Voltage-out DAC without an output filter

[mudihan]

As a purist approach, one can connect the outputs of DACs with voltage out (such as TI's PCM1753) directly to the next stage (a preamp, a volume control, or a power amp). I was reading this article:

focus.ti.com/lit/an/sbaa055/sbaa055.pdf

and it talked adding an output filter (P6). Two of them are active, so I am not considering them. But the first one, the 1st order passive filter, involves only a resistor in series and a cap parallel. Is such a passive filter necessary? What are the advantages and disadvantages of using/not using such filter?

Since I am using a 10k stepped attenuator, I do not want the source output impedance to be to high (may change the attenuation curve).

Thanks!

[janneman]

Red Book CD performance specs include the filtering to obtain the specified signalk to noise ratio. How much you have to filter depends on the sampling rate of your DAC. With a high sampling rate, the sampling noise is at a higher frequency so you can maybe get away with a simple RC filter.

Otherwise you need more steep filtering, either active or poassive L-C sections. You probably should research this a bit more.

Jan Didden

[ttan98]

I use a single low pass filter after the I/V amp., and then a buffer, it seems to work very well.

I think if you pick the correct I/V amp and buffer combo, it will sound very good.

I think the THS4031 used as I/V is ideal and LT1010 as buffer. Others think 4031 and buf such as BUF634 or OPA627(expensive) or AD825 are also applicable.

2 poles low pass filter need to use accurate and or non preferred values of capacitors, they are sometimes hard to get or matched esp. those that come from discrete comp not so much as from SMD cap. Jan may shed some light on this.

[Extreme_Boky]

Originally posted by mudihan
As a purist approach, one can connect the outputs of DACs with voltage out (such as TI's PCM1753) directly to the next stage (a preamp, a volume control, or a power amp). I was reading this article:

focus.ti.com/lit/an/sbaa055/sbaa055.pdf

and it talked adding an output filter (P6). Two of them are active, so I am not considering them. But the first one, the 1st order passive filter, involves only a resistor in series and a cap parallel. Is such a passive filter necessary? What are the advantages and disadvantages of using/not using such filter?

Since I am using a 10k stepped attenuator, I do not want the source output impedance to be to high (may change the attenuation curve).

Thanks!

I don't us any filtering at the DAC output, in particular if it is current out. Later on yes, but not at the I out.

In your case (V out) I suppose you can always check the amount and spectrum of digital rubbish you are getting, and act accordingly. Your amplification may be more sensitive to this rubbish, and could cause some overheating (in extreme cases) - but this can be tested as well: load the output with capacitive load (worst case) and check the amp for stability - see what comes thru to the speaker's end and how the amplifier handles this with difficult loads (don’t forget that the speaker cables if capacitive, will make things even worse!)...

(Scared enough?)

Good luck,
Boky

[hifimaker]

A couple of factors to consider...

1) Delta Sigma DAC's create a lot of high frequency noise that your tweeters must deal with, and a resulting loss of the contextual dynamics due to the the noise.

2) No active filtering will result in drive problems with the preamp. The DAC to preamp connection becomes VERY dependant on cables. With the right active filter design, much less exotic cabling is needed.

Just my own experience.

-David