General Impedance Converter GIC Low Pass Filter DAC

This post is about understanding the General Impedance Converter or in short the GIC filter. This is the type of audio filter that has been forgotten in few decades.

So here I am, back again to the DAC topic and will only focusing on the specific analog section.

I thought I will fully stop to make any DAC project but in fact it is hard for me to do it. Especially like in this post, the GIC type of filter has ben around my head since the very first time I built my own DAC last year, so I think I will do it now.

But before I am going straight to this GIC filter topic, perhaps for clearer understanding about this, I should first introduce the various type of audio filter in the DAC analog section.

First things first, the analog filter section that will discuss in this post is the circuit that separate from the DAC IC itself. This commonly found on the vintage multibit DAC.

In contrast, the modern delta sigma DAC has include this analog filter inside the same IC die. The coming of this delta sigma DAC on early 90's is to integrate highly complex digital system using low cost CMOS IC technology. This low cost approach including to the DAC process that only involves maximum 4 bits with high interpolation rates (oversampling), including the noise shaper and simple analog filter in one IC die.

Back to the analog filter, the rule of thumb is the highest the order/slope of the filter, the complex components will be use. Take one example for the simplest first order low pass filter (LPF) can be use either with RC filter, means only one resistor and one capacitor in series with the audio signal, or only using a capacitor across the feedback resistor on the I to V converter. This simple first order LPF method can be found on the Sony CDP-750 here.

So why we want the highest order of LPF?

In fact you don't need that high order LPF if you already using the oversampling or the digital noise shaper before the DAC. This oversampling will remove large amount of digital quantization distortion that will appears on the output of the DAC. Theoretically the 4 times oversampling only required second order LPF and 8 times oversampling only required first order LPF to make the analog output clean, free from the digital glitch that naturally appears from the DAC. But when it implement on the real audio performance, it seems that the higher order number of LPF, the better the sound quality.

How about the non-oversampling DAC?

The oversampling DAC become very popular now. The LPF require for this non-oversampling DAC is the high order filter (above third order) to smoothing the audio frequency band. It is quite funny to see some of non-oversampling DAC design using only the first order LPF, or even doesn't use any filter at all. This high frequency noise will appear and effecting to the sound quality of the DAC.

What I will explain next in this post is the LPF that construct with the op-amps circuit. As you may already know that I always using op-amp because it is widely available, better on specification, easy to use and fail proof.

The very common third order LPF can be achieve by using a simple first order LPF and in series with second order multiple-feedback (MFB) architecture filter like shown on the AD1865 DAC schematic bellow.

The green circle is the first order LPF and the blue circle is the second order MFB LPF.

The detail schematic for the second order MFB filter is shown on the picture bellow.

This LPF circuit is commonly use if the cost and the space required for the components layout is limited. This LPF circuit also doesn't need high current and voltage to make it works, typically it only require dual rail 5 volt same as the voltage to the DAC IC. This circuit do the job very well and the sound quality result is very good.

The next circuit is in my opinion the better LPF design. The overall concept is same as above but on the second order circuit is now using unity gain Sallen-key LPF instead of MFB LPF. The detail schematic is shown on the picture bellow.

Although this unity gain Sallen-key LPF seems more simple than the MFB filter, but to implement correctly need extra attention on the parts use and overall design.

First, the op-amp require for this circuit is should be unity-gain stable. The oscillation will occurs if the op-amp chosen is not design for this task, resulting audible distortion and potential damaging the op-amp.

Second, this LPF circuit required at least dual rail 9 volt to the op-amp. Optimal voltage require is actually dual rail 15 volt. This means a new pair of voltage regulator is require only to supply to this circuit op-amp.

I use this LPF circuit on every optimized DAC I made, like in the PCM63, AD1862 and AD1865 Ultimate DAC. The sound quality is more superior, means more dynamic and smooth on the audio band.

Then when I want more improvement, then I should look at to this GIC LPF design.

Hope you don't mind to look first the schematic on the picture bellow.

Now I know you will start to scratch your head.

This GIC LPF circuit like on the schematic above is the third order filter. This is far complicated than the previous LPF circuit because there is additional op-amps on it. In fact, if you take a look closely to the schematic, it is the same circuit as the unity gain Sallen-key design above but replace the R-C-R-C network to this op-amps circuit.

PCM1702 and PCM63 on their datasheet has recommend to use this GIC LPF. I share the complete schematic on the picture bellow.

This GIC LPF can be found on high-end CD player on early years of the digital audio golden era. What I heard that Accuphase CD player also using this GIC LPF on their non-oversampling design as well.

More interesting is the Nakamichi 1000-P which using 3 stages of GIC filter at the analog to digital conversion section, creating above 9th order LPF, while on the DAC analog section they start to using a discrete transistor circuit.

The photo bellow is the Nakamichi 1000-P internal DAC components.

Later on, the popularity of this GIC LPF are vanished, replaced with more simple LPF circuit like what I previously explain.

Personally this what really makes me curious why this GIC LPF is now forgotten? Is it because of the audio performance? I don't think so.

Or because the manufacture now aiming for simple and economic design of the DAC and LPF considering to the oversampling digital circuit that already takes the place. This is I think make more sense.

The only answer for this GIC filter performance is by build it and test it by my own.

As you can see from the cover photo of this post, I have going over there and I will saved it for the next few post.

I will share my new DAC build that using this GIC LPF filter. This new DAC will be compare to the same DAC that I have previously built. All of it will cover on the next post. Please stay tuned:)

Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.