around DAC's do give material improvements, no matter what they are.

Sayonara

[ash_dac]

Hi,

I was going to say that, but then I know fully well that if he's asking this quesiton in the first place then he's bound to be wanting to use "designer" caps

Apologies, I should have referred to a capacitor property and not 'make'!

Thank you for the information on the tda1545, and vref.




Regards,


Ashley