Passive Low Pass Filter for my DAC Chip

Hari Iyer

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This post is in continuation to the below post


After almost 2 years of tinkering with my Marantz Blueray player DAC board,, i finally am able to conclude on them. I have wasted 2 years of my audio life chasing better opamps, better capacitors and resistors from the stock which the player had. But as there was an inherent design flaw in the active LPF (conceptually), changing better components would have not mattered much. Infact, i have already received my modding componets last week, but something in the back of my mind prevented me from doing this replacement and i avoided.

In continuation of my search, i thought of a passive low pass filter design instead of an active filter and finally came up with a transformer based design. The schematic which i came-up for my DAC chip is as below,

1654831276329.jpeg

The +ve and -ve Iout from the DAC chip is fed to a blanced transformer and the secondary with the load termination is the output for the DAC. The resistor network for the primary and secondary side needs to be computed based on the DAC chip used and cannot be universal values. We need to check the Iout and the corresponding Vout from the DAC datasheet and compute the load impedance required for the DAC chip. The transformer primary becomes the load for the DAC chip and the resistor network shown in the schematic is required for impedance matching to the required load. For my DAC chip it was 820 ohms. The primary end of the transformer center tap is grounded for a current sink.
If not matched the DAC chip could damage and the I/V conversion would not be 100% accurate. The load resistor in the secondary is required for the required Vp-p. I have computed this value for a Vp-p of around 1v. This is how the final assembly looks.

1654831320167.jpeg


As this is a prototype - cheap chinese carbon resistors are used for testing. This may later get replaced by a thin film resistor. This is how they look when installed in the Blueray player.

1654831376183.jpeg


90% of the DAC board is now redundant and i have used only 10% of the board till the DAC chip.
Listening test revaled that they are way superior to active filters and very coherent and accurate. Lows and Highs were excellent from turning them on and mids required 4hr of break-in before they could sound great. The entire FR is now flat. The transformer that i have used is an NEC audio transformer salvaged from a telecom equipment (courtesy my elder brother). What i have realized from this experiment that even the best of op-amps, best of capacitors and resistors which are 100+ in numbers in an active filter can have its own sound signature and color the sound from the original recording. This was the first thing i noticed vis-a-vis active vs passive filter. Even the best R2R DAC will suffer from this problem as there are too many active devices in an R2R DAC giving a chance for coloration. In the passive LPF the SQ is limited by the quality of the audio transformer and the resistors used. I have used a transformer ratio of 1:1. Other ratios like 1:4 or 1:10 will also work equally good.

Any FM who dreams of owning an R2R DAC but did not pursue due to high cost, this is the best economical option and i strongly believe will sound even better than an R2R DAC (imo - as i have never heard an R2R DAC yet). Any one interested to know more can contact me with a PM.

Thanks for looking.
 
Further listening reveals that the passive LPF has excellent micro detail revelation that were masked previously by my active LPF. Also the passive LPF is more dynamic and fast compared to my active filter.
 
@Hari Iyer : this network also does the I/V function?
Yes, the impedance matching with the DAC chip is critical to preserve the I/V conversion, else there could be some error. I think the active filters are not able to manage this correctly and there is so much coloration in the sound. Even R2R DAC has to compensate for this error in the LSB and MSB areas.
 
Should be attempted, definitely. ( I don’t like to indulge in opamps, anyways)
I should have couple of more transformers and can make a pair for you to test. It's not for the faint hearted as it's not a plug and play device like any commercially available product. You need to study your CD Player schematic, identify DAC, compute the resistor network. Also isolate the current active filter from the DAC chip else that will also load the chip. Not for novice but easy for experienced DIYers.
 
After listening for a week, i get an impression that the passive LP filter is the way to go. I am not sure if any FM has ever attempted this or listen this before. It has an ability to sound almost 99.99% close to studio recording. Moreover the SQ is very organic like listening to a R2R master tape. I doubt if any R2R DAC can match this. Only an A/B can decide and conclude. Above all the cost of upgrade is low and outcome is phenomenal - like buying an state of the art product. This keeps me wondering why do companies waste so much of time, money, resources putting an active filter and colouring and ruining the sound stage? I think most of the designs are dictated by the marketing department to make them look fancy to increase cost of the product. If I can design this successfully, i am more than 100+% sure the manufacturer can do this too. Wondering.

Next project after this successful implementation is a passive RIAA phono stage which could be challenging.
 
Finally could trace some theoretical document to support my above DIY effort. Am happy that i am doing something correct. Check out the attachment.
 

Attachments

  • AN_912.pdf
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In my DAC I have added a resistive network to achieve this 2 years ago. I shall now remove this resistive network and add a termination resistor at the transformer secondary so that the reflected impedance in the primary of the transformer matches the DAC output impedance. This will have maximum power transfer and minimize any losses. Work for coming weekend :)-
 
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